[Elecraft] Receiving and demodulation

Don Wilhelm w3fpr at earthlink.net
Mon May 14 02:03:32 EDT 2007


Brian,

The 1st IF (hardware) AGC is highly dependent on the width of the 
'roofing filter'.  We do not have the relevant figures for the relative 
dynamic range of the 8.215 kHz IF vs. the 15 kHz DSP portion of the 
receiver.

When those specifics are available, your question can be answered.  For 
now, I only assume that a narrow filter will be the best solution for 
AGC 'pumping' when operating digital modes.

The capability to demodulate PSK31 and RTTY signals and the AGC 
'pumping' situation are entirely two different animals.  One thing is 
for certain, the AGC cannot be totally controlled by the DSP section, so 
the real answer depends on the actual dynamic range that the A-D 
conversion can handle - we don't have that answer yet.

73,
Don W3FPR

Brian Lloyd wrote:
> In playing with PSK31 recently using a *very* suboptimal receiver (my 
> old Icom IC-706mkII) I realized how important it is to couple the 
> demodulation system with the the rest of the receiver. For example, I 
> can watch the AGC respond to unwanted signals and make the desired 
> signal uncopyable. So it seems to me that, in the process of 
> demodulating a signal the demodulator has information about the energy 
> in the desired signal that should be used to drive the AGC. That implies 
> to me that you really cannot effectively separate the two. The "sound 
> card plugged into the radio" approach is interesting as a way to start 
> but it is not the final solution.
> 
> 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?
> 
> As we come up with more and more interesting digital modulation schemes 
> for HF I can see the desire to closely couple the demodulator with the 
> rest of the receiving system. The K3 has some real possibility here. 
> Will it be incorporated?
> 
> How about an API to let other programmers add demodulators and couple 
> them to the rest of the receiver, controlling roofing filter selection, 
> AGC, etc.?
> 
> 73 de Brian, WB6RQN
> Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com


More information about the Elecraft mailing list