[Elecraft] [K3] FM and AM Bandwidth

[Lyle Johnson]

> Typical FM bandwidth is 5Khz, so the DSP really needs to be changed as 
> in its current state the K3 is chopping off 1Khz of received signal in 
> FM, or 20% of the bandwidth.

There is more than one FM standard in use in Amateur radio around the 
globe.  In some parts of the U.S., 2 meter channels are on 20 kHz 
spacing, some are on 15 kHz spacing.  In some parts of the world, 
channels are 12.5 kHz apart, or moving towards 12.5 kHz spacing.

The rule of thumb for FM signals is that the bandwidth is (2 * 
deviation) + Fmod where Fmod is the highest audio frequency.  This is an 
oversimplification, but serves to illustrate the idea.

In the K3 we have a 13 kHz wide IF filter.  By limiting deviation to 4.5 
kHz and Fmod to 3 kHz, we occupy about 12 kHz.

If we allowed 5 kHz audio and 5 kHz deviation, we'd occupy 15 kHz.  Such 
operation leaves no room for frequency error or adjacent channel 
rejection in a 15 kHz channel region.

By limiting the audio response to about 3 kHz, we ensure that we are 
compatible with existing channels and will be compatible with future 
narrower channels.

In the case of AM, we will be increasing the upper audio frequency response.

73,

Lyle KK7P