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Notes on the Icom Dual Watch Feature

Figure 1: Simplified IC-756Pro II receiver block diagram, showing Dual Watch front end (on left).
Simplified receiver block diagram. Image courtesy Icom Inc.

The Icom "Dual Watch" architecture, pioneered on the IC-781, is a very practical compromise which offers limited dual-receive capability. It eliminates the complexity and costs associated with a second IF chain and demodulator. Referring to Figure 1, a power splitter following the preselector and RF preamplifier feeds two separate 1st mixers, A and B. Each mixer receives local oscillator (LO) injection from a dedicated synthesiser (1st LO A and B).  PIN-diode attenuators at the mixer outputs allow adjustment of the relative A and B signal levels via the BAL control. A power combiner following the 1st mixers sums their IF outputs. The combined IF signal is fed to the 64.455 MHz roofing filter, and thence down the IF chain. Setting 1st LO A and B to separate frequencies allows simultaneous reception of two signals on different frequencies (A and B channels).

The RF preselector filters and preamplifier, IF chain, DSP demodulator and audio chain are common to both the A and B channels. The operator hears both signals mixed at the common receiver audio output.

The upshot of all this for the operator is that the Icom topology limits dual reception to two signals in the same mode, IF bandwidth and band (the B channel can be in an adjacent band, e.g. A on 20m and B on 17m, without too much loss of sensitivity). In addition, the common demodulator and audio chain preclude "stereo" reception or separate audio in L & R headphones. To provide "stereo" capability, the radio would need two independent IF chains, each with its own DSP board. This would have a major impact on the price of the radio. Icom's product-management people probably felt that the demand would not justify the increased cost.

When combined with Split, Dual Watch enables the operator to maintain a "listening watch" on the distant station's listening (receive) frequency, whilst monitoring that station's transmit frequency. This is very useful when working a DX station which is operating split. I find the Dual Watch/Split option very useful when working half-duplex on 40m and 20m. 

The reason why a little more band noise is audible when using Dual Watch is that the idle-channel noise power in the B channel  is summed with that in the A channel, thus raising the IF noise floor by 3dB. (Admittedly, the effect would be a little less obvious if the A and B channels had separate audio outputs.) The operator can counteract this effect to some degree by turning the BAL control to favour the channel tuned to the weaker of the two signals.

When the A and B signals are closely-spaced (offset < 1 kHz),  the operator may hear a peculiar beat-note effect. This is particularly noticeable with two CW signals. The reason for this is that the common audio output presents both signals binaurally to the user. Assuming insignificant intermodulation in the IF/audio chain, the inherent nonlinearity of the human ear causes the user to perceive the A/B difference frequency. This is audible as a beat note on CW, or a "growling" effect on SSB. (Contributed by Matt, KK5DR).

The IC-781 was originally designed under a defence contract; the Dual Watch feature is quite well suited to guard-frequency monitoring in the same frequency range, or the monitoring of both frequencies in a half-duplex (split) situation. These scenarios often arise in some military/government HF operations. The utility of Dual Watch in the amateur radio service could well be incidental to the original design intent.

The term "Dual Watch" is probably an inheritance from the original contract under which the IC-781 was developed. "Watch" = "listening watch", also termed "guard". For example, a ship's radio operator maintains a "listening watch" on 2182 kHz, the distress and calling channel. 

One can assume that Icom carried Dual Watch over to its later "high-end" amateur HF transceivers, partly because of its acceptance by amateur IC-781 owners, and partly due to demand from non-amateur purchasers. Icom by no means limits the sales of its HF radios to the amateur marketplace. (Type-acceptance is not nearly  the "big deal" in other parts of the world that it is in the US.)

Icom appears to have done some fairly thorough market research during the progression from the IC-756 (which was based on IC-775 and IC-781 architecture), through the IC-756Pro to the IC-756Pro II. The areas of improvement in the Pro II relative to the original Pro tend to reflect this - particularly as Icom's product literature and ads indicated that they obtained customer feedback during the product-planning phase of the Pro II.

A poll of "how many buyers would make a purchase decision based on the availability of dual receivers, each with its own audio output" most probably yielded sufficiently low numbers to deter Icom from embarking upon the significant redesign effort, cost increment and packaging changes involved in fitting the radio with an additional IF chain and DSP board. The second receiver would require a dedicated DSP to provide IF filtering and demodulation independently of the main receiver.

Copyright  2003, A. Farson VA7OJ/AB4OJ. Images courtesy Icom Inc.
Last updated: 11/27/2015

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