Geoff, GM4ESD wrote: Digressing from the subject of audio output for a moment, having the "RF Gain" control the gain of the IF amplifier only can lead to some undesireable effects unless the IF amplifier design is done very carefully using suitable devices. The problem is that the linearity of some of the more popular low cost amplifier chips actually gets worse as their gain starts to be reduced, although some types of these chips will recover as the gain is further reduced. Assuming a situation where the Preamp is "Off" and the Attenuator is "In" leaving two or more signals are in the passband, some strong and the one that you want is weak, any negative effect on linearity caused by changing gain is of course unwelcome. The overall effect is determined by the receiver's Gain Distribution and the characteristics of the stages both before and after the controlled IF amplifier.
Many modern (not ham) HF receivers designed to handle *very* strong signals while allowing copy of an adjacent signal close to the noise still do use "RF Gain" to control some type of front end variable attenuator, and a strong RF amp with fixed but very low gain embedded in reasonably high loaded Q variable frequency tuned circuits. But these receivers consume a lot of power. ================== Quite right, in my experience. That's true of tube-type amplifiers as well, which is why the really low-noise state-of-the-art tube RF amplifiers were fixed gain as well. They were most common in VHF/UHF applications. I believe that's why the K2 uses a fixed-gain wide-dynamic range I.F. preamplifier (Q22) ahead of the I.F. filter, and puts the variable gain MC1350 (U12) after the filter where it's reasonably well protected from off-frequency signals. Of course, the K2 has a switchable RF amplifier and a switchable attenuator which, while needing manual control by someone who understands when to use them, go a long way toward making the K2 stand up well in extreme conditions. It helps, too, that the K2 is a straightforward single-conversion design. More complex multiple-conversion receivers need more protection to do as well because of all the extra mixers and amplifiers they have. Of course, that complexity allows for some nice gadgets like front-panel "passband tuning" but it puts a huge load on the design to stay competitive with a more simple, straightforward single-conversion superhet like the K2. One of my homebrew designs from the 70's, which also used the MC1350 (that chip has been around a l-o-n-g time!), used pin diodes to control a variable attenuator at the antenna input to the receiver in addition to varying the gain of the MC1350. After fiddling with it for a long time, I reverted to a simple step attenuator at the antenna input much like the K2 has. That choice speaks to a basic question in the design of anything: which is the more eloquent and desirable, extreme complexity that does anything and everything as well enough and fully automatically, or a simple design that does specific things very well but which may require a more knowledgeable operator and greater operator intervention? I don't think there's any one answer to that question. If there were we'd not have both automatic and manual transmissions in cars. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
