Author Topic: Re: FeelElec New Arrival FY-6900 Signal Generator  (Read 31674 times)

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Offline flannp

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #225 on: March 03, 2020, 07:43:50 pm »
I was using a high pass filter.

I just did it again with a 10k resistor and 100nf RC low pass filter.
 

Offline flannp

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #226 on: March 04, 2020, 02:24:28 am »
In one case the sweep is fast and another it is slow.

That is just the end of the previous sweep that wasn't fully captured. It was at the same speed
« Last Edit: March 04, 2020, 06:26:49 pm by flannp »
 

Offline Mouse69

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #227 on: March 13, 2020, 04:54:33 pm »
Hi Guys,

Well I'm going to start modding my fy6900, first was the earth mod, but what follows here trying to keep it cool.

948648-0
This is the spray painted black (stove paint) heat sink, I'll be adding a very small scrape of thermal compound under it

Next the lower grills in each side, I used a Stanley knife, gently pushed with my thumb (sharpe bit on the grill, safe back of knife against my thumb) to cut them away, to allow a little more air in

Then marked out some 5mm spaced holes in the top casing and drilled (note to self, next time idiot put your glasses on)

And added a fan at the back

Fan not connected yet, I'm doing the SDG PSU swap out, it will get wired in at this point to suck the air out the back (I used an MTB bike tube to make a gasket and yes I can see it's not perfect at the top, I will try and address that - to keep the vibration and thus noise down).  I'll get the thermal camera on it and see what it looks like once it's done
« Last Edit: March 13, 2020, 04:56:47 pm by Mouse69 »
Cut towards your chum, not towards your thumb
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #228 on: March 14, 2020, 08:59:45 am »
I've received mine... seem a decent improvement over the previous. Space for a fan, iec connector, ground attached to the gnd of the output at the psu.
The psu is missing ycap and primary side of psu totally floating. Edit: there is an ycap between negative after the rectifier bridge and ground. On the input there is a little isolation transformer.
The output is above +/-15.5v, with 16v rated caps. Seem ideal to place an LDO.
The XO is 10mhz, and is easy to find a TCXO with lower 1ppm.
As usual space for better opamps instead of the 3002 parts.
There is also the space and hole for putting a cooler above the LDO.
I think that with 2 LDO at the output the smps ripple can be totally eliminated. Remain the emi created by the smps.
In any case not seem as bad as the other psu that i have of the fy3200
« Last Edit: March 14, 2020, 09:19:18 pm by masterx81 »
 

Offline AlMuz

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #229 on: March 19, 2020, 05:58:48 pm »
Here is my mod of FY6900 with linear power supply having total TDP around 4W.

In terms of TDP to design fully linear power supply for this device (without any DC-to-DC switching converters, like done by SDG Electronics) it is hard to go with a single transformer that has single output.

The requirements for analog output circuitry is bipolar +/- 13.5V producing solid 250mA (that is if you connect outputs to a normal 50 Ohm terminated load).
If you want short-circuit both outputs - you need at least 0.5A. Both outputs have 50 Ohm in series, so in worst-case scenario when you short-circuit them to the ground at peak +/-12V output voltage the either of supply rails will be looking into the load of two parallel 50 Ohm resistors (so that is 25 Ohm). This results in 480 mA max current consumption (disregarding other losses in output circuitry of FY6900 to power OpAmps, LDOs etc).

So bipolar supply with max 0.5A on each rail is more than enough for normal use.
On eBay I found a perfectly fitting toroid transformer for $25 shipped, producing 26V RMS with center tap at 0.76A (20VA).
That means at max load will feed 13VAC RMS at 0.76A to power each rail.



In normal use of FY6900 (meaning both outputs conected to 50 Ohm terminated loads, so 250mA total) the output RMS of this transformer is around 18V (between center-tap wire and either of two "power rail" wires).

The LM317/LM337 module from eBay ($10 assembled and shipped) produces almost no-heat (radiators barely warm).



Most heat (around 0.5W) is dissipated by 1V drop on each diode in rectifying bridge, so that leaves 17V of DC with some ripple after capacitor to LM317/LM337.
The output voltage of LDOs should be configured at 13.5V, so that is perfect 2.5V voltage drop close to the spec of LM317/LM337.
So total TDP of LM317/LM337 module is at most 2W spread on three componnents (full-bridge rectifier and 2 LDOs with radiators).

I was hoping that the remaining power of this transformer will be enough to get additional +5V rail to power digital circuitry of FY6900.
Connecting another LDO to +13.5V rail produced a lot of heat. That is because digital part (FPGA + LCD) consumes around half an amp (530mA in my case).
(13.5V - 5V)x0.5A = 4.25W TDP reuquires an LDO module with larger radiator which barely fits inside remaining space of FY6900.

But the worst part was that digital side sucking 0.5A brings positive "rail" of transformer output down to its nominal voltage (of 13VAC RMS).
That brought the voltage at the input of LM317 to be too low (no overhead room for required drop to provide regulation).
So the positive output rail of 13.5V started sagging (at 50 Ohm load on both outputs) passing through some ripple left after full-bridge rectifier and smoothing capacitor.
The FY6900 could not output perfect sine wave with 24V peak-to-peak swing, positive half-wave was distorted. To be said - even at 16V peak-to-peak output the sine wave was distorted.

I've tried to replace an 5V LDO with a decent switching DC-to-DC module, hoping that instead of heat produced by LDO that voltage-drop energy will be converted into more current for 5V rail. The situation improved a lot, got perfect sine at 20V peak-to-peak, but 24V was still distorted.

Started thing about should I go and look for another transformer, more powerful one. Or the one that maybe has two outputs - one with center-tap to create bipolar +/-13.5V and another set of output wires to create for +5V rail. Another option was find one more tiny transformer for 5V specifically.

It was easier to find one more small transformer to power +5V rail specifically.
6VAC RMS 1A for $16 from eBay:



Also got EMI filter for $10. Did not do any comparison tests on how it improves the situation, but "if it fits - let it sits":



A cheap $2 LM317 module from eBay did not fit well with this smaller transformer:



For peace of mind I've put a requirement of 1A output on +5V rail (600mA for digital components of FY6900 and some 200...400mA leftover for cooling fan).

LM317  by its spec requires 3V drop to provide regulation. At 1A of load the transformer output RMS gets down to 6V, voltage drop after full bridge rectifier leaves almost nothing for a linear regulator to deal with:



  • Green and Blue - transformer AC-output lines referenced to the ground of LM317.
  • Pink - full-bridge rectifier with smoothing 1000uF capactor.
  • Yellow - LM317 output which is expected to be clean 5V at 1A load.
  • White - is a difference between Blue and Green computed by the scope (expected be the an actual AC sine wave of transformer output, but actually distorted at its peaks whenever full bridge rectifier turns on)

At 0.5A of load  (so nothing left for cooling fan) this module still produced considerable ripple on its output:



So, decided to make my-own LDO module:
  • In my parts bin I've found another 5V LDO with rated 0.8V voltage drop at 1A load.
  • Also decided to use more beefier smoothing capacitor on full-bridge rectifier (10000uF).

The situation became much better (at 1A load):



Both transformers with EMI-filter left no space to fit LDO modules. So LDO boards were mounted to the top surface (upside-down), hanging on top of main FY6900 board.
Also painted plastic case with MG Chemicals Nickel infused conductive coating to reduce EMI from outside.





Here is output before (square wave at 0.0000V amplitude, sorry did not make this one as a normal screenshot from the scope):


And after:

« Last Edit: March 19, 2020, 06:45:36 pm by AlMuz »
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #230 on: March 19, 2020, 11:54:10 pm »
Doh! Logic part consumes that much? I'm using a 2x15vac 20va transformer, and almost done a 317+337 + small 5v buck. So i'm out of luck and i need also a bigger transformer (or a second transformer) like you.
Sorry, but what is the purpose of the opamp in the 317/337 board?
« Last Edit: March 20, 2020, 12:18:01 am by masterx81 »
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #231 on: March 20, 2020, 06:01:52 am »
@AIMuz, if those last two 'scope images are referencing a before you replaced the original SMPSU board with an after you'd installed your replacement analogue PSU, there seems to be very little improvement. However, when you turn the level down to less than 501mV p-p (plus or minus 250mV on the "DC Waveform" setting), you need to keep in mind that the output from the generator's first stage opamp is routed via a 20dBish attenuator (possibly still with a Z0 of 85 ohms as per the 6600 and 6800 models) which will reduce any PSU noise and ripple to a tenth of the voltage you'd otherwise see for voltage settings greater than 5.001V p-p.

[EDIT 2020-03-31 Definitely the same 85 ohm attenuator! See https://youtu.be/kA6uxUDg55M?t=366 for proof!]

 You should be testing using the "DC Waveform" setting at voltage levels in excess of +/-250mV to prevent the attenuator being switched into the circuit (and above the +/- 2.500v setting if you want to examine the effect of the TH3002i +12db boost amplifier on the psu noise level). That avoids any 'leak-through' of whatever waveform you've set it to output (in this case, it appears to be a 1KHz squarewave) making it easier to identify the psu originated noise components that are polluting the signal generator's outputs.

 I presume you're using good quality RG58CU patch cables to link the output to the 'scope's input and have them properly terminated. One thing to watch out for when testing with the "DC Waveform" at the higher voltage levels is to avoid overheating the terminating resistors (6Vdc - half of the 12v is lost in the 50 ohm output resistors in this case -  represents some 720mW of dissipation in the termination resistance load which just might be more than what they're rated to handle for any extended period).

 Apropos of this, you miscalculated a little on the worst possible case scenario loading on the +/- 13.5v rails which comes not only from the need to supply an extra 25% to the 1st stage opamps which drive the final stage opamps along with some additional 'vampire loading' allowance (somewhere in the region of another 50mA on each rail at a best guestimate) but also the fact that when you set both channels to output the maximum 12v amplitude of the "DC Waveform" at the same polarity (either both -12 or +12 volts simultaneously into 50 ohm loads), one or the other 13.5 volt rail will then need to supply some 350mA or so (the 50 ohm loading reduces the output voltage to 6 volts - it's only 12v for hi-Z loads (I have to keep reminding myself every time I'm trying to calculate the worst case abuse of the +/- 12v rails - this bites me almost every time I try to calculate this  >:( )).

 If you really want to drive into short circuits indefinitely, you'll need to supply an additional 240mA making the total, absolute worst case demand some 590mA, say 600mA to be on the safe side (though not so safe for that beleaguered THS3002i (or a pair of THS3491 / 3095 / 3091 opamps if that upgrade mod has been applied)).

 It's only when you're outputting repetitive waveforms that the 13.5 rails get to share the loading at an average of half of this worst case demand. Even so, each rail may suffer transient overloads with very low frequency waveforms (say 120Hz or lower). Maximum voltage square waves being the most stressful on the PSU at any frequency up to the 20MHz limit before the  limit is reduced to a mere 5V p-p thereafter (the 24V p-p limit is reduced to the 20V p-p limit at the 5MHz threshold ISTR).

 Can you retest using the "DC Waveform" option at above the 250mV threshold to switch the attenuator out of the circuit? I'm sure you'll get a different picture. I certainly did when I ran those tests on my FY6600 and it wasn't a pretty sight when I upped the voltage above that 250mV limit. :( I'm still using the original, if modified, smpsu board so the 80 to 95MHz 3mV ripple on top of the other psu noise wasn't entirely unexpected - just a disappointment to see it breaking through onto the outputs like this.

 I suspect that at this frequency, this is most likely to be directly radiated RFI leaking into the main PCB rather than ripple on the 12v rails getting through all the filtering and LDOs onto the internal voltage rails (not that any of the LDOs will be able to block such ripple passing straight through them to any meaningful extent at these high frequencies).

 I have my own thoughts on upgrading to a less noisesome psu. Essentially a similar setup to yours but without the analogue voltage regulators. I'll be using a 15 to 20VA R transformer with two separate 15v secondaries (I'll have to float a second 13v output positive buck converter so I can use it for the negative rail (inverting output buck converters are not only expensive (and less efficient) but as rare as unicorn droppings on Ebay and the like).

 The switching noise of a small buck converter or three is a lot easier to deal with than that which blasts out of an unshielded mains voltage smpsu as I discovered when I chose to use a 7805 sized 1.3A 24 to 5v (with higher output voltage settings available) dc-dc buck converter in my my homebrewed GPSDO rather than an actual 7805 to drop the incoming 9 or 12 v wallwart supply down to the required 5 volts.

 However, before I test the depth of the psu modification waters with both feet, I'm going to try the effect of using battery power (a 12v SLA for the +12v rail with a 7805 off of that for the +5v rail with a pair of 6v lantern batteries for the negative rail). That'll dispose of the psu noise source and verify that a quiet psu replacement will actually rid the generator of this presumed source of noise and give me a benchmark to compare against whatever design of PSU I ultimately cook up (I hate 'flying blind' with this sort of project).

 I've had the lantern batteries for several months now - I just haven't got a round tuit since I've kept myself occupied with other projects (that GPSDO for one, and the addition of a 10MHz reference input socket with frequency injection locking module to the FY6600 for the other).

 Since those projects have now finally been completed, I may actually pull my finger out and run that battery power test in the next few days, assuming I don't manage to find yet another project to keep me from this laudable task. The main obstacle is the lack of a suitable 6 pin connector to reduce the risk of damage from operator error. The alternative would be to solder flylead wires onto the psu's output pins (the rectifier diodes conveniently eliminate the back-feed risk) which I can connect to a switching control circuit to connect/disconnect the battery power to all three rails simultaneously (something that's required to play it safe regardless of how it's connected anyway - or maybe not; see my 'train of thought' comments below).

 Testing with battery power might, on the face of it, seem a simple enough task but it could prove a little more complicated than just connecting the batteries one by one, assuming the sequence is of no importance when it might well be critical. In this case, discretion is the better part of valour - I'd rather be safe than sorry, especially after all the hard work I've put into turning this sow's ear into something resembling, if not a silk purse, then at least a rayon facsimile of such.

 I'd like to replace the psu board with something a lot less noisy before I finally restore the case fixing screws to their rightful place - they've been kept aside in a safe place for just over a year now. ::) I'd just  like to confirm in my mind that the existing psu board is not masking out another source of noise that simply can't be eliminated before embarking on what could land up being a folly of a project if that proves to be the case.

 You can't get any quieter than battery power (and a 7805 for the 5 volt rail) so it's the ideal way to test for any other hidden noise sources that may yet defeat the whole purpose of such a project. I'd prefer to know exactly where this will end up rather than proceed on blind faith alone.

 As regards the business of applying battery power to all three rails simultaneously, it's just occurred to me that I could run it up on the mains psu, connect the 12v batteries in parallel to the psu rails via the soldered on flyleads and connect the 5v regulator via a Schottky rectifier diode and then shut off off the mains power to make a smooth transition to battery power for the few minutes required to test the noise levels on the output before reversing this procedure to go back to mains power before safely shutting it down.

 The only extra complication in my case being the OCXO's independent 12v mains smpsu which has an unswitched connection to the C6 mains socket. Fortunately, for this test, the regulation of this 12 volt supply won't be critical so I can just use a couple of 1N400x diodes in series from the 12v SLA battery to avoid any backfeed issues whilst getting it all set up (I'll just have to unplug the whole generator from the wall outlet to cut the mains power rather than just using the rear panel switch in order to transition it to battery power). I may have a result sooner rather than later, in which case, I'll post a report to both this and the FY6600 topic threads.

 This "train of thought" portion also includes some "train of action" involving recording screen captures by way of a "Before" record to eventually compare against my franken PSU (a chimera that will be made up of a conventional analogue mains transformer with rectifiers and BFO capacitor smoothing, feeding a bunch of dc-dc converters).

 I've attached all 20 screen captures, including the very first test one made after being forced to power cycle reboot the 'scope (a Siglent SDS1202X-E, btw - yours appears to be either the 1204 or the 1104 model going by the image you apologized for - no need for any apology).

 This was the first time ever in over a year that I've managed to crash the 'scope so thoroughly that only a power cycle reset could fix it. I'd plugged my usb thumb drive in about an hour earlier (I'd had to nip out on an errand) and it was only when I pressed the 'Print' button that everything froze up. The next four images led to my experiencing a minor epiphany which, if you care to take a look right now before reading any further, will put the observant 'way ahead' of me in regard of the cause of this event...

====SPOILER SPACE====






=====================

 I was expanding the X axis out in search of the 40 to 70 KHz switching ripple artefacts before it occurred to me that the 10MHz was an improbable switching artefact (not that such fixed and stable frequencies, even up to 20MHz, aren't uncommon in custom high grade switching regulators/dc-dc converters for use in high grade T&M kit) before the penny dropped.

 I'd been putting a half volt p-p 10MHz Sinc pulse out on CH1 before I'd set up the 0.3v DC test signal on CH2 and forgot to turn it off to avoid unwanted crosstalk. Once I'd belatedly turned CH1 off, the picture became a lot clearer as you can see from the following sequence of screen captures.

 Using scope traces to analyse switching noise in mains voltage smpsus (particularly the universal voltage kind) is a far from ideal method but if you don't happen to have a spectrum analyser to hand, then at least it offers a way to get some insight into the nature of the problem (it does rather better when looking at dc-dc converters that run at a fixed switching frequency once loaded up sufficiently to exit their voltage maintaining 'hiccup mode' - hint: don't waste time examining this type for noise and ripple when they're in hiccup mode through insufficient loading).

 You'll have noticed my switching between the 20MHz and full BW options to demonstrate the huge difference in results. The 20MHz 'standard' for noise measurement 'scope traces originated back in the days of analogue PSUs and the rarity of lab grade 'scopes that actually had wider bandwidths. These days, with high frequency kit powered from an smpsu, that 20MHz standard has become somewhat obsoleted. You really aught to be doing these tests on the full bandwidth setting if you don't want the true nature of smpsu switching noise hidden by an artificially imposed 'modesty filter'.

JBG

« Last Edit: March 31, 2020, 12:01:58 am by Johnny B Good »
 

Offline Adrian_Arg.

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #232 on: March 20, 2020, 03:17:48 pm »
I have a 20Mhz fy6900, just put a fan in your space, and a heatsink in the fgpa, if I made all the modifications that other users make, it would be convenient to buy a siglent or rigol, here in Argentina it is too expensive to assemble a good source. Also for my hobby is (for now) excellent.

in spnish
yo poseo un fy6900 20Mhz, solo le coloque un ventilador un su espacio, y un disipador en el fgpa, si le hiciera todas las modificaciones que le hacen otros usuarios, me convendria comprar un siglent o rigol, aca en Argentina es demasiado caro armar una buena fuente. Ademas para mi hobby anda (por ahora) excelente.
 

Offline AlMuz

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #233 on: March 21, 2020, 07:55:24 am »
Sorry, but what is the purpose of the opamp in the 317/337 board?

Had the same question when I've got this module in mail.
The double opamp sits in a feedback of adjustment pins for LM317/337.
Looks like some improvement over basic LDO schematic with resistor divider, but not sure what exactly (better ripple rejection?):

 

Offline AlMuz

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #234 on: March 21, 2020, 09:04:38 am »
@JBG


I am not sure if your 2-channel scope supports this feature, but I have mine connected to home Ethernet and really enjoy its embedded web-browser interface (scope hosts a web-server): https://youtu.be/Cxh_Liay09E?t=161
No hassle with USB sticks.


Thank you for the notes on FY6900 output scaling. That indeed expected and it is my fault I did not think of that during my initial tests.

Assessing noise characteristics by signal oscillogramms is a bit tricky and unrewarding, but what else can we do without having an access to a decent spectrum analyzer?  I know there is an FFT option inside our scopes, but its precision is not right where I wish it to be.

Looking onto noise oscillograms at nanosecond scale can be deceiving. Oscilloscope trigger might be hitting tiny sub-section of signal that has noise components of much lower frequencies but much higher amplitude. And you wont see that unless you zoom out timescale. So I decided to stick with millisecond scale to cover broader spectrum when making these screen shots.

But first - establish the baseline!

Here is the scope output with no probes connected at predefined settings (2ms time scale, 1mV resolution):


Here if I attach the regular 300MHz probes and shield their tips (but not short to the ground):


Coaxial cables are not ideal, and some of them more susceptible to EMI than others.
Here is with setup of regular BNC-coax on one input and RF-coax RG316 on another, both 50Ohm terminated at the scope inputs.
Having other ends wrapped in foil they still catch a lot of EMI (my scope SDS1104X-E was fully hacked, so bandwidth goes into 300MHz, RG316 is upper signal and regular BNC-coax which comes with FY6900 is lower):



Here is what I get if I connect other ends of two coaxes to the inputs of FY6900:



Coax type makes a lot of difference!

RG316 looks cleaner, but that might be not because of it is of higher quality or better shielding.
But rather higher capacitance due to its smaller diameter, so that works like low-pass filter.


So, to mitigate effect of the cable characteristics and its EMI susceptibility (if any) I've decided to switch the scope into 20MHz bandwidth:



Now they become more or less equal. So that we can now focus on FY6900 output than on test setup.
RG316 was replaced with another RG58/59 BNC-coax similar to one which came in a box of FY6900.
First check with DC 0V, as expected, nothing new. Basically the same I've posted earlier.


And here is at 1V (somewhere after first relay click):


Beyond second relay click, 5V:


And at 10V, big OH SH!T, I see 120Hz mains coming through:


My PSU calculations apparently were way too optimistic.
20VA feeding two channels of two 50Ohm-terminated loads leaves no more headroom for voltage regulators.

Here is more prominent at 11V:


Negative reail is even worse, problems start at -9V with some strange curves:


Very terrible -12V:


It is unlikely I will ever need such DC offsets in my real projects, so I guess I will leave it as is.
I really wanna see similar picture of 5V DC with original supply, but not willing to disassemble my FY6900 just to "check it out"

@Johny B Good or anyone:
Can you please measure and post the same screenshot of coax connected and 50 Ohm-terminated scope with 2ms/div horizontal and 10mV/div vertical resolution setting?

Thanks,
Alex
« Last Edit: March 22, 2020, 01:23:40 am by AlMuz »
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #235 on: March 21, 2020, 01:18:14 pm »

Had the same question when I've got this module in mail.
The double opamp sits in a feedback of adjustment pins for LM317/337.
Looks like some improvement over basic LDO schematic with resistor divider, but not sure what exactly (better ripple rejection?):
Seem configured as active lowpass filter. Maybe for have a more stable reference at adjust pin?
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #236 on: March 22, 2020, 09:01:16 pm »
@AIMuz,

 Thanks for the speedy response considering the mass of screenshots you'd probably had to peruse in some detail. In hindsight, I realise it would have been better for all concerned if I had actually been a little more selective over which screenshots to attach to my reply. Usually, the limiting factor is in the total file size but, in this case, the whole collection was less than a MB's worth which wasn't the best criteria in choosing what to send. Apologies for the screenshot overload.  :-[

 The output scaling is easy to overlook, especially if you're unfamiliar with the basic circuit layout as I had been prior to downloading DerKammi's reverse engineered circuits, some 12 months ago now, for the FY6600 which he'd posted to github for the benefit of the members contributing to the FY6600 topic thread.

 Sadly for me, the 1202X-E doesn't have that web server facility built in. A later firmware update might add this feature (I think I've applied two updates so far and I don't think there has been another one since).

 Your comments regarding trying to assess noise characteristics are ever so true.  :( The built in FFT SA feature is limited by the use of an 8 bit ADC but using the ENOB option might improve the limited dynamic range (I haven't tested whether this is actually compatible with the FFT function yet).

 I started using it just after I posted all those 'scope traces and with some experimentation with the settings and winding the  timebase down to 20μs per division, along with an averaging factor of 16, I eventually got a decent looking spectrum covering the 0 to 100MHz range. I'll attach a couple of screenshots (the first with the generator switched off, leaving only the OCXO powered up, with the second showing the noise spectrum when turned on at a 0.3v DC output setting). You can be the judge of the usefulness of my FFT settings, which to me, seem to be the best I've ever managed to to set it up to so far.

 Even after getting the FFT settings optimised, I was still seeing inexplicable spectra which led me, belatedly, to look for a baseline.  :-[  In the process, I discovered that one of my BNC dummy loads (the first one I happened to pick out of the tray) was acting like an antenna - it actually raised the baseline noise spectrum above that from an empty BNC socket contrary to my expectation of seeing a slight reduction from the 50 ohm input loading.

 The rest of my dummy loads however, were free of this 'antenna effect' so it seems to have been just a weirdly faulty dummy load (testing the resistance with a DMM shows the expected 50 ohm resistance value (51 actually but all the others showed readings in the range of 50.4 to 50.8 ohms so, allowing for the test lead resistance, within a 1% tolerance range). I've put that weirdly broken dummy load to one side for a later more detailed examination.

 Apropos of the quality of BNC patch cables, I've also discovered some serious problems. The half metre BNC to BNC cable supplied with the FY6600 is no shining example and, what's worse, the BNC plugs on my collection of 3 metre 'Cheapernet(tm)' patch cables are out of spec on the outer barrel dimensions, making them a little on the iffy side in regard of getting a good contact with the ground return on most sockets I plug them into. However, with a bit of adjustment on 'The Angle of Dangle', they're still an improvement over that supplied patch cable.

 The actual cable quality might be ok (I'll have to chop the plugs off one to check - no great loss in this case). I've got a small collection of good quality BNC plugs I can use as replacements if the quality of the cable proves good. I guess even just over 20 years ago, the quality of Chinese manufactured patch cables was even worse than it is today.  ::)

 Not being sure about how much noise the little 12v half amp smpsu board that powers the OCXO module might be contributing, I landed up pulling the plug to completely isolate it from the mains supply. When I was still seeing interference artefacts in the FFT display, I landed up disconnecting the 9999 counts DMM I use to monitor the OCXO's tuning voltage from my homebrewed GPSDO along with the BNC patch cable plugged into the 10MHz output socket.

 This merely reduced the level of 10MHz spaced spurs rather than totally eliminate them so I pulled the plug on that as well which only gave a slight further reduction. In the end, I unplugged the mains cable from the C6 socket on the signal generator, leaving the connection to the 'scope as the generator's one and only connection to the outside world. Even in this state, I was still seeing noise that only reduced by unplugging it off the end of the scope connecting cable which then only completely disappeared once I'd unplugged that from the 'scope's CH1 input socket.

 It seems that the plastic case of the FY6600 makes a very lousy shield, leaving the circuitry inside exposed to external interference. I plan on taking it and the 'scope (plugged into a long mains extension cable) outside, weather permitting, to repeat this test outside of what I suspect is a very polluted electromagnetic environment. It could turn out that a "Noisy PSU" may prove the least of this signal generator's woes although I rather doubt that eliminating this noise source won't at least offer a significant improvement.

 If this proposed "Outdoors Test" demonstrates a significant (unpowered state) noise reduction, I'll have to run my battery power tests outdoors too if I want to collect any meaningful test data. It might seem to involve some effort just to get a baseline but I'd rather know the score before upgrading the PSU in the blind hope that I'll get a tangible improvement rather than discover that I've actually made things worse (or at least no better) for all my trouble.

 Regarding what you said about setting too fast a timebase to see the slower yet higher amplitude mains frequency related ripple component is ever so true. It's a problem I became all too painfully aware of about a year ago when I first started trying to diagnose PSU noise and ripple from waveform traces alone. This sort of investigation is where the services of a half decent spectrum analyser comes into its own. If the use of ENOB is compatible with the FFT SA function on these DSOs, it might just provide a poor man's version of a half decent SA. I'll do some testing later today (it's now 04:27 UTC this Sunday morning right now).

 Picking up on the points you raised over the BNC patch cables, your hypothesis in regard of the RG316 having a higher capacitance per metre due to it being thinner than RG58 doesn't apply. Being 50 ohm cables they will both show close to 100pF per metre (if you have an LC meter, you can test this quite easily). Although the conductor diameter does have some effect on its per unit length inductance, at this scale of physical dimensions seen with typical co-axial cables, it's an order or three less in magnitude (I wasn't able to find any figures in the literature on this aspect of co-axial cable properties - it's just a gut feeling) compared to the changes in capacitance per unit length that result from altering the diameters ratio and dielectric permitivity.

 Since the impedance of co-axial cable depends on the ratio of L (which only changes slightly) and C which can be radically altered by the conductor diameter ratio, the measured capacitance for any diameter of 50 ohm cable remains very close to the 100pF per metre I've measured with various sizes of cable. Obviously, the capacitance value is lower for 75 and lower still for 93 ohm cables. I don't know the figures off the top of my head but comparing the capacitance (using an LC meter or DMM with a capacitance measuring feature) against length offers a convenient way to quickly ascertain an unknown (dirt cheap) cable's nominal impedance.

 In this case when using a dummy load matched to the cable's impedance, you're using it as a transmission line and the LPF effect simply doesn't exist (other than for the unfortunate effect of the 15 to 20pF loading on the scope's input in the 100MHz and beyond range when it has no built in 50 ohm termination option and you're obliged to use an external in-line terminator). The lower noise level using the RG316 cable will simply be on account of its much better quality in regard of screening over that of the cheap cable that was used in the manufacture of that half metre BNC patch cable supplied with the signal generator.

 All this discussion of patch cable quality (lack of in my case) has made me realise that I urgently need to upgrade my collection of test cables one way or another. I'll probably land up assembling my own set of patch cables unless I can track down a source of reasonably priced ready made patch cables of acceptable build quality which, these days, is easier said than done.  :(

 The following attached screenshots include the requested traces (but on the 500μV per division setting needed to raise a usable trace in my case) as well as the FFT plots I mentioned above.

 The first two traces after the FFT images were captured with nothing plugged in and then with only a dummy load (I can't recall which way round they were captured - since there's no perceptible difference, the distinction seems rather academic). The fifth shows the Cheapernet(tm) 3 metre patch lead unterminated with the far end not plugged into anything whilst the sixth is terminated at the scope end. In both cases, the cable is simply coiled up on the test bench. The triggering source was CH1.

 For the last four captures displaying the results of using the supplied half metre patch cable, I decided to use the AC line triggering option (hence the hardware frequency counter readings of 49Hz in the top right corner - obviously limited to truncating to the nearest whole Hz). In this case, this short cable was simply laid out on the bench in a straight line.

 The first of this final sequence depicts an unterminated cable and I was obliged to reduce the Y axis sensitivity to 1mV per division for this capture. Grabbing the middle of the cable increased the amplitude by some 2 to 3 dB, confirming the abysmal quality of the cable itself. Holding onto the BNC plug at the far end reduced the level. The source of the external interference - the induced 50Hz voltage that my body was picking up - was being shunted to ground via this contact so no great surprise here.

 The second image shows the result of connecting a 50 ohm terminator at the scope end of the cable, the third image shows the result of transferring the terminating load to the far end (scope end unterminated) and the fourth image shows the result when both ends are terminated. As before, I was once more obliged to switch back to the 500μV per division sensitivity setting for these last three screen grabs.

 There was no discernible difference between the final three screen captures, confirming that the ingress of the 50Hz interference was due to incomplete coverage by the screening braid, allowing the E field component to leak through and induce a voltage onto the Hi Z input (1M ohm with circa 18pF shunt capacitance) when unterminated. Connecting a single 50 ohm terminator at either end was more than sufficient to attenuate it well below the 'scope's input noise level. Terminating both ends would be just "Gilding the Lilly" as far as this source of interference is concerned.

 It's clear from these results that my Cheapernet patch cables could prove to be worthy of a connector upgrade. I've got enough BNC plugs to upgrade three patch cables at a pinch, two of which are of the screw onto the cable type (one with spring type centre pin socket making it solderless, the other needing the centre pin contact to be soldered for best reliability of this connection), the other four being the more conventional braid clamp and soldered centre pin "Burndept" style.

 I've just replaced the bad plug on the cheapernet(tm) patch cable I'd been using with the solderless screw on plug and it seems to be quite acceptable (the outer screen proved to be mylar foil overlaid with a tinned copper braid with the centre wire being stranded tinned copper). I'll check out the other five patch cables. They might not all be as bad as the one I've just repaired and some, if not all, of the bad ones might only require replacement of a single plug to effect a repair. My patch lead situation may not be quite so dire as it first seemed. I guess you could include verifying your cables as a vital part of establishing your "baseline". The refurbished patch cable hasn't altered the original spectra I'd previously collected so I guess I must have got the "Angle of Dangle" just right on the earlier runs.

JBG
« Last Edit: March 22, 2020, 09:03:43 pm by Johnny B Good »
 
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Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #237 on: March 23, 2020, 04:32:24 am »
Hello all,

 I've been experimenting with my SDS1202X-E's FFT function beyond those noise spectra plots I'd taken of my FY6600's DC waveform at 0.3V output setting. This time, I've used the FFT function to examine a 10MHz sine wave at 1V rms (a p-p voltage setting of 5.9V to get the peak of the 10MHz lined up to the 0dBV mark in the FFT graph.

 Rather gratifyingly, the resulting plot looks just like the classic response you typically see for a single tone carrier wave, complete with sideband noise pedestal. It seems the FFT function, if you have the patience to wait for it to average 16 scans at a timebase speed of 10μs per division or slower, is a half decent "Poor Man's SA" after all. It might even be possible to improve the quality further if the 'scope allows for the use of the ENOB feature with the FFT function. I haven't tested this yet (I have a sneaking suspicion that the FFT function will preclude the use of the ENOB feature).

 My first attempts at setting the correct levels to produce a zero dBV had resulted in a carrier wave at -6dBV in the FFT plot and it was only after turning the "Measure" feature on that I realised I'd overlooked the need to account for the voltage halving effect when driving 50 ohm loads when calculating the p-p voltage setting required to translate the 1.414 peak amplitude of a 1 volt rms sine wave up to 2.828 volts. I'd already applied a twofold increase, forgetting the need to double up yet again when driving the 50 ohm loading at the 'scope end of the cable. :palm:

 Anyway, I've attached two FFT plots so you can judge for yourself the efficacy of the Siglent 1000 series' built in FFT function's ability to produce meaningful spectral data. The response curve for the 10MHz sine wave disappears off the top of the plot by 20dB on account of my having to set the reference to -20dBV to bring the noise floor into view.

 I could go into further detail but I think this truly is a case where "A picture is worth a thousand words." so I'll leave it at that and let the pictures speak for themselves.

JBG
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #238 on: March 26, 2020, 02:15:20 am »
 Following on from reply (#236) where I mentioned patch cable issues, the discovery of my unpowered FY6600 generating noise and the need to test outside of a noisy environment, favourable weather permitting, well, the long overdue 'favourable weather' finally materialised here in the UK this Tuesday the very next day!

 The afternoon temperatures rose to the giddy heights of 17 to 18 deg C for the first time in over 5 months, motivating a foray into our garage to dig out the patio and garden furniture which proved rather handy as far as my 'outdoors testing endeavour' was concerned. As a result, I managed to gather ample data, proving my hypothesis that my FY6600 was as rainproof as a colander in a rainstorm as far as keeping out the EM interference that my hobby room seems to be awash with.

 I've descriptively renamed the screen captures to keep the verbiage toned down to a minimum. The first seven images are from the outdoors test, the next four attempt to demonstrate the effect of loading the +12 rail with some 250 to 400mA or so of loading on the smpsu ripple voltage artefacts in the FFT scans, the next two are baseline noise floor scans of the DSO to determine the origin of the -75dBV at the extreme LHS every FFT plot (it seems to be just a plotting display artefact - you can just detect the 50Hz peak which disappears completely when the input is terminated).

 The last 14 images (blast EEVBlog's 25 attachment limit! - it's just 12, the last two will be attached to a second post) show the effect on the noise floor of the FY6600's use of the 12dB boost amp and the attenuator as you select each of three output voltage ranges with examples of the effect this has on a 10MHz sine wave over a range of level settings from a low of just 5mV p-p right up to the maximum of 20V p-p (unloaded Hi-Z voltages - the voltage is halved when driving 50 ohm loads).

 The problem with my patch cables seems to be a combination of dimensional tolerances stretched to the extreme opposite limits between the BNC connectors on the patch cables and the slightly just too undersized barrel diameters of the sockets used by Feeltech. More than likely, the main culprit is Feeltech rather than those patch cables. I might be able to bodge a work around by squeezing the sockets into a slightly oval shape. The only worry is that I might damage them or just make things worse. Has anybody tried this bodge with any success?

 For anyone thinking of following along, keep in mind that I've modified the 85 ohm attenuator pad used in the 6600 and 6800 models (and possibly in the latest 6900 one as well, I can't recall whether this question was ever answered let alone what the answer may have been).

 I'd been attempting to convert it to a 50 ohm pad matching the actual Hi-Z voltage attenuation Feeltech had corrected for in the firmware but the best I could manage was a 45 ohm pad matching the required attenuation. It had been such a brain bursting exercise trying to calculate the actual voltage attenuation of this whacky oddball 85 ohm attenuator pad that I gave up trying to fine tune it any further until I'd managed to recover from all the effort I'd expended in getting it this good (45 ohm is, at least, some improvement over the 85 ohm monstrosity Feeltech had created so taking some rest on my laurels seemed justified).

 I can always have another go at cracking this problem any time I feel I'm up for yet another round of busting my brain. Anyhow, the point is that you'll get slightly different results from mine at settings below the 500mV p-p threshold (or the equivalent DC voltage settings within the range of +/-250mV)

JBG
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #239 on: March 26, 2020, 02:17:36 am »
 Here are the last two image files EEVBlog prevented me from including on account of their 25 file attachment limit.

JBG
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #240 on: March 31, 2020, 04:26:29 am »
 In my previous post, regarding the effect of choosing dc voltage levels to avoid switching the attenuator into the output circuit, I pondered as to whether this attenuator network had been corrected from the 85 ohm pad used in the previous two models or not. Well, I came across incontrovertible evidence that this "Skoolboy Howler" remains uncorrected in the FY6900 when I chanced upon the following YT review video https://youtu.be/kA6uxUDg55M?t=366.

 That link takes you to the point in the video just before it pans to reveal the resistors identified as RS1 through to RS6 in between the relays just behind the output sockets which can be seen to be the same 100 and 510 ohm values used by its predecessors.

 It looks very like the original design brief had been to switch a 20dB (50 ohm) attenuator pad into the output for p-p voltage settings below the 500mV threshold and compensate in the firmware with a 20dB boost so as to improve the S/N ratio at the bottom end of the range. Somehow or other (with the FY6600 model at least - possibly an inheritance from an even earlier model), they'd cocked up the BoM for this attenuator network and used a firmware bodge as a quick 'n' dirty fix to save having to re-work the boards. They've obviously chosen to correct for the unterminated Hi-Z case (such a quick 'n' dirty fix won't work for both cases) and hope no one would get wise to their kludge.

 For such a cheap function generator, this could be excused as a temporary solution until the next generation, the FY6800 in this case, could provide a golden opportunity to correct this mistake. However, since they were aware of this issue (they had to be in order to apply a firmware fix), ignoring this opportunity becomes completely inexcusable and since they also carried on perpetrating this error into their next 'golden opportunity' with the FY6900 to put things right, one has to assume the design/production "team" at FeelTech are just a bunch of retards going all out to destroy what little credibility that may remain of their employer's reputation.

 Of course, since this is so obviously targeting the cash strapped hobbyist market demographic, it's not inconceivable that the designers may have deliberately planted all these errors and omissions not only to save costs but also to provide "Easter Egg" like challenges to their target demographic for them to test their expanding electronics knowledge upon.

 I've offered this theory before in the FY6600 thread which was regarded then as being a rather too fanciful explanation for all of these readily avoided defects. Now however, with the earth loop fiasco and attenuator nonsense (and other deficiencies) still being carried through to the FY6900, this theory (which originally started out as a "Joke") starts looking ever less fanciful.

 TBH, I suspect the truth of the matter is somewhere in between. The designer(s) probably reported the E&Os to production who went, cap in hand, to management who probably then said something along the lines of, "Sorry, at the price we're selling these things, we simply can't afford the expense - besides which, we're pretty sure our target demographic will be more inclined to have a go at fixing these minor issues in view of its low price point and the cost of shipping it back, thereby ensuring voided warranties. Leave it be and see how it goes before we splash the cash remedying what may prove to be a non-existent problem.".

 I know it might be hard to believe but I think the EEVBlog membership forms only a tiny fraction of Feeltech's customer base so that imagined scenario might not be quite so far from the truth as you might think. The apparent influence the FY6600 topic thread had on Feeltech's rethinking the use of the more expensive C14 socket over the original 'cheap as chips' C8 mains socket and 2 wire mains cord was probably more to do with highlighting the litigation risk for consequential ESD damage to DUTs due to this negligence.

 That mains socket upgrade was about as brutally pragmatic a solution as it could possibly have been with absolutely no thought for the consequences of introducing the deleterious effects of ground loop noise and mystery dc offset voltages. They couldn't have done less work in replacing the C8 with a C14 socket if they'd tried. The C14 and matching mains lead costs were unavoidable (and likely cheaper than the C6 based alternative). This knee jerk reaction to all the complaints about the ESD risk in the FY6600 thread speaks volumes about Feeltech's attitude to their customers.

 Mind you, having said all that, if I was in the market for my very first cheap function generator and wasn't already in possession of a FY6600, I'd certainly buy the FY6900-60M as a 'starter' generator. My FY6600 has been as much a project as a T&M tool so I don't regret any of the time and effort I've invested over the past 12 months or so in improving it beyond its original 'stock' condition.

 The one remaining question I'd like to answer however, is whether the smpsu board is as detrimental to this function generator's performance as almost everyone seems to imagine. I have my doubts but the only way to determine just how big of a problem this is, is simply to run it off battery power with only a 5v analogue regulator in the mix to power the logic rail.

 If the battery power test shows a tangible reduction of spurious noise and ripple, I'll  build a test PSU based on a conventional step down mains transformer to power dc-dc buck converters for comparison against analogue voltage regulators.

 I have a strong feeling that the noise and ripple from three small dc-dc converters will be far more manageable than that of the original smpsu board, hopefully to the point where the noise and ripple performance of analogue regulators will become so marginalised as to make them (and their heatsinks) totally redundant appendages.

 Whatever the outcome of this final psu testing project, my next function generator is going to be a model from a more upmarket manufacturer such as Siglent or one of their competitors. As much as I've enjoyed tinkering with and modifying the hell out of this toy function generator, there eventually comes a time to move onto bigger and better things.

 Once I've dealt with the psu issue one way or another, I'll have pretty well exhausted all of the possible enhancement opportunities so generously offered by FeelTech's FY6600, leaving me free to consider a possible more upmarket replacement at my leisure. I may well be able to manage quite nicely with the FY6600 for a little while longer yet before the need for something better forces me to fork out another wad of cash for a bigger and better function generator.

JBG
 
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Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #241 on: April 01, 2020, 07:00:42 am »
Someone can explain me, please, what is this problem of the 85 ohm pad? As i've not well understood it. Thanks!
 

Offline Johnny B Good

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #242 on: April 01, 2020, 09:06:48 pm »
Someone can explain me, please, what is this problem of the 85 ohm pad? As i've not well understood it. Thanks!

 The short explanation is that it changes the output impedance of the function generator from the specified 50 ohm standard to a non-standard 85 ohm impedance at amplitude settings of 500mV p-p or less. The function generator otherwise behaves like all normal function generators at amplitude settings above this 500mV p-p threshold.

 This technique of switching an attenuator into the output of the primary amplifier stage to keep noise levels at bay for sub 50mV p-p settings is universally applied by all makes and brands of signal/function generators from the cheapest to the most expensive.

 The difference between Feeltech (God bless 'em) and every other make, is that every other make employs 50 ohm pads (typically 20dB or thereabouts) in order to maintain a consistent 50 ohm output impedance specification.

 BTW, if you're only using this to generate test signals at audio frequencies, this anomaly is usually of very little significance. It only becomes an issue when dealing with MHz frequencies where you have to start treating your interconnecting cables as the transmission lines that they are.

 Pondering the almost zero cost of using the correct resistor values to create a 20dB (or thereabouts) 50 ohm pad, I plugged the slightly misremembered values for this resistor network into the Pi attenuator calculator and was startled to see a 26dB 50 ohm pad result pop up! This led me to have another look at that YT video to check and was relieved to see the 100 and 510 ohm markings instead of the 56 and 510 values I'd plugged into the calculator. I guess the 56, rather than the actual 100 ohm value must have been a memory of previous 'test' values I'd used over six months ago when trying to calculate a solution to Feeltech's error.

 The best I had been able to manage back then had resulted in a 45 ohm pad with the appropriate unloaded attenuation to match Feeltech's firmware bodge solution. Even then, I had been relying on test values of the series pass element to trim the unloaded attenuation to exactly match the existing attenuator.

 Somehow or other, my initial calculations had been so far out, that this had resulted in a Zo of 45 ohms which no two recalculation attempts of the many I tried were ever able to resolve. In the end, I just gave up busting my brain against the problem and simply accepted that whilst a 45 ohm pad wasn't perfect, it was at least far better than the 85 ohm one it now replaces. I think that, six months on, I'm now sufficiently recovered to risk another go at solving this puzzle once and for all (fingers crossed).

JBG
« Last Edit: April 01, 2020, 09:21:16 pm by Johnny B Good »
 
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Offline bdunham7

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #243 on: April 02, 2020, 08:20:39 pm »
This isn't a criticism of anyone, especially all the Feeltech modifiers out there, but it does occur to me that the FY6600/6800/6900 are the most highly polished turds in history.  :-DD

Keep up the good work, guys!
 
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Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #244 on: April 04, 2020, 12:06:29 am »
Thanks Johnny B Good.... i still miss what is a "50ohm pad" but i've roughly understood the problem, that if i've well understood was fixed in the 6900.

Another thing, someone know what calibrate every trimmer onboard? In the fy2300 was easy as the circuit was much simpler. I see 3 of them per channel. On the 3200 there was the offset, zero adj and gain, i suspect that on the 6900 are the same...
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #245 on: April 05, 2020, 10:37:53 am »
 So, regarding this argument, i've discovered something.
If i set the waveform to dc, it uses the offeset circuit to output the signal.
So, setting the signal to square wave and dc 99.999%:
  • If amplitude is > 5v / with 6v signal i get +3vdc without load. With 50ohm load i get only 1v instead of 1.5v expected
  • If amplitude is > 0.5 and <= 0.5 / with 5v signal i get +2.5vdc without load. With 50ohm load i get  1.25v as expected
  • If amplitude is < 0.5 / with 0.5v signal i get +0.275vdc without load. With 50ohm load i get  0.102v

I know that 50ohm, must be the impedance and i'm measuring pure resistance, but it's normal?
« Last Edit: April 05, 2020, 10:43:29 am by masterx81 »
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #246 on: April 05, 2020, 11:05:52 am »
Taking this schematic of the fy6600 as good also for the 6900:
963700-0
The output relays can do a lot of combinations, but i can hear them clicking only in the 3 ranges that i've listed. I suppose that the ths amplification takes place only with Vout > 5v, between 0.5 and 5 i have direct output from the opa, and under 0.5v enter the attenuation. But why in the > 5v range i not have the 50ohm output resistance?
 

Offline nctnico

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #247 on: April 05, 2020, 12:25:18 pm »
So, regarding this argument, i've discovered something.
If i set the waveform to dc, it uses the offeset circuit to output the signal.
So, setting the signal to square wave and dc 99.999%:
  • If amplitude is > 5v / with 6v signal i get +3vdc without load. With 50ohm load i get only 1v instead of 1.5v expected
  • If amplitude is > 0.5 and <= 0.5 / with 5v signal i get +2.5vdc without load. With 50ohm load i get  1.25v as expected
  • If amplitude is < 0.5 / with 0.5v signal i get +0.275vdc without load. With 50ohm load i get  0.102v

I know that 50ohm, must be the impedance and i'm measuring pure resistance, but it's normal?
What is the current limit of the opamp? And what is the open loop output impedance of the opamp? If you dig deeper into the specifications then you'll see that a surprisingly large number of opamps can't drive a lot of current.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online masterx81

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #248 on: April 05, 2020, 12:48:03 pm »
One opamp is the upgraded ths3491. But seem that in some ranges it uses an opa686n.
 

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Re: FeelElec New Arrival FY-6900 Signal Generator
« Reply #249 on: April 05, 2020, 01:02:14 pm »
Errata corrige. On 6600 is the opa, on 6900 there is an ad 8009 that can supply enough current for a 50ohm load at 5v
 


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