Ok, so i could not get the variable gain to work as desired the last time I updated this. Now it worked, I was getting Oh i dont know -3db of gain attenuation, but that was not anywhere close to where I wanted to be, IE to have 0 to -20db attenuation. So I have given up the ghost on that idea and put it back in the sometime when i know more basket.
Ok, so I still need to be able to vary the gain of the RF Preamp, well, rather than vary the gain, lets attenuate the crap out of it. So after a lot of googling about trying to find solutions, i came across a circuit that is a variable attenuator. It should attenuate things by a minimum of -3 to -20db, so that’s in the ball park, its just a handful of resistors, caps and diodes, and by varying a supply voltage with a pot, you can change how much signal passes through the circuit.
Its simple enough, that even I can do it. About the only thing it requires are PIN Diodes, which if you are a popcorn kind of guy like me you wont have, but i have plenty of signal diodes, so off to LT Spice to build and evaluate a the circuit to see if it will do the job I require and then, if it works ok, build a prototype on the breadboard to make sure its going to work for reals.
UPDATE: Well I am thinking that LT Spice might not be the best tool for this job, by their nature diodes are non linear and a pin diode is linear and it is a non trivial matter to do linear artistry in LT Spice. The outputs i am seeing are quite wrong the higher the attenuation becomes as can be seen by the following image, however, when i ramp up the voltage, the output trace does track in input trace perfectly, so i think i might need to just breadboard this one and see for reals what is going on.
Will update again after I have breadboarded the circuit.