wrote in message
oups.com...
Not exactly twice, but I know what you mean. It would be at 20455Hz


Nope, either 10910 kHz or 9090 kHz, i.e. 10000 kHz plus or minus 2*455 kHz.
The LO is at 10000 kHz plus or minus the i.f. of 455 kHz. If it's at 10455
kHz, a signal at 10910 kHz will also mix to produce a 455 kHz i.f.; if 9545
kHz, a signal at 9090 kHz will also mix to 455 kHz.

[snip]
But then they realized a problem with that. Every time two frequencies
are heterodyned together, the sum and the difference result. This means
that there is an image frequency, ie one that is incidental to the
heterodyning but which you you don't want. If your IF frequency is
455KHz, and you want to receive WWV at 10MHz, there will also be an
image twice the IF frequency away (higher or lower depending on
whether the receiver's local oscillator is higher or lower than the
signal frequency).
[snip]