[Elecraft] Receiving and demodulation

[Brian Lloyd]

On May 23, 2007, at 9:33 AM, lyle johnson wrote:

>> One of the interesting features of the K3 is that PSK31 and RTTY   
>> operation are part of the radio. Does that mean that the PSK31  
>> and  RTTY demodulators in the K3 control the AGC so that the  
>> desired  signal and only the desired signal controls AGC?
>
> The use of an SSB bandwidth "waterfall" display for PSK31 is a  
> great example of using the wrong filter for the job, at least after  
> you've established the QSO.  The Rx AGC gets pumped by strong  
> signals inside the "roofing filter" bandwidth, as you've observed.
>
> On the K3, when I am copying a PSK31 signal, I spin the selectivity  
> right down to 50 Hz.  Then, the PSK31 signjal I am copuying is the  
> only one inside the fitler bandwidth, and is the one driving the AGC.

Thanks Lyle. That helps. Sounds like AGC is derived from the envelope  
energy in the passband. I was thinking of actually looking at the  
energy contained in the desired signal, in this case the PSK- 
modulated carrier and the associated sidebands, and using that to  
control the AGC even tho' there are other signals in the passband.  
This could result in the AGC being set such that the A:D saturates  
but that's life.

Thought: set the Eb/No of the desired signal to the desired 12dB or  
so using the AGC and then track it. That would guarantee the maximum  
headroom regardless of filter selection.

And narrowing the roofing filter as much as possible is also nice ...  
unless you are trying to copy more than one signal in the passband  
simultaneously.

73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com