Hi, George,
I think that I just earned one typo in that last message, unless you count the
spelling of discernible! I should have said:
I have used the attenuator approach, but it DEGRADES the noise figure or minimum
discernible signal of the receiver.
In some cases the use of the attenuator is just fine. I am using 36 db of
outboard
attenuation right now on 75M to cut the static crashes back. Of course this
only
lets me listen to the strong signals, which is ok. If the objective is be able
to handle weak and strong signals in the same passband on 20M (say for a 12 or
15 kHz
roofing filter), then good dynamic range is critical. That is where removing
the
bypass capacitor helps. This could be switched in and out using a transistor
as a
switch, but since the parts are surface mount, it was not worth the effort for
me!
Now, to an example on how this works our with some illustrative numbers that I
hope
will make it across the Internet intact: Using the formula for Noise Figure of
cascaded amps, we get the following. I hope I am not preaching to the choir
with this!
Ftotal= F1+ (F2-1)/G1 + (F3-1)/G1*G2 +... (Fn-1)/G1*G2*...Gn
F is the Noise Figure of the individual stages, and Ftotal is the total for the
stages under examination.
Noise
Noise Gain Figure Gain
Fig(db) (db) (ratio) (ratio)
1 Front End L/C bandpass filter: 2 -2 1.58 0.63
2 FET Mixer 6 6 3.98 3.98
3 Xtal Filter 2 -2 1.58 0.63
4 JFET amp 5 18 3.16 63.10
5 Diode double balanced mixer 8 -8 6.31 0.16
Total gain is 12 db.
Ftotal (ratio)= 7.96 Noise figure
NFtotal (db)= 9.01 db Noise Figure in db
If the gain is changed on the JFET amp from 18 db to 8 db, with no change in
Noise
Figure for that stage, then the Total Noise Figure changes to the following:
Ftotal (ratio)= 8.44 Noise figure
NFtotal (db)= 9.26 db Noise Figure in db
Total gain would then be reduced to 2 db, which is less likely to clobber the
stages that follow, but the noise figure is still good.
Adding a 10 db pad in front between the antenna and the first example would
increase the Noise Figure from 9.01 db to 19.01 db. The total gain would
be 2 db, but with a much higher noise figure!
And yep, I too think this is fun stuff! Thanks for the discussion.
Regards,
Jack, KE4ID
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