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*Subject*: R: [amsat-bb] Downconverter noise*From*: "i8cvs" <domenico.i8cvs@xxxxxx>*Date*: Wed, 28 May 2003 06:37:41 +0200

----- 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 ---- 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

**Follow-Ups**:**Re: R: Downconverter noise***From:*Scott Townley

**Re: R: Downconverter noise***From:*Edward R. Cole

**References**:**RE: Downconverter noise***From:*Howard Long

**Re: Downconverter noise***From:*Scott Townley

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