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
them.
Regards and 73,
Gary, ZL1AN.
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