[Elecraft] K3 pwr out on digimodes

[Geoffrey Mackenzie-Kennedy]

Pete F5VNB wrote:

> Joe Subich, W4TV wrote:
>>
>> Absolutely not!  When conventional rigs are generating ALC
>> their final amplifiers are ALREADY in compression and they
>> are generating IMD in the PSK31 signal.
>
> A bit sweeping, Joe. Surely any sane engineer would put the onset of
> system ALC at a level *before* the PA goes into compression? (Such
> systems would not include grid current derived ALC from some class AB2
> PA of yesteryear.)

The problem with conventional closed loop ALC systems which sample at the PA 
is of course that the PA, or the PA and the Driver, can be driven into 
compression before the ALC system "catches up" and reduces drive, the point 
Joe is making. While it is tempting to set the ALC threshold well below the 
point at which PA compression takes place, the PA could still be driven into 
compression during the ALC "catch up" period. Reducing the attack time 
constant of the ALC system below some minimum can breed its own set of 
problems.

Because this type of ALC system was first intended to be used as part of a 
transmitter's protection system and not as a processor, one method of drive 
level control frequently used samples at some point in the transmitter chain 
and adjusts the gain of some stage AFTER the sample point. This method 
ensures that there is no "catch up" problem. If used in a rig such as the K3 
a suitable sample point would probably be just after the bandpass filters.

A parallel exists in receiver AGC systems.

73,
Geoff
GM4ESD



Peter Connors, F5VNB wrote:


> Joe Subich, W4TV wrote:
>>
>> Absolutely not!  When conventional rigs are generating ALC
>> their final amplifiers are ALREADY in compression and they
>> are generating IMD in the PSK31 signal.
>
> A bit sweeping, Joe. Surely any sane engineer would put the onset of
> system ALC at a level *before* the PA goes into compression? (Such
> systems would not include grid current derived ALC from some class AB2
> PA of yesteryear.)
>
> To avoid the well-known problems of selecting suitable ALC time
> constants and loop gain, the K3 performs ALC at the DSP level: for any
> signal, SSB or PSK31, doesn't this demand a flat transmit passband in
> the system following the processing?
>
> Good thread!
>
> 73, Pete F5VNB/G4PLZ
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