Re: Measuring Noise Figure

Christopher Bartram <cbartram@...>

Like Andy, a long while ago (late '70s) I also used the hot/cold load method to estimate NF. In my case at 144, 432, and 1296MHz. As they for the purpose of my measurements they were relatively easy to define physically, I used the temperatures of the triple-point (ie. effectively melting ice) and boiling water. All good standard school physics lab. stuff.

The load - which remained connected - was an 1/8W metal film resistor de-leaded and suitably waterproofed. No change of return-loss was detectable over the 0 - 100C range.

My noise receiver at that time was a linear 2ishMHz-wide IF strip with a square-law detector at 29MHz preceded by a 2m converter. It was easy to check the linearity of the detector, while the assumption that the noise spectrum at the amplifier output didn't change between the two source temperatures seemed reasonable - and still forms part of the assumptions made by modern, high-end NF-ometers.

That system was sufficiently accurate to confirm the measurements of the 144MHz amplifiers which I'd designed made with a commercial saturated thermionic diode noise source based NF-ometer. This unit effectively used the same technique as Chis Trask outlined. The performance of the commercial unit wasn't good enough to make realistic measurements at higher frequencies, but I was able to get a friend, working in the research labs of a semiconductor company, to measure my other amplifiers, and there was also good agreement.

A few months later, I brought (brand-new!) an HP346 source. That still forms the basis of my NF measurement kit, and is still producing readings which correlate well with other sources/instruments to beyond 10GHz - including modern Keysight demonstrators. Nowadays I use the source with an HP8790B. The home-brew IF strip still exists, and is just about to to be used as part of a new microwave EME system in order to enable accurate cold sky/Sun, cold sky/Moon and cold sky/ground measurements.

The hot/cold source technique is still probably the only way of making realistic measurements, and I'm very dubious indeed about noise figure '_measurement _' systems which don't use some form of calibrated source.

If you want to learn more about NF measurement, I very strongly suggest a careful read of the HP application note written by Ian White, GM3SEK and David Stockton, GM4ZNX.

Vy 73


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