Six modules located on the bottom chassis deck accomplish the analog frequency
conversion functions. These interconnected modules are the FIRST, SECOND, and
THIRD CONVERTERS and the FIRST, SECOND, and THIRD LOCAL OSCILLATORS.
Signal interconnection between modules is by miniature semi-rigid coax.
Two interconnected PRESELECTOR BOARDS located on the top foldout chassis deck
perform frequency band pre-selection and signal pre-amplification between the antenna
input and the FIRST CONVERTER module.
The DSP BOARD located on the inner chassis deck performs all digital signal
conditioning, detection algorithms, and logic control functions. The DSP BOARD also
contains the audio/video output amplifiers and the TCXO master frequency reference.
Distribution of control signals and power to the various radio modules and boards is via
connectorized ribbon cable between the DSP BOARD and the target board/module.
All DC power is supplied by a shielded universal AC input switching supply located on
the inner chassis deck. The DSP BOARD receives the outputs from this main power
supply for post-regulation and distribution to the other radio modules.
A front-mounted fan supplies forced-air ventilation. Cooling air enters at the front-
mounted filter and exits through vent holes around the rear perimeter of the chassis.
The front panel board provides both high-speed and low-speed fan connections. To
change fan speed from fast to slow, see paragraph 9.5 of this manual.
5.2
CIRCUIT DESCRIPTIONS
The rear panel antenna input is protected by a spark gap and filtered by a 3 GHz low-
pass filter on the MID/HIGH BAND PRESELECTOR board, then routed to the common
terminal of a 4-position switch. Below a 88 MHz receive frequency the RF signal from
the antenna is connected to the LOW BAND PRESELECTOR board, either directly or
through a 20 dB resistive pad.
5.2.1 Low Band Preselector
Dual PIN diodes on the LOW BAND PRESELECTOR select one of eight sub-octave
bandpass filters. Each of the eight filters selects a frequency band approximately 2/3
octave wide to cumulatively cover the Low Band range of 2 to 88 MHz (slightly over 5
1/3 octaves). After bandpass filtering, the selected fractional-octave band is amplified
by and passed through a 140 MHz band-reject filter. This band-reject filter provides
additional attenuation of the Low Band 1st IF frequency. Buffers pass serial data to a
PIN diode driver Commands enables power to the RF preamplifier.
PN 74446
5-2
