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

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soundtec:
Im just trying to visualise the circuit ,
Its essentially three -10db sections ,with the 1db range always in circuit , seeing as the rotary switch is 2x12pos I was thinking h pad type network on the 1db range ,then the three way toggle switch offers gain ranges of, unity to -10 db , -10 to -20db and -20 to -30db in one db steps.
Maybe frequency switchable  low pass LC sections could be cut in and out across the three attenuation  ranges ,that way the more attenuation you can live with, the better the out of band rejection of the harmonics .

Little passives gadgets that 'do exactly what they say on the tin', are uncomplicated and instantly feel intuitive to use , Im a huge fan of .
I could think about making a small smd board kit including toggle switch coarse range ,1db fine range control ,frequency cutoff control ,enclosure ,bnc's and maybe selected or even paralelled 0.1% resistors throughout  . Might not be too difficult to to end up with a very close tollerance network that fits on a postage stamp ,good layout with low incidental capacitance would ensure flat frequency responce to very high frequencies .It would  be possible  to use Lorlin rotary 1x12way for fine and 2x6 for frequency cutoff controls ,
hardwired 0.1% throughhole resistors  could be used without having much effect at multiples of any audio frequency of interest .An analog attenuator post output definately beats any form of digital control of output level ,if levels less than -30 db are  required you could always add an inline attenuator with a given loss ahead of the unit, and you'd still have your precise 1db steps .

DPA31:
Hi Soundtec,

I see your project is progressing. Nice cabinets.

On my side I have finalised THS3091 with 18k and 27 K in piggy back mods on R54 and R45. see attached picture.

I am now looking at the output attenuator for 0-0.5 V range.

Looking forward to seeing your progress Soundtec.

Dom.

 

soundtec:
Added a  scale ,poor quality print as you can see,and there are some mistakes ,but for now it will do .



soundtec:
Just looking at the schematic , I see the gain and dc adjust pot are symetrical on the OPA686 ,this means the extra capacitance should balance out and have little effect on the signal .

Simply putting a pot in place of our 1.5k feedback resistor on the 3095 or 3491 would cause extra capicitance that might upset our output stage . What if we   replace the 499ohms with 470 +  a 50 ohm preset to ground, now any extra capacitance caused by the pot is 'hidden'   by the ratio of resitance  of  30:470 ohms  and it allow us precisely trim the output stage gain so whats coming out of the generator on both ranges corresponds exactly to the numbers on  the display.

Heres a nice attenuator calc that allow you specify impedence and it works out the values in the chain, you can rejig the impedence to get it to coincide better preferred resistance values.Shunt calc by the same author. 

http://rssconsultancy.co.uk/atten.html

I found an good op amp gain calculator here ,  its quite comprehensive compared to most ,

https://www.allaboutcircuits.com/tools/op-amp-voltage-and-gain-calculator/

You can easily add in the measured values from your Fy6800 and calculate the discrepency to expect . This software computes in the effects  of series and input to ground resistor too ,in our case 50 ohms and 10k , even 1% tollerance components in this position  will have  a very little effect on the circuit calibration, as pointed out earlier . A perfect 499:1500 ohms resistor set will have a margin of error ,but as little as a few ohms either way in the resistors can get you an exact gain of four . maybe if you had a batch of 1.5k and 499 ohm resistors to choose from you could find  closely matched pairs ,so that if you have a high 499 ohm you could match it with a slightly high 1.5k to maintain our 4:1  gain ratio.

soundtec:
I found some very high spec resistors at 1.5k ohms , .02% Beyschlag mini melf,
The op amp calculator shows with exactly 1.5k in rF position with 50ohm series resistor and 10k input loading a 497 ohm is needed in rG for a  perfect x4 gain .
Maybe a batch of 499 1% could be selected from to find 497 ohms +or- a margin that works with our fine tollerance component  to produce a very tight calibration on the output of the FY66/6800 . Set and forget with very low drift components (15-50ppm)beats even a preset ten turn adjustment ,Its probably less that 10 euros worth of parts and a little work to make the FY into something approaching a voltage reference , so Im gonna give it a try,  as someone way back at the start of this forum page  asked,
'Whats the point in a screen displaying  10.0000 volts if the last few digits are are garbage?'

I found an op amp noise calculator last night ,it allowed tuning in of the circuit values and noise parameters of the op amp ,I tried a THS 3491 in the circuit of the FY66/68 , I got a whopping noise figure of around  -128 db with x 4 gain ,thats within a few db of the theroretical limits imposed by the laws of physics itself , it also means any noise contribution from the op amp is very much below whats produced by the dacs ,always a good thing .

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