Products > Test Equipment
FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
DPA31:
I got it |O, Thks Soundtec.
I forgot the switch relay effect on range over 5V
@ 50Hz I measure 0.181 Vrms for 0.5 Vpp but 0.177 Vrms for 0.51Vpp
1.769 Vrms for 5 Vpp but 1.766 Vrms for 5.1 Vpp
I envisage to put a piggyback SMD resistor in parallel either on R63 & R74 (1.5 K) or R45/R54 to adjust THS3091 outpout stage gain. But it will not correct the DAC non-linearity. So the question is at what frequency and what amplitude to adjust the final stage ?
BTW I 've got an adjustment range of 24 mV for half a turn on the gain trimpot on a 5V amplitude signal. I was able to adjust equally the gain of both channel.
Another point : what is the purpose of DC_OFFSET_Channel_CAL pot ? Does anyone know or test it ?
Cheers.
Dom.
DaveR:
Hi Dom,
As far as I could tell when I adjusted mine a few weeks ago, the DC_OFFSET_Channel_CAL pot adjusts the vertical symmetry of the sine or square waves. I used it in conjunction with the GAIN pot to get the waveforms on both channels identical by overlaying them on the scope. Only the top half of the waveform was affected by adjusting it, and its range was quite small, but it served its purpose (if that's what it's meant for). The name on the schematic is rather cryptic, but it's safe to give the pot a wiggle and watch what happens.
Regards,
Dave
soundtec:
Ok thats all good information to get in ,
Word from the mouth of someone with 60 years experience in electronics is you use finer tollerance resistors to make up the potential divider across the output IC . Each IC has a range of specific resistor values ,not simply any resistors of a certain ratio,that ensure best performance ,generally between gains of about 2 and 5 best performance is had .
When the drive to the 3095 is engaged the signal is fed from the opa686 outputs , my guess is theres an offset built into the software calibration which tries to make up for the slight gain differences as you switch between high and low range ,if we have calibration 'flat' and can adjust amplitudes independantly between opa and ths op amps externally we could essentially do away with using some calibration features in the digital domain ,maybe this is the issue as when you switched to 5.1 volts the output level actually went down DPA31.
So were essentially getting 12 bit performance , good op amp technique should be able to squeeze those last two bits out easy ,better sorting the amplitude differences between ranges in the analog domain as much as possible and not relying on bit munching internal calibration .
Theres a few nice theories for you to mull over in any case ,
Your idea to piggy back two resistors has merrit too Dom , if you picked x2 the required values ,then selected from a bunch of 0.1% tollerance in both rg and rf , you could get the resolution of the display to read bang-on to better than what we see now , if the range was a few mV off you could simply select from your bag of components a marginally lower value for each channel individually until its right 'on the money' ,in the words of Zappa .
Kindest regards Gents ,
Its Friday and the beer is flowing ,I hope your taking a dose of your fav poison and kicking back like me ........
as is as we saw its maybe a few tens of mV off across the ranges at a volt or two amplitude .
DPA31:
Hi,
BTW, another mean for adjusting the gain had been described before in the blog:
"With regard to the circuit FY6600_Main_A0.pdf at the THS3002 amp, the gain figure is 3.986 with a 10K input termination resistor and 4.006 without this resistor; changing this resistor to 33K results in a 3.999951 gain factor which is very close to a perfect 4."
These 10K are R44 & R53.
They act as a voltage divider on the THS 3095 input. The effect is very light.
Dom.
soundtec:
Yeah I can see the effect your talking about alright Dom . Thing is, raising the input impedence of the 3002 wiring and all its associated components to 33k will most likely effect the high frequency performance to some degree ,higher load probably does reduce harmonic distortion from the opa686 to some degree but the effect of any stray capacitance in the wiring etc could be magnified .
My guess is pots over time and temperature are subject drift , probably a lot more than a 0.1% resistor , if we use the doubling up or paralelling idea for the resistors we might be able to select much much better than 0.1% tollerance .The two 100 ohm resistors in parralel at the output of the 3002 are pretty low spec ,into high impedence like a meter a few ohms difference here wont make any odds ,but say if your driving 50 ohms ,or the relay clicks in the output attenuator ,any minor discrepency in the 50 OHm value will cause an error in the values displayed .
the use of 499ohms and 1.5k seems like a reasonable compromise value that works with most op amps ,but I wonder to get the very best performance in 3095 or 3491 op amps do we need other values with a 3:1 ratio .
What tollerance are you guys seeing on the 499, 1.5k ,10kand 50(2x100ohm) parts ?
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