SECOND MIXER AND CRYSTAL SWITCH OPERATING EXAMPLE

TM 11-5820-670-30

1-16. CRS SECOND MIXER A3300 AND CRYSTAL SWITCH A2000. (CONT)

SECOND MIXER AND CRYSTAL SWITCH OPERATING EXAMPLE Radio Frequency Selector Switches

Set to 30.00 MHz

EL1AT092

Two strong frequencies enter the second mixer from the 53 MHz filter, 53.50 and 52.50 MHz. These

signals are heterodyned with the output from the interpolation oscillator.

With the radio tuned to 30 MHz, crystal Y2006 is selected. This crystal sets the interpolation

oscillator's output at 46.850 MHz. When 46.850 MHz is mixed with 53.50 and 52.50 MHz, several

strong frequencies are produced. However, difference frequencies are closest to the 5.625 MHz

resonant frequency of A3300 tank circuit.

The two difference frequencies are produced as follows:

52.500

53.500

-46.850

5.650

6.650

Of the two difference frequencies, 5.650 MHz is coupled across the tank to the next module. The other

frequency, 6.650, is beyond the tank's bandwidth and, therefore, is attenuated.

The example presented assumes the local oscillator is running exactly at its correct frequency. In this

case, exactly 5.65 MHz is passed by the second mixer's tank circuit. However, if the local oscillator is

running above or below 41.5 MHz when the radio is tuned to 30.00 MHz, heterodyning occurring in

mixer A3200 and in second mixer A3300 causes the second mixer to output a signal that differs from

5.65 MHz by the amount the local oscillator differs from 41.5 MHz. When the second mixer's output is

not exactly 5.65 MHz, an error signal is generated to correct the local oscillator.

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