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*Subject*: Re: R: [amsat-bb] Downconverter noise*From*: Scott Townley <nx7u@xxxxxxxx>*Date*: Tue, 27 May 2003 22:57:11 -0700*In-Reply-To*: <007301c324d2$e6c35940$bcc7b650@b3o7f1>

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 > > > > > > >---- >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 http://members.cox.net/nx7u ---- 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

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

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

**R: Downconverter noise***From:*i8cvs

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