Circuit Described at: http://everycircuit.com/circuit/6652344659345408
To determine true signal generator setting for 455-kc signal (outside of AM broadcast band) we can use "harmonics" or whole number multiples within the signal generator outputs. A few examples:
455-kc times 2 = 910-kc 2nd Haromonic
455-kc times 3 = 1365-kc 3rd Haromonic
455-kc times 4 = 1820-kc 4th Haromonic
Let's use 2nd harmonic as 910-kc falls in broadcast band.
Set-up example: Broadcast stations on AM radio closest to 910-kc are 880-kc and 970-kc. If we start with 880-kc station and remove beat sounds with signal generator setting around 440-kc and we see dial setting of signal generator reads 445-kc or 5-kc above 440-kc generator output. Verify this with beating 970-kc station 2nd harmonic of 485-kc and dial having the +5-kc reading of 490-kc. Then, set signal generator dial to 460-kc to give 455-kc signal generator output.
SPECIAL CAUTION USING IF BANDS:
If radio receiver has IF tuned to 455-kc a confusing double beat may be heard when signal generator beats with signal in IF amplifier as well as test station.
If receiver has RF stage and IF wave trap little chance of signal generator output beating with IF amplifier signal and this receiver can be used to to set 455-kc output on signal generator. Also, could use receiver where IF amplifier frequency
is quite different from test signal we want to set. A Tuned Radio Frequency receiver (or TRF receiver) could be used for calibration.
Proper settings and list of important area radio frequencies should be made for service man to have available for future use.
