Some thoughts on the IC-7700 single-RX architecture

by Adam Farson, VA7OJ/AB4OJ

The question has frequently arisen as to why the IC-7700 has only one receiver, and does not support Dual Watch. Here is a possible technical rationale:

Apart from the cost considerations involved, the IC-7700 receiver has a fairly narrow front end similar to that of the IC-7800. The receiver has a bank of half-octave bandpass filters with steep skirts, followed by a tuneable preselector (Digi-Sel) with one high-Q tuned circuit. The preselector will attenuate signals well removed from the frequency on which it is peaked; if Dual Watch were fitted as implemented in the IC-756Pro series, the preselector would degrade the sensitivity of the "SUB" channel at wide offsets. Such degradation does not occur in the IC-756Pro3, as it has no preselector. (Of course, if Dual Watch were provided, one could switch out the preselector when Dual Watch was active.)

Note that the roofing filters play no role in Dual Watch, as it is at the 1st IF output of the combiner following the MAIN and SUB 1st mixers. The 1st IF signal reaching the roofing filter is a composite of the MAIN and SUB 1st mixers' IF outputs. Any filtering which might affect Dual Watch performance has already occurred in the RF preselector and BPF's.

Another consideration with Dual Watch as implemented in the IC-756Pro series is that an RF amplifier is placed at the input of each 1st mixer, and an IF preamplifier at each input port of the 1st IF combiner, to make up the 3 dB loss of the RF splitter and 1st IF combiner respectively. These amplifiers can generate additional IMD under strong-signal conditions. Such make-up amplifiers are not required in the IC-7700 front end.

A secondary receiver can easily be tied into the IC-7700 by connecting a 3-port hybrid splitter as follows: Input to IC-7700 RX OUT, Output 1 to IC-7700 RX IN, Output 2 to the auxiliary receiver's RF input. The splitter will typically have 3 dB insertion loss per port and > 30 dB trans-hybrid loss.

If the secondary receiver is fitted with a mute line, this can be controlled from the SEND line (or ACC1/2 Pin 3) on the IC-7700.

Copyright  2007-2012 A. Farson VA7OJ/AB4OJ. All rights reserved. 
Last updated: 09/25/2019

Expression Web 4