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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator

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soundtec:
Page 35 of the THS 3491 data sheet explains things clearly in relation to rF and rG, layout and incidental capacitance etc .
An adjustment pot could cause extra pF's to be added to the feedback network and hamper high frequency stability .
Maybe a bag of 50 each value 1.5k and 499ohm 1% parts could be selected down into matched pairs with very close to 1:3
ratio or I do see some 0.1% in batches of ten that might be worth a try . Im unsure of the wattage rating and size of the resistors needed also.


DPA31:

--- Quote from: soundtec on August 25, 2018, 12:39:57 pm ---
What tollerance are you guys seeing on the 499, 1.5k ,10kand 50(2x100ohm)  parts ?

--- End quote ---

Hi Soundtec,

Here are the resistor values measured in situ on channels 2 & 1:

498 & 497
1492 & 1485
10.15 K & 9.99 k
50.4 both

Dom.

soundtec:
Those figures are interesting Dom ,thanks .
I think it shows why the output from the final stage is off by a margin , slightly low compared to where it should be .
To be honest Im not very wall practised in SMD rework , I look at the size of those resistors and the density of components I the area ,and get a bit scared by thinking about how easy it would be to mess up the board . Still I very much would like more accuracy from the amplitude display ,so Im strongly cosiddering upgrading all the resistors to 0.1% or better if I can through selection of components .

In the mean time ,I put together an enclosure for my post attenuator , it has a toggle switch with  three positions , -10db, -20db, -30db , I also have  a rotary two pole switch with 12 positions I'll set up as 10 single steps of 1db , a unity gain position ,and an off position that shorts the DUT input to ground through a 150 ohm resistor for noise testing . As far as precision is concerned ,if I can make the 10 db ranges accurate ,I dont really mind some 'slop in the gears' on the 1db range switch .

As you will see from the photos Im all done with the casework and constructional side ,she's looking good  ,I have an idea the form the attenuators will take in my head but Im still not sure which is the most appropriate style of attenuation ,ladder or H network etc .
I also found a very neat stepped attenuator calculator ,you fill the blanks in terms of charachertistic impedences and attenuation  you want and the software calculates the values , I think you can model several different kinds of attenuator topologies with it too . I'm unsure about the exact form both attenuation circuits need to be to function properly in series , and any advice people can offer would be welcome .

Its all a bit chicken and egg  as far as choosing impedences , I need it for audio signals primarily ,but Id also like the attenuator to be usable at least up to a couple of hundred khz sine, I'll need to be able to feed single ended line level audio inputs of 10 or 20 kohms as well as high impedences  directly from the output ,in the case of balanced connection  being required a mumetal transformer can be used across the outputs ,I have a 1:1 10k-10k sowter transformer thats good up to about +10dbu and 50khz of bandwidth feeding 20 kohms input impedence. For mic level inputs which are around 3kohms normally, maybe a mic input transformer used in reverse(10k-600ohm) might work well.


I need more specific advice before I start building the attenuators really  ,so I dont end up having to re-do it again ,you know the old saying 'measure twice cut once' , but my head is a little fuzzy on the maths   :palm:

Pictures to follow soon , coffee time . I'l post too another forum about the specifics of the impedences and see if anyone likes the idea enough to fill in the blanks for me .

DPA31:
Hi Soundtec,

You have an interesting project. Could you please put the link for your attenuator calculator.

You mentioned attenation of -10dB, -20db, -30db : are they dBV (20lov V2/V1) or dBW (10log V2/V1).
I understand you keep 50 ohms as output impedance, am I right ?

To continue on THS3095 gain adjustment :
I set 5.0V sine 50Hz and I measure 1.768Vrms that is 5V pp
Then I set 5.1V and I got 1.7655 V but at OPA 686N output I got only 0.446V rms which multiply by gain 4 gives only 1.784Vrms instead of 1.803V rms. Before to

Dom.

soundtec:
 A little passive test Jig set for audio signals comprising the attenuator ,a transformer , switchable low pass filter  (to improve matters when making THD measurments) and some patch cables makes a nice addition to the FY6800  .

Maybe a simple LC low pass could be integrated into the 10,20,30db attenuator network ,then either switched to one of an handfull of known frequencies or out of circuit as required ,might afford a reduction in the harmonics of 40-50 db or more from the raw sig gen output. If we're at .03%THD at full output from the gen  ,if we could mange 60 db of attenuation say at 2 khz ,driving a 1khz signal ,wouldn't that give us .00003%THD (with some attendant insertion loss in amplitude terms of course).

Im not sure  100% sure about setting the characteristic impedence of the attenuator ,50 ohms ,its always handy to have low -Z output capabillity ,but this will come at some penalty in load on the output IC's and distortion produced  , it will also cause -3db output due to the series output resistor ,on the plus side with a 50 ohm characteristic bandwidth will probably be in the many hundreds of khz. Maybe 600 ohm might be a good compromise for general audio work , I do see an implementation of the 3491 with 2,3 and 4 op amps per channel in the spec ,at high frequencies the thd is reduced by more than 30-40 db over the case of a single chip , thats at 50 ohms ,if your opperating into higher impedences at audio frequencies THD dissappears off the scale to -100db or more down from fundamental .

dbu or dbv might be the correct expression,  I think dbu relating to a given impedance and reference level,dbv relating purely to voltage  ,Im a bit sketchy about that ,so double check for yourself in anycase .

I think the effect your seeing on the output levels is the purely down to 1% components on the network surounding the output ic  Dom ,broadly the results of the voltage checks agree with the ohms your measuring on the individual components . Im just looking for a bit of software I found last night which allowed allowed statistical evaluation of rF and rG and its outcome on the accuracy of op amp gain setting. It looked really usefull for us ,I'll find it again dont worry ,and the attenuator step calc  |O

Good to see a few old regulars turning up for a look at the topic again ,no need for names , :phew: ,saluts .

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