Products > Test Equipment
FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
Johnny B Good:
--- Quote from: rhb on December 04, 2018, 01:01:17 pm ---
--- Quote from: Johnny B Good on December 04, 2018, 04:04:17 am ---....snip...
BTW, I still think the JDS6600 is a bag of shite. It's merely a tarted up poor man's FY6600 imho and not worth its £62.48 plus £6.99 economy delivery price tag.
Regards and a merry Xmas, Johnny B Good
--- End quote ---
I'm going to have to read through all your ramblings on the F***Tech. It's both entertaining and educational. But you still didn't give a reason for your denigration of the JDS.
Still, it's a great pleasure to encounter someone who both knows what they are writing about and writes well. My BA is in literature, so I have done a lot of reading.
If I didn't have an Instek GDS-2027E sitting open on my bench while I try to sort the JTAG and TTY port I'd have the F***Tech open and follow along.
--- End quote ---
I had to google "GDS-2027E" to check what it was you were working on. No sensible results until I tagged "DSO" on the end. First hit was Farnell/E14's details page on this stock item (one in stock at £462 plus the 20% VAT). Now that's more within my spending range! :) However, now that I have an SDS1202X-E (a snip at £365, VAT and free delivery included), I'm not in the market for another 70MHz DSO.
The big downside to the JDS version is the use of R2R ladder networks to save the cost of a couple of high speed 14 bit DAC chips. I did see the expected artefacts demonstrated in a tear down video but can't find it on Youtube. Almost certainly it was a link in an EEVblog posting but a quick search for threads mentioning the JDS6600 failed to reveal where I'd seen that particular video and another search on youtube was just as fruitless as was a search through my browsing history.
If you want your full amplitude sine waves to be free of glitches at the zero crossing and half amplitude points, it's best to avoid assembling an R2R ladder network out of even the highest precision resistors available unless you're only going for an 8 bit resolution in your DAC circuit (you'll need to specify resistor values to a precision better than 0.1% for the ninth bit then 0.05% for the tenth and ever tighter by a factor of two each time you add successive bits of resolution to your R2R ladder network).
You wouldn't expect any sane EE to try and outcheap Feeltech's FY6600 design but the JDS6600 represents a reduced BoM costed version that sacrifices performance out of all proportion to the minuscule cost savings involved (it's essentially the core of the FY6600 with the DAC chips substituted for by four dozen or so (high precision?) resistors.
Just how cheap do you want your poor man's version of a £360 Siglent dual channel 30MHz AWG to be? I'd happily pay another tenner for a version of the JDS6600 that used proper 14 bit DAC chips... oh wait, I did! :) Not quite an extra tenner though, just an extra 6 quid as it happened. :)
Regards, Johnny B Good
PS I've not had any more problems with the PSU board since I dialled the capacitor upgrade back a couple of notches (the 470μF substitutions for the original 220μF caps on the 12v rails don't seem to causing any problems for the driver chip). That's not to say I've stopped seeing that small third of a watt variation in its consumption. It just means I'm happy to accept that it's a characteristic of the generator's operation when supplying steady output signals into 50 ohm loads rather than some sign of impending PSU failure.
PPS That kakamaimee output attenuator resistor network seems to have been designed as a 20dB pad for an 85 ohm impedance line. :wtf: The firmware mediated compensation for the insertion of this pad for 500mV and below is only correct for the unloaded Hi Z case. What is required is to calculate the voltage attenuation ratio for this Hi Z case and design a 50 ohm attenuator pad that gives the same unterminated output voltage level and the extra 6dB of attenuation that's expected when properly terminated. I've only gotten as far as determining that it's an 84.755 ohm line attenuator.
I'll try doing the rest of the calculations later on. I should be able to come up with a corrected BoM list for the attenuator resistor network compatible with their 'quick fix' firmware compensation for the Hi Z error condition[1].
[1] They were probably aiming to introduce a 20dB drop in output level, ie a reduction to 10% of the (terminated) output voltage with an expected reduction of attenuation to 20% in the Hi Z case. What they actually got was a much greater reduction to 10.33% in the Hi Z case which they must have 'corrected' for in the firmware (but only good for the Hi Z output case). the 6dB drop expected with a 50 ohm load becomes an 8.6 dB drop in this case (I think).
Trying to reconfigure for a 50 ohm attenuator pad that provides the same open circuit voltage attenuation and hence the expected 6dB drop when terminated with a 50 ohm load is doing my head in, even with the help of an on line dB calculator and a Matching Pi Attenuator Calculator. Never mind, I'm sure I'll find a solution... eventually! :-\
[EDIT 2020-04-14]
I did find a solution! It turns out I'd been overthinking the problem and giving F***tech far too much credit for ingeniously disguising the cheap choice of resistors used in their attenuator networks (RS1 to 6) by a 'firmware bodge' when in fact all they'd done was simply pick a cheap resistor combo that would provide the 20dB voltage reduction in the Hi-Z out[put case only (and let the Devil take the hindmost - those users relying on the output remaining at a 50 ohm impedance - for settings at or below the 500mV p-p mark).
Using preferred values from the more expensive E192 range, substituting the 510 ohm with a 249 ohm series element and each of the 100 ohm shunt elements with 61.2 ohm resistors produces a 20.03dB @50.11 ohm attenuator pad. Even relying on the +/-0.5% tolerance of the E192 range limits the variations to a worst case low of 19.96dB @50.28 ohms and a worst case high of 20.1dB @49.94 ohms, still plenty good enough for use as 'drop in' replacements.
Anyone looking for a 'quick fix' to this 85 ohm attenuator issue, only needs to know that the required replacement is exactly a 20dB @50 ohm attenuator pad. The existing resistors (W1 to 6) can be shunted with additional smd resistors (easily calculated) soldered over the top, saving having to deal with the ticklish business of extracting the originals without risking harm to the board. At the modest 60MHz upper frequency limit, the small additional stray capacitance introduced by doubling up resistors in this way can safely be ignored.
JBG
bugi:
--- Quote from: Johnny B Good on December 05, 2018, 03:23:48 am ---If you want your full amplitude sine waves to be free of glitches at the zero crossing and half amplitude points, it's best to avoid assembling an R2R ladder network out of even the highest precision resistors available unless you're only going for an 8 bit resolution in your DAC circuit (you'll need to specify resistor values to a precision better than 0.1% for the ninth bit then 0.05% for the tenth and ever tighter by a factor of two each time you add successive bits of resolution to your R2R ladder network).
--- End quote ---
I have some vague memories of reading mentions that the resistor network on JDS isn't all R2R, but something else at one end, apparently exactly for the reason to make it a bit more accurate.
Also, IIRC, the manufacturer itself mentioned something about them doing something with the R2R/resistors stuff so that the accuracy would be better than expected with standard way to putting one together. Which I read in between the lines that either it refers to that something-else-than-R2R, and/or perhaps they use binned components (i.e. higher tolerance relative to each other, but not necessarily closer to the nominal value). Maybe... maybe just marketing talk.
I bought such JDS unit (didn't even know about the FY-stuff at the time). The little I have used it so far, the biggest problem isn't tiny errors in the DAC, but larger issues at higher frequencies/amplitudes caused probably by the amplifiers, and power supply noises etc. But I guess the biggest issue depends on what the device is used for.
ledtester:
On the subject of replacing the power supply, here's a video of one guy experimenting with using DC-DC converters:
Johnny B Good:
--- Quote from: bugi on December 05, 2018, 08:37:06 am ---
--- Quote from: Johnny B Good on December 05, 2018, 03:23:48 am ---If you want your full amplitude sine waves to be free of glitches at the zero crossing and half amplitude points, it's best to avoid assembling an R2R ladder network out of even the highest precision resistors available unless you're only going for an 8 bit resolution in your DAC circuit (you'll need to specify resistor values to a precision better than 0.1% for the ninth bit then 0.05% for the tenth and ever tighter by a factor of two each time you add successive bits of resolution to your R2R ladder network).
--- End quote ---
I have some vague memories of reading mentions that the resistor network on JDS isn't all R2R, but something else at one end, apparently exactly for the reason to make it a bit more accurate.
Also, IIRC, the manufacturer itself mentioned something about them doing something with the R2R/resistors stuff so that the accuracy would be better than expected with standard way to putting one together. Which I read in between the lines that either it refers to that something-else-than-R2R, and/or perhaps they use binned components (i.e. higher tolerance relative to each other, but not necessarily closer to the nominal value). Maybe... maybe just marketing talk.
I bought such JDS unit (didn't even know about the FY-stuff at the time). The little I have used it so far, the biggest problem isn't tiny errors in the DAC, but larger issues at higher frequencies/amplitudes caused probably by the amplifiers, and power supply noises etc. But I guess the biggest issue depends on what the device is used for.
--- End quote ---
It's easy for me to point out the deficiencies of the poor man's version of the FY6600 (not that that didn't have its own defects including the very serious issue of the early firmware version 3.0 bricking the AWG). However, since people are still being tempted by its lower price point (about £6 cheaper than what I paid for my FY6600 just over a month ago) and improved 'haptics', ignoring the loss of the additional features of the FY6600, I think it's well worth reminding anyone, who has yet to sample one of these gloriously cheap AWGs for themselves, of this particular deficiency for which no practical DIY remedy exists (unlike the other issues of the PSU, the clock chip, the high voltage output opamp and the kakamaimee output attenuator that's automatically switched in for the sub 501mV p-p output amplitude range - not forgetting the need to add a small 40 or 50 mm cooling fan to stop it cooking itself into an early grave).
I most definitely saw a video where the entirely predictable glitches at the zero crossing and half amplitude points of its sine wave output being unambiguously displayed on a 'scope trace and commented on by the reviewer. Unfortunately, I haven't yet been able to track down this review and tear-down video for all to see for themselves rather than simply just having to take my word for it. >:(
Regards, Johnny B Good
Johnny B Good:
--- Quote from: ledtester on December 05, 2018, 10:54:17 am ---On the subject of replacing the power supply, here's a video of one guy experimenting with using DC-DC converters:
====link removed====
--- End quote ---
I've seen this guy's earlier videos on the subject. I can't recall watching this video before so thanks for that link. It was an interesting insight into how a project can get out of hand (BTDT>BTS!). This is yet another video on how not to eradicate the half mains leakage of a cheap class II smpsu free of the HF switching transients that get through via common mode conduction path on the output wiring when you remove that bloody 1 or 2nF class Y capacitor responsible for the problem as a "quick 'n' dirty fix".
He's on the right lines but I do have to question the wisdom of incorporating the mains transformer and rectifier pack inside the box. It strikes me that a better solution would be to use an external 16 to 20 vac 15VA transformer supply, leaving the rectifier and smoothing inside from which to feed the +/-12 or 15 volt and +5v dc-dc converter modules.
Such modules are quite tiny so could be mounted clear of the original PSU board to leave the way open for an improved smpsu board equipped with a ferrite transformer blessed with the shielding foil layer to screen the secondary windings from the high voltage switching transients on the primary winding, neatly obviating any need for that accursed class Y emc bodge capacitor to produce a supply with a similar to, if not better than, isolation of a supply using a small conventional mains transformer.
If he'd taken that approach to keep his future PSU upgrade options open and placed the dc-dc converter modules in the space between the existing PSU board location and the front panel, he could have made a neater job of adding fan cooling as I've previously described using a slimline 50mm 12v fan powered off the 5 volt supply mounted onto the base between the PSU and the rear panel (and blocking off those useless rear vent slots for good measure).
The side vents are actually quite effective once you remove reliance upon the thermo-siphon effect alone as a passive cooling solution and provide just a modicum of forced ventilation in this way. It also helps to "gasflow" the accidentally provided exhaust leakage path provided by the RHS front panel clip slot in the base of the case to help dissipate the heat which builds up under the main board. :)
Regards, Johnny B Good
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