"Dual-Watch" 20m SSB Transceiver
Iulian Rosu, YO3DAC / VA3IUL, http://www.qsl.net/va3iul
ICOM introduced for the first time "Dual-Watch" architecture in IC-781 model. This 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 the Block Diagram below, a power splitter following the RF preamplifier feeds two separate 1st mixers, Mixer RX1 and Mixer RX2. Each mixer receives local oscillator (LO) injection from a dedicated Variable Frequency Oscillator (VFO-1 and VFO-2) .
A power combiner following the 1st mixers sums their IF outputs. The combined IF signal is fed to the first IF amplifier on 9MHz and thence down the IF filter and IF chain. Setting VFO-1 and VFO-2 to separate frequencies allows simultaneous reception of two signals on different frequencies (A and B channels).
"Dual-Watch" function allows you to receive two signals in the same band simultaneously. For example, you can listen to a DX station transmitting on 14.225MHz while also listening to the pileup calling him on 14.230MHz.
The IF filter and preamplifier, IF chain, 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.
When the A and B signals are closely-spaced (offset < 1 kHz), the operator may hear a peculiar beat-note effect. The reason for this is that the common audio output presents both signals binaurally to the user.
The receiver LNA's use two 2SK1740 FET's in parallel to increase the intermodulation performances.
The first IF amplifier using a 2SC5551 transitor is followed by the 9MHz SSB crystal filter. The 6dB bandwidth of the filter is about 2.4 kHz.
The last two IF amplifiers use dual-gate MOS-FET's BF998 controlled by AGC.
The SSB demodulator and DSB modulator use the same circuit SA612 followed by the 2W audio amplifier LA4425A.
On the TX side at the modulator output can be find the Dual Side Band signal on 9MHz. This signal is amplified by a FET before routed through the SSB crystal filter, which duty is to attenuate one of the side bands. After the filter the SSB signal is amplified and up-converted using an SBL-1 mixer. The 14MHz signal is amplified by few power stages reaching at the end 25W RF output power.
The main VFO-1 works from 10MHz to 10.7MHz, which is double of necessarily frequency needed for mixing 14MHz RF signal with 9MHz IF.
A divider by two is used after the main oscillator to convert the signal to desired frequency. The reason of this frequency division is to avoid the beating between the two VFO's (1 and 2).
The two receivers front-ends can use the same antenna when RX switch is connected to ANT-1, or can use different antennas when RX switch is connected to ANT-2.
To get good spatial diversity is recommended to have at least λ/2 (10m) between the two antennas. The antennas could have also different polarizations, helping in this way to demodulate even weaker signals.
Dual VFO Schematic