NOISE SOURCE CIRCUITS FOR MICROWAVE

Hi George,

Cable capacitance is not an issue. This is 50 Ohms throughout: Matched source, cable and load.

Something that may be an issue, is cable *loss*. It takes a bit of getting used to, but that actually *increases* the amount of noise that comes out of the cold source. It makes the amplifier under test look noisier than it really is. I use the shortest possible length of semi-rigid coax to minimize that problem.

Jeroen Belleman

Cable loss = warm resistors added to your cold one. Of course it increases noise.

Presumably you could correct for that, but you'd need to do a ton of math, and know the temperature distribution of the cable. It's easier to do what you suggest and just use a short cable.

The HP346B uses an audio range clock into a current limiter and to drive an Avalanche Diode to yield an excess noise ratio of 15 dB ENR. It is AC coupled before the output impedance matching to 50 Ohms output and is powered by a switched 24 Vdc supply.