General coverage / Receive-only bands

[dj2mg]

This adds support for a "general coverage" receive-only bands. Adding more specific receive-only bands should be straight forward.

• Adds a new band labelled "GEN"
• Transmit is enabled for ham bands (that is all the previously existing bands) and disabled for receive-only bands (via the Mode state machine)
• For receive-only bands a low pass filter is selected based on the tuned frequency; no change for ham bands (it appears to be fixed for ham bands)

[KI3P]

Thank you very much for the pull request, Michael!

I wrestled with the question of whether to disable transmitting out-of-band. The arguments against it are:

• If you travel with your radio between ITU zones, but aren't able to update the software with the new band edges, then you might not be able to transmit at legal frequencies in your new zone.
• In cases of emergency, radio amateurs are permitted to transmit at any frequency necessary to secure help. This is the case in the US, at least -- it may be different elsewhere.

In the end, I felt that it was the responsibility of the operator to use their equipment in a compliant way and decided to not restrict the transmit frequency. What do you think about this?

[Edit]: Upon testing, I see that this version of the code still allows transmitting out-of-band when a ham band is selected -- it's only when GEN band is selected that transmit is disabled. This makes sense to me.

[KI3P]

The code updates the LPF selection as you tune around the bands when in the new GEN receive-only mode. It leaves the BPF in bypass mode.

However, the LPF filter is not in the receive signal path, so this does not affect the received signal:

If you want to have an effect on the received signal quality, you need to switch the BPF instead of the LPF.

[dj2mg]

Hi Oliver. You are right about the low pass filters. I looked at the schematic, but somehow didn't see it, until you pointed it out.