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: [email protected] > [mailto:[email protected]] On Behalf Of Peter > Connors, F5VNB > Sent: Saturday, January 17, 2009 4:34 AM > To: [email protected] > 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 _______________________________________________ > 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 _______________________________________________ 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
