[Elecraft] K3 pwr out on digimodes

[Joe Subich, W4TV]

Peter, 

> Surely any sane engineer would put the onset of system ALC 
> at a level *before* the PA goes into compression? 

Not necessarily.  Some incidental compression in the final
amplifier is not necessarily a bad thing for CW or SSB 
systems.  If the ALC is derived from something like grid 
current that does not begin to appear until the amplifier 
enters the compression range and includes a suitable time 
constant the compression will be minimal and only occur 
on an occasional peak.  

This form of ALC is suitable for CW, SSB and FSK operation 
in that compression is of no consequence for CW and FSK 
which suffer no IMD issues in even saturated operation 
and of minimal consequence for SSB since the peaks are of 
sufficiently brief duration and the compression is quite 
mild (one or two dB).  

> 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?  

Again, an absolutely flat passband is not necessary for a 
CW, FSK (including AFSK) or SSB system.  With CW and FSK 
the only concern is the system loss at a single frequency 
(or a very narrow range of frequencies for AFSK).  The power 
calibration process compensates for the loss at the single 
frequency for CW and the rate of change is sufficiently low 
that the compensation will be correct +/- a few Hertz of the 
compensated frequency needed for narrow FSK. 

With SSB, one or two dB of passband ripple is not an issue. 
The passband of the human voice has far more "ripple" than 
any moderate quality filter ... ten dB or more of high 
frequency boost is often applied through equalization or 
designed into microphone responses to increase intelligibility.

Again, these issues are only important when transceivers 
designed for CW, FSK and SSB are used for modes (e.g., 
multiple tone digital modes) they were not designed to 
handle or operated in a way (e.g., "click on the waterfall") 
they were not designed to be operated.  

Multiple tone digital modes are essentially a full-time 
multi-tone signal (e.g., two-, three- or more-tone IMD 
test).  Successful operation of multi-tone and multi-
carrier digital systems requires that the passband be 
absolutely flat across the operating range and that any 
amplifier have no compression (amplifiers are generally 
specified for the 1 dB compression level).  These are 
criteria are significantly more stringent than normal 
amateur equipment.  

There are two approaches for successful amateur digital 
operation.  First: use digital modes that are narrow 
compared to any "ripple" in the passband, operate the 
power amplifiers at least 3 dB plus the peak to average 
(crest factor) of the particular digital mode below their 
SSB PEP rating and reset the power level for any change 
in "subcarrier frequency" relative to the SSB dial 
frequency.  Otherwise: utilize on frequency RF generation 
(do not employ any intermediate frequency stages, mixers 
and filters in generating the RF) and use a power amplifier 
chain that is capable of much higher saturated power output. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Peter 
> Connors, F5VNB
> Sent: Saturday, January 17, 2009 4:34 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
> 
> 
> 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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