[Elecraft] beat freq method

[Scott Ellington]

This brings up an interesting point, though it doesn't affect the method at all.  The "beat" Fred refers to results from adding of the sidetone to the signal tone:  When they happen to be in phase, the volume goes up 6 dB, when they are exactly out of phase they cancel, so you hear the volume go up and down.  It is a completely linear process.

This is not what produces the sum and difference "beat frequencies" in a mixer, which requires nonlinearity or multiplication.  If you look at the audio coming out of a receiver with a spectrum analyzer, with two audio tones present, you will see only those two frequencies, not sum and difference frequencies.   (Except for some tiny traces due to inevitable distortion.)   

So, for example, if you tune in a signal for a 400 Hz tone, and the sidetone is 600 Hz, you will NOT hear a 200 Hz beat, unless you turn the volume up so high that your receiver and/or ears produce distortion products.   (Don't do that!)  When you get close, though, you will hear that fluctuation in volume, which we can use to match frequencies.

73,

Scott  K9MA


On Mar 28, 2011, at 2:18 PM, FredJensen wrote:

> If it is close, you'll 
> hear the combined audio signals fading up and down rather than a "beat 
> note."  That fading *is* the beat note but it's too low a frequency to 
> hear it as a tone.

Scott Ellington
Madison, Wisconsin
USA