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Old 13th May 2022, 10:49 pm   #30
nemo_07
Triode
 
Join Date: May 2022
Location: Cologne, Germany.
Posts: 28
Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
The 2nd and 3rd pictures are my test rig for "hot" frequency sweep of double tuning+ single tuning with a dual gated mosfet BF981. The overall gain is 19db and bandpass Q = 159. The "cold" Q (no amplifier) of the double tune stage is about 85 and single tune stage Q is 30.
...
In the Pioneer TX8500 service manual, it stated that "the noise figure is determined by the insertion loss of the antenna input tune circuit",

insertion loss = ( 1 - QL/Qu)**2

In my measurement Qu = 100, QL = 30, IL = 0.49db
I'm sorry to say that, but your interpretation of result is incorrect.
The cited formula gives an absolute value of insertion gain (here 0.49).
Insertion loss expressed in dB should therefore look as follows:

IL[dB] = -10 log {( 1 - QL/Qu)^2]} or
IL[dB] = -20 log ( 1 - QL/Qu).

The minus sign is to denote insertion loss as opposite of insertion gain.
For your data we have
IL = -20 log (1 - 0.3) = 3dB.
This is not a drama in terms of S/N ratio, as the noise present at the antenna terminal will experience the same treating.

Adam
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