Today, a rather ordinary envelope arrived, still it feels a bit like xmas, because of the contents….

…. a HP (Agilent Keysight) 346B Noise source, with nominal 15 dB ENR. I have long been looking for one, at a reasonable price, and finally scored this unit on xbay.

The calibration sticker shows good flatness, especially, in the 0.01-1 GHz region, which I need most, it is perfectly flat.

Prior to having it re-calibrated, a good opportunity to look inside. There are two sections: the current source, and the RF noise source assembly (which is hermetically sealed, and you better don’t touch!).

These are some close-ups of the 00346-60001 power supply and current regulator board.

The current regulator inside of the 346B has always been a big mystery to me, because no schematic has been published by HP, in any of the service manuals. How it works, check out the schematic. The incomming 28 V (which is provided by the noise figure meter) is converted to a square wave, about 7-8 kHz, using a LM311H comparator. This is then converted to a negative voltage, about -20 V, absolute value varies a bit with loading condition.
The negative voltage is then used to sink current from the noise source assembly. The current setting of my unit is about 18.7 mA, programmed by the “10 Ohm” resistor.
Why the negative voltage? To simplify the design of the noise diode.

Why did HP use a 5V6 Zener for the current reference, well, this is fairly obvious, when looking at the datasheet of such diodes.

Around 5.6 V, the temperature coefficient virtually vanishes (this is why such voltages are also used for voltage reference circuits). The other Zener diode, unfortunately, I was not able to identify. It has 24 V drop, fair enough, any regular Zener should to the job to keep the base bias of the output constant.

For a quick burn-in, the source has now been connected to a 8970A Noise Figure Meter, and output appears to be very steady, less then 0.1 dB drift over a few hours. More noise measurements and calibration tasks to come, let me know if you need any noise sources measured, might be able to help.