[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

2.4 GHZ Down Converter Noise VS IF Gain??

The combination of events has caused me to ponder the following any easy 
answer I hope for the RF noise figure Guru's

Many people including my self put attenuation between their receivers and 
their downconverters if for no other reason than to get the better apparent 
( to the ear) signal to noise and to keep the downconverter from driving 
their receivers into clipping or at least into the non linear portion of 
the AGC's range.

Recently as some of you may recall during in a bout of experimentation I 
had a "stupid moment" and transmitted into MY Cal Amp 31732 frying the two 
IF transistors. While trouble shooting and reverse engineering the Cal Amp 
circuit ( I'm still looking for a schematic diagram) and ultimately 
replacing the IF transistors it became obvious to me that the IF circuit 
lends itself to having the gain adjusted as there are resistors from both 
emitters to ground. Increasing the value of these resistors  from their 
current vales of about 2 ohms in the first IF and 5 ohms in the second IF 
to a higher value would certainly lower the gain  (a relative easy thing to 
do). Both of the IF transistors are pretty "beefy" little jobs rated at 
around 1W dissipation each and in their current bias configuration they get 
warm to the touch!

So my question is this. What happens to the actual noise figure of the 
Downconverter if I lower the gain of the IF amps by increasing the  values 
of the emitter resistors? I know what happens at the interface to my 2M 
receiver/ IF it is much happier with out all that hiss in its face same as 
when I put in the attneuator.


"A man who picks a cat up by the tail learns a lesson he can learn no other 
way" .... Mark Twain

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org