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IF output signal switch reduces noise in shortwave receivers

27 Jan 2014  | John Dunn

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This item is a description of a noise suppression strategy, not about a specific design. I've done this thing twice, once on an old Hallicrafters S-20R short-wave receiver as my senior project at Brooklyn Polytech and again sometime later, on an R-390A receiver.

In both cases, the receiver was rendered inoperative by pulse noise. The S-20R was being impinged upon for demonstration purposes by an HP pulse generator while the R-390A was catching it from a very noisy switch mode power supply that was under development in the next room. Similar interference could be expected from a 1957 Ford station wagon with straight wire connections from its distributor to its eight spark plugs. (A personal experience there!)

IF output signal switch reduces noise in shortwave receivers

Figure 1: This sketch shows what in both receivers was going on and what was done about it.

With a quiet signal, reception worked just fine, but with pulse noise interference, reception could be and was completely jammed.

One problem was that with noise impulses present as shown in the middle trace above, the receiver's automatic gain control (AGC) would very rapidly lower the signal path's gain (fast attack) on each pulse and then very slowly restore that gain back to normal again (slow decay) which put a time gap in the signal path that was of far longer duration than that of the interfering pulse had been. That extended signal gap being repeated at the pulse repetition rate undid the listener's ability to hear the intended signal.

The effect was mitigated by putting a signal switch in the mixer's IF output as shown above and keying that switch off for each noise pulse so that no noise pulse could make its way into the IF strip and into the AGC circuitry. The signal gap from the switching was too quickly over for the AGC's slow decay characteristic to respond while the short durations of missing signal did not obstruct someone's ability the hear the intended messages, meaning, to hear the local news, the Brylcreem commercials and so forth.

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