Products > Test Equipment
FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
DaveR:
Hi josh - BIAS is just the screen shorthand for the modulation frequency of the output, which can be set from 1uHz up to 1MHz. As for the default output voltage, there is no way to change it from the front panel, but it could be changed by tweaking presets W5 (for Ch1) and W3 (for Ch2) on the main board. I couldn't say offhand over what range you'd be able to adjust it, though, and you'd obviously have to live with incorrect screen vs actual figures until you recalibrate it.
Regards,
Dave
yo0:
--- Quote from: DaveR on November 04, 2018, 01:33:58 am ---Hi Pio - those screenshots are probably just taken from an early model as the 6800 went into production, and still used as examples for advertising on ebay etc.. I doubt very much whether you'll get anything other than v1.6 or 1.7 if you buy one now.
Regards,
Dave
--- End quote ---
Thank you Dave!!
Best regards.
Pio
Johnny B Good:
--- Quote from: Vytautas on November 14, 2017, 10:41:16 am ---Hi, cybermouse,
Just to be sure, I tried powering the modified board with new op amps from the original switching power supply. Sure enough, the waveform is clipped and heavily distorted in high amplitude settings. So, the voltage from that supply is not enough. +-15V is a must (no wonder, this is to be expected).
--- End quote ---
That's a surprise considering the 5v p2p limit above 20MHz. I wouldn't have thought the +/- 11.5 ish volts of an unmodded PSU board would have been a problem at a mere 5v Pk to Pk setting. The voltage swing limits pretty well just about save the generator from gross distortion of the sine wave output even for the THS3002 dual current feedback opamp chip that Feeltech have stuck with 'through thick and thin' despite the need to swap over to the later single CFB opamps they'd made provision for in the unpopulated U21 and U22 locations on the main board once they went beyond the original 15MHz limit in the first version of the FY6600.
Apologies for the *very* late response (and first posting) but I didn't discover this EEVblog forum until about a fortnight ago as a result of a posting in the sci.electronics. design (sed) news group (Usenet) under the thread titled: "amazing ARB pricing" way back in October 2018 (the 25th) where my initial contribution had been to seek advice on sourcing a cheap oscilloscope.
This set me off on a quest to buy myself a cheap Chinese 2ch DSO which I resolved by purchasing a nice SDS1202X-E from Siglent's UK agent (Labtronix) just 5 days later which was delivered just two days after that. It was only then that it occurred to me that a signal generator might be worth obtaining as a useful accessory to my DSO purchase, hence my taking note of the link to this forum that had been provided by the OP in the sed "amazing ARB pricing" thread.
At that point, this forum thread had grown to a massive 69 pages and I read every last page (three days!) before deciding, in spite of the few serious negatives with the earlier firmware, to purchase an FY6600-60M model for the princely sum of £75.66 delivered.
FYI, the model I received reports firmware version 3.3 which seems to have addressed a lot of the issues mentioned such as the long press of the "OK" button (the rotary control knob) to set the current settings as subsequent defaults on power up failing to provide a feedback 'beep' - it now does so (after about one second's worth of keeping the knob pressed).
The first mod I applied was to add a 47K resistor to the PSU to boost the 4.95v (loaded) to 5.49v so as to raise the 12v rails from circa 11.5 up to circa 12.5 volts. A few days after that I decided it might be a good idea to replace the 12v rectifier diodes with something better than those 1N5819s (not the best choice imo) with some proper 30A 45v rated dual shotky rectifiers (TO220 package - it was a bit of a tight squeeze!) recovered from scrapped smpsus which gave me a quite surprising boost to 13.7v on the +12v rail and 14.05v on the slightly less loaded negative rail.
I didn't bother improving the 5v diode since this will reduce the +/-12v rail voltages. The just below 9 watts consumption 'flat out' has now dropped by about a 100mW in spite of the voltage boost which indicates a reassuring improvement of PSU efficiency. :)
I haven't bothered trying to tackle the half mains voltage 'tingle current' issue since I know this is the inevitable consequence of the mandatory EMC 'grounding capacitor' (typically a 1nF Y class) in a class II double insulated PSU. The best solution imo, is to track down a better class of smpsu where the need for such a capacitor has been obviated by the use of an HF ferrite switching transformer with the necessary screening foil built in to shield the secondary winding(s) from the HF voltage switching transients on the HV primary.
In a class II double insulated PSU, this shielding foil will still be connected to the 'half mains' voltage grounding reference but will only present a few pF of leakage coupling of the 50/60Hz half mains voltage rather than the 470pF or higher of a class Y capacitor.
Since I did these PSU mods prior to recording my DSO traces as a 'before the opamp upgrade' record, I can only show attached jpg images for the 20Mz at 20v into 50 ohm load after having improved the PSU, so I have no proof that my mods are of any benefit with the existing opamp chips. Nevertheless, I've attached them for reference.
I ordered a couple of THS3491 opamp chips from Farnell/E14 last Thursday and I'm expecting them to finally get here, as promised by UPS, this Monday ("By the end of the day"). It'll be interesting to see whether these will offer any further improvement over that seen with the lower spec THS3095s that had been used by one of the posters to this thread earlier this year.
The problem in this case being the performance limitations of the OPA 686N chips driving them and the relay switching which bypassess the THS3002 (or 3091/3095/3491 upgrade) for frequencies above the 20MHz limit. It could well be that the crafty relay switching employed in Feeltech's design to minimise the shortcomings of the THS3002 chip will probably turn the THS3491 upgrade into an 'overkill solution'. Still "Better safe than sorry.", after all, there wasn't that big a price premium in opting for the 3491s over the slightly cheaper 3091 or 3095 chips and, who knows, I or someone else might come up with a mod to take full advantage of these opamp upgrades. :)
Regards, Johnny B Good
Johnny B Good:
--- Quote from: Vytautas on November 13, 2017, 06:17:40 pm ---Hi, cybermaus,
The problem is mostly due to the "30021" op amp. As you say, it is only just above 7Vpp into 50 ohms. And I had tried at first powering the generator from my lab supply with +-15V. The improvement was miserable if any. So, it must be the op amp.
With the THS3095 the generator easily outputs 20Vpp (10Vpp into 50R), no problem. Still, if to be picky, the behavior of the THS3095 is in line with what the manufacturer shows in their datasheet for the component. The pictures bellow are taken from the datasheet. At high frequencies, as those graphs also show, I see some distortion (not clipping though but exactly the same as manufacturer graphs show). The manufacturer suggests properly paralleling two op-amps for each channel (second picture) to remove most of the distortion. That is what major manufacturers, like Siglent, and others do in their designs. For the sake of interest, I might have a look at that later (accidentally, I do have two more THS3095s in my drawer). By the way, Texas Instruments have recently introduced a new power opamp - THS3491. If what they say is true, that one is a real beast with unmatched performance. But they do not have them yet.
Sure, the generator steps down to 5Vpp at 20MHz. But that is to be expected. Normal behavior. Difficult to achieve frequent swings at high voltages.
--- End quote ---
Although the subject of replacing the THS3002 dual opamp seems to be just over a year old (I've not been able to track down later opamp references), rather than directly follow up on my previous, first time post, I thought a 'reply' to this post would be quite apposite in view of the fact that I've just replaced the stock 3002 opamp in my FY6600-60M (purchased almost three weeks ago) with a pair of THS3491s and so can verify that 'what they say' is indeed true! :-)
I've taken the liberty of reposting Vytautas's oscillographs showing the improved 20MHz, 20V p2p waveforms along with my own 'before and after pics' which do show a further quality improvement over the earlier 3001 and 3095 chips. Unfortunately, this modification doesn't improve the quality of square waves beyond what was available with the original dual opamp. I can only suppose this is a limitation imposed by the anti-aliasing filter.
Regards, Johnny B Good
DaveR:
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
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