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R: Re: SNR measurements



Hi  Ken,

Turning down the RF gain control with the audio wide open effectively
disable the AGC but now it is necessary to measure the new receiver
sensitivity.

The sensitivity of  FT-736 is -15 dB uV for (S+N)/N= 12 dB
corresponding to 0.177 uV effectively applied to RX input or
0.354 uV measured at the generator with its open output.

The above sensitivity is corresponding to that of a receiver
with NF= 6.27 dB

If  turning down the RF gain the above original sensitivity do not
changes than the overall noise figure of the system including
2400 MHz is not affected.

If turning down the RF gain the above original sensitivity decreases
than the FT-736 noise figure increases degrading the overall noise
figure of the entire system including 2400 MHz

This is why it is better to switch OFF the AGC control and not reduce
to much the RF gain of the second  IF receiver.

If the AGC control is swithed OFF in a HF 28-30 MHz receiver after
a new third conversion 144/28 MHz than any gain reduction at this point of
the 28 MHz amplification chain do not affect  anymore the overall noise
figure of the system because the gain is reduced in a term of the equation
in wich the value of  gain G3 is less important as can be seen making a
simple calculation.

                     F2 - 1          F3 -1             F4 - 1
Ft  = F1 + ------------ + ---------- + -----------------
                       G1            G1xG2       G1 x G2 xG3

Where:
Ft = overall system noise factor
F1 = noise factor of the first stage
F2 = noise factor of the second stage
F3 = noise factor of the third stage
G1 = power gain of the first stage
G2 = power gain of the second stage
G3 = power gain of the third stage

For more detailed informations on this matter i suggest to read
 the article written by James R. Fisk, W1DTY
"Receiver Noise Figure Sensitivity and Dinamic Range
       - WHAT THE NUMBERS MEAN"-
Ham Radio Magazine october 1975 pages 8 to 25

73" de i8CVS Domenico

----- Original Message -----
From: kgshutt <kgshutt@cox.net>
To: i8cvs <domenico.i8cvs@tin.it>; AMSAT-BB <amsat-bb@amsat.org>; Ronald
Long <rlong3@columbus.rr.com>
Sent: Thursday, February 20, 2003 4:38 PM
Subject: Re: [amsat-bb] Re: SNR measurements


> Hi all,
>
> I have made sun noise measurements using my FT-736.  I was under the
> impression that turning down the rf gain control with the audio wide open
> would effectively disable the AGC.  Am I wrong?  Any comments?
>
> 73 de Ken - K5GUU
> ----- Original Message -----
> From: "i8cvs" <domenico.i8cvs@tin.it>
> To: "AMSAT-BB" <amsat-bb@amsat.org>; "Ronald Long"
<rlong3@columbus.rr.com>
> Sent: Wednesday, February 19, 2003 4:14 PM
> Subject: R: [amsat-bb] Re: SNR measurements
>
>
> > Hi Ron W8GUS,
> >
> > Your suggestion to use the output transformer in the audio
> > circuit loaded as you describes is exactly what i do even using an old
> > DRAKE R-4C HF tube receiver with an additional  144-146 to
> > 28-30 MHz additional conversion to take the advantages of
> > the AGC switching ON and OFF facility.
> >
> > 73" de i8CVS Domenico
> >
> > ----- Original Message -----
> > From: Ronald Long <rlong3@columbus.rr.com>
> > To: <amsat-bb@AMSAT.Org>
> > Sent: Wednesday, February 19, 2003 9:17 PM
> > Subject: [amsat-bb] Re: SNR measurements
> >
> >
> > > NX7U wrote:
> > > Once again, Domenico is right on the money--the key is a constant
> > bandwidth
> > > measurement that includes the noise in the bandwidth.
> > > ------------------------------------------
> > > One other possible source of error is the fact that the output
impedance
> > of
> > > modern receivers is not constant or accurately known. An article I
read
> > > suggested connecting an output transformer in reverse to the speaker
> > > terminals, say 8 ohms to a known value, R. Connect a 1W resistor of
> value
> > R
> > > across the R winding and connect the audio voltmeter across R.
> > >
> > > ron w8gus.
> > >
> > > ----
> > > Sent via amsat-bb@amsat.org. Opinions expressed are those of the
author.
> > > Not an AMSAT member? Join now to support the amateur satellite
program!
> > > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> >
> >
> > ----
> > Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> > Not an AMSAT member? Join now to support the amateur satellite program!
> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>







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