On 11/14/2010 12:08 PM, Magnus Danielson wrote: > In the first stage, the input signals is mixed by the average > frequency (37 MHz in this case) causing the beat frequencies to become > roughly the same (27 MHz in this case). The second stage would then > act as as the normal offset local oscillator and beat-frequency mix-down. Magnus,
I'm writing you directly because I want to avoid a nitpicking flame war or I agree with your math, but your nomenclature seems a bit confusing. The arithmetic mean or average is 27 Mhz. It seems that for the general case is the local oscillator is the arithmetic mean plus the lower frequency. You want to have high side injection for the low frequency and low side injection for the high frequency. I'd call the average frequency the "intermediate frequency" and define it as the arithmetic mean of the two frequencies of interest, because it literally is and because it conforms to common use for superheterodyne receivers. Likewise I'd call the injection the "first local oscillator" or "first injection" and define it as the intermediate frequency plus or minus the frequencies of interest for comparison. The "normal" DMTD oscillator could then be called the "second local oscillator" or "second injection" One side effect of this approach is that common mode variations on the oscillators being compared will either be enhanced or reduced because those effects are inverted in frequency for the lower frequency. I haven't run the math to prove this, but it looks like it'll be proportional to the ratio of their frequencies. I need to think more about the impact of variations between the first and second LOs. -- mailto:[email protected] Oz POB 93167 Southlake, TX 76092 (Near DFW Airport) _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
