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Old 7th October 2011, 03:07 PM   #41
klewis is offline klewis  United States
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Hi Dick,

Just before receiving your note I had a similar revelation, how does the spectrum analyzer know what the level of the fundamental is if I'm sending it to the analyzer nulled? The analyzer must be using something like 1 Vrms as 0dB. So, when I turn the volume nob up or down it's looking at the harmonics against this fixed assumption on input voltage. So, I need to figure out what reference voltage it's using for 0dB and then set the signal sources to that level for future tests or figure out how to have the anlayzer use what ever the un-nulled fundamental voltage is as 0dB. Does this methodolgy make sense? I need to crack the manual open and have a look.

Ken
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Old 7th October 2011, 08:02 PM   #42
richiem is offline richiem  United States
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Methodology makes sense.

The common level references are:
0dBV = re 1VRMS = voltage level
0dBm = re 1mW into spec'd Z; typ. 775mVRMS into 600 ohms = power level
0dBu = re 775mVRMS into a high impedance = voltage level

HP has used dBV and dBM (600 ohms) variously for audio and spectrum analyzers, but watch out for dBm (50 ohms or 75 ohms) for many units depending on application.
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Old 8th October 2011, 01:20 AM   #43
klewis is offline klewis  United States
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Quote:
Originally Posted by richiem View Post
Methodology makes sense.

The common level references are:
0dBV = re 1VRMS = voltage level
0dBm = re 1mW into spec'd Z; typ. 775mVRMS into 600 ohms = power level
0dBu = re 775mVRMS into a high impedance = voltage level

HP has used dBV and dBM (600 ohms) variously for audio and spectrum analyzers, but watch out for dBm (50 ohms or 75 ohms) for many units depending on application.
Hi Dick,

I set the output to 1Vrms and the spectrum analyzer scale read approximately 0dB. So, with the output at 1 Vrms at 10kHz I nulled the fundamental by about 60dB and got the following spectrum plots for the 4402B and the second one for the 8903A.

To get the full spectrum to plot, I will have to tweak my VEE program. That will take a few days.

What do you think about the results?

Ken
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File Type: png 4402b_10k_1_v_0dB.png (52.5 KB, 214 views)
File Type: png 8903a_10k_1v_0db.png (55.4 KB, 213 views)
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Old 8th October 2011, 05:19 AM   #44
richiem is offline richiem  United States
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About what I would expect from the 4402B at 10kHz, and the 8903 is worse than I expected.

See below the comparable 1VRMS @ 10kHz spectrum (up to 40kHz -- as high as my computer can see) for my HP 239A -- the fundamental is at -60dB re the 1V input. (I rescaled the spectrum to dBV)

Note that the calculated THD of about 0.5%, re the -60dB fundamental, fairly accurately reflects the RMS sum of the visible harmonics, roughly 0.0006%
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File Type: jpg HP239A-10kHz-0dBV.jpg (27.6 KB, 213 views)

Last edited by richiem; 8th October 2011 at 05:37 AM.
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Old 8th October 2011, 03:38 PM   #45
klewis is offline klewis  United States
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The 4402b calcs out at .00091% the manual says it should do .0005% maximum in the 1 to 10 k range and typically .0002%.
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File Type: png 2011-10-08_0835.png (9.9 KB, 200 views)
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Old 8th October 2011, 03:41 PM   #46
klewis is offline klewis  United States
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The HP8903A is at .0019%.
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File Type: png HP8903A Thd.png (10.6 KB, 193 views)
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Old 8th October 2011, 09:36 PM   #47
richiem is offline richiem  United States
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On the 4402B, check the DC voltage at TP4, and adjust R145 to get 0VDC (the allowable range at TP4 is +/-1VDC -- that's a lot). Small changes here may have an affect on the residual THD.

As to the 8903A, I'm not sure that there is anything you can do -- looks like the oscillator and its AGC either work or they don't....

For comparison, I've attached the spectrum of my State-Variable Cordell oscillator at 10kHz and 0dBV -- 0.0025% -- worse than your 8903A.
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File Type: jpg Cord-10kHz-0dBV.jpg (32.1 KB, 66 views)
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Old 9th October 2011, 12:54 AM   #48
klewis is offline klewis  United States
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Hi Dick,

I found the adjustment as you described in the manual. I think the tolerance is +-0.1V I got it down to 0.015V. The 2nd and third improved. I also figured out how to get the spectrum analyzer to average the values, instead of just getting a snap shot at a specific instant. Here are the new calibrated values for the 4402b.

Ken
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File Type: png 4402b_calibrated.png (10.2 KB, 69 views)
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Old 9th October 2011, 02:50 AM   #49
richiem is offline richiem  United States
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Much better but still a long way from the spec and typical values that K-H list.

Have you checked the accuracy and linearity of the analyzer's amplitude scale? I would definitely do that -- you need a known good attenuator with 10dB or 20dB steps plus maybe an additional fixed attenuator of say 40 or 60dB to give you the dynamic range. I don't remember the attenuation available in the 8903, but it or the 4402B may have sufficiently accurate attenuation to do the job for your 80dB range analyzer.

The issue here is about potential compression of the amplitude range near the bottom of the displayed range so that the displayed products are actually lower than displayed values. Back in the day I used both an HP 3580 and 3581 and had to do a very careful alignment of each to get accurate results, and still had to be careful with the 3580 about products showing up that had much lower amplitudes than seen in the display.

Just a thought.
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Old 9th October 2011, 03:02 PM   #50
klewis is offline klewis  United States
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Hi Dick,

I have a stepped attenuator that I can measure with the 8903a so that I have known values. Are you suggesting that first set the input to the signal analyzer to 0dB and then attenuate the signal by around 80db (with a known or verified attenuation value) and see if the analyzer shows this same value of attenuation?

Ken
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