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Re: R: Downconverter noise



Thanks Dom,
Yes, I was being a bit sloppy on my nomenclature between "factor" and 
"figure".  Since the equation was written algebraically (rather than 
logarithmically) I figured it should be apparent; but it is good practice 
to use ones terms precisely.
As to the last bit, yes, about 10 min. after I wrote that I realized I was 
backwards.  Thanks for the correction.\
CU on AO-40 if/when I get my antennas finished!




At 21:37 2003-05-27, i8cvs wrote:

>----- Original Message -----
>From: Scott Townley <nx7u@arrl.net>
>To: Jon Ogden <na9d@speakeasy.net>; Howard Long <howard@howardlong.com>
>Cc: Phil <phil@spiderweb.com.au>; <amsat-bb@AMSAT.Org>
>Sent: Tuesday, May 27, 2003 2:46 PM
>Subject: Re: [amsat-bb] Downconverter noise
>
> > Available noise power output from any active device can be stated as:
> > FGkTB
> > where F is the noise figure referred to the input (which is how it's
> > usually specified), and G is the available gain.
>
>Hi Scott,
>
>I aegree with you but in the above equation F is the noise factor and if the
>noise figure is NF=1 dB than F= 10E(1/10) = 1.258 in factor
>
>T in the equation is the equivalent noise temperature of a 50 ohm resistor
>at room temperature of 290 kelvin because the input of a device under test
>in the lab must be terminated in to a 50 ohm impedance of a generator so
>that the noise produced by any active device i.e. the downconverter  in
>this case with no antenna connected and open input is a wrong procedure.
>
>You got a correct available noise power of -102 dBm because you uses
>290 kelvin for T in the above formula and so your input is terminated in to
>a 50 ohm resistor at room temperature.
>
>
> > For the AIDC-series, assuming available gain=37dB and the noise figure is
> > ~1dB (but you can see from the equation that a with a gain of 37dB, that
> > even several tenths variation in the NF makes no difference) the available
> > noise power is -102dBm (2.5kHz RX bandwidth).
>
>I aegree with you and infact if you make your calculations using the above
>equation introducing:
>
>F= 1.258
>G= 10E(37/10) = 5011.87 time in power
>k= 1.38 x 10E -23
>T= 290 kelvin
>B= 2500 Hz
>
>Than Pn1 = FGkTB= -101.99 dBm or about -102 dBm
>
>This is the available output noise power or the output noise floor of
>the AIDC downconverter in the lab when its input is connected to a
>source of 50 ohm at room temperature of 290 kelvin.
>
> > What your S-meter does with this obviously varies, but -102dBm is
> > significantly higher than your RX noise floor!
>
>I aegree:
>
>For example the SSB sensitivity of a RX like a FT 736 in to a BW of 2.5 KHz
>is -15 dB uV for a (S+N)/N= 12 dB
>
>The above sensitivity can be converted very easily in a noise figure
>NF= 6.27 dB and consequently the computed input noise floor is -133.73 dBm
>when its input is connected to a source of 50 ohm at room temperature
>of 290 kelvin like an attenuator or the output of a downconverter.
>
>In this conditions, if  the above AIDC downconverter has the input
>terminated to a 50 ohm resistor and if its output is directly connected
>to the  FT 736 input than the S meter will show a noise level that is
>the difference between -101.99 - ( -133.73)= 31,74 dB
>
>The standardization of the S meter readings for frequency bands above
>144 MHz states that the S-9 reference level is -93 dBm available signal
>power at the receiver input.
>
>If a noise level power of -101.99 dBm is injected in a receiver with a well
>calibrated S-meter than the signal level reading must be
>-101.99 - (- 93)= -8.99 dB belove the S9 level .
>
>Since the S-point standardization recommended by IARU to manufacturers
>of equipments is 6 dB for the S-point, than the noise level for a -101.99
>dBm noise level input power will be between S7 and S8
>
> > With the dish fitted, the available noise power should increase slightly,
> > as antenna/sky noise have been added.  But for a "good" system it will not
> > be detectable on your S-meter.
>
>I disaegree here:
>
>When the downconverter input is connected to the feed  than the 50 ohm
>resistor at 290 kelvin in the lab is removed and the downconverter input
>sees only the equivalent noise temperature of the antenna/sky wich is
>generally lover than 290 kelvin so that the antenna/sky noise is added but
>the 50 ohm resistor equivalent noise temperature is subtracted and the power
>noise should decrease, but how much ?
>
>To better understand what happens i suggest to convert the above formula
>Pn1=FGkTB in the following one:
>
>Pn2=Gk (T1 + T2) B  where
>
>T1= equivalent noise temperature of the AIDC downconverter
>T2= equivalent noise temperature of downconverter input source
>
>Since NF= 1 dB and F= 1.258 than T1= (1.258-1)x 290 = 75 kelvin
>
>T2= 290 kelvin if the downconverter input is terminated in a
>         50 ohm resistor in the lab.
>
>T2= 20 kelvin about at 2400 MHz if the dish is looking at the cold
>         sky depending on feed spillover.
>
>Replacing the numbars the output noise level of the system is still -101.99
>dBm if the downconverter input is terminated in a 50 ohm resistor but it
>decreases to -107.84 dBm if the input is connected to the antenna looking
>at the cold sky and it make an improvement in sensitivity of
>-101.99 - ( -107.84 )= 5.85 dB
>
> > All that noise is why on some receivers, it's not a bad idea to pad down
> > the IF.  Remember that the bandlimiting devices in your receiver are
> > *after* the RF front end--so if you're the RF amp/1st mixer then B is much
> > much higher, and the noise power seen is proportionally much higher
> > too.  Too much noise power leads to saturation of the mixer and lots of
> > other bad things :-)
> >
>Aegree
>
>73" de i8CVS Domenico
>
>
>
>
>
>
>----
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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

----
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