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Icom R71 Synchronous detector [message #1083] Tue, 15 November 2011 01:44
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Registered: December 2004
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Author: Dayle Edwards

The Icom R71 in all its flavours is one of the best performing receivers of all time, and its AM reception can be improved enormously with the inclusion of a synchronous detector. As it happens, this is very easy to do, as much of the necessary circuitry is already present, in the form of a balanced SSB detector and carrier insertion oscillators, used in the existing SSB modes.

Looking at the SD circuit, it comprises nothing more than a dual opamp. The Icom SSB product detector output, when close to zero beat with the wanted AM signal will contain the difference frequencies, either a audio heterodyne, or when locked, a DC voltage. The DC voltage is a function of any phase difference between the two inputs to the mixer, the carrier insertion oscillator, and the IF ouput. This voltage is heavily filtered to remove as much audio as possible , amplified by opamp A, and the resultant is fed to the varicap diode frequency shift of the 30.2 MHz master oscillator on the Icom PLL board, thereby correcting any phase error to the received signal. Any mistuning will result in a hetero-dyne from the mixer output, this is amplified by opamp B, rectified and switches the display "tune" segment via TR1/TR2. Diode D3 allows the Synch board to coexist with the FM option if fitted... this also uses the "TUNE" segment.. The "TUNE" display segment will activate within 20 cycles or so from the center frequency, giving a lock indihoneyion when correctly tuned.

The Synch detector will lock well on signals below the noise, and can exhibit a dramatic improvement on many marginal signals. With stronger signals, the audio quality can be very much improved, selective sideband fading or cancellation can be almost eliminated, and any noise is held to the level of the wanted signal. The Synch detector uses selectable sideband, simply by selecting either USB, or LSB, all installed filters able to be used with this design, as too the Pass Band Tuning and notch filter. It is important to note, the capture range of the design is limited to around 20 Hz, it is only required to lock the phase, not the frequency, so correct initial tuning is critical, the lock range being approximately 80 Hz. The limited bandwidth also allows normal SSB/CW modes to be used with no interaction from the synch detector which is permanently active. Normal AM remains selectable from the front panel.

The board will mount on the two pillars used when installing the FM option, or if this is already installed, it will piggy back above using longer screws and spacers. The unit is wired as follows.


The signal input is got from R157, rear right of main circuit board.
The minus 10 volts is got from R197, rear center between the crystal and FM option connector.
The 8 volt plus is found at jumper W46, center on the circuit board opposite the regulator chip.
The tuning voltage will connect to R105 extreme right lead on the PLL board, rear right hand side.
The indihoneyor voltage connects to the RED wire in the two pin connector for the FM option The circuit boards need not be removed for these connections to be made, it is sufficient to carefully solder the wires to the exposed resistor leads. All added wires are at DC only, so no additional birdies, spurs or noises are created.

Initial setup:

Connect a voltmeter to IC1 pin 1, and with the radio tuned off a station at around 25MHz, slowly adjust VR1 to obtain a reading of near 3.5 volts. The unit is now ready for use. Tune in a weak AM station in SSB mode, slowly tune for a zero beat. At some point, the carrier will lock, the audio will become clear and natural sounding, and the "TUNE" segment should light. Unless the ICOM high stability VXO option is fitted, its likely each station will need to be manually tuned slightly up or down from the set frequency indihoneyed. Remember, the R71 tunes in 10 Hz steps, these are not recorded on the frequency display.



[Updated on: Tue, 15 November 2011 01:45]

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