Welcome to the 6600 Club, Johnny! You've just reminded me that I still haven't got round to fitting the 3491s I bought for my FY6800 several weeks ago. I'd always planned to do a side by side comparison with the 3095s I put in the FY6600, but I got sidetracked onto other projects and forgot about it. It's now back on the list of jobs to be done! I'd recommend doing the D75J TCXO upgrade to your 6600 as well - it gives a huge improvement to frequency accuracy and stability.
Regards,
Dave
Hi Dave and thanks for that welcome.
I've had the benefit of a
lot of hindsight (over 12months of it!) by the time my interest had turned to the FY6600. :-)
Reading all 69 pages took about 3 days on my initial full read through in order to glean as much guidance in my decision as to which cheap Chinese ARWG to blow my hard earned upon (it was quite a compelling read!). I really owe everybody who has contributed to this discussion over the past 12 months or so a debt of gratitude so, thanks to everybody for your invaluable insights and suggestions. As a matter of interest, after having gotten hold of an FY6600-60M, I reread the whole of this discussion thread again! I just wanted to clarify a few of the points made in the various upgrades I was interested in.
I might be an old hand on Usenet but I'm a bit of a newbie when it comes to web based forums (fora). My one experience of participating in web forums was about a decade ago when I tried to stay in contact with the exiled zetnet clientèle after Breathe Networks Ltd (BNL) hoovered up the last of the decent small ISPs when the original owners were forced to sell up/their customers down the river (take your pick).
Zetnet had set up several private news groups on their news server (this was back in the day when many such small ISPs ran their own news servers complete with a few private news groups to serve their customers needs). Zetnet had its own USP to which BNL were entirely oblivious, so much so that I got myself a whole year's worth of free service through their total and utter technical incompetence.
Zetnet had a very loyal following in their customer base of which a large part had developed a strong sense of community through the private/closed zetnet news groups. When the end finally came, a web forum had been set up for the "Zetnutters", as we referred to ourselves, to maintain contact. The problem was, as it generally is, the rather klunky interface and the need to log on every time. Web fora are, in general, a piss poor substitute for Usenet imo, and I soon gave up on this rather klunky substitute for the old zetnet news groups.
Since then, my main experience of web forums has been in the form of anonymous guest read only sessions typically as the result of a google search for answers to various technical issues, where, as a silent witness, I''d often find myself screaming inside my mind at the abysmal lack of expertise and ignorance being demonstrated before my very eyes. Sometimes I'd find inspiration for solving a problem but most times, I'd just find the experience an exercise in frustration and move on.
Even where I feel I could join in simply to straighten out a few misconceptions, there was very little point since the thread I'd tracked down through a search had run its course, often several years earlier, leaving me with a sense of frustration that I've since come to terms with as the unfortunate side effect of such guest visits.
Here, in this case, the situation is very different. For one thing it wasn't the result of a search engine's attempt to direct me to a particular topic - it was a side note in a usenet posting in SED about the remarkably cheap price point of modern Chinese AWG signal generators where one of the contributors (possibly the OP himself) had posted a link to this discussion thread. The other point being that despite this thread's remarkable age, it was quite obviously still ongoing and a quick skim had revealed a very high signal to noise ratio with a high quality of technical competence which, for me at least, is an extremely unusual experience when it comes to web forums. Anyhow, here I am, actually participating (in my own gauche way) in a web forum! :-)
This leads me to explain that I'm currently seeing an error message (which I suspect is no longer actually relevant - I hope!) in this reply form, stating:-
"The following error or errors occurred while posting this message:
The message body was left empty."
as a result of my wanting to cancel my reply so that I could go back to click on the "Insert Quote" button by way of starting over again. Having opened the reply page, I could see no way of adding 'quotage' (hence this attempt to start over) other than to ignore the 'error message' and go back to your post and click the "Insert Quote" button in the hope that it would clear the error message - which it hasn't. I figure this is a a basic bug or 'feature' of the data entry input system that's handling the 'reply to function'. It's probably waiting on my hitting the 'send button' to recheck for content before clearing the error message. [Actually, all it needed was for me to press the preview button after having written something for it to display. :-)]
The point I'm making is that you might be (actually, you are!) reading a copy of what I've been typing into the message reply box (I'm going to copy and paste my efforts into a backup text file just in case :-) ). Also, of course, in my haste to make my first posting, I'd been anxious to get something written up to test my understanding of the reply mechanism, hence the terse nature of my introduction to the group.
Anyhow, now that I've explained my distracted first posts and sorted out the distracting error message in this attempt to reply, I can provide a little more info about my opamp upgrade. Having managed to cobble a special de-soldering bit (short length of 2.5mm solid copper wrapped around the heating element of my 25W Antex) to retrieve the original 3002 chip non-destructively (I did manage to slightly dislodge a tiny smd resistor in the removal process but I was able to to resolder it back in place with the aid of a tooth pick and my 18W Antex). I was able to solder the 3491s onto the U21/U22 locations with some difficulty (and trepidation) using my smaller 18W Antex.
I was aware of the issue with the extra Vref and /PD pins on the 3095 and the 3491 chips occupying isolated pads on the board. Not a problem for the 3091 but a problem certainly for the 3491 (can't recall if it was the same for the earlier 3095 chip) so I had a very careful read of the data sheet to confirm that whilst the /PD could be left floating, the Vref pin couldn't - it must be connected to ground or -Vss - I chose to strap it to the -Vss pin.
The 3491 has the added complication of the thermal pad on its underside. I was considering cutting out a tiny piece of thermally conductive silicone rubber to act as a 'gasket' but decided this was an unnecessary complication since it's electrically isolated anyway (although there is a circuit track or two in the landing zone that would only have the solder mask to electrically insulate them from the thermal pad - not a problem once the chip is firmly soldered into place) so I decided the best compromise was simply to place a very thin coat of heatsink compound in the landing zone between the solder pads. I also dabbed tiny blobs of heatsink paste on the opamps and the third chip covered by the heatsink prior to reassembling it back to the board. It's worth noting that the heatsink just about fully covers the U21 and U22 chip locations rather than just 60% of the original dual opamp chip - just as well since at full chat (20Vp2p 50 ohm load at 20MHz) the total mains input wattage has gone up from 8.8W to 9.9W measured with my trusty Metrawatt analogue watt meter.
I've more or less had the FY6600 running flat out since I completed the mod around 3:30 Monday afternoon which, as I type this, represents a 12 hour 'soak test'. It runs a little warmer but I've seen no sign of the 3491's fabled thermal protection kicking in.
Using a cheap IR thermometer, I got readings of 50 or so deg C off the base of the heatsink as well as the top of the Cyclone FPGA. Aluminium has a lower surface IR factor than most other materials hence my pointing the IR thermometer at the base where I'm probably picking up the PCB temperature (closed up, the base of the case feels just as warm as the two warm spots on the lid). I can touch the top of the FPGA with my finger without any discomfort but the aluminium heatsink is almost too hot to maintain finger contact with - but this is to be expected anyway, even when both items happen to be the same temperature.
With regard to that D75J TCXO upgrade, that's also on my to do list. I've been checking out availability of 0.1ppm 50MHz TCXOs and the cheapest units on Ebay claiming 0.1ppm, are all from the same Chinese based trader, yuyan-industrial with 3 different models ranging in price from £17.90 to £22.0). The question in my mind is, "Are there such TCXOs with 0.1ppm stability and accuracy to be had from Chinese suppliers via the auspices of Ebay, or is it case of the impossible AH claims made for Surefire 18650 Lithium cells and I simply land up with a 5 or 10 ppm "TCXO" (if I'm lucky)?".
TBH, I don't want to rush into spending any more money just yet until that FY6600 has survived the "Infant Mortally" phase of its service lifetime. Those THS3491IDDAT chips from Farnell/Element14 have already added an extra £27.36 onto the original £75.66 I paid for the sig generator in the first place, turning it into a £103.02 sig generator (still cheap for the improved performance, mind you). A two or three orders of magnitude improvement in frequency accuracy and stability would be a nice touch, especially if it can be done using a simple drop in replacement for the existing clock generator module. Shoehorning an OCXO module into the box is something I'd prefer to avoid if possible.
I think I should be looking for a better quality smpsu which uses the slightly more expensive type of ferrite isolating transformer with a screening foil layer to shield the secondary windings from the HV switching transients that get capacitively coupled onto the output terminals rather than rely on the EMC capacitor bodge responsible for the unconscionable levels of half mains voltage leakage current.
The real hazard of using the EMC cap bodge lies with its potential (pun intended) to discharge a high voltage into the DUT at the moment of contact - the 'half live tingle current' is merely an annoyance otherwise. If you want to retain the option to eliminate ground loop currents by letting the generator float free of any local safety earth connection, a properly designed smpsu is the best way to go imo (even if you do intend to make this a switchable option). I suspect the existing smpsu is now running close to its maximum rated output so worth an upgrade anyway.
Right now, I'm looking to source cost effective TCXO modules (preferably 'drop in replacement if possible) and a decent 15 to 20 watt rated three rail smpsu so I'd welcome any advice in regard of achieving those goals. :-)
Regards, Johnny B Good
[EDIT for future reference 2019-10-22]
A lot of water has since flowed under the bridge and many discoveries from hard won experience have led to a change of mind over wasting time in a holy grail search for any type of class II smpsu that exchanges the cost of a class Y2 cap for that of the penny expense of a shielding foil in the HF ferrite stepdown/isolating transformer to eliminate this common mode interference on the outputs free of any such 'touch voltage leakage' from the mandated by EMC directive Y cap to short the high voltage switching transients back to their origin to attenuate such unwanted switching noise by some 10 to 20dB (if you're very very lucky!).
In the end, after trying out some rather fanciful 'nulling out' schemes, done in the name of avoiding an earth loop issue, it became all too obvious that the only practical way to "Have your cake AND eat it!" was to fit a 3 pole mains socket (C6 or C14 - I chose a C6 to minimise the "Tail Wags Dog Effect"(tm) with this extremely lightweight signal generator) purely to give access to a connection to ground to permit a 3 to 10KR 'static drain' resistor connection to the common return rail (0v and BNC shield ground returns).
On a 240v mains supply, this knocks the 80 to 90vac leakage voltage seen with a typical DMM down to just half a volt rms (and less with a 3K resistor). There is absolutely no need for a hard earth connection in this class II mains powered device and the 10 to 3KR 'drain resistor nicely drops the Y cap leakage voltage to well below the safe limits for even the most delicate of electronic components you might otherwise damage with a 170 to 340vdc transient discharge from that accursed Y cap at the moment of contact at the worst possible point in a cycle of 50 or 60Hz mains voltage.
Assuming a BNC cable shield resistance as high as one ohm between the signal generator and the DUT's ground reference points, a 10KR will give you some 60dB and a 3KR some 50dB reduction in unwanted earth loop interference from noise sources in the mains earth wiring (neutral to ground potential difference voltages due to volt drops from the total house load current on the incoming supply, including possible thermocouple effects from hot junctions and the more likely galvanic dc sources of interference through electrolytic corrosion from damp ingress which could easily compete with any dialled in DC offsets in the tens of millivolts range).
Also, it's worth pointing out that the 'static drain' resistor will also attenuate any common mode switching transients being conducting back into the mains safety earth wiring, reducing radiated interference from the HF switching transients leaking onto the output ground return via the smpsu's transformer's inter-winding capacitance.
If you want to avoid replacing the existing smpsu board with an analogue bodge psu (for the time being at any rate whilst you choose the most optimum analogue based design), I suggest you upgrade the output rectifier diodes to proper Schottky types, double up the 220μF 16v caps on the +/-12v rails to 470μF 16v 105 deg C types (voltage rating _isn't_ the issue here) and wind a single turn buck winding on the transformer to buck the 5v winding voltage (half a dozen single turns of thin insulated wire in parallel to give better coupling and minimal loss) which neatly forces the +/-12 rails to something like 13.5v each on load without resorting to increasing the 5 volt rail to 5.5v as per the earlier trick to get a decent voltage level on the 12v rails. You may have to reverse the connection between the lifted out cathode of the 5v diode and the pcb hole it was disconnected from.
The details were posted here, with pictures on page 74 of an 86 page (at the time of this edit) topic thread:
https://www.eevblog.com/forum/testgear/feeltech-fy6600-60mhz-2-ch-vco-function-arbitrary-waveform-signal-generator/msg2310768/#msg2310768 I'm still using the original (if many times modified smpsu board) at this time but I do have plans to replace it with an analogue hybrid (classic 3 terminal voltage regulators for the +/-13.5v rails powered from 15 or 16vac secondary winding(s) for lowest ripple noise on the analogue voltage rails with a 5v 1.5A rated output dc-dc converter connected to any ac winding voltage in the range 7 to 18 volts (to give me more transformer secondary options - ideally a separate 8vac winding to maximise isolation of the digital and analogue rails and optimal voltage choice for use of a 7805 regulator if deemed beneficial in spite of the modest additional heat load this would introduce).
However, I do want to test the benefit of completely eliminating the switching noise of the existing psu by putting a recently acquired pair of 6v carbon zinc lantern batteries in series for the -12v rail and a 12AH SLA to power the +12v rail and a 7805 regulator for the 5v rail to provide noiseless sources of dc power to allow me to determine the potential benefit of an analogue psu upgrade. It's now just a matter of finding a Round Tuit to run such a test setup.