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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator

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Miti:
O dear... I thought American people have no clue they were doing this... Considering they keep doing it after they realize the mess they're in...  It's sad.

Zenith:

--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm ---
 I bought my Siglent SDS 1202X-E from their UK agent (Labtronix, if anyone's interested) a lttle over 3 months ago as a result of my attention being drawn to this thread by a usenet posting into SED. It was the startling revelation of the amazingly low prices for brand new modern T&M kit that prompted me to replace my very neglected "Boat Anchor" 'scope and sig generator with high performance replacements I could carry around one handed.

--- End quote ---

I hadn't taken much interest in electronics for about five years. The scope I used most was a Tek 475 which is a beautiful instrument, but it's failed and I've had to fix it a couple of times and it's a bit big. I really ought to get a scrapper as a spares mine, as some of the parts are becoming hard to find. The same as you, I looked around and found that DSOs had come of age were quite cheap, also CRT based scopes are disappearing from the market.

I bought the Siglent for about £275 because 100MHz is enough for most things, and the Tek goes to 200MHz if needed. The Siglent offers a lot for the money, but most importantly, they seem to have a good reputation for reliability. I might have spent more and bought one with higher bandwidth, protocol decoding and four channels, but I don't find myself hankering after that. The only real problem is that sometimes it becomes confused and only shows what looks like a distorted 50Hz signal, but the Default Setup button clears that.  The manual could easily be better.


--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- A week after getting hold of the 'scope, I realised I needed a signal generator to complement it so seriously contemplated Siglent's SDG1032X (just a fiver shy of the 365 quid I'd already shelled out on the 'scope). When I compared the specs, I realised I could get a poor man's version (the FY6600) which offered similar features (some better, others not quite so good) for just 21% of the SDG1032X's asking price, so re-read the whole of this thread... (twice!) before placing an order with a Chinese supplier who held stock in the UK and less than a week later, I had myself an AWG of my own. From then on it was downhill into the depths of DIY electronics improvement.  :-\

--- End quote ---
I have a number of signal generators and a couple of analogue function generators. I bought the FY6800 largely because I wanted a sweep generator for the visual alignment of the IF stages of valve comms receivers. I've never seen much in the way of sweep generators on the second hand market. I've come across various wobbulator projects, but they seem limited and a lot of trouble for not much. The analogue function generators won't cut it. I could just cut to the chase and buy a spectrum analyser with tracking generator, but it seems a lot to pay for the amount I'd use it. Apart from that the FY6800 is a nice, compact and versatile signal source. Function generators are things I've always regarded as handy, but rough and ready. The FY6800 by comparison with analogue function generators, is very accurate and very stable.

The sweep function on the FY6800 looks like a marketing feature. No output sweep waveform, not even a sync pulse and you can't supply a sweep. Not much use really, unless I've overlooked something.

The VCO function is getting there, but it's not quite linear, so it would involve a custom non-linear sweep to compensate, which can be done. Here, the desperately bad PC software has enough functionality to be able to load a text file created by a program and load it as an arbitrary waveform. Also the VCO, on my FY6800 does not work from 0 to 5V, it works from around -0.9V to about 5.3V. A lot of effort to get it to work, but practical.

The FM function seems to work extremely well and is the answer. Using an 11 step staircase sweep arbitrary waveform created via a C program and the ropy PC software, and a frequency counter says that that for a sweep from 400KHz to 500KHz, all the points are where they should be to within close limits. Furthermore the modulation source can be Ch2 routed internally, so there isn't the slight nuisance of using a cable to connect to the VCO connector. I think it will do what I want.

It's this thing all over, like the 85 Ohm pad, the rubbish software and all the rest. It's a mixture of the outstanding for the price, and ridiculous failings caused by extreme penny pinching or thoughtlessness.


--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- As you've noted with the supplied 'scope probes, they leave a little to be desired (certainly with regard to their BNC adapter tips) but aside from that niggling shortcoming, they (and the 'scope itself) appear to meet their specifications just fine. Of course, as I've pointed out several times here and elsewhere, all test kit has limitations, even the expensive kit costing ten to a hundred times more.

--- End quote ---
The probes are justifiably described as 100MHz and they were thrown in with the scope.  There are limits to what you can reasonably expect for a certain price. I don't know what Keysight, LeCroy and Co., throw in as probes. A pair of decent quality leads for an inverter welder will cost £40 and up. Cheap to moderately priced welders come with leads which are grudgingly up to the job. Expensive models often come with no leads, because "our customers are very discerning as to leads and electrode holders and prefer to source their own". Penny pinching IMHO.


--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- With regard to the limited resolution of the voltmeter function, that's inherent to the 8 bit high speed ADCs used in these DSOs to capture the signal waveforms. With only 8 bits resolution, you're only going to see a maximum of 256 steps (twos complement signed integer - -128 to +127) in the Y axis of the 'scope's display. The ENOB setting using oversampling can extend the resolution by another 1 to 3 bits but that introduces a compromise which may or may not be acceptable depending on which aspects of the waveform you're trying to examine.

 Luckily, for the purpose of displaying a waveform on the screen, this is generally ample resolution. Even luckier in the case of DSOs, if you need to see finer detail, you can alter the gain and dc offset so as to concentrate the view over a limited Y axis range of interest in the waveform. However, as I've already mentioned, this technique has its limits and is yet another compromise (one, however, which does extend the capability beyond that of the traditional CRT based scope).

--- End quote ---
Scopes don't make great voltmeters, despite what teachers said in A Level Physics years ago,  but the feature on digital scopes is damned convenient and by playing with the offset as you describe, they can produce better resolution. For instance, I have a Li-ion battery that a meter says is at 3.98V. Casually checking with the scope gives between 3.92 and 4.00. Averaging gives 4.00. increasing V/Div and putting in an offset of 3.0V and the scope says 3.98V, but it's a bit of messing around. If you used an analogue scope to do the same, you'd probably say it's at 4V, rather than 3.8 or 4.2 and leave it at that, as a quick and dirty measurement. One feature I'd like to see on this scope is a coarse vertical shift control setting.

The number of digits after the decimal point offered by digital scopes and instruments like the FY6800 tend to seduce you into taking them at face value, but when you look closer, it's different story.



--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm ---
 Although my FY6600 has some DC offset, it's mercifully only a matter of a millivolt or so in the sub 500mV range (the range where that silly 85ohm pad is switched into the output circuit) and still low enough to be insignificant at the higher output ranges so I've never been tempted to tweak the trimmers (let sleeping dogs lie undisturbed lest they bite you in the bum). I may have a go at trimming out such residual offsets later on when I'm better tooled up to make sense of any such adjustments but, for now, they're not a problem and I don't see any benefit in taking the risk of making things worse.  ;)

--- End quote ---

A 100mV ptp unterminated signal is reduced to about 39mV when a 50 Ohm terminator is added. That's consistent with an 85 Ohm output impedance. A relay cuts in at over 500mV and the ptp on the scope jumps up to half the ptp given by the FY6800, so the silly 85 Ohm pad is still there.

It's my experience that messing about trying to get rid of something like a 1mV offset, which you can live with, is likely to end in tears. You adjust the pot and get 15mV, then adjust it and get -11mV, then with safecracker fingers you can dot around zero, but never hit zero, and eventually settle for something over -2mV, which is worse than when you started. Then you can break something, involving much more work, or you find that the offset drifts over time because of thermal effects etc.



--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- Being mindful of what I'd said in my previous post about the impedance of the generator being immaterial when driving a load matched to the impedance of the connecting transmission line (in this case, a 50 ohm dummy load hanging off the BNC T adapter plugged into the 'scope's Y channel input socket using the very short 50 ohm BNC patch lead supplied with the FY6600), I've just checked the frequency response of my FY6600 and see 74 and 75 percent of the amplitude (or, in this case, the Peak to Peak) at the 60MHz limit compared to the amplitude produced at 1MHz (the 100KHz amplitude was just a tiny fraction higher by way of a sanity check).

--- End quote ---

I have a collection of BNC patch leads, terminators and I usually terminate things properly. Are we going to see much in the way of transmission line effects with a 100KHz signal and about 1 metre BNC cables?


--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- This is less than the 29% drop associated with the -3dB bandwidth point of 70.70707% voltage amplitude. Since this frequency is well within the 'scope's claimed 200MHz bandwidth, it would seem that my FY6600 example comfortably meets its own bandwidth specification. However, I'm just waiting for the half hour screen saver setting to time out before running the self calibration routine to repeat those tests (which event has occurred just now). I'm now waiting for the self calibration to complete and I can then repeat the bandwidth test...

 I've gotten pretty well the same figures on both channels of the generator and ditto for the 'scope channels, doing only one channel at a time. If anything, there seems to be a slight discrepancy with the 'scope's frequency response where it rises slightly below the 1MHz mark on the Y2 channel compared to the Y1 channel. Just a small fraction of a dB, nothing to get worked up about.

 All in all, the FY6600 seems to be performing to its specification on sine wave bandwidth at least. Since the PSU, opamp and TCXO mods, it now exceeds its original DC offset/signal clipping limits and frequency stability capabilities (and the impedance mismatch below the 500mV setting isn't quite so shabby as it originally was - 45 ohm versus the original 85 ohm, whilst still not ideal, is a noticeable improvement just the same).

 Since the FY6800 is essentially a revamped FY6600, I'd expect that to likewise exceed its sine wave output bandwidth claims, barring any out of the ordinary manufacturing defects (they have enough ordinary cost cutting induced deficiencies to begin with).

--- End quote ---
As far as I can tell after checking with an HP8640A sig gen, which has not been calibrated for many years, this scope exceeds the maker's bandwidth claim and I recall was pretty much flat up to about 60MHz. According to the scope the FY6800 is 1.7dB down at 60MHz and 1.1dB down at 40MHz. I find that remarkable for a unit at this price.

I must look into the the PSU, opamp and TCXO mods. There's something about this contraption that starts the urge to tinker. I think it's because in some ways it's outstanding, but it's got so many daft faults. It's a bit like the £3 radio sold in SuperDrug a few years back. People started reverse engineering it and fitting product detectors and so on. There was also an astronomical telescope sold by Lidl for about £50, which had people doing detailed investigations and finding various ways to improve it.


--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm --- I've only addressed the matter of the frequency bandwidth aspect, ignoring the question of output voltage level calibration which seems to be within +/- 10% (according to my 'scope which, for the moment I'm assuming to be within its own +/-3% tolerance range - ICBA to knock up a simple peak voltage detector using a point contact diode to sanity check the voltage levels right now).

 It's all too easy to obtain misleading results when checking out such kit when you overlook the transmission line effects of the connecting cables at radio frequencies and neglect the need to terminate the cables with a resistive impedance matching that of the cable (50 ohm dummy loads with 50 ohm BNC cables). It's also worth keeping in mind the limitations of your test and measuring equipment (DSOs don't, for example, make a very good substitute as a digital voltmeter).

 If you're seeing unexpected results when checking out new kit, it's more than likely you have overlooked something vital like properly terminating a test cable. Take a pause from what you've been doing and consider the possibility that you've overlooked something in your test setup before concluding you have been 'sold a pup'.  If that doesn't resolve the issue, you can always seek advice from any technical discussion group, whether it's EEVBlog or a usenet group just as Andreax1985 has done in this case. There's nothing wrong in seeking expert advice or a second opinion from your peers.

JBG

--- End quote ---

I started messing with this after seeing Andreax1985's problem with the offset, which seemed a lot to me. I'm fairly new to DSOs and looked into it in a fairly lazy way, using the cursors, forgetting to force the scope to recalibrate first. The DVMs with a DC waveform (which I've found useful) showed an offset of under 1mV. I assume the offset is independent of the waveform. Returning to it with the scope and proceeding more carefully and using the voltage measurements, it seemed to be about 1mV. I learned something about the scope and the FY6800, and I hoped my observations were useful to Andreax1985, at least in that he didn't send back a sig gen which was behaving as expected.

It's my experience that when investigating things with a scope on its highest sensitivity range, you can see strange things, like differences between two BNC patch cables of the same length. That's at DC. It wouldn't take much say, by way of a voltaic effect to do it. Cheap Chinese BNC cables are definitely suspect.

EEVBlog looks like a thriving community full of knowledgeable people and I shan't hesitate to either ask advice or give advice when I think I have something to contribute.

Andreax1985:
Thanks all for your suggestions, a lot of food for thought. So I did some further testing and I gathered the following evidences:

FY6800 set to DC, 10.0mV output. Here my measurements:

i) My cheapo digital multimeter: 9.5mV
ii) My DSO using a (cheap) direct BNC-BNC coaxial cable: 11.49mV
iii) My DSO using a (cheap) 1X probe, clipping the probe ground lead to FY6800 CH1 BNC ground and touching the FY6800 CH1 BNC signal lead with the probe tip: unable to get a stable measure, I read from 16 to 23mV, depending on where exactly I clip the probe ground lead on FY6800 CH1 BNC ground.

FY6800 set to DC, 0.0mV output. Here my measurements:

i) My cheapo digital multimeter: 0.2mV
ii) My DSO using a (cheap) direct BNC-BNC coaxial cable: 1.70mV
iii) My DSO using a (cheap) 1X probe, clipping the probe ground lead to FY6800 CH1 BNC ground and touching the FY6800 CH1 BNC signal lead with the probe tip: unable to get a stable measure, as before

Questions:

1) Method iii) appears totally unreliable: why?
2) First thing that comes to my mind is that if my multimeter is right, my scope has around 2mV DC offset (which, btw, is within the specs). But then why, if I switch off FY6800, the scope correctly measures around 0.1mV (whereas when FY6800 is on and set to DC 0.0mV the scope reads 2.10mV)??

Zenith:
I see much the same. Try lengthening the earth lead from the probe with a jumper and you get some really strange results. The answer is that although it's easy to assume that ground is ground and all at the same potential, but conductors act as antennas and the longer they are, the more RFI and induced voltages they can pick up. If you are looking at the waveforms produced by a 555 oscillator at a few volts it doesn't matter, but if you are looking at low level signals, you have to pay attention to earthing and ground loops. There's a reason why they include the little spring earth connector with scope probes and there are Tektronix technical notes on probing.

As for using a DVM with unshielded leads connected to a BNC/croc clip cable, that's not really ideal for looking at voltages of around 1mV. We have to work with what we have and it seems to work fairly well, but if there are a few inconsistencies we shouldn't be surprised.

I've also noticed that it's as well to self-calibrate the scope regularly. Every time it recalibrates the no signal levels can be very slightly different. Don't forget that we are talking about measuring voltages towards the limits of what the scopes and normal, cheap DVMs can do.

What I have seen is that sometimes the FY6800 gives about a 13mV offset according to the DVM. The scope shows the same as a mean of about 13mV with a noisy waveform superimposed. The noisy waveform has spikes at around 65KHz. Powering off the FY6800 for a minute then powering it up clears it and it returns to having an offset of around 1mV. It reminds me of your original post about a 20mV offset and it looks like a definite flaw in the product. I wonder if it's been covered somewhere in this long thread.

Andreax1985:

--- Quote from: Zenith on February 14, 2019, 01:23:55 pm ---I see much the same. Try lengthening the earth lead from the probe with a jumper and you get some really strange results. The answer is that although it's easy to assume that ground is ground and all at the same potential, but conductors act as antennas and the longer they are, the more RFI and induced voltages they can pick up. If you are looking at the waveforms produced by a 555 oscillator at a few volts it doesn't matter, but if you are looking at low level signals, you have to pay attention to earthing and ground loops. There's a reason why they include the little spring earth connector with scope probes and there are Tektronix technical notes on probing.

As for using a DVM with unshielded leads connected to a BNC/croc clip cable, that's not really ideal for looking at voltages of around 1mV. We have to work with what we have and it seems to work fairly well, but if there are a few inconsistencies we shouldn't be surprised.

I've also noticed that it's as well to self-calibrate the scope regularly. Every time it recalibrates the no signal levels can be very slightly different. Don't forget that we are talking about measuring voltages towards the limits of what the scopes and normal, cheap DVMs can do.

What I have seen is that sometimes the FY6800 gives about a 13mV offset according to the DVM. The scope shows the same as a mean of about 13mV with a noisy waveform superimposed. The noisy waveform has spikes at around 65KHz. Powering off the FY6800 for a minute then powering it up clears it and it returns to having an offset of around 1mV. It reminds me of your original post about a 20mV offset and it looks like a definite flaw in the product. I wonder if it's been covered somewhere in this long thread.

--- End quote ---

I think I got around the 13mV offset, try if this works for you:

i) Make sure that in your "SINE" waveform settings, the default value for amplitude is 5V.
ii) Switch off FY6800 using the main switch on the rear (not the standby button on the front).
iii) Switch on FY6800, then select the "DC" waveform (using the wave button). Set "volt" to 0.000mV.
iv) Read the actual voltage with your multimeter: you should read the infamous 13mV offset.
v) Now select the "SINE" waveform (using the wave button). Set "amplitude" to zero (and while you are at it also set "offset" to zero).
vi) Go back to "DC" waveform (using the wave button). Set "volt" to 0.000mV.
vii) Read the actual voltage with your multimeter: the abnormal offset should be gone and you should read around 0mV as expected.

So, this seems a firmware flaw to me (I'm using 1.7.1 fw). Can anyone confirm?

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