Note on dual-diversity reception with the IC-7800

by Adam Farson, VA7OJ/AB4OJ

Dual-diversity reception is a configuration which allows the operator to usetwo receivers, connected to separate antennas but tuned to precisely thesame frequency, to select the signal with the best S/N for uninterruptedreception in the presence of selective fading etc. Its main use is for thereception of RTTY or other data modes, but it also works with SSB signals.

The two antennas are physically separated and/or pointed in differentdirections (space diversity), or vertically and horizontally polarisedrespectively (polarisation diversity). This antenna arrangement increases the statisticalprobability that a selective fade will only affect one, but not bothreceiver systems. The receiver audio outputs are fed to a circuit known as abaseband diversity combiner, which automatically switches the signal withthe best S/N ratio to the line output. 

Usually, the mixers of both receivers are fed from a common local oscillator (LO) toensure phase coherency between the two received signals. From a first lookat the IC-7800 user manual, it is apparent that the IC-7800 does not offerthis LO routing feature - at least not for the 1st LO. The 2nd LO sources feeding the A and B second mixers (I/Q down-converter)are derived from a 40 MHz source which is  common to both receivers. (Refer tosimplified block diagram of IC-7800 front end.)

All the DDS frequency sources in the IC-7800 are clocked from the precision(5 x 10-8) internal reference oscillator or anexternal reference. I believe that the DDS LO system in the IC-7800 is sostable that equalising main and sub frequencies by pressing the [M=S] keyshould be sufficient to allow good dual-diversity operation. The main and sub (Aand B)receiver audio outputs are fed to separate audio channels. Thus, the operatorcan listen to the main and sub receivers on stereo headphones. Anexternal diversity combiner can also be connected. 

In a typical fixed-station operation, the two antennas used can be twohorizontal dipoles 90 degrees apart, or a horizontal dipole and a verticalground-plane. Diversity reception is probably of greatest value on 7 MHz andbelow. 

Q: Do I understand that you would not be able to tune the band withdiversity active, but would need to press the [M=S] button to put both receivers on the samefrequency?

A: Yes, this would be a limitation - but not a serious one. The main purposeof dual-diversity reception is to be able to select the receive channel with thebest S/N, as a means of offsetting signal-quality degradation due to selectivefading. Thus, you would only need the diversity function once you had tuned inthe desired station. In practice, you would tune the main receiver to thedesired frequency, then press [M=S]. In the commercial HF installations with whichI was involved in the past, the diversity combiner was enabled only once thereceivers had been correctly tuned to the frequency of the distant station.

Copyright  2004, A. Farson VA7OJ/AB4OJ. All rights reserved.
Last updated: 09/25/2019

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