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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.

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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".

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