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Old 21st Nov 2022, 12:06 am   #244
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by Radio Wrangler View Post
A diode is a one-port device. Mixers rely on the LO being much larger than the mixture of signals, so that the LO dominates the signals and drives the diodes as much like switches as possible.

The period while the diodes are transitioning between the 'as much off as the drive allows' and the 'as much on as the drive allows' is when they are most amenable to having their switching time modulated by incoming signals. Hence intermodulation.

So we want to get the diodes to go as quickly as possible between the saturated states. If you read data from packaged diode ring mixer manufacturers, they offer mixers with better intermod performance at higher LO drive levels. Essentially they employ measures to increase the LO level at which damage occurs and you have to provide the LO power.

All this assumes sinewave LO.

But the benefit of the higher power becomes more a case of the speed in crossing the active region and less in how hard they are turned on/off.

Having a squarewave drive has the faster dV/dt to get across the transition, but doesn't apply as much heat and stress as a sine with a comparable dV/dt.

Making the LO drive a current source rather than a voltage source also helps manage stress and can lead to a faster transition.

So that's how to soup up a diode ring mixer to the hilt. Guy Douglas wrot it up in the HP journal in April 1982

http://vtda.org/pubs/HP_Journal/HP_Journal_1982-04.pdf

Racal had done a mixer for their RA1772 HF receiver based on a 1968 paper by P R Rafuse which embedded DMOS FETs in a ring like the diodes used. In this they missed a massive trick. The MOSFETs are TWO port devices and give separation of LO and signal paths. There is no need to use the circuit topology optimised for one-port devices.

Several people eventually saw the next step. A colleague at HP came up with it as a suggestion and it got used. A little later someone else contributed it to the 'Tech Topics' column in RADCOM. He named it the 'H-mode Mixer' and it's worth looking at.

David
Thanks. The paradoxical nature of ring diode mixers is that they are supposed to improve "linearity" even they work through the non-linearity mixing action of fast switching diodes.

I have found both Rafuse's IEEE paper and RA1772 ring diode-DMOS schematic. I have only skipped through the articles quickly. Rafuse mentioned that the LO current should be as large as possible to force the series diodes into their linear region. It can be seen from the attached figure LO voltage vs finite rise time from 1982 HP paper that the bigger the slope dV/dt, the less phase modulation.

I cringe every time when I see the terms "current source" is being mentioned. An ideal current source has infinite slope in the V vs I graph (or infinite resistance). Many active transistor configurations can be regarded as a current source with high output impedance with relatively constant current. In this context, the hard switching with square waves (dV/dt = infinity) behaves a constant LO current source. Rafuse wrote that the non-linearity of ring diodes during the OFF state is responsible for ALL of the intermodulation if the IF and LO ports are perfectly matched. The use of 1:1 Baluns are for RF and LO isolation, reducing the intermodulation products.

In some way, this is almost opposite to the zero current switching of IGBTs at the zero crossings of Double Resonance Solid Tesla Coils( the OneTesla which I still own) but the goal is the same to reduce power losses:

https://onetesla.com/tutorials/intro..._store=default
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