On 1/29/2025 3:19 PM, Mark Holt / W7EAZ
via groups.io wrote:
A user recently posted his "BUSY" light being illuminated all
the time preventing his transmission.
He showed his waterfall and I asked why he had a notch filter
in the middle of his pass band.
When he eliminated the notch filter he was able to transmit
because the "BUSY" indication would go out.
?
So, my question is: Why would a notch filter inside the pass
band cause a "BUSY" indication?
I experimented and found that if I turn on my notch filter
outside the 800Hz bandwidth and slowly tuned it toward the VARA
800Hz pass band it would trigger the "BUSY" indicator.
See the attached screenshot of my VARA waterfall.
I slowly tuned the notch from the high-side of the pass band
until the "BUSY" lit up.
Mostly curious for my own benefit on signal processing that
is going on.
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If this is not the correct forum, please indicate where this
question needs to be asked.
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Mark /
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The "BUSY" indicator is similar to the noise-activated squelch on
an FM rig.?? It doesn't actually detect specific tones in the
passband as an indication of "signal present".? Rather, it looks
at the total noise power in the passband.? When the noise power
drops due to a signal being present, the channel is considered
"BUSY".?? Tuning a notch filter into the passband also decreases
the amount of noise in the passband and "fools" the program into
thinking the channel is "BUSY".
Stephen H. Smith??? wa8lmf (at) aol.com
Skype:??????? WA8LMF
EchoLink:? Node #? 14400? [Think bottom of the 2-meter band]
Home Page:?????????
New? 30 Meter?? APRS-over VARA? Frequency
As of 1 Jan 2025.?? Details Here:
????
- APRS over VARA? --
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"Studio B" Ham Shack on Wheels
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