开云体育

When regulating Vcc with a zener we automatically insert a simple cap-input Pi network in the Vcc line. I'd check with a scope to determine the before and after noise reduction and to establish the values.

The automatic go-to standard we use for sensitive front-end preamps is 1 to 3.3 uF and 30-100uH. The choke should be Hi-Q (80-100+), but in low current circuits the resistive loss will be minimal and 3.3V might end up being something like 3.25 at a low mA draw. But always build a test circuit on a punch board first to get a feeling of what's going on.

I've done this on many WJ8711 and 8718 receivers along with a 500 Ohm trim pot on the output to establish the fixed gain ... usually 6 to 18 dB net. The WJs don't mind any impedance changes since they are never 50 Ohms (exactly) on the input anyhow. Actually, the receivers without the factory preselector act more like a constant voltage source and don't sag the source (antenna) voltage assuming your input signal is somewhere around 50 Ohms. I personally have used antenna input impedances of 25 to 150 Ohms (resistive) and cannot detect any problems.

Additionally, having some kind of circuitry ahead of the WJ's front end (non-preselector types) is added protection from static discharges at the antenna. The WJs do not necessarily have much protection in that regard anyhow. We've done this with Racal, Harris, and ITT receivers as well.

Good signals to all in 2025,

Bob, N1KPR