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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
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Johnny B Good:

--- Quote from: Dbldutch on September 02, 2019, 03:46:14 am ---Good question, I probably should have elaborated on that.

I used the two seperate windings of the transformer to my advantage and make it easier to create a single “star” groundpoint by creating two separate supplies. With center-tapped transformers it’s easier to create grounding issues when using perf or protoboard and not a real pcb. Another reason was that I could use full bridges and therefor smaller buffer electrolytes. Diodes are cheaper than good buffer caps.

The only connection of the two supplies is at the connector to the main board.

Thirdly, I wanted to isolate the high frequency noise that inevitabilly comes from the 5V DC-DC convertor away from at least the negative supply as much as possible.

--- End quote ---

 Thanks for that enlightenment (and for your critique of my writing style). I must admit that whenever I dare to review my own scribblings, some of the more effusive of my postings do make me cringe a little - I think you may have a point. :-[

 Anyway, aside from the fact that a centre tapped bridge rectified transformer winding is effectively a full wave bi-phase supply to each of the positive and negative supply smoothing capacitors. you make a valid point in regard of minimising noise injection into the common ground reference point.

 It was just that, on the face of it, the use of separate windings and bridge rectifiers seemed a little unnecessary for a bi-polar dc supply. Obviously, there can be good reasons for such duplication. In your case, the issue of noise injection through a common grounding link between the ground point of a centre tapped transformer to a relatively remote grounding point on the regulator board, and in my case, contemplating a means to float a positive dc-dc converter regulator so as to connect it in reverse to the common ground point on the PCB carrying the other two conventionally wired +5 and +12 dc-dc converters.

 I guess when we're dealing with eliminating noise from a nice and quiet replacement PSU, your reason for the separate secondaries and rectifiers trumps 'convention' every time.  :)

JBG
CDaniel:
If you are very worried about the 5V converter noise than you would choose a transformer with a separate winding for that ...

But , I use a linear regulator for the 5V rail + a dropping resistor and the power dissipation is not that big , it is doable . So , you are not bound to use a switching regulator .
Johnny B Good:

--- Quote from: CDaniel on September 02, 2019, 08:18:55 pm ---If you are very worried about the 5V converter noise than you would choose a transformer with a separate winding for that ...

But , I use a linear regulator for the 5V rail + a dropping resistor and the power dissipation is not that big , it is doable . So , you are not bound to use a switching regulator .

--- End quote ---

 That's a neat way to shed some of the heat dissipation burden off the regulator. Ideally you need to determine the maximum, worst case current loading on the regulator so you calculate the exact resistance value required to still leave a safety margin over and above the regulator's dropout voltage.

 Unless you're trying to design for a "Universal Mains voltage PSU" with a 240v 50Hz stepdown transformer that will still provide sufficient secondary voltage(s) at 110 volt input without manually switching a split primary between series and parallel, you might want to test at the lower mains voltage limit (216vac) and check the 100Hz ripple to ensure you still have a small (half volt or so) margin over and above the regulator drop out voltage. You can then calculate the resistance required, picking the nearest preferred value that's just less than this and select a resistor with a wattage rating to suit.

 Obviously, you can just make an educated guess, erring on the low side to be safe or, alternatively, just use a dc-dc switching converter to provide a fixed voltage to the regulator some 2.5 to 3 volts higher than its output voltage. If this trick is good enough for high grade commercial test gear, it should be good enough for these cheap signal generators. :)

 Having said all that, I don't think there's really any need to eschew the use of a switching regulator on the 5v supply since the only components which make direct use of this PSU voltage are the relays. From what I can see in the reverse engineered circuit diagram for the main board, the rest of the digital supply voltages all come from the three LDO regulators supplying 3.3, 2.5 and 1.2 volts. All the critical analogue supply voltages come from the +/-12v rails which only directly drive the high level THS 3xxx opamps, another 3.3v LDO regulator feeding the analogue Vdd pins of the DACs and the +/-5v LDO regulators feeding the OPA686Ns.

 If you're going to use a dc-dc converter for the 5v supply and ground return noise is a concern, you might want to rejig that 6 pin ribbon cable connector to route the 12v analogue supply grounds to the main board end of the connector independently of the 5v ground return (put the join as close to the main board connector as practicable).

 This ideal can only be fully realised when all three rails are fed from three separate transformer secondaries, each with their own independent bridge rectifiers and smoothing caps. If the +12v and +5v regulators are fed from a common rectified and smoothed supply, the best you can do here to is keep the common ground wire as short and as heavy gauge as possible.

 There was very good reason for doubling up on those ground wires in that overly long ribbon cable which Feeltech undid when they swiped one as a convenient way to provide a hard earthing connection to the C14's PE tag in the 6800 and 6900 models.  :rant:  >:D

 If you're going to go to as much trouble as building an analogue PSU to replace the smpsu board used in these signal generators, then you might as well do the best job possible or not at all imo. This means specifiying a high quality mains transformer with a 16VA minimum rating with a couple of 18v ac 300mA minimum rated secondaries and a single 8v 500mA secondary.

 If you're contemplating an OCXO upgrade you'll also need to make allowance for the additional 100 to 300mA at 12v loading typical of a warmed up OXCO which might best be served by a separate analogue PSU which will allow you switch the generator off completely but allow the directly connected OCXO supply to carry on drawing power whilst it remains plugged into a wall outlet.

 It will be difficult enough to find a mains transformer with all the required secondary windings let alone one with two 18v secondaries and a centre tapped 16v secondary that can be used to power 5 and 12 volt analogue regulators. Alternatively, you could choose a higher VA rated transformer with 500 or 600mA rated 18v windings and just hang another 12v regulator off the rectified and smoothed +20v output to power the OCXO[1]

 Using the front panel on/off button to put these generators into standby only saves a couple of watts at most, leaving it to consume some 5 to 6 watts in this state so there's good reason to keep that OCXO module powered up independently of the signal generator if possible since it's not only the matter of the three minutes or so warm up delay but also that of "retrace" that's at issue here. Of course, if you're going to add a socket for an external 10MHz reference, the issues of warm up and retrace pretty much go away.

[1] If, when you've managed to succeed in finding a suitable transformer from which to power everything, the thought of having an additional 5 or 6 watts going to waste in Feeltech's version of 'Standby' when the OCXO can keep itself nicely warmed up and on frequency without this 'help' is any cause for concern, you can always fit powerFET switches into the DC rails feeding the regulators and either repurpose the mains switch to control them or add another separate 'standby' switch to turn everything except the OCXO off (assuming the several hours long 60ppt 'warm up drift' after an overnight shutdown escapes your OCD attention - if it doesn't, I suspect you might prefer to forego such standby economy in your quest to achieve the best possible frequency stability out your OCXO investment  ;) ).

 TBH, the only reason I have such a facility at all is simply because I hadn't upgraded the psu to one capable of handling the extra load from my OXCO modification so had no choice but to add a small half amp rated 12v smpsu board which gave me the opportunity to wire it directly to the mains connections on the C6 mains socket, bypassing the on/off switch.

JBG
Dbldutch:
While playing with the Lars DIY GPSDO, I came across a funny problem that I have not found reported about the FY6600 yet.
https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/

I wanted to use the FY6600 to mimic the signals for the gps 1pps clock and the 10Mhz oscillator. I used ch1 set at 1 second pulses, and ch2 for 10Mhz pulses. The Lars circuit uses a HC390 counter to create 5 and 1MHz pulses. The 1Mhz pulse and the 1 pulse per second go the the inputs of a HC4046. the 4046 resolves the phase difference and shows that as a pulse relative the the phase difference on the output. I'm elaborating here, because I initially though that I could use the phase setting of the FY6600 to create a difference in the edges and observe the working of the circuit.

What I found was a continuous stepped phase shift between the channels.

After some diggin' around, I attached the FY6600 outputs directly to my scope and observed the same thing. This phase shift is a function of the FY6600.
My version is pretty old, V2.9 software and V1.41 for the main circuit board. I did modify the power supply, see an above post, and I upgraded the main clock to a D75J-050.0 part, which is a 1ppm precision part. However, both mods are unrelated to the problem I see.

Anyway, as long as both channels are within one decade of each other, the edges are aligned. However, when one channel is more than one decade apart from the other, there is a continues and stepped phase difference.

This can be easily reproduced on my unit:
Set ch1 to a 1 Mhz pulse, and set ch2 to a 10KHz pulse. Connect both channels to a scope with the same length BNC cables and trigger on ch2.

Set the scope timebase to 10ns and closely examine the phase of the two signals, they are perfectly aligned (if you used the same length cables  ::)).
As a test, use the phase adjustment (but also notice the funny behavior on 4, 7, 10 degrees, etc - also a bug?) Apart from the jumps, all is in order it seems. Set the phase back to 0.

Now set ch2 to 1KHz (or lower). Note the stepped drift of about 2 ns between the channels about every 5 seconds.
Set the scope timebase to 200ns and observe that the trace of ch1 seems to "walk" to the left of the screen continuously.
Next, set ch2 to 100Hz. Note the much faster stepping with larger jumps and at a rate of 1 second.

This effect is probably due to the way the FPGA is programmed.
Needles to say, this makes the FY6600 pretty much useless for these kind of measurements.

Can somebody with newer hardware and/or software confirm this behavior or point me to a post that already described/reported this?

Tks!

pantelei4:

--- Quote from: Dbldutch on September 06, 2019, 11:30:06 am ---Can somebody with newer hardware and/or software confirm this behavior or point me to a post that already described/reported this?
--- End quote ---
FY6800
Yes, the generator cannot synchronize for multiple frequencies. Phase steps 4ns.
It cannot synthesize the frequency exactly, the jitter is also 4ns. He cannot divide the frequency by an integer and maintain synchronization. :horse:
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