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Circuit Notebook

Synchronous AM detector for improved shortwave reception

This simple add-on synchronous AM detector will significantly ease the reception of difficult shortwave AM signals by reducing background noise, holding the signal and noise at a more constant level, and reducing selective fading and distortion. The wanted signal will always be above the atmospheric noise level and artefacts such as heterodyne whistles and added noise are not introduced.

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Transistor Q1 functions as a buffer, to eliminate any loading on the host receiver’s circuitry. Q2 is an amplifier/limiter which, because it is driven hard into clipping, strips any AM modulation from the received carrier. The phase of this signal is shifted by the required 90° by inductor IFT1 and the signal then used in a balanced diode ring mixer (D1-D4) to demodulate the original carrier which is injected via the two 220W resistors. The diodes are switched at the intermediate frequency (IF), thereby synchronously demodulating the wanted signal.

The circuit is designed to replace the existing diode detector in the host receiver. This is done by removing the diode and its filter circuit and then connecting the input and output of the synchronous detector module in its place (see diagram).

You will need to check the circuit of the host receiver to make sure the AGC is supplied by a separate detector diode. If this is not the case, then the input of the module can be connected in parallel with the existing detector circuit, with the original diode being left in-situ. The output of the module is then taken to the appropriate input of the audio section; usually the top of the volume control, the mode switch or even to a separate amplifier.

The input connection to the detector will need to be very short and shielded cable will be required to prevent oscillation and feedback problems. IFT1 is chosen to suit the IF, either 455kHz or as with most Icom equipment, 9MHz. In the latter case, a 10.7MHz IF coil can be used, with a 50pF capacitor across the tuned winding to allow it to resonate at the lower frequency. No other changes to the circuit are required.

To set up, tune the receiver to a weak, noisy signal and adjust IFT1 for maximum signal and least noise. The rough "frying egg" sound of background noise will be reduced to a soft hiss, making the received signal easier to listen to. Lightning crashes and other "pops" will be reduced to the same level.

The IF coils used can be obtained from a defunct radio-tape player and the circuit can be made small enough to be easily installed in almost any receiver.

Dayle Edwards,
Taylorville, NZ. ($80)

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