Peter Connors, F5VNB 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.) > A sane engineer surely would; but the huge majority of transceivers are trying to use the ALC loop for too many different functions including gain levelling between bands, peak limiting and also power control.
That is NOT how ALC was originally intended to work. The basic idea was invented and refined by the Collins company more than 30 years ago, but was intended only to handle *small, dynamic* variations in SSB output power (mostly due to filtering a clipped SSB signal, which re-introduces a few dB of amplitude variation). But ALC is also being used to achieve much larger output variations, notably for the front-panel RF PWR control. The ability to vary the output power all the way from 100W down to say 1W requires a very high gain in the ALC loop. As well as producing instability and overshoots, such a high loop gain ensures that any activation of the ALC will cause hard limiting - with obvious consequences for IMD. SM5BSZ has published a strong critique of the way that most rigs MISuse the ALC loop. Instead of ALC helping to prevent IMD, it has become a major potential *source* of IMD. SM5BSZ's criticisms are backed up by measurements on a range of different rigs: Real life Dynamic Range of Modern Amateur Transceivers http://www.sm5bsz.com/dynrange/dubus205/dubus205.htm Speech Processing for SSB Transmitters http://www.sm5bsz.com/dynrange/dubus405/dubus405.htm The conclusion of SM5BSZ's papers is that rig designers need to stop demanding the impossible from the dynamic ALC loop, and go back to the original Collins concept. The big variations required for power control are "static" settings that do not require a dynamic control loop. They should be achieved by calibrating and then pre-setting the TX exciter gain to give the required power level for each band. Then the ALC can return to its original role of handling small dynamic variations, which only requires a low-gain ALC loop with a much lower potential for re-introducing IMD. Does this sound familiar? It should, because the K3's power control and ALC system has been very much influenced by the ideas from SM5BSZ and originally from Collins. Only Elecraft know the intimate details of how these concepts have been implemented in DSP code, and the extent to which the K3's ALC differs from traditional rigs. The bars on the K3's ALC display have been programmed to show something very different from the ALC meter in a traditional rig. Bottom line: don't make assumptions about ALC behavior in the K3, based on the widespread misuse of ALC by other rigs. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
