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Old 30th October 2007, 03:53 AM   #101
anatech is offline anatech  Canada
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Hi Demian,
Quote:
The HP3904 is just a little klutzy to use (poor UI) and its performance isn't much different from an HP339A.
I think you mean the 8903A? I've never heard of a 3904 (transistor?). The 8903 is the beginning of the all digital display stuff. At that time everyone was trying to do this. I haven't used an 8903, but I've been told the 339 is a better product. Isn't the residual noise and THD worse on the 8903? I thought it was.
Quote:
The HP339A commands a higher price partly from familiarity with the name.
Yes and no. The HP product holds cal extremely well and is one of the more reliable products out there. Their analog meters are scary accurate as well. The price does reflect the quality. Expectations are higher also. It's true that Boonton is not as well known and it is also very high quality. I really don't think you can go too far wrong with an HP product. This from much experience. I used to work in a calibration lab as well.

These days I expect THD to be analyzed in the digital domain. So a DSO with these capabilities would be the direction. Otherwise you are looking at full blown spectrum and signal analyzers for your work.

Yes you have a big stack there. But for $400 I would still go for the 339A.

Hi Stefano,
Just starting out, try to grab an HP 334A or 333A. It's an instrument that you will use a lot and you may find it does everything you need. What is nice is the extra bandwidth. That is why I still have one. You can pick them up for less that $100 USD sometimes. Keep an eye out.

-Chris
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Old 30th October 2007, 04:13 AM   #102
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Sorry about the 3904 - I was buying transistors this afternoon. I don't remember how low the residuals are. I could drag mine back home and check it, but then I need to find a place for it.

If I had to maintain one of these with limited resources the HP339 would be the first choice, being the simplest.

Its interesting the Shibasoku 725D a current product with GPIB etc uses a digital readout for frequency and an analog meter for level & distortion. And the meters response is a little strange- very fast and doesn't wander. Like its on the output of a D to A converter.
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Old 30th October 2007, 09:06 AM   #103
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A stupid question but....

to calibrate the distortion analyzer ST i would need a low distortion wave generator source but....i don't have it.
I was wondering indeed if a signal generator PC-based would work out as signal reference.
If so, what program can i use? i have a demo-trial version of audio tester but after few minutes it shuts down...'cause it's trial

Can eather way be used such a sistem as wave gen ref?
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Old 30th October 2007, 09:35 AM   #104
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"Goldwave" have a nice waveform generator , which you can use to make wav-files containing you signal..
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Old 30th October 2007, 09:40 AM   #105
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i'll google this program.
BTW is it free?

What i was...and am....wondering is the distortion level and/or noise floor of a pc-based signal generator is high or low.
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Old 30th October 2007, 02:55 PM   #106
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Yes and Yes. . . computer sound cards are all over the map.
Calibration of a distortion analyzer coveres several areas- voltage sensitivity, frequency response, filter response, notch centering (optimizing the notch nulling circuitry) and when an oscillator is in the unit the oscillator level, response and distortion.
The sound card can be useful as a source for a low distortion sine wave but if it has significant ultrasonic noise, and many do, its usefulness is reduced.
The internal oscillator and an accurate AC meter can do most calibration stuff for an analizer. Optimizing the notch really requires a wave analyzer or equivalent to see just the depth of the notch at the fundamental. However you can usually get a pretty good start at it on a loop back test. However when you are within a few dB of optimum the harmonics of the source and internals in the analyzer will dominate the readings and you will go in circles with adding and subtraction of signals. The FFT on the output of an analyzer can give you the same info as the wave analyzer.
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Old 30th October 2007, 08:23 PM   #107
anatech is offline anatech  Canada
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Hi Demian,
Yes, the calibration of the AC voltmeter is the main thing. The rest of the procedure is rather beyond most people and possibly myself with the gear I do have. Instruments with frequency response to the MHz will certainly respond to any digital noise form a sound card if it exists.

Hi Stefanoo,
In my experience, the alignment of an HP THD meter should not be touched. It is probably more accurate than any meter you have unless you have a newer Agilent like a 34401A. Even then, a 334A has higher frequency response, so .... Just don't touch it. Clean switches carefully. Lubricate the shafts and clean out old gunk. That is mechanical work. Replace filter caps at the most.

I have yet to find one out of alignment much unless an aspiring tech has been in there. You can buy better meters, but I suspect you will hold on to this one anyway.

-Chris
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Old 31st October 2007, 05:14 AM   #108
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Has anyone used a "Lock-in Amplifier" to assist in their measurements?

I've read a little about this, but have not personally tried it yet, one reason....I don't have one.

http://www.cpm.uncc.edu/programs/Explore_1.pdf
http://www.cpm.uncc.edu/lock_in_1.htm

=RR=
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Old 31st October 2007, 01:50 PM   #109
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Thanks for the tip. I will try it. I have a KH equivalent I can play with. I think thats the method that Shibasoku uses for its internal distortion analyzer. Certainly an interesting read.
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Old 31st October 2007, 05:41 PM   #110
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Quote:
but I've been told the 339 is a better product
I have both of these (more precisely the 8903B).
I don't see any aspect in which the 339 (btw excellent instrument) can do better than 8903B.
The THD loopback analysis can easily be better than -92 .. -94dB using the HP - LP filters in a correct way. The 8903A/B do offer more measurement capabilities which I find very useful.
Marcello

P.S. The 8903B generator's THD is typically better than -105db for reasonably output levels...
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