UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - MuTek Pre-amps for the FT290R

Radio Wrangler · 19th Oct 2018, 1:43 pm

The 8970 uses a diode detector and its response is a bit affected by the probability density function of the noise, and not all noise creation mechanisms are dependably Gaussian.

Resistor thermal noise is pretty Gaussian stuff, but noise from avalanche mechanisms gets a bit interesting at lower levels. So I stuck a proper true RMS detector in the noise figure analyser that replaced the 8970. People had driven the noisiness of a lot of devices down so low that the difference was beginning to show.

I had an interesting moment when evaluating some new noise diodes. Their output level was increasing as I decreased the bias current. Increasing quite dramatically. What would happen if I turned off the current? would the noise density go to infinity and cause a new big bang setting off the next universe? That was happening was the bandwidth of the noise was coming crashing down, so the total output power wasn't doing anything silly... but I was looking at the density and at a fairly low frequency.

Life has its little surprises!

David

19th Oct 2018, 1:43 pm	#18
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: MuTek Pre-amps for the FT290R The 8970 uses a diode detector and its response is a bit affected by the probability density function of the noise, and not all noise creation mechanisms are dependably Gaussian. Resistor thermal noise is pretty Gaussian stuff, but noise from avalanche mechanisms gets a bit interesting at lower levels. So I stuck a proper true RMS detector in the noise figure analyser that replaced the 8970. People had driven the noisiness of a lot of devices down so low that the difference was beginning to show. I had an interesting moment when evaluating some new noise diodes. Their output level was increasing as I decreased the bias current. Increasing quite dramatically. What would happen if I turned off the current? would the noise density go to infinity and cause a new big bang setting off the next universe? That was happening was the bandwidth of the noise was coming crashing down, so the total output power wasn't doing anything silly... but I was looking at the density and at a fairly low frequency. Life has its little surprises! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done