Noise in receivers

[Radio Wrangler]

Over the years, I've built myself a monster spreadsheet. All the stages in a receiver strip get a line each. There's no limit on number of lines, I can model individual connectors and cables where necessary. Each stage has gain, noise figure, bandwidth and 1dB compression point.

A number of rules of thumb for performance estimation have been included and I've verified these over many years. I can put in signals in the channel, offset, or I can even put in noise power densities.

I get graphical representations showing where the bottlenecks are for different performance parameters. I can see what each stage's contribution is to gain, noise figure and compression.

I used this to design the receiver structure of the Agilent Noise Figure Analyser.

Shall we say it works.

The NFA has switched gain stages throughout its length rather than variable AGC gains. It operates these switched gains (attenuators) in patterns to optimise the ratio of the noise it's measuring to the noise it itself makes (what a concept!) and also keeps an eye on overall compression because that really upsets Y-factor type Noise Fig measurements. A lot of care had to go into making sure that gain steps added arithmetically, without combination-dependent errors creeping in (One of the big problems in the HP 8970 series noise figure meters)

You might think variable AGC for a shortwave receiver that sold for a hundred quid would be easier?

Nope.

David

[Wendymott]

Well. After a number of wasted hours... I have finally found the problem. Maybe "wasted" is the wrong term...Learning may be better. The upshot of the problem was that say Receiver 2, was 12 db down in sensitivity, compared with Sample 4. I removed the AGC control, by grounding the AGC amp with a 1K. Thus I knew that the gain differences should show, where the problem lay. The signal levels... 20uv @ 10 mhz... were constant to the vol control slider. OK so far. So I injected audio..... same levels to each radio..
RX 2 was 12 db down on the audio level meter at the speaker terminal. Nothing in the schematic showed a reason. The audio o/p was monitored .. both identical. Speaker impedance ?? NO. Then I measured the resistance of the speaker pin to ground. RX2 was showing a 6.2 ohm difference. Chasing the wiring, there was an extra wire on RX2 to the rear of the chassis, to a DPDT slide switch, which is only accessible internally. Attached are two resistors to ground. This was shunting the audio.....its connected to the extension speaker jack, I guess to limit the drive to an extension speaker. Switching the switch to the other position cured the problem. It is obvious that Yeasu removed this stupid idea on later versions, and the schematic was for the later version. Maybe I can get on with my life.

Sorry David.... Should have mentioned your comments... I also have a spread sheet of all 4 FRG 7's RF sensitivity, Front end tuning etc. It is interesting to compare receivers like for like. Good ol Excel