Hi Demian,
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
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.
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
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.
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.
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?
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?
"Goldwave" have a nice waveform generator , which you can use to make wav-files containing you signal..
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.
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.
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.
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.
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
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
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=
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=
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.
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...
I agree!! As a general rule, it is better not to touch any instrument calibration (or other...) unless you have experience, reference calibrated instruments and of course all the service documentation to perform it correctly.
Marcello
Hi guys,
I'm looking at a HP 334a distortion analyzer. I have no idea how to use it but want 'to get my feet wet'. Like I said- I'm a total newbee with such an item so I have a question. What other equipment will I need to make use of the item? Function generator?
Thanks a lot.
I'm looking at a HP 334a distortion analyzer. I have no idea how to use it but want 'to get my feet wet'. Like I said- I'm a total newbee with such an item so I have a question. What other equipment will I need to make use of the item? Function generator?
Thanks a lot.
You will need a low distortion oscillator to get utility from the HP334. Typically they were used with HP 200CD oscillators. Thay combination was what McIntosh used for their traveling clinic.
... or, in you decide to use one of the above mentioned DF meters, you do not need anything else, since they all have a very low THD oscillator inside themselves.
Hi mpmarino,
There are the 654, 652 oscillators with a meter, your sound card and a low distortion oscillator can be built. Nice because it is a single frequency. You can make one for 1 KHz and another for 20 KHz if you want.
So, you probably have what you need to start already. An external oscillator is good because you can test filters and frequency response with them.
The 334A is also a really good wide band AC voltmeter also. Flatter than most and it's response exceeds 1 MHz. The 65xA oscillators will match the range of the distortion analyzer, so you can work on AM radios with them also.
-Chris
There are the 654, 652 oscillators with a meter, your sound card and a low distortion oscillator can be built. Nice because it is a single frequency. You can make one for 1 KHz and another for 20 KHz if you want.
So, you probably have what you need to start already. An external oscillator is good because you can test filters and frequency response with them.
The 334A is also a really good wide band AC voltmeter also. Flatter than most and it's response exceeds 1 MHz. The 65xA oscillators will match the range of the distortion analyzer, so you can work on AM radios with them also.
-Chris
anatech said:
Thanks Chris,
I currently do not have an oscillator, is the 65x your recommendation? Please keep in mind the el-cheapo budget😉
unlikely, see your sig
P.S.: while I have your attention, are you familiar with the insides of the Nak LX-5 tape deck? If so, would you field a question or two if I brought it up elsewhere?
Hi mpmarino,
For THD measurements, variable frequency oscillators are not always the best. I love my 339A, but really you only do THD at a couple frequencies. So a few fixed frequency oscillators would economically fit that bill.
The first thing to do with one of these is to resist the temptation to turn the frequency adjust. There is a nylon coupling in there that may break (I got one broken already). The shafts tend to get sticky and seize. Thankfully it's easy to disassemble and clean / lubricate the assembly. Then it's nice and smooth again. This is also a good time to clean all the knobs and the faceplate. I hate working with instruments that look like heck. Clean switch contacts next (avoid sprays if you can. It gets into trimmer caps and other bad places.
-Chris
Send me the link & I'll have a look. I hope I can be of help.
Yes. A super low THD oscillator is not so important and these have the advantage of being pretty good in that department and more importantly, pretty flat response. The meter is very helpful as they are accurate. The 654A has a great attenuator. Trust me on this. The other ones are okay but the 654A is really useful. The one thing that always drove me nuts was the lack of a meter on the several oscillators I have owned. With it's high frequency ability, you can tease audio circuits in the ultrasonic region where you often see problems. In the 100 KHz to 800 KHz region.
For THD measurements, variable frequency oscillators are not always the best. I love my 339A, but really you only do THD at a couple frequencies. So a few fixed frequency oscillators would economically fit that bill.
The first thing to do with one of these is to resist the temptation to turn the frequency adjust. There is a nylon coupling in there that may break (I got one broken already). The shafts tend to get sticky and seize. Thankfully it's easy to disassemble and clean / lubricate the assembly. Then it's nice and smooth again. This is also a good time to clean all the knobs and the faceplate. I hate working with instruments that look like heck. Clean switch contacts next (avoid sprays if you can. It gets into trimmer caps and other bad places.
-Chris
Here's a link to the very low distortion oscillator in the Krohn-Hite 6900B DA -- this will easily better the 0.001% mark but only one frequency:
http://www.tech-diy.com/TestEquipment/Oscillators/KH_6900B.gif
I posted some pix of my Krohn-Hite museum a while back -- finally got around to popping the cover on the KH4024 oscillator -- this cost me about $9 a couple years ago -- distortion is around 0.0017% at its minimum and for most of the range is around 0.0077% -- the 4024 has quadrature output and a pulse generator -- not bad when you consider the trivial cost. This oscillator will not see any diagonal wire clippers, it's just a museum piece!
http://www.tech-diy.com/TestEquipment/Oscillators/KH_6900B.gif
I posted some pix of my Krohn-Hite museum a while back -- finally got around to popping the cover on the KH4024 oscillator -- this cost me about $9 a couple years ago -- distortion is around 0.0017% at its minimum and for most of the range is around 0.0077% -- the 4024 has quadrature output and a pulse generator -- not bad when you consider the trivial cost. This oscillator will not see any diagonal wire clippers, it's just a museum piece!
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Hi gang
Would somebody mind to post the spectrum after the notch of a perfect HP-334 ( think, there is a dedicated output for ) ?
Interested in 40Hz, 1kHz and 10kHz/ input level ca. 1V.
I'm curious about the devices own distortion; a TEK SG505 as source would be great.
Thanks, greetings from Vienna
Gary G.
Would somebody mind to post the spectrum after the notch of a perfect HP-334 ( think, there is a dedicated output for ) ?
Interested in 40Hz, 1kHz and 10kHz/ input level ca. 1V.
I'm curious about the devices own distortion; a TEK SG505 as source would be great.
Thanks, greetings from Vienna
Gary G.
Another Krohn-Hite collector! I have a fair assortment of their stuff as well. I have learned a lot from studying their techniques. I have one of the 7500 amps waiting for repair. I have been reluctant to work on it because of the high voltages but soon I want to get it off of the bench.
There is a very important trick in the 4024 amp circuit- they have an active drive to a cascode on the input transistors. This is a key to reducing the distortion of the system. In the case of the oscillator they can predict all of the operating points so they can really optimize the circuit. The flatness of the response and stability of the output of those oscillators is exceptional. The 4400 is in the same league with less output- shows the benefits of opamps.
There is a very important trick in the 4024 amp circuit- they have an active drive to a cascode on the input transistors. This is a key to reducing the distortion of the system. In the case of the oscillator they can predict all of the operating points so they can really optimize the circuit. The flatness of the response and stability of the output of those oscillators is exceptional. The 4400 is in the same league with less output- shows the benefits of opamps.
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