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Old 7th March 2013, 05:03 PM   #11
RNMarsh is offline RNMarsh  United States
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You know you can buy an IC that does all you want - a complete function generator. More than one mfr but here's one: XR2206. And a complete kit of it is available if you just want to get down to business and use the instrument.... but you wont learn as much...

Go to www.Electronics-diy.com
Such as this simple circuit ready made on pcb:

XR2206_Function_Generator_Schematic.jpg

It wouldnt be too hard to design a ss variable R that would allow you to sweep the oscillator (maybe auto switching C as well). AND, a tracking BPF to reduce thd to very low number? Sorry.... getting carried away.

Thx-RNMarsh

Last edited by RNMarsh; 7th March 2013 at 05:22 PM. Reason: Function generator IC -
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Old 7th March 2013, 06:12 PM   #12
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Thanks! Yes, I actually have an XR2206 chip, but I am enjoying this project as more of a design challenge/learning experience. It's actually functional now and meets my original specs but then I started thinking about optocouplers. A "tracking BPF," huh? I googled it and it looks like something I will not be able to figure out
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Old 7th March 2013, 07:14 PM   #13
paba is offline paba  Canada
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After a quick look at the datasheet of that chip, I don't think the sine distortion is low enough and it does it produce FM stereo?, kind of mportant, FM mono is nice but it is really L+R, L-R and such that you need for tuner work.
Paba
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Old 7th March 2013, 07:58 PM   #14
RNMarsh is offline RNMarsh  United States
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Just a thought. And, swept signal are a nice addition to spot freq. for FR tests.

For distortion of an FM tuner, i would think .1% or less THD is needed.

-RNM

Last edited by RNMarsh; 7th March 2013 at 08:00 PM.
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Old 7th March 2013, 10:54 PM   #15
paba is offline paba  Canada
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Quote:
Originally Posted by RNMarsh View Post

For distortion of an FM tuner, i would think .1% or less THD is needed.

-RNM
I respectively disagree except for tube tuners of course and some really poor performing solid state ones.

Thanks
/paba
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Old 8th March 2013, 02:55 AM   #16
RNMarsh is offline RNMarsh  United States
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Poor tube or ss tuners? Like the Marantz 10B (0.5%) or the ss Magnum DynaLabs at 0.1%
If you are well under 0.1% in any tuner you are exceptional. I always try to be at least 10 times under the thd of what I measure so as to trust the test numbers I get. For tuners, that would put it at .01%..... your stated target.
You can also buy used commercial units off eBay for a couple hundred bucks with .002% or less. Just some more ideas for you. Plus some web sites that have schematics for low distortion oscillators and mfr maintenance manuals with oscillator schematics. For <$40 you can get a SOTA oscillator from Viktor on eBay at .00005%. Already made.

Thx-RNMarsh

Last edited by RNMarsh; 8th March 2013 at 03:11 AM. Reason: Low thd FUNCTION generator--??
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Old 8th March 2013, 04:16 AM   #17
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FWIW, the XR2206 is a nice but obsolete chip. The ones easily available today seem to have some problem meeting their voltage specs. Over about 11 VDC they won't cover their range, if they oscillate at all. Under that they seem to work OK.

Though I don't know the distortion that can be achieved, the Analog Devices DDS chips seem to be the way to go these days in terms of function generators.

For a nice low THD analog circuit, I like the generator Bob Cordell uses in his THD analyzer. It's a state variable circuit using common opamps that performs very well.
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Old 8th March 2013, 02:00 PM   #18
paba is offline paba  Canada
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Quote:
Originally Posted by RNMarsh View Post
Poor tube or ss tuners? Like the Marantz 10B (0.5%) or the ss Magnum DynaLabs at 0.1%
Thx-RNMarsh

I just checked 3 low cost tuners from my collection for kicks and specs are
@1K, 100% mod:
Sony ST-S555 ES (0.08%), 1985
Sony ST-J60 (0.08%), 1978
Kenwood KT-900 (0.04%), 1981

Most japanese tuners from this period will have distortions specs in this range. Say up to the late 80's.

Quote:
Originally Posted by RNMarsh View Post
I always try to be at least 10 times under the thd of what I measure so as to trust the test numbers I get. For tuners, that would put it at .01%..
Thx-RNMarsh
fully agree on this. That's why the XR2206 won't cut it with .1% spec.

But an FM stereo gen with 0.01% out of the RF output is really best in class and meguro and other instruments that can do that are pricey. I think finding an affordable stereo gen that does 0.05% is more realistic and would cover a great deal of tuners. the 80%-20% rule I guess.

cheers
paba
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Old 8th March 2013, 04:29 PM   #19
RNMarsh is offline RNMarsh  United States
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If the spec were from Tek or HP etc I would believe them. Competitive products for consumers... and relatively cheap ones, I dont believe until they are measured.
Back then, what instrument/test equipment did they use to measure those numbers with?
Not saying they are not well below .1% but asking. I might trade-in my MagnumDyna Lab.

Thx-RNMarsh

Last edited by RNMarsh; 8th March 2013 at 04:35 PM.
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Old 8th March 2013, 05:05 PM   #20
paba is offline paba  Canada
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Like I said a few posts back, some tuner manufacturers had suspicious specs but if we read the 1978 Yamaha T-2 owner's manual they say they used the following instruments to measure their tuner.

VHF SG: Meguro MSG-297 or 296
VHF SG: GE501
stereo moduluator: Meguro MSG-211 (one of the best ever btw)
Distortion analyzer HP4333A

All very credible.

and finally the
The T-2 spec is 0.05%

hope that clarifies
paba
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