Thanks to Ron for his excellent explanation and all the others who replied. Now that I know about the 1200 hz difference I will keep an ear out for this phenomenon.
Do you think this would happen in the K1 also, or are the K1 filters sharper? I'm much more used to hearing very weak signals rather than very strong ones -- after all I did answer the supressed image first :-) 73, Mike N2HTT > Yes, you heard an "image". All superheterodyne receivers (like the > KX1) that > use a BFO for CW/SSB reception have two of them because they have two > mixers: one at the input to convert the signal frequency to the > intermediatefrequency (I.F.) and another at the detector to convert > the I.F. to audio. > > It works like this. When you inject a signal and a second local signal > (local oscillator or BFO) into a mixer, the mixer reproduces the > signal at > its output on frequencies that are the sum and difference of the input > frequencies. > > The KX1 uses an I.F of about 4913.6 kHz. All signals coming in at the > antenna are converted to this frequency, amplified, filtered and > passed to > the detector which is a mixer. A 'beat frequency oscillator' (BFO) > producesa signal at 4913.0 kHz. That means the detector puts out > two signals, the > sum of these two frequencies and the difference between these two > frequencies. The sum is a very high RF frequency - over 9 MHz. > That's easily > filtered out. The difference frequency is what we're interested in: > 600 Hz. > That's the audio frequency you hear in the phones. > > Now suppose you tune slightly so the same signal produces an I.F. > of 4912.4 > kHz. That mixes with the 4913.0 BFO to produce 600 Hz again! You'll > hear the > same signal at two places on the "dial" separated by exactly twice > the beat > frequency. In this case, they're 1200 Hz apart. > > All superhets do this. In earlier receivers before modern crystal > filters,it was a very well understood phenomena. As the I.F. > filters got better, > they would reject one of the signals. In this case, a high-performance > crystal filter (such as in the K2) would reduce the signal at the > I.F. of > 4912.4 so much you'd not likely hear it at all. Manufacturers > called this > "single signal" reception: each signal now only appeared once as we > tunedacross the bands. Amazing! <G>. > > We've come to expect "single signal" reception in all modern gear > thesedays, but it depends entirely on how strong the signal is and > how good the > I.F. filter is. The tiny filter used in the KX1 is not as good as > the those > used in larger and more expensive rigs like the K2. Its "stop band" > attenuation isn't as good. That, coupled with your hearing a very > strongsignal on the band, allowed you to hear that "other" signal > that's normally > suppressed. > > Even with the simple filter used in the KX1 you should only be able > to hear > the second signal when listening to a very strong station! > > 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 > _______________________________________________ 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
