On a sunny day (Fri, 1 Feb 2013 03:05:43 -0800 (PST)) it happened RealInfo
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Hi all
What is the "right" total amplification factor of an IF amplifiers chain in a radio receiver ?
For example a receiver employs 4 IF amps cascade , what is the proper total
voltage amplification of that IF cascade ?
Thanks in advance
EC
Just multiply the voltage gain of each stage, or add the db gain of each stage.
I reality IF gain will likely be under influence of some AGC[1], and vary
between 'next to nothing[2]' and 'a lot[3]'.
[1] Automatic Gain Control.
[2] Can be less than 1.
[3] Depends..
There are also limiting IF amplifiers, for example for FM,
so then output no longer increaes when the input increases,
you can than speak of a minimum signal level required for limiting.
duh
So, if the smallest signal input is 1 microvolt, and you want 1 volt
out, the gain is 120db? And that includes audio gain of maybe 20db?
But, but, but dB only refers to power, right? ;-)
Decibels are fertile ground for confusion, much like percentages are
for bankers. The basic definition is 10*log10(P1/P2). _If_ both are
electrical signals working into the same impedance, this is the
same as 20*log10(V1/V2). The trouble is that not everyone is equally
careful with this.
For example, control system engineers employ 20*log10(V1/V2) and
totally ignore impedance. They even use dB to express ratios between
different units, e.g., as the ratio between say, voltage and pressure
of a transducer. (I once got into an argument over that with Dr.
Middlebrook.) RF engineers will usually, but not systematically,
take the impedance into account. For example again, my Rohde &
Schwartz 50<->75 matching pads are marked as having the same
attenuation in both directions, while my Radiall matching pads
have two different values, despite doing exactly the same thing in
the same way.
Then there is the jungle of reference levels, with an abundant number
of definitions, sometimes conflicting, often illogical. For example,
dBm is dB referred to 1 mW, but dBu is referred to 1uV. I wouldn't
be at all surprised if with some digging, we could come up with over
a hundred different definitions of reference levels that are in
common use in one corner or another.
Jeroen Belleman