RF FREQUENCY SELECTION

TM 11-6625-2975-40

1.18. RF SECTION OPERATION. (CONT)

Synthesizer assembly A4 generates the if carrier signal at the frequency specified by the rf tune signal

from counter I/O board A3A3. The synthesizer assembly A4 employs a phase-lock loop to provide a

stable rf output frequency. A variable oscillator in the synthesizer assembly provides the means for

slewing the frequency as requested by the keyboard entries.

The rf output from synthesizer assembly A4 is supplied to rf strip line assembly A3A1. The audio

modulation signal from demodulation/ale assembly A3A2 is also supplied to rf strip line assembly A3A1

and used to modulate the rf carrier signal. The modulated rf signal is then supplied to the digitally-

controlled attenuator A6AT1, which is set to the correct attenuation for the output level selected.

Counter I/O board A3A3 contains the I/O ports for connecting the rf section to the control bus. It also

contains an rf frequency counter that provides rf frequency information to the CPU to be displayed on

the front panel.

RF FREQUENCY SELECTION

The cpu sends rf frequency control data over the control bus to I/O ports on counter I/O board A3A3.

Counter I/O board A3A3 routes this control data to synthesizer assembly A4. This digital rf tune signal

tunes synthesizer assembly A4 to the selected carrier frequency. Synthesizer assembly A4 then

generates the selected rf carrier frequency and supplies the signal to rf strip line assembly A3A1.

When a sampling of this rf carrier frequency indicates that the frequency is phase-locked to a 25 kHz

loop-lock frequency, a phase-lock status signal is supplied to counter I/O board A3A3. The CPU also

sends data over the control bus to the I/O ports of A3A3 indicating the mode selected. Counter board

I/O A3A3 converts this mode selection data into I/O control signals and supplies them to demodulator/

alc assembly A3A2. The I/O control signals adjust the audio modulation levels from analog board A2A2

to a correct level, providing a standard percent of modulation for each mode. To change the percent

of modulation to other than the standard percentage, the audio modulation signals in the audio

generator circuits must be adjusted. Demodulator/alc assembly A3A2 checks and automatically

adjusts the level of modulation and then supplies the signal to rf strip line assembly A3A1.

The composite audio modulation is combined with the rf carrier signal on rf strip line A3A1 to produce a

modulated rf signal. This signal is supplied through the digitally-controlled attenuator A6AT1 to the RF

OUT connector on the front panel. The digitally-controlled attenuator sets the rf output level. The CPU

also transmits data over the control bus to the ports on counter I/O board A3A3 for selecting the

desired rf output level. The data on the control bus is routed to the digitally-controlled attenuator

A6AT1 by counter I/O board A3A3. The binary-controlled signal sets the attenuator to the correct level

for producing the selected rf output level at the RF OUT connector on the front panel. When microvolt

readings are being displayed on the RF LEVEL display, an additional 6-db attenuation is inserted. This

additional 6-db attenuation makes the signal output at the RF OUT connector one-half of the displayed

RF LEVEL value. This results in an output which is hard microvolt, without the use of an external 6-db

pad. When the rf output is being displayed in db mw, the additional 6-db attenuation is removed, and

the signal output at the RF OUT connector is absolute, the same RF LEVEL is being displayed on the

front panel.