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R: Downconverter noise




----- 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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