SECOND MIXER AND CRYSTAL SWITCH OPERATING EXAMPLE

TM 11-5820-401-34-3/0967- LP-432-3060

1-17.

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

Radio MC-TUNE-KC Control Set to 30.00 MHz

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 30.00 MHz, the Y2006 crystal 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, the difference frequencies are closest to the 5.625-MHz

resonant frequency of the A3300 tank circuit.

The two difference frequencies are produced as follows:

52.500

53.500

- 46.850

6.650

5.650

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 that 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, the heterodyning

occuring in the A3200 mixer and in the A3300 second mixer 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.

1-13