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CHAPTER 3
GENERAL THEORY OF OPERATION
3-1 INTRODUCTION: The TEN-TEC Model RX-340 receiver combines a high,
dynamic range front end with a versatile DSP back end to provide extraordinary
performance and flexibility. Refer to the overall block diagram Figure (10-1) and
interconnect diagram Figure (3-4).
RF signals applied to the receiver's Antenna Input (J5) are preselected by a switchable
1/2-octave bandpass filter network. Balanced amplifiers and a high-level 1st-mixer
preserve 2nd and 3rd order intercept points during conversion to the 1st-IF frequency of
45.455 MHz. A pair of 2-pole crystal filters provide a 1st-IF selectivity of 16 kHz to
reject 1st-mixer spurious products and the 2nd-mixer image at 910 kHz offset. Both
mixer outputs are available at rear panel connectors J9 and J10.
After conversion to the 2nd-IF frequency (455 kHz), signals are filtered for a 16-kHz
bandwidth and applied to the AGC-controlled 2nd-IF amplifier, which provides up to 80
dB gain. Amplified signals are filtered again for 16-kHz bandwidth and split off to the
3rd-mixer stage and Signal Monitor output (J3).
The 3rd mixer converts signals to a center frequency of 16-2/3 kHz where they pass
through a low-pass filter and are applied to an analog-to-digital converter. The A/D
converter produces a serial data stream at a 66-2/3 kHz sample rate for input to the
Digital Signal Processor.
Serial data from the DSP (at a 133-1/3 kHz sample rate) is applied to a digital-to-analog
converter. The D/A output samples are de-multiplexed into two or three output channels
depending upon mode selection. Half of the D/A output time is devoted to the DSP's IF
output, which is first converted back to 455 kHz by mixing with the third LO, then
filtered for a 16-kHz bandwidth and finally made available at the IF Output connector
(J4).
The other half of the D/A bandwidth is separated into USB and LSB audio channels in
ISB mode, or into a single audio channel in all other modes.
3-2 PRESELECTOR (81878): Refer to Figure (10-8). Eight 1/2-octave bandpass
filters spanning 500 kHz to 30 MHz are selected by diode switching (D1–D16). Switches
are biased into conduction by control voltages supplied by the DSP/CPU board (81807).
A high-level, push-pull parallel FET amplifier (Q7-Q12) compensates for filter and
switching losses prior to the first mixer.
3-3 PREAMP/ATTN: For normal reception, switching diodes D19-D20 provide a
straight-through signal path to the FET pre-mixer amplifier. For weak signal reception,
D21-D22 route signals through preamplifier Q13 for 10 dB added pre-mix gain. For very
strong signal reception, D17-D18 route signals through a 15-dB attenuator prior to the
pre-mixer amplifier.
3-1
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