View Single Post
Old 9th Mar 2021, 4:24 am   #17
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,802
Default Re: Noise in receivers

The diode in your AM detector is a non-linear element.

Your incoming signal is a carrier along with a pair of symmetrical sidebands. The idea is that the carrier is stronger than the sidebands and so dominates the action of the diode. Seen in the limit of a very strong signal, the carrier component turns the diode hard on and hard off. So the diode operates as a gate. This on/off gating modulates the passage of the sideband signals into the output of the detector. It's the same as mixing the DSB components in the IF with the carrier, and the result is the wanted audio. THe carrier also mixes with itself. It is, of course, perfectly in phase with itself, and so it also produces a DC output... the rectified carrier, seen another way. With the diode turning hard on and hard off, the conversion of IF sideband power into audio power is quite efficient.

So how about a very small signal in the IF reaching the detector diode?

There is less voltage in the sidebands, so you get less audio. But there is also less carrier, so the diode is not being turned on very hard. So its efficiency at converting sideband voltage into audio voltage is degraded and gets worse the smaller the signal.

So these two effects combine, as your signal gets smaller the signal gets smaller and the detector gets less efficient, so the amount of audio recovered falls disproportionately.

So now you keep the low level signal, but you add a signal generator operating dead on the carrier frequency and at a level bigger than the signal. This works like the signal's own carrier, but being bigger, it operates the detector harder, making it switch more strongly, making it more efficient! so the amount of audio you perceive increases.

The added carrier could be a little off-tune if the beat frequency is below the cut-off of the audio stages. This is called Exalted Carrier reception. It's more sensitive than a plain old diode AM detector. The added carrier could be phase locked to the carrier of the incoming signal if there is enough of it for a narrow band PLL to acquire it. This in the Synchronous detector found in some receivers - particularly ones intended for HF broadcast DXing.

Anyway, that's an explanation of why the audio improves when you add an artificial carrier. You can add it at RF or IF frequencies.

Phase noise effects only come into serious play when there are multiple signals or whn you tune close to DC.

If your LO has bad phase noise sidebands, the principle of reciprocal mixing makes things work as if your LO was clean, but every signal in the band had sidebands on it just like your LO. In bad cases these can overlap and add. So your receiver experiences a worsened noise floor. Thke the signals away and try to measure the floor, and it's OK. Drives people round in circles!

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now