Low Distortion Signal Generator

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Re: CFG253

EchoWars said:
Interestingly enough, I was able to find an older gentleman who did contract work for Tek back in the day, and still has complete information on the unit, as well as his own adjustment procedures, worked out after repairing dozens of these things. He has promised me that he will get them copied and sent. Very cool.

Sorry for the post necromancy... I fully realize that you posted this nearly three years ago, but did you ever get the service info for the CFG253 from this guy? If you did, and still happen to have it, I'd be eternally grateful if you would share a copy with me! :)
 
I just received an

"AMBER 5100 Programmable High Performance Audio Generator. Ultra-low distortion sinewaves (-106dbV THD+N), 10Hz-110kHz. Balanced and single-ended output with 50/150/600 Ohm impedances, self-cal function, GPIB "
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I don't have the manual.
It works, but freq is off by about 50Hz @1k, and the buttons stick a bit.
My dist. test measures at 0.05% at 1k, 1v.
(should be lower, no?)
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I opened it up, and on the oscillator board there is a "bulging" electrolytic cap (100u 25v).
I marked the polarity of the cap, removed it, and noticed the "board marking" does not coincide with the way the cap was installed (polarity).

As installed, negative went to ground.
As marked on the board, positive is to ground.

This looks to be the original factory cap.

This cap comes off a 311 comparator's VEE pin (-v), and goes like this:
VEE pin - thru a 100 ohm resistor - thru this bad cap - to ground.

So if it's coming off the neg voltage rail, positive should go to ground.... Does sound right?....and it was installed wrong at the factory ?

=RR=
 
I should have more results for an easy bandpass filter to lower distortion of a .01 % source in a couple days. This technique was already reported by another post above.
It easily takes a .01% source to .002%, maybe more. To build it you need one op-amp, 2 caps, and 3 resistors to make the circuit. Surprisingly, or not, I could not get an AD797 to work in it without oscillating. The oscillation made huge harmonics, which gave it poor performance compared to other op-amps. The basic circuit is here
http://sound.westhost.com/project63.htm

I tried two versions, Q=9, and Q=6. The Q=6 worked better. I used common values for all parts, carefully matching the two polystyrene .01uF caps. For about 2kHz, I used R1=47K, R2 = 680, R3= 100K.
It ended up being 1.91kHz. I determined this by feeding the sine source in to the filter then adjusting the frequency while watching the output on a scope. My source was an HP8904A.

I use my ono sokki CF6400 to test a few op-amps. There was about 3dB difference from the best to the worst. The best were the LME49710, and AD847. After that, the OPA134, OPA604, and TI NE5534 were all also decent. Any of these would work fine, at least they did for me, without oscillation problems, in a fairly crude proto board. I used panasonic FM 33uf 35V +.1uF ceramic mono for bypass on +/- rails. Also used a copper foil ground plane. see pic below.
 

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Below is a the log-log response of the first filter, Q=9, built with R1=68K, R2= 470, R3= 150K. Both used cap values for C of .01uF

The Q=9 version had slightly better measured attenuation, but had higher residual distortion on the initial readings made by the Ono Sokki 16 bit analyzer. I plan to use my M-Audio Audiophile 192 next and see how that fairs with internal and external sources.

Here's the attenuation measured at offset from fundamental where these harmonics would be

Q=9

2nd -21.3dB
3rd -26.2dB
4th -29.1dB
5th -31.3dB

Q=6

2nd -18.4dB
3rd -23.4dB
4th -26.3dB
5th -28.4dB

the Q=9 plot is below
 

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Here is the KH4400 (I haven't figured out how to attach two images to one post yet.)

Both read .0008% THD w/ 30 KHz low pass filtering, using inverting input and all of the other tricks I have figured out for lowest readings.

The display is 0-25KHz and the marker is at 2 KHz.

Krohn Hite:

I just picked up a Krohn-Hite 4400 -- plugged it in and was dis-heartened that the distortion was coming in around 6% -- so off went the lid -- I hooked up the oscillator to my Tektronix PS5010 and found that the distortion was 0.0008% with an external supply. The positive rail was also running "hot", there was a lot of ripple on both rails and the output was clipping under all circumstances. First thing I noticed were about a half-dozen 6.8uF 35V tantalum caps -- I replaced these with 100uF/100V Panasonics, replaced the filter caps (800uF Mallory's) with 1,000uF Panasonics, and replaced the positive 7815 regulator. Now the oscillator runs at 0.00022% when the 22kHz LPF, about 0.00046% with the 80kHz filter.

I had to pull out the Weller 1200 to solder this stuff up -- they weren't into dainty soldering pencils back in 1980.
 
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