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Author Topic: HP 8903B "Audio Analyzer"  (Read 796 times)

Frank Koenig

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HP 8903B "Audio Analyzer"
« on: September 20, 2017, 03:42:53 pm »

I recently had come into my life this piece of gear. It's basically a low-distortion sine-wave generator, self-tuning notch filter, and an AC voltmeter. As such it can measure frequency response magnitude; signal, noise, and distortion (SINAD); and distortion (THD). Under normal circumstances, distortion can be measured down to .003%, which is good enough  8)  What's pretty cool is a post-filter "monitor" output that allows you to see the residual noise and distortion on a scope (or spectrum analyzer).

While not doing much you can't do with a computer/audio-interface/FFT rig, it might have a potentially useful combination of precision and convenience. I'm curious to see if it will work for looking at transducer distortion.

I'd be happy to hear from anyone who has experience with this type of box, and any neat hacks you know.

Thanks.

--Frank

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Scott Holtzman

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Re: HP 8903B "Audio Analyzer"
« Reply #1 on: September 20, 2017, 04:21:31 pm »

I recently had come into my life this piece of gear. It's basically a low-distortion sine-wave generator, self-tuning notch filter, and an AC voltmeter. As such it can measure frequency response magnitude; signal, noise, and distortion (SINAD); and distortion (THD). Under normal circumstances, distortion can be measured down to .003%, which is good enough  8)  What's pretty cool is a post-filter "monitor" output that allows you to see the residual noise and distortion on a scope (or spectrum analyzer).

While not doing much you can't do with a computer/audio-interface/FFT rig, it might have a potentially useful combination of precision and convenience. I'm curious to see if it will work for looking at transducer distortion.

I'd be happy to hear from anyone who has experience with this type of box, and any neat hacks you know.

Thanks.

--Frank



The cool thing about SINAD is it's a systemic measurement.  In the RF world it was used as a benchmark for receiver sensitivity.  The old salts would tune with a milliwatt tone (1004hz) and slowly reduce the input signal as each stage of the preselector and filters were brought into alignment. 

By the time I was trusted with a non-inductive tuning tool (about 1982) SINAD measurements were incorporated into service monitors (signal generator, scope, receiver and in some cases spectrum analyzer) all in one convenient 94lb case that apparently was luggable up 26 flights of stairs to work on a transmitter on the room (but I am digressing now). 

My point is that you could tune the receiver simply by the SINAD value.  I am not sure how you are going to use this for AF transducers.  With a measurement microphone?


 
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John Roberts {JR}

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Re: HP 8903B "Audio Analyzer"
« Reply #2 on: September 20, 2017, 05:02:55 pm »

I recently had come into my life this piece of gear. It's basically a low-distortion sine-wave generator, self-tuning notch filter, and an AC voltmeter. As such it can measure frequency response magnitude; signal, noise, and distortion (SINAD); and distortion (THD). Under normal circumstances, distortion can be measured down to .003%, which is good enough  8)  What's pretty cool is a post-filter "monitor" output that allows you to see the residual noise and distortion on a scope (or spectrum analyzer).

While not doing much you can't do with a computer/audio-interface/FFT rig, it might have a potentially useful combination of precision and convenience. I'm curious to see if it will work for looking at transducer distortion.

I'd be happy to hear from anyone who has experience with this type of box, and any neat hacks you know.

Thanks.

--Frank


I don't know about hacks but that was standard test bench fare before the AP became popular. Good solid bench standard.

JR
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Frank Koenig

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Re: HP 8903B "Audio Analyzer"
« Reply #3 on: September 20, 2017, 10:22:10 pm »

My point is that you could tune the receiver simply by the SINAD value.

Scott, that's interesting. Thanks. I think it explains why there is an analog SINAD meter in the upper right corner of the panel -- much easier for finding a peak than flickering numbers. It only goes up to +24 dB, and that makes sense, too, since anytime SINAD gets bigger than that you just back off on the RF input to keep the receiver near the limit of its sensitivity.

Quote
I am not sure how you are going to use this for AF transducers.  With a measurement microphone?

Yes. Signal chain: pwr amp -> speaker -> acoustic -> mic -> pre amp. We'll see when I find time to play with it.

--Frank
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Scott Holtzman

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Re: HP 8903B "Audio Analyzer"
« Reply #4 on: September 20, 2017, 10:32:16 pm »

Scott, that's interesting. Thanks. I think it explains why there is an analog SINAD meter in the upper right corner of the panel -- much easier for finding a peak than flickering numbers. It only goes up to +24 dB, and that makes sense, too, since anytime SINAD gets bigger than that you just back off on the RF input to keep the receiver near the limit of its sensitivity.

Yes. Signal chain: pwr amp -> speaker -> acoustic -> mic -> pre amp. We'll see when I find time to play with it.

--Frank

Yes, that's right.  Digital meters suck for tuning any kind of analog filter AF or RF, your are past the peak or dip because of the latency so you end up having to lead and lag your tuning to account for the hysteresis of the device.

12db sinad was the standard for quoting RX sensitivity.  You are exactly right if you were changing frequency quite a distance you would have to set the generator 50db or more above the .02 uV you would expect and then get a rough tune, then keep repeat until the desired sensitivity was reached.

Much more fun was adjusting oscillators with lissajous patterns.  On an analog scope you could get the pattern to lope around 1hz when you were dead nuts on.

The are so many analogs from the RF world in audio.

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Scott AKA "Skyking" Holtzman
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Rex Ray

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Re: HP 8903B "Audio Analyzer"
« Reply #5 on: September 22, 2017, 10:30:37 am »

I love the 8903B. We have a few here in the Nashville shop. They were at least $10k new,I found ours on the interwebs for about $1500 each. I have an Tektronix AM700 in my office now that needs a memory battery. XLR in/out,dual FFT measurements. Woot!
Rex Ray
Sr. Engineer
Clair Nashville
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Bruce Burke

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Re: HP 8903B "Audio Analyzer"
« Reply #6 on: November 11, 2017, 10:36:23 am »

SINAD is used in determining the overall health of FM radio receivers as it is a measurement of
Signal to Noise + distortion products. So while you might get a good reading on a receiver just doing a quieting (no modulation)measurement, the radio could still be way off frequency, whereas SINAD might fail, because the distortion would be up from the modulated signal running into the skirt of the receiver IF filters.

The 8903 (like most HP equipment) uses an HPIB interface to run it remotely. You would need a card in your computer to connect to it(and an HPIB cable, of course). The most popular programming software is National Instruments Labview, which allows you to do automated testing and data recording.

You most certainly could do transducer testing, but all your interfaces would need to be calibrated and the calibration profiles loaded into Labview.

Where I once worked, we used B&K equipment to test transducers. That included the electronics, calibrated microphones, calibrated "voices," and even calibrated "heads."

-Bruce
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