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Old 23rd September 2004, 02:35 AM   #21
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Default Re: 8038 way too much THD

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Originally posted by thespeakerguy
about 5% typical.

1 - wein bridge - agreed I've made these with very low THD. Frequency accuracy is poor, but that's not usually an issue.

2 - A uController driving a CODEC or DAC. You can hard code a sine wave into memory. Use the timer to set up different freqs. Battery powered, portable, dead on freq accuracy. THD equal to DAC's capability

3 - Take a crystal-based oscillator. Divide to 100x desired frequency, then divide the 100X. Use the 100x signal (i.e. 100kHz) to clock a switched-capacitor filter. Apply the 100x divided signal (in this case, 1kHz square wave) to the input of the SC filter. The output is a very pure sine wave with exact frequency. You will need to add an external filter to remove the SC clock feedthru.

3 - All methods. Add an LC based low pass filter to the output. A third order filter suppresses the second harmonic by 18dB, and the subsequent harmonics by even more. This will make a good sine wave source better.

The problem with an LC LPF is that you have to know the impedance it is going to work with, and every time --

The problem with switched cap filters is the "clock feedthrough" and other noise components -- even in the case of the lowest noise filters this is some tens of uV. You can use gangs of digital pots and make linear filters (on the Analog Devices website) but you will need to do the decade filtering by switching caps. Despite these limitations you can get a digitally filtered oscillator under 0.01% THD.

EBay really seems to have dried up -- if you see the Krohn Hite THD analyzer it has a very low noise, very low THD oscillator. Output is 5.1 V so you will need some attenuation.
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Old 24th September 2004, 07:41 AM   #22
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Default eBay

As I said earlier if you wait they will appear. There is an HP 239 oscillator on eBay. Its not cheap however. Just keep looking.

The Tek 4084 is very good and should be used with a very low distortion oscillator.

I had to deal with a digitally synthesized LDO source a while back. Finally scrapped it and made an airplane PCB with an analog oscillator to replace it. The radiated noise made it unuseable. It was a turntable motor driver.

Soon (months) I will go through my collection of audio analyzers and see which I can part with. If there is interest I will post the list when I get to it. I have lots of other interesting stuff for audio DIY as well.
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Old 24th September 2004, 01:04 PM   #23
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Default Re: eBay

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Originally posted by 1audio
As I said earlier if you wait they will appear. There is an HP 239 oscillator on eBay. Its not cheap however. Just keep looking.

The Tek 4084 is very good and should be used with a very low distortion oscillator.

I had to deal with a digitally synthesized LDO source a while back. Finally scrapped it and made an airplane PCB with an analog oscillator to replace it. The radiated noise made it unuseable. It was a turntable motor driver.

Soon (months) I will go through my collection of audio analyzers and see which I can part with. If there is interest I will post the list when I get to it. I have lots of other interesting stuff for audio DIY as well.
-Demian
Sometime this winter I will take Crawford's IM Analyzer from AudioXpress and simplify it -- this is a project in need of DSP.

The 4084 is a ruggedized version of the AA501 (I think) -- I have the 4084 and it works very nicely.

I was also thinking that the SG505 oscillator could be modded pretty simply to reduce the distortion.
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Old 24th September 2004, 05:22 PM   #24
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The SG505 is already very low distortion at .0008% or less. Improving this is very difficult.

As for DSP, the relatively simple analog systems used by Sound Technology and others get to less than .001% pretty easily. I would think the A to D would be the limit of performance for doing it digitally. Is it worth the effort?

And today there is an HP 339 needing knobs and ?? for cheap on eBay.

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Old 24th September 2004, 08:00 PM   #25
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I mis-spoke, I have an SG502 -- like comparing a Ferrari with a Trabant.
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Old 24th September 2004, 11:51 PM   #26
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Quote:
There is an HP 239 oscillator on eBay. Its not cheap however. Just keep looking.
Guess I will. Bummer. The HP 239 seems to be exactly what I'm looking for after doing some research. Not as cool looking as my old Waveforms tube gen, but small and low distortion. $200 does seem to be a bit high, but I need to replace the Waveforms badly...and patience isn't one of my virtues...
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Old 4th October 2004, 12:34 AM   #27
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A Sound Technology 1700A just appeared on ebay and its cheap. John Curl uses one still with an opamp upgrade and finds that its performance is as good as it gets. (I don't know which opamps he switched it to.) Stock its oscillator is .001% or less. Not bad for a buck! Will probably end at less than $100.
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Old 4th October 2004, 09:39 PM   #28
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Thanks! I'm watching it, but I may also have a lead on a Tek SG505, which is all I really need. If the 505 deal falls through, then maybe the 1700A....
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Old 18th October 2004, 11:16 PM   #29
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The 1700A went for a bit under $200, which would have been OK, but I threw in a max bid of $180 and lost it. There was also a very nice 1701A that I was going for, but again, my max bid wasn't enough, and it sold for well over $300. I would have bid higher for it, but could not be at the computer for the end of the auction.

There's another 1701A on the 'Bay for a starting bid of $450 (!!). However, it's beat to hell, and 'tested to power up only'. Not a chance I'm bidding on that one.

However, it appears that I have found a SG505 with opt-2 for $300. The last few that have sold on eBay went for well over this amount, and they did not have the balanced output option. Funny that the prices for these SG505's varies all over the map...this is the cheapest, and I've seen them offered at test equipment resellers for as much as $1100. What a scam!

Now I need to find a TM power module...

PITA, and getting expensive, but hopefully this will end my search.
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Old 19th October 2004, 12:41 AM   #30
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Thje Sound Technology stuff goes for prices all over the map. The details to look for are on the analyzer side- Auto Set Level is very useful. And the IM analyzer is also very good. The ST1700A doesn't have some of the features of the B and the 1701 is very similar to the B. I have a 1700B and a 1710 that has 15V output to meet some GSA specs.

The SG505's are always expensive for what they are. You can also look for the SG5050, the same with programmability. I have lot looked intot he cost of a TM frame for a long time. I don't need more holes for Tek stuff. I already have too many (7 scopes at last count).

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