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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
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rhb:
Don't get the 6600 as it has mains leakage voltages on the BNCs.

The sine wave performance of my 6600 at low output voltages compares very favorably to my Keysight 33622A which is a 120 MHz, 1 GSa/S AWG with a list of around $6500.  I got a used one from the Keysight eBay store for 1/2 that, but still my most expensive piece of T&M kit.  Ironically, I got a replacement front panel to fix my borked V3.0 unit just after I got the 33622A.  After 9 months of trying to get F***Tech to replace it.

At the moment I've got my 6600 which only has the grounding mod generating a 10 nS pulse at 1 V (2 V setting) into 50 ohms on a 1.5 GHz Lecroy DDA-125.  It's got a little bit of ringing, but the pulse is clean enough to use for time domain reflectometry and vector network analysis using a DSO with 10 Mpts or better sample depth.

That setup will give about 70-80 dB of dynamic range with an 8 bit DSO and suitable software which I plan to write.  It will provide VNA functionality up to the 6 m ham band at 50-54 MHz.  The same software will provide similar quality spectrum analysis.  It will only run on a PC, but it will do the job, albeit somewhat awkwardly.

A more ambitious project of mine is to develop FOSS DSO FW for COTS DSOs based on the Zynq and Cyclone V chips.  That will take a couple of years.  Right now I'm working on getting the data from the BNC to  memory with pipelined math operations in the FPGA PL on the way.
beanflying:
There is still some very good deals on what I assume is the last of the 50MHz 6600's on evilbay if your budget is lean. Forget the Linear supply upgrade and earth it like I did on about page 6 of this thread (I think about there). Treat it as a 'reasonable' Signal Generator and don't waste to much time and money on trying to make Champagne from Pig Swill.

Otherwise head for the 6800 as it seems to be firmware upgradeable at least  :horse:
d3m0:

--- Quote ---Treat it as a 'reasonable' Signal Generator and don't waste to much time and money on trying to make Champagne from Pig Swill.

--- End quote ---

Yeah of course I'm conscient of what I'm buying. I don't want to waste time and that's why I'm asking here. I think changing opamps for a few bucks is worth the time and money. The improvement looked in pictures I think is great.

So... going for a 6800 and only change opamps? What about firmware upgrade and open  bandwith?


Thanks again!
DaveR:
Neither the 6600 nor the 6800 is a waste of time or money (negative comments on here usually relate to problems with the earliest 6600s from 2017), but both can be improved quickly and cheaply.  Go for the 6800 (it has a better front panel) and fit new opamps.  Also fit a TCXO to give much better frequency accuracy and stop drift.  If you want to push the boat out, modify the existing power supply or fit a linear.  The upgradability is nice, but it's only relevant as long as Feeltech bother to produce new firmware for it, which probably won't be for very long - but there's nothing wrong with the existing firmware, so why worry about it?  That's really all there is to consider, unless you want to pay several times as much for a "branded" item which does much the same things.

Johnny B Good:

--- Quote from: d3m0 on March 08, 2019, 11:42:40 am ---
--- Quote ---Treat it as a 'reasonable' Signal Generator and don't waste to much time and money on trying to make Champagne from Pig Swill.

--- End quote ---

Yeah of course I'm conscient of what I'm buying. I don't want to waste time and that's why I'm asking here. I think changing opamps for a few bucks is worth the time and money. The improvement looked in pictures I think is great.

So... going for a 6800 and only change opamps? What about firmware upgrade and open  bandwith?


Thanks again!

--- End quote ---

 It all depends on what you're planning to use it for. If it's just for audio work or you're hardly going to push it beyond the 5v p-p level, upgrading the opamp and the PSU will largely be wasted effort. The dual THS3002i opamp only gets switched into circuit at settings above the 5v p-p "amplitude" mark and the 11.7ish volts on the +/- "12 volt" rails only becomes an issue with amplitudes close to the 20v p-p limit when you need to apply DC offsets of more than half a volt or so.

 At audio frequencies, the existing 50ppm rated XO chip is more than adequate for the job so upgrading that to a 0.1ppm TCXO would offer very little tangible benefit. However, regardless of all this, what is worth doing is the fitting of a small cooling fan to reduce the rather high internal temperatures due to the vent slots not being placed to make use of passive convective cooling to any effect (tilting it up on its kick stand actually makes the situation worse in this case).

 Although Feeltech seem to have taken notice of all the vociferous complaints about the issue of the half mains live 'touch voltage' and "done something about it" by upgrading the mains socket from the cheap 'n' cheerful IEC C8 unpolarised two pole connector, I'm afraid to say they've been a little slapdash over inflicting a heavy duty IEC C13/14 3 pole earthed connector on this 'tiny little doggy', giving it a tail that will wag it rather than it doing the tail wagging.

 Worse still, not only have they introduced a low impedance earth loop with its stray interference voltages and DC offsets from galvanic and thermocouple effects at millivolt output settings, they've compromised the ground return connection between the main board and the PSU zero volt rail by simply chopping one of the two ground wires to divert to the safety earth pin of the mains connector. It's as if they have no understanding of the pros and cons of the original floating class II arrangement, concentrating on fixing the "con" at the expense of the "pro".

 What this means for someone purchasing an FY6800 is that they need to undo the damage inflicted by Feeltech's bodging of the IEC C13/14 connector if they want to avoid seeing the mystery DC offsets (and ground loop interference) that Andreax1985 had reported when checking out his recently acquired FY6800.

 In short, this means disconnecting the safety earth wire from its connection into the chopped wire in the ribbon cable connector between the PSU and the main board, making good the damage inflicted by Feeltech by rejoining the cut ends. At this point, you can either add a 10K resistor between this joint and the disconnected earth wire from the IEC C13/14 connector's earth pin or else, find a better way to connect this resistor to the zero volt/ground rail to free this ribbon cable of Feeltech's bodgery.

 The PSU is still a class II item as before so doesn't require such a safety earth connection. All that was ever required of a polarised 3 pole mains connector upgrade with the FY6600 (and the FY6800) was a means of eliminating the half mains 'touch voltage' leakage via the EMC mandated Y class capacitor.

 In this case, a 10K resistive connection serves this function very nicely, attenuating as it does, the unwanted stray galvanic dc voltages and mains borne interference by some 60 to 80 dB, neatly limiting the half live mains voltage to, worst case on 240v mains, just half a volt ac which neatly addresses the issue of damage to any devices being tested through high voltage discharge from the class Y emc bodge capacitor in the PSU.

 If you feel that even half a volt ac is still too much of a risk, you can use a 1K resistor instead to get this down to 50mV ac and still attenuate the earth loop induced galvanic and noise voltages by some 40 to 60 dB. This resistive earth connection is essentially a very low impedance static discharge drain connection rather than a class I (III?) protective earth.

 Of course, the problem of doing any mods like this, no matter how urgent or essential, is the issue of "Voiding The Warranty". Unless you've bought this via a local Agent bound by your country's consumer protection laws, even when you have a valid warranty claim, the extra pain of dealing directly with Chinese suppliers can be sufficient to put you off making a claim when the original cost is so low as to make any out of pocket expenses (whether temporary or not) look out of all proportion to the benefit.

 Although the cost of these products is 'chump change' compared to the cost of  just the next quality level up (Siglent territory), it's right on the edge when it comes to deciding whether  to "Collect on the warranty or just write it off?". The sensible approach in this case would be to allow enough time for any manufacturing defects to show themselves within the 30 or 60 day return period before embarking on any warranty voiding modification work even if you've already decided that you'd write it off rather than face the hassle of trying to collect on a "Chinese Warranty" in the event it fails outside of that initial period.

 Although the earth loop and fan cooling mods are urgent and necessary mods, you can provide non invasive fixes by making up a special mains (extension) lead with a 10 or 1 K resistor in series with the earth wire and use a small desk fan to blow cooling air over the back of the generator to mitigate the excessive temperatures until you're ready to take the plunge into more elegant, yet warranty voiding, fixes once the honeymoon period is over.

 All of the improvements that were applied to the FY6600, are all equally applicable to its successor (including the half live mains leakage earthing mods - the bulk of the effort has already been applied by Feeltech, it just needs to be refined to eliminate the mains earth loop issue they'd introduced). It's basically just a question of deciding which, if any, of those mods are worth doing.

 Regarding the possibility of firmware updates/hacks to expand the frequency range past the current 60MHz limit, that seems rather doubtful since beyond 60MHz, you'll need to modify the 5th order anti-aliasing LPFs on the main board to nudge them closer to the 125MHz Nyquist limit.

 Being an arbitrary wave function generator, there's nothing to stop you loading a sine wave that's all 2nd harmonic and no fundamental to double up the output frequency to test the response of the on board LPF to identify exactly where the cut off frequency has been set to. ISTR a figure of 75MHz being mentioned a year or so back in this thread.

 This trick of loading a custom sine waveform into one of the arbitrary wave slots might be a handy work around to the 60MHz limit to get a useful signal output in the 4 metre amateur radio band (and possibly even into the VHF/FM broadcast band if all you need are a few tens of millivolts of signal output).

 The chances of Feeltech ever doing a firmware upgrade to raise the frequency limit to 75 or 80MHz seem pretty slim in view of the existing LPF's turnover frequency somewhere around the 75MHz mark. If they CBA to fix their 85 ohm attenuator goof, what chance is there that they'd modify the LPF filter components? In any case, such a raising of the turnover frequency would also require a steeper roll off to avoid aliasing problems above the 125MHz Nyquist limit so they'd need to upgrade it from its current 5th order design to a 7th or 9th order design which would probably only be practical using separate filter modules in place of the existing filter network components.

 Experimenting with a custom arbitrary sine waveform to explore the frequency response of that filter should give you some hint as to whether Feeltech are likely to extend the frequency limit with a firmware change in a later version of the FY6800 as they've done in the past with the FY6600 models. The existing filter might just allow Feeltech to offer a 75 or 80 MHz option but don't expect to see a 120MHz version in the same sort of price bracket any time soon.

 At the end of the day, these Feeltech signal generators are what they are, damn good value for the money 'wartified' versions of their more expensive cousins with the bonus of being amenable to several cost effective enhancements to cure it of the worst of its wartiness.

 The biggest danger for most of its users is that of getting carried away with the scale of these attempts to turn this particular "Sow's Ear" into a "Silk Purse". If you can dial your ambitions down from "Silk Purse" to "Rayon Purse" in this particular pursuit of perfection, that should save you from losing all sense of proportion and getting sucked into a never ending upgrading project that's taken on a life all of its own.

 The trick to this is knowing when to call time on the project and say, "Enough is enough! My work here is done.". For something as cheap as this, I'd say a good rule of thumb to apply is that once you realise that you've spent as much on the upgrade components as you've done on the whole generator itself, you've gone about as far as is sensible, if not further, and it's now time to insert those long case screws (mine have been a permanent workbench exhibit for the past 4 months now) for the very last time and be done with it.

 If your requirements for perfection increase with more ambitious projects later on, let the quote from the "Jaws" movie, "We're going to need a bigger boat" be your guide as to the solution and move upmarket to something better again. Your old,Feeltech AWG can always be boxed up and put into storage as an emergency generator - after all, it's not exactly a "Boat Anchor" that's likely to give you a hernia in ten years' time when you're eventually forced to have a clear out. :-)

 BTW, I thought I'd attach a picture or two of interest. The first one shows my permanent bench exhibit of the four case screws to the bottom left in the foreground. The u-Blox NEO M8N GPS module is just to the right, powered from a 5v wallwart with a 14cm  3/4 wavelength wire antenna plugged into the SMA socket (the built in Patch antenna only works sporadically due to a suspected error in the LNA 'reference circuit' used by the manufacturer) to receive some 6 to 12 satellites whilst a good 16 feet away from the window.

 It's set up this way because the eight metre (correction: 5 metre) active antenna cable won't stretch to the workbench. The scope is displaying a perfectly jitter free 2MHz squarewave on the PPS line. The 'ringing' as anyone observant enough to spot, arises out of the excess 10 inches' worth of earth clip lead between the probe and the zero volt rail of the GPS module.

 The second photo is a screen capture showing the rather horrific level of jitter you get when dividing a 48MHz TCVCXO clock  down with a non-integer division ratio to obtain a 10MHz clock. This demonstrates why I have three ON SEMICONDUCTOR - NB3N502DG - PLL CLOCK MULTIPLIER, 8SOIC chips winging their way to me as I type.

 The plan being to multiply the already jitter free 2MHz back up to 10MHz with just a mere 40ps p-p jitter on the clock multiplier's output (interestingly, with rise and fall times specified at just 1ns) to make a "Poor Man's GPSDO frequency calibration reference source" that's good enough to use as an external 10MHz clock supply for anything that has such an external 10MHz reference socket (currently nothing right now until I modify the FY6600 with such an option).

JBG

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