Distortion Analyzer's

bktheking

bktheking

Gitter Done!
I'm getting into the FM alignment mix and i'm working on the last piece of gear I hopefully need. With Marantz receivers for example, adjusting the IF is looking at distortion. They call for an audio distortion analyzer. I was given a heathkit IM5248 which is a intermodulation distortion analyzer. Can I use this for alignment? IMD is talking about 2 frequencies vs 1, i'm thinking I need THD not IHD.
 
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bktheking said:
I'm getting into the FM alignment mix and i'm working on the last piece of gear I hopefully need. With Marantz receivers for example, adjusting the IF is looking at distortion. They call for an audio distortion analyzer. I was given a heathkit IM5248 which is a intermodulation distortion analyzer. Can I use this for alignment? IMD is talking about 2 frequencies vs 1, i'm thinking I need THD not IHD.
Click to expand...

You will need a standard THD analyzer like a HP 333A or a Sound Technology 1700A. What are you using for your AM and FM signal sources?
 
I think it would be interesting to use the IMD analyzer for the alignment.
Your RF generator needs to support external modulation sources.

I'd think IM distortion might be at it's worst where THD is at it's worst.

There may be some on-line Distortion measurement applications that use a soundcard. You can use a spectral/FFT analysis application, pump out 400Hz, and look for rises in harmonic content (800Hz, 1200Hz, 1600Hz, ....)
 
You may find this an interesting read.


IMAexplanation.jpg
 
That is an interesting article but for discriminator alignment you are trying to get the best gain, symmetry and linearity out of the discriminator and I don't think twin tone analysis applies here, nor will it work. The IM method looks at distortion caused by the mixing of the 2 tones. I doubt that a misaligned discriminator would cause IM distortion, but what I am more concerned about is that using IM distortion you may not be able to properly tune the discriminator at all.
 
Download a free copy of Visual Analyser and you get spectrum analysis and THD% via your sound card. You may have to work up a cable with some padding resistors to go into the card.
 
The Sound Technology 1000A FM alignment generator is essentially an IM analyzer. It strips the LF tone from the tuner's output and presents the HF modulation envelope on a scope. One simply adjusts the demodulator for the flattest upper or lower envelope edge, and voilà: minimum distortion.
 
OK so the im approach is the best one according to the article. How would you set everything up? I'm using a sencore sg165. Take a marantz for example, which is what I work on for the most part, what do I do?
 
bktheking said:
OK so the im approach is the best one according to the article. How would you set everything up? I'm using a sencore sg165. Take a marantz for example, which is what I work on for the most part, what do I do?
Click to expand...

No, it's not. See Dr*audio's post above.

As for your marantz receivers. Do what it sez n the Marantz manual. No more, no less. I use an audio freq. spectrum analyzer running on a pc thru a soundcard to measure & observe THD. It has some added benefits over the typical metering arrangement.

These guys used to have a nifty tutorial on their site to get you started:
http://dummyload.steinbay.com/marantz/schmain.html
Contact them and see of it's still around.

RF/IF alignment is no place to improvise. So be careful.
 
You do have to watch out for high input levels. As for the thd of a soundcard. Most I have tried to use, Creatives, have been fine. I use it for discriminator alignment and you never get vanishingly low readings. It doesn't have to be precise either. Low is Low regardless of whether it matches spec or not. Often there is some other "noise" in the spectrum that can skew what you are measuring . With a Spectrum Analyzer you can see it.
 
ConradH said:
Download a free copy of Visual Analyser and you get spectrum analysis and THD% via your sound card. You may have to work up a cable with some padding resistors to go into the card.
Click to expand...

Neato. :thmbsp:

Going to have to give this a spin on my Mac. The headphone jack does optical input, too, so I can get even more precise readings :thmbsp:











(he said, jokingly)
 
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