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Old 21st October 2004, 04:34 PM   #51
1audio is offline 1audio  United States
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The problems you mentioned with the function generator are probably more than withing spec, except for the 50 mV offset. The sine wave is made from the triangle wave probably with a series of diodes. There are usually 1 - 3 pots for trimming the wave. The discontinuity at the top may be endemic, getting rid of it will be hard if even possible.

Is the 50 mV always there independent of the level setting? These usually have an output amp and there is an offset trim at the input of the amp. There will also be trims for the integrator, adjustments for the tracking of the panel/voltage to frequency conversion etc.

I would shop for a good Krohn Hite function generator before putting much energy into this one. You can get a KH 2200 lin/log 30 MHz generator for less than $100 on eBay. They can get below .1% THD with careful trimming (until the tem changes). The digitally synthesized generators are nice with very good specs, but expensive.
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Old 21st October 2004, 06:27 PM   #52
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Old 22nd October 2004, 03:30 AM   #53
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Default wondering about KH tuneable filters

there have been quite a few up for sale recently (I bought some from Kodak of all people) -- interesting that they have ganged precision pots and switchable capacitor banks -- sounds like a device in need of "repurposing" . fwiw, they seem to add a few 0.01% distortion to the output of my 1120 generator.

the SG505 (just got a manual) -- uses a dual 10K pot.
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Old 22nd October 2004, 05:20 AM   #54
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The KH filters are very useful. They are not ultra low distortion (actually I think the VCVS filter has a limitation there) but they are otherwise very good. There are many models and different configurations. I have one that is battery powered (dead nicads after 20 yrs of course) but switch selected 48 dB/*ve filters that can be used independently or together and another that is adjustable slope and dials. And a few others. Good for testing speakers and subwoofers. I would be reluctant to use one for listening, but it may be real good.

The dual pot is part of a classic state variable oscillator. The same as the KH and the Boonton. The Boonton is electronically tuned and phase locked to a crystal reference making it in effect a synthesizer but with .0008% THD.
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Old 23rd October 2004, 09:45 AM   #55
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Quote:
Is the 50 mV always there independent of the level setting? These usually have an output amp and there is an offset trim at the input of the amp. There will also be trims for the integrator, adjustments for the tracking of the panel/voltage to frequency conversion etc.
The offset is about 5mV on the low volt output setting, and 10x that at the high setting. Yes, I can see the output amp, but unless I can tell which pot to fool with, I'd rather leave it alone (there are 4 or 5 in very close proximity to the output amplifier).
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Old 25th October 2004, 03:24 PM   #56
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Old 29th October 2004, 12:50 AM   #57
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Default CFG253

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.

Just as cool, my Tek SG505-02 arrived today! Looks like new, and cost me less than any eBay auction I've seen in a long time (<$250). Now, only if the power supply would show up.
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Old 29th October 2004, 01:32 AM   #58
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This is a bit off topic,

Ive been looking at this forum, and I want to ask If anyone has the schematics for a low-distortion sinewave generator? I have built the one in rod elliots page and If there is any better circuit I can try.
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Old 29th October 2004, 02:06 AM   #59
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go to Linear Technology's website -- they have an oscillator which uses two gain stages so you don't overdrive them. unfortunately they have changed the "look" of their website so it is missing some of the former functionality.

also http://www.amplifier.cd/Technische_B...clean_sine.htm

i can also scan the SG505 schematic --
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Old 29th October 2004, 02:18 AM   #60
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Go to http://www.linear-tech.com/ and do a search for AN43. The oscillator section begins on page 27.

The lowest distortion one they show (Fig. 48, THD of 0.0003%) has a range of only 1.5KHz to 15KHz. A couple of problems with building it:

1. You would need to provide switching of two film caps in the circuit (C1 and C2) from 0.1f to 1f, and that would give you two ranges...150Hz - 1.5KHz, and 1.5KHz - 15KHz.

2. Adjusting frequency with a standard potentiometer will be very very touchy. You would need a precision 10-turn dual potentiometer with very good channel-to-channel tracking. Not too hard to find, but they are NOT cheap.

3. The optical photocell might be a PITA to find. The Clarex CLM410 seems to be out of production, but the looks like the VTL5C10 is still made by PerkinElmer. I don't know who's backside you'd have to kiss to latch onto one of these devices, but it might not be hopeless.

4. The output of the circuit swings almost rail-to-rail by default. You would need another pot and likely an output buffer to make it really usable.

I was ready to go this route until I found the SG505 for a good price.

Edit: jackinnj, very cool!! Never saw that before. However, it's not exactly simple, and fixed at 10KHz freq. Still, unbelieveably low distortion. (it's AN67, BTW)
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