5.0
THEORY OF OPERATION
5.1
OVERVIEW
The Ten-Tec model RX-400A is a DSP-based receiver with a wide-band IF output
available at 21.4MHz. The superhetrodyne architecture is triple-conversion to the IF
output for received frequencies of 88 MHz to 3GHz, and double conversion when tuned
below 88 MHz. The IF output bandwidth is 6 MHz for tuned frequencies of 20 MHz to 3
GHz, and 2 MHz when tuned below 20 MHz receive frequency. Tuning step size is 10
kHz (1 kHz optional) at the 21.4 MHz IF output. Conversion gain from antenna to IF
output is 3 dB nominal. The block diagram in Figure 5-1 outlines the signal path and
major functions throughout the RX-400A architecture.
The 21.4 MHz 3rd IF is bandpass filtered to 300 kHz bandwidth in the 3RD CONVERTER
module, AGC leveled, and presented to the DSP board. On the DSP board, a high-speed
A/D converter drives an AD6620 Digital Down Converter which applies fine tuning,
further bandpass filtering, and decimation to the digitized IF signal to prepare the data
for a Digital Signal Processor. Final selectivity filtering and mode-specific detection
functions are accomplished by the DSP.
Audio/Video outputs are available via connectors on either rear or front panel. The
outputs are designed for a nominal 600-ohm load.
5-1
FIGURE 5-1 RX-400A BLOCK DIAGRAM
Low Band
nverter
HF Converter
WBIF 1
WBIF 2
RF Input
Preselector
Mid 1
Mid 2
High
Ref.
First Converter
1
st
LO
2
nd
LO
3
rd
LO
BPF
BPF
Second Converter
Third Converter
BPF
BPF
A/D
CONV
DDC
DSP
D/A CONV
ANALOG
Reconstr.
USB Interface
ETHERNET
Interface
COMMAND &
STATUS DATA
COMMAND &
STATUS DATA
AUDIO
Ext. Ref.
21.4 MHz IF
2528-4090 MHz
2300 MHz
950 MHz
117-119
MHz
4090 MHz
2528 MHz
BPF 6 MHz
BW
BPF 2 MHz
BW
140 MHz
PN 74446
