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SECTION 10
RECEIVER (A4)
10-1.
General.
The Receiver down converts the 10.7 MHz first IF signal to 455kHz. Following the down
conversion a linear or a logarithimic IF amplifier provide the gain prior to AM and FM detectors or the spectrum
analyzer detector respectively. Post detection filtering provides the wide or narrow band responses for the
audio outputs. The audio amplifier for the speaker and the alarm generator are also contained on this module.
A block diagram of the Receiver is shown in figure 10-1 and its schematic in figure 10-2.
10-2.
Down Converter.
The 10.7 MHz IF signal is converted to 455kHz by mixing with a 10.245 MHz local
oscillator. The local oscillator is phase locked to the system 10 MHz frequency standard. A sample of the
10.245 MHz VCO signal is output to the Processor 1/0 module. There the VCO signal is mixed with 10 MHz, the
difference is divided by 49, and the result compared with a 5kHz reference obtained from the 10 MHz. Any
frequency difference causes a correction to be made to the VCO frequency via the 10.245 MHz VCO TV line
through the Loop Filter.
10-3. The IF filter following the mixer provides the selectivity for the system. Two bandwidths, ±100 kHz
wide band and± 13kHz narrowband, are processor selectable to correspond the front panel bandwidth control.
10-4.
linear IF Amplifier and Detectors.
The linear IF Amplifier amplifies the 455kHz signal to the AM and
FM detectors. The DC signal from the AM detector is fed to the AGC Amplifier and Squelch Detection circuitry.
There it is compared to the AGC reference with the resulting AGC signal controlling the gain of the IF
Amplifier. For signal present indication and squelch operation the SQUELCH LVL from the front panel is
compared to the AGC voltage. When the AGC voltage fall below the squelch level, indicating a strong signal,
the SIG PRESENT line is activated. With the SIG PRESENT active the audio is allowed through the select
switch and the signal present light on the front panel is illuminated. To warn the operator when the IF input
level is beyond the linear range of the IF amplifier, the AGC voltage is also compared to a fixed IF overload
level. When this level is exceeded, the IF OVLD line is activated causing the processor to flash the warning on
the CRT display.
10-5.
The AC component from the AM detector is buffered by the Audio Buffer and then passed to the Audio
Select switch. The lower 3 dB corner on the AM audio response is approximately 100Hz.
10-6.
Frequency modulation is recovered by a dual bandwidth phase locked loop discriminator. The
bandwidth, wide or narrow. is selected coincident with the IF Filter bandwidth. Audio from the discriminator is
applied to the Audio Select switch.
10-7.
A 455kHz Buffer amplifier provides an interface between the IF Amplifier output and the IF processing
circuits on the Scope/DVM Control module.
10-8.
Audio Switching and Filtering.
The output of the AM or FM detector or the SSB AUDIO signal from the
Scope/DVM Control module can be selected as the demodulated audio output. Selection is made by the
processor depending on the operating mode and the presence of the active state on the SIG PRESENT line. If
the SIG PRESENT line is not active, the Audio Select switch is opened squelching the audio signal.
10-9. The Audio Filter provides either wide or narrow band filtering on the recovered audio. For wideband a
0.5 dB bandwidth of 100kHz is provided.while narrowband has a 0.5 dB bandwidth of 3kHz. The output of the
filter is separately buffered to three signal lines. The DEMOD CAL AUD signal is used on the Scope/DVM
10-1
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