噪声系数测量

Technical data is subject to change. Copyright@2004 AgilentFundamentalnoise conceptsHow do wemakemeasurements?What DUTscan wemeasure?What influencesthe measurementuncertainty?What is Noise Figure ?NoiseOutNoise inMeasurement bandwidth=25MHza) C/N at amplifier input b) C/N at amplifier outputNinNoutGa RsTwo examples of Noise FigureExample 1: In a receiver, the LNA is connected to an antenna which points to earth’s atmosphere (290K) and the LNA has 3dB NF and 10dB gain. Noise power at LNA output is: -174+10+3=-161dBm/Hz Example 2: In a transmitter the modulator noise floor is -140dBm/Hz. The modulator output is amplifier by a linear amp with 3dB NF and 10dB gain. Noise power at amplifier output is: -140+10+3=-127dBm/Hz-140dBm corresponds to a noise source with a temperature 700 million K, i.e. DUT input is not Standard Temperature and Example 2 is wrongJust to emphasize this point, noise figure only represents the noise added to the input noise referred to the DUT output when the noise into the device is thermal noise at the standard temperature. So the first example here is correct. In the second example, the noise going into the device is much higher and therefore the noise figure of the amplifier cannot be added to the noise out of the DUT from the modulator. In reality if the noise of the amplifier is only 3dB then it will add practically no noise to that generated by the modulator.11An Alternative Way to Describe Noise Figure: Effective Input Noise TemperatureNinNout = Na + kTB GaRsOutput PowerGa , NaSlope=kBGac isti ter c arais NoC ree eFhNa -Te Te Source Temperature (K)Let’s now plot the output noise power as a function of the temperature of the noise source. In the equation for Nout I have substituted Nin for kTB where T now varies from absolute zero upwards. It’s a linear curve as we are dealing with very low power levels so all devices are operating in their linear regions. Actually the line is a very standard ‘y=mx+C’. M is the gradient in this case kBGa and c is the point at which the curve intersects the y axis. C is equal to Na. What you can say at T=0 is that no power at the device output comes from the noise source. All the output power at this point is generated within the DUT. This gives us another figure of merit for describing the noise performance of active devices. If you look at the graph I have drawn the characteristic of a noise free device. If you transpose the added noise Na through this line on to the x axis you arrive at Te, the effective input noise temperature. When you multiply Te by the gain bandwidth product of the device you get the amount of noise added. It’s a useful figure of merit because it is independent of the device gain (unlike Na).12Effective Noise Temperature relation to NFNa + kToBG F= kToBG = Therefore Te = (F-1) . To Na Assume Na = 0 Ts Te kGBTe + kGBTo kBGTo = Te + To ToTsGain GGain GWhat is Te if the NF is 3dB?13Te or NF: which should I use?Use either - they are completely interchangeable typically NF for terrestrial and Te for space NF referenced to 290K - not appropriate in space If Te used in terrestrial systems and the temperatures can be large (10dB=2610K) Te is easier to characterize graphically14Friis Cascade FormulaGa1Ga2F1 F2-1 Ga1F2Σ FN+1 = Σ Fn + Fn+1 - 1 ΣGNF12 = F1 +Where Σ Fn is cumulative NF up to nth stage and Σ FN+1 is cumulative NF up to (n+1)th stageNoise figure can be used for much more than just characterizing a single stage. If you know the noise figure and gain of each stage you can calculate the noise figure of a cascade of devices. This equation is known as the cascade formula or Friis formula. F12 is the noise figure of the 2 stage system. G1 is the gain of the first stage, F1 is the NF of the first stage and F2 is the NF of the second stage. The formula clearly shows why you must put your best noise figure devices at the front of the chain. Also the higher the gain of the first stage, the less the noise figure contribution from subsequent stages.15Receiver Modelling using Excelstage 1 stage 2 stage 3 stage 4 TOTAL NF AMP1 2.00 14.00 2.00 14.00 AMP1 2.00 9.00 AMP2 4.00 16.00 2.00 9.00 AMP2 4.00 16.00 2.16 30.00 AMP3 5.00 20.00 2.49 25.00 AMP3 5.00 20.00 2.17 50.00 AMP4 10.00 30.00 2.51 45.00 AMP4 10.00 30.00 2.171NF gain cummulative NF cummulative gain1 22 3 4NF gain cummulative NF cummulative gainstage 1stage 2stage 3stage 4TOTAL NF 2.51NF gain cummulative NF cummulative gainAMP1 4.00 16.00 4.00 16.00 LOSS1 4.00 -4.00 4.00 -4.00AMP2 2.00 14.00 4.03 30.00 AMP1 2.00 14.00 6.00 10.00AMP3 5.00 20.00 4.03 50.00 AMP2 4.00 16.00 6.16 26.00AMP4 10.00 30.00 4.03NF gain cummulative NF cummulative gainAMP3 5.00 20.00 6.1710*LOG((10^(F22/10))+(10^(G20/10)-1)/10^(F23/10))Here is an example of how useful the cascade formula is in the estimation of receiver sensitivity. I’ve used EXCEL to illustrate the example as EXCEL is a very simple and powerful way of performing linear calculations. Both examples have four system components. In the first one I have my low noise amplifier at the front followed by a linear gain block followed by 2 further gain stages. My best noise figure device is placed first as it will dominate the noise figure performance of the system. You can see that the overall noise figure performance is little more than the noise figure of the first stage. The second example is identical, except for the fact that the LNA has lower gain. This mean that the noise contribution of the following stages is more noticeable. The point to make here is that the noise figure of a device is important - but so is its gain. In the third one I have swapped the first two amplifiers around and you can see the difference his has made to the overall noise figure - although the cumulative gain is the same the noise figure is dominated by the first - and now poorer - noise figure performance. The last example is similar to the very fist one except that now4 dB of loss have been introduced. This is common in receiver systems and could represent the cabling between an antenna and the LNA or a front end duplexer. The noise figure of a passive lossy device is equal to its loss. Overall you just add front end losses to the system noise figure to get the overall noise figure The noise figure of a passive device can be seen to be same the magnitude of the insertion gain. For example, a 6dB attenuator will have a noise figure of +6dB, but an insertion gain of -6dB. This can also be seen from standard calculation as well. As an example : if Noise Factor = N out / Gain x N in, and if Noise_out = Noise_in for this case, and Gain = 1/4 then Noise Factor is 4 and the noise figure is the log of this at + 6dB I’ve shown the cascade equation in slightly modified form. This is what you would type into excel. Fn is the cumulative noise figure up to the nth stage and sigma Ga1 is the cumlative gain.16Why do we measure Noise Figure? Example...Transmitter: ERP Path Losses Rx Ant. Gain Power to Rx Receiver: Noise Floor@290K Noise in 100 MHz BW Receiver NF Rx Sensitivity -174 dBm/Hz +80 dB +5 dB -89 dBm + 55 dBm -200 dB 60 dB -85 dBmERP = +55 dBmPatC/N= 4 dB:sses h Lo200dBChoices to increase Margin by 3dB 1. Double transmitter power 2. Increase gain of antennas by 3dB 3. Lower the receiver noise figure by 3dBReceiver NF: 5dB Bandwidth: 100MHz Antenna Gain: +60dBPower to Antenna: +40dBm Frequency: 12GHz Antenna Gain: +15dBHere is an example of why we need to know the noise figure of a device. In this example, we have a satellite that transmits with an effective radiated power of +55dBm, and is transmitted through a path loss, of +200dB, to a receive antenna with gain of 60dB. The signal power to the receiver is -85dBm. The receiver sensitivity is calculated here using kTB is at -174dBm /Hz and the noise power in a 100 MHz bandwidth you add 80dB. The noise figure of the complete receiver is +5dB. So the receiver noise floor is at -89dBm. S we currently have a 4dB carrier to noise ratio in our 100MHz channel. If we wanted to double the link margin to get improved receiver reliability, then we could double the transmitter power. This would cost millions of dollars in terms of increased payload and /or higher rated, more expensive components and more challenging engineering issues. Another way is to increase the gain of the receiver. This would cost millions in terms of size and mechanical engineering, and the debates over local environmental issues and planning permissions. While lowering the Noise Figure of the front end would be a fraction of this, and is the more attractive economically. Noise figure is a $$$ figure.17What Noise Figure is Not…Not a figure of merit for different modulation techniques use BER instead Not a quality factor for one port networks e.g. synthesizers, power supplies Not a useful quality factor for high power stages use transmitter testerWe have discussed what noise figure is. It is maybe usefully to briefly describe what noise figure is not. It does not give any indication of the efficiency of the modulation scheme chosen. In digital receivers this is done by BER. BER and noise figure have a nonlinear relationship where as you gradually decrease the signal to noise ratio you will suddenly see a rise in BER as 1’s and 0’s become confused. Noise figure is a two port figure of merit. It does not describe one port networks such as terminations or oscillators. Oscillators do generate noise and will affect the sensitivity of receivers but noise figure is not a means of measuring oscillator quality. Here phase noise measurements would be more appropriate. High power stages imply nonlinearity and noise figure is a function of strictly linear systems. Also high power stages implies high levels of input noise, so the added noise of the of the high power stage is likely to be very small - remember noise figure is defined where the input power has an effective temperature of 290K.18Summary of Noise FundamentalsThe Origins of Noise Signal to Noise ratio Definition of Noise Figure Effective Noise Temperature Friis Cascade Formula Using Excel in Rx modeling System Sensitivity Calculation19How do we make measurements?Fundamental noise conceptsHow do we make measurements?What DUTs can we measure?What influences the measurement uncertainty?Now that we have seen the basic concepts of noise, let’ now look at how we make those measurements.20Nout = Na + kTBGaGa , NaRsNout = Nh or Nc RsXXX YYY ZZZ AAABBBCCC......ENR dBFrequency Excess Noise Ratio, ENR (dB) = 10 Log 10( T h -290)290Fundamentalnoise conceptsHow do wemakemeasurements?What DUTscan wemeasure?What influencesthe measurementuncertainty?Fundamentalnoise conceptsHow do wemakemeasurements?What DUTscan wemeasure?What influencesthe measurementuncertainty?ResultsN8970 Series Noise Figure Analyzers•Fast, accurate and repeatable noise figure measurements up to 26.5 GHz (higher frequency also possible)•Simultaneous noise figure and gain measurements.•Compact and portablePSA Series Spectrum Analyzers •Industry’s highest performance spectrum analyer•Now with Noise Figure personality.Noise Sources•Up to 26.5 GHz and 15dB ENR •Calibration data is automatically down-loaded from the SNS series sources to noise figure analyzer.Technical data is subject to change. Copyright@2004 Agilent。