[time-nuts] Noise down-converter project

[ed breya]

The noise converter project based on the Scientific Atlanta 4647 is 
moving along nicely. Still no luck in finding any more info about this 
unit, but I did quite a bit of digging in the guts, and figured out 
enough to make progress.

The noise generator, to my surprise, is not a typical noise-diode-based 
type, but an all-amplifier deal, and apparently the fundamental noise 
source is a 75 ohm resistor in conjunction with the input noise of a 
2N5179 amplifier front end. The first few stages are broadband, followed 
by maybe eight bandpass stages, to craft the power level and shape, 
resulting in the 50-90 MHz noise signal, which gets passed to the noise 
amplifier box.

The noise amplifier is broadband again, then feeding a CATV type hybrid 
power amp for final output, which goes through a ferrite part, which is 
either a splitter or directional coupler, for leveling, then on to a 
decade step attenuator using Teledyne TO-5 style relays. The leveling 
signal from the local detector is sent back to the noise generator box 
where it somehow does the gain control. Altogether, a couple dozen or so 
transistors are used in the gain stages.

The step attenuator output is sent to the last box, the "C+N amplifier," 
where the external carrier input is attenuated with a step attenuator, 
then amplified up and leveled in similar fashion (including another CATV 
hybrid PA), then through its own step attenuator, and added to the noise 
through a reactive power combiner. So, the noise and carrier signals are 
each at least 3 dB bigger than the spec output levels, to accommodate 
the adding process.

I added a small board into the noise amp module, with an RF relay to 
pass the signal as normal, or route it to the new converter. The maximum 
PSD of the noise available there is about -70 dBm/Hz, versus the -73 
dBm/Hz at the normal C+N output.

The rest of the action is all built into the 70 MHz oscillator/agc amp 
module now. I sacrificed the agc amp function, and utilized the space 
for the mixer and LPF, and added yet another CATV type PA in the 
oscillator section, for the LO. More on this in the next installment.

Ed