Re: R: [amsat-bb] SNR measurements

[Scott Townley <nx7u@xxxxxxxx>]

Once again, Domenico is right on the money--the key is a constant bandwidth 
measurement that includes the noise in the bandwidth.

I might add that after Step 7, you can calculate your system G/T with a 
simple formula.  That quantity is the *definition* of your downlink 
ability.  (Try it on Gene's link budget spreadsheet.  No matter what G or T 
are, for a given G/T, the received S/N of both the beacon and your own 
downlink are the same!)

Unfortunately Step 1 is not nearly as easy as it sounds; of all the radios 
I've ever owned in 27 years, only my (HF) TS-850S allows you to turn off 
the AGC.  My FT-726R doesn't do that.  Hopefully the other popular radios 
(FT-847 seems to be it) do.

At 02:02 2003-02-19, i8cvs wrote:
>Lee KU4OS,
>
>The metod that i use for (S+N)/N  measurements on sun noise or any other
>signal at the output of my receiver is quite easy and accurate:
>
>1)Switch OFF the AGC of your receiver
>
>2)Switch the receiver to SSB or CW wich works linearly using a product
>    detector
>
>3)Reduce to a minimum the RF gain control (all CCW)
>
>4)Increase to maximum the audio gain control (all CW)
>
>5)Connect an AC voltmeter 5 volt f.s to the audio output (best high
>    impedance)
>
>6)Put your antenna to the cold sky  and increase the RF gain until you get 1
>    volt AC indication and call this voltage V2 (this is your noise reference
>    baseline)
>
>7)Without changing any adjustements put your antenna to the sun and read the
>    new indication and call this voltage V1
>
>Your  (S+N)/N =  20 log  (V1/V2)
>                                         10
>
>For example, if  V1 reading is 1.42 volts than your (S+N)/N = 3 dB
>
>Remember that your S meter or voltmeter are measuring (S+N)/N and not S/N
>and for this concept i recommend to read "On Decibel and Noise" by W3GKP
>published  in QST january 1968  pages 34 to 36
>
>If you want to convert (S+N)/N in to S/N use the formula S/N= (S+N/N)-1
>where (S+N)/N is in factor and not dB
>
>For the above example (S+N)/N = 3 dB = 2 in factor
>and S/N= 2-1 =1 or 0 dB
>
>So when you are measuring 3 dB of (S+N)/N your S/N is 0 dB or signal=noise
>
>Increasing the value of  (S+N)/N the difference between (S+N)/N and S/N
>became smaller
>
>For example if (S+N)/N= 10 dB than S/N= 9.54 dB
>
>For measurements of  (S+N)/N on AO40 MB or any constant carrier in to the
>passband,tune the audio output for maximum V1 using the above procedure
>with Rx in SSB or CW and make the same calculations as above.
>
>The above measurement is accurate because signal and noise are measured in
>to the same passband of the receiver IF
>
>In order to simplify the above measurement and calculations i have built a
>linear dB meter using a RMS to DC chip Converter AD 636 from Analog Device
>and i have linearly calibrated for it a 100 uA meter from 0 dB to 20 dB
>
>If you or any other is interested in this matter i can send as an
>attachement both the QST article and the dB meter diagram wich is actually
>widely used by the EME operators.
>
>W0LMD Robert is using as well the above instrument with a chip AD 636
>
>73" de i8CVS Domenico
>
>----- Original Message -----
>From: Lee <lee-fl@cfl.rr.com>
>To: <amsat-bb@AMSAT.Org>
>Sent: Monday, February 17, 2003 5:05 PM
>Subject: [amsat-bb] SNR measurements
>
>
> > I've been trying to make signal to noise measurements on AO-40 over the
> > weekend and seem to be getting numbers better than I would expect.  As a
> > result I'm inclined to believe that my technique is in error since sun &
> > ground noise measurements result in fairly reasonable values for system NF
> > and gain.  What would be the best approach or software to use for making
>SNR
> > measurements?  I would particularly like to find a technique for measuring
> > the SNR of the MB's BPSK signal since that eliminates a lot of uplink
> > related variables.
> >
> > Using Spectran, I tried measuring from the null in the center of the
>middle
> > beacon to the peak during the idle period between blocks and get a value
>of
> > 30 dB when the transponder is on and 33 dB during the mode-K window from
>MA
> > 124-126.  While I suspect the absolute numbers, the change is consistent
> > with the drop in noise floor that I see when the receivers are
>disconnected
> > from the S2 tx.
> >
> > Following the mode-K break I looked at CW carriers at two power levels.
>14w
> > EIRP resulted in Spectran showing a carrier peak 11 dB above the noise
> > floor.  Increasing the power to 140w EIRP resulted in an increase of the
> > peak to 16 dB. During the measurements the U-Rx AGC varied from 6 to 13 dB
> > with an average value of 10 dB.
> >
> > Comments and suggestions would be greatly appreciated.
> >
> > 73,
> > Lee-KU4OS
> >
> > ----
> > 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!
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>
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>----
>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

Scott Townley NX7U
Gilbert, AZ  DM43di

----
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