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[TenTec] Passband measurements

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Subject: [TenTec] Passband measurements
From: g.bold@auckland.ac.nz (Gary E. J. Bold)
Date: Fri, 10 Oct 1997 12:56:49 +1300
Hi all,

Thanks for the various comments re the passband measurements I make, and the 
effect of speakers/mikes etc: 

Re the method/signal source I use: (psuedo-white RF noise)

Yes, it DOES measure the "whole radio".  If there is "leakage" around an IF 
filter somewhere, that will show up.   (Thus, what you find may be quite 
different from what you'd get by extracting a filter and measuring it on the 
bench).  If there's hum, or hum harmonics (not always easy to hear with small 
speakers/phones) that will also show up.

The measurement environment closely approximates that in which we normally use 
our filters (high background noise contaminating a small CW signal).   The 
"simulated white noise", (RF noise), which is energising the filter contains 
all frequencies simultaneously, so the result includes any inband 
cross-modulation effects.

Can I compare results "absolutely"?

No.  Of course it IS better to have a genuine, calibrated white noise rf 
generator, and calibrate the mike (if used) with another audio white noise 
generator if you want "directly comparable" measurements, but few of us have 
those.  RF noise IS pretty white, if you get away from static-crash 
contaminated low bands.  

But the bottom line is that with a laptop computer I can make "reasonable" 
passband measurements in 5 minutes in a strange shack, and show the results 
immediately.  None of my measurements CLAIM to be absolute.  I just normalise 
all results to 0 dB at the passband maximum.  What we see correlates well with 
what I hear.  

Those here who have volunteered their radios for testing have all found that 
the measured responses which INCLUDE the speaker/enclosure system (through the 
mike) are illuminating, and usually surprising.  There may be 5 - 10 dB dips 
and resonances which the radio's owner didn't know about.   If you normally 
listen through the speaker, this may explain why your radio "sounds quite 
different to the one down the road".  Of course, it is equally easy to cut out 
the speaker and digitize directly from the phone jack, and I do that too.  

This method is not new. VK3BID was doing it in 1982, but he used a high-end 
hardware spectrum analyser.   What IS new is that we can do it all now with 
software.   The digitizing package I use is shareware, and anybody with a PC, 
Win95, and a soundcard should be able to run it, digitize their radio, and 
email me the resulting record for analysis. 

If you want to see some samples, I can email you a text file with a table of 
typical results.   You can cut these out, import into a spreadsheet, and plot 

Regards and 73,
Gary, ZL1AN.

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