Revisiting the FY6600 FY6800 FY6900 attenuator problem. - Page 1
Forum section: electronics / testgear

author: Chris Learing
author_rank: Contributor
timestamp: November 22, 2021, 03:08:49 pm
content: New member here, so be gentle.I saw several suggestions with regard to the horrible attenuator in these AWGs. In particular the 85 vs 50 Ohm issue. Not only that, but the firmware appears to have been cobbled because the attenuator is not exactly 20dB but it still produces the correct output for the display (into a Hi Z). There were several solutions suggested in the various threads, but as far as I could tell none of them would work properly. They would either give the correct output voltage into a Hi Z, or into a 50 Ohm load, but not both. I may have missed some other suggestions in the very long threads. Anyway I decided to look at it again.If the input shunt resistor is changed to 115R, the series resistor to 301R and the output shunt resisor changed to 115R||118R (giving 58.24R) all appears to be well. Correct voltage into Hi Z, correct 6dB further attenuation into 50R. Give or take a small error.This was worked out by calculating the attenuator to give a 19.7dB loss (the same as the existing one) but with input and output impedances of 85R and 50R respectively.Forgive me if this has already been suggested, but if it has, I couldn't find it.Chris

author: Chris Learing
author_rank: Contributor
timestamp: November 24, 2021, 11:12:38 am
content: For anyone interested (which appears to be nobody) the parts arrived this morning and I've just carried out the mod in practice. It works perfectly. Correct voltage at Hi Z and at 50 Ohms.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 24, 2021, 12:40:37 pm
content: Hi Chris, thanks for providing this and welcome to the forum. I suspect your thread could not be found in searches due to its naming: perhaps put the exact model numbers that are affected by this? I don't think a search with correlate FY6X00 with the FY6600/FY6800/FY6900 models. Also, perhaps post on the other discussion threads (necroposting, I know) which will trigger a notification to everyone interested in these funcgens.Regarding the problem at hand, I was not paying attention to the entirety of the other discussions and therefore not entirely familiar with the problem: my FY6600 indicates "Amplitude" but it is in fact peak-to-peak. Is this what your solution addresses?I just learned to live with this idiosyncracy on my generator...

author: Chris Learing
author_rank: Contributor
timestamp: November 24, 2021, 12:49:01 pm
content: Thank youNo, the peak to peak remains. Nothing else really makes any sense on waveforms other than a sine wave. I think Pk Pk is pretty normal on AWGs, not sure.The problem was the output impedance. 50 Ohms (as normal) on two of the ranges, but 85 Ohms on the lowest range. So when it was loaded into the normal 50 Ohms the output voltage fell by a completely different amount than most people would expect. It seemed to have irritated several people in the other thread. It certainly irritated me.I've changed the title. That probably makes more sense. Ta.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 24, 2021, 02:34:51 pm
content: Thanks for the clarification; I haven't realized that but will test and eventually apply your fix to my funcgen. I am lucky enough to have another excellent HP3314A, where I can always keep the FY in check.Regarding the terms, Amplitude is relevant for all periodic ordinary waveforms (sine/square/triangle/sawtooth) and should reflect half of peak-to-peak. That is why I ended up just living with that...

author: mawyatt
author_rank: Super Contributor
timestamp: November 24, 2021, 03:27:56 pm
content: Not knowing the whole story behind this, so with that in mind:Couldn't one measure the AWG output resistance using an accurate resistor of somewhere near 50 ohms to load the output. Using a DC output from the AWG a DMM could measure the open circuit output voltage, then load the AWG output with the mentioned resistor and measure the DC voltage again.Rgen = R(Voc/Vload -1), where R is the applied "load" resistor, Voc is open circuit voltage and Vload is voltage with load resistor applied.Best,

author: Chris Learing
author_rank: Contributor
timestamp: November 24, 2021, 06:12:43 pm
content: Quote from: mawyatt on November 24, 2021, 03:27:56 pmWell, yes, but we already know the output impedance. It is 84.5 Ohms. What my suggestion does is correct this to the 50 Ohms it should have been when it was made.

author: mawyatt
author_rank: Super Contributor
timestamp: November 24, 2021, 07:43:06 pm
content: Quote from: Chris Learing on November 24, 2021, 06:12:43 pmIs this because the AWG has incorrect attenuation resistors installed at build by OEM, and just a build error on yours?Best,

author: Chris Learing
author_rank: Contributor
timestamp: November 24, 2021, 07:58:59 pm
content: Quote from: mawyatt on November 24, 2021, 07:43:06 pmThey are all built like that. All three models going back years.

author: mawyatt
author_rank: Super Contributor
timestamp: November 24, 2021, 08:33:33 pm
content: Quote from: Chris Learing on November 24, 2021, 07:58:59 pmWow, that's really not a glowing report for the OEM!!Best,

author: normi
author_rank: Regular Contributor
timestamp: December 31, 2021, 10:09:15 pm
content: Chris,Do you have any pictures of these resistors and their board location.

author: theHWcave
author_rank: Regular Contributor
timestamp: January 02, 2022, 06:48:10 pm
content: Hi Chris, can I second norml's request for pictures and board locations please (I have a 6600)

author: rsjsouza
author_rank: Super Contributor
timestamp: January 02, 2022, 11:29:27 pm
content: Additional details about the attenuator resistors can be found at:https://www.eevblog.com/forum/testgear/feeltech-fy6600-60mhz-2-ch-vco-function-arbitrary-waveform-signal-generator/msg2014117/#msg2014117

author: Chris Learing
author_rank: Contributor
timestamp: January 04, 2022, 10:42:01 am
content: I stole the attached photo from another thread and added two red circles.Left resistor is the input shunt resistor (1), middle is the series resistor (2), right resistor is the output shunt resistor (3). Bottom set for channel A, top set for channel B.(1) - Change to 115R(2) - Change to 301R(3) - Change to 115R in parallel with 118RThis fixes the problem.Edit: Correct typo.