6/6/2014UPR, Mayagüez Campus Radiometers Radiometers are very sensitive receivers that measure thermal electromagnetic emission (noise) from material media. The design of the radiometer allows measurement of signals smaller than the noise introduced by the radiometer (systems noise).
6/6/2014UPR, Mayagüez Campus Topics of Discussion Equivalent Noise Temperature Noise Figure & Noise Temperature Cascaded System Noise for Attenuator Super-heterodyne Receiver System Noise Power at Antenna Radiometer Operation Measurement Accuracy and Precision Effects of Rx Gain Variations
6/6/2014 Topics of Discussion… Dicke Radiometer Balancing Techniques Reference -Channel Control Antenna-Channel Noise-Injection Pulse Noise-Injection Gain-Modulation Automatic-Gain Control (AGC) Noise-Adding radiometer Practical Considerations &Calibration Techniques
6/6/2014 Radiometers Task: Measure antenna temperature, T A which is proportional to T B, with sufficient radiometric resolution and accuracy T A varies with time. An estimate of T A is found from V out and the radiometer resolution T. Radiometer TATA T A V out TBTB
6/6/2014 Noise voltage The noise voltage is the average=0 and the rms is
6/6/2014 Noisy resistor connected to a matched load is equivalent to… [Z L =(R+jX)*=R-jX] Independent of f and R!,
6/6/2014 Equivalent Output Noise Temperature for any noise source T E is defined for any noise source when connected to a matched load. The total noise at the output is Ideal Bandpass Filter B, G=1 ZLZL Receiver antenna
6/6/2014 Noise Figure, F Measures degradation of noise through the device is defined for T o =290K (62.3 o F!, this = winter in Puerto Rico.) Total output signal Total output noise Noise introduced by device input signal input thermal noise
6/6/2014 Noise Figure, F Noise figure is usually expressed in dB Solving for output noise power
6/6/2014 Equivalent input noise T E Noise due to device is referred to the input of the device by definition: So the effective input noise temp of the device is Where, to avoid confusion, the definition of noise has been standardized by choosing T o =290K (room temperature) Examples: 1dB NF is and 3dB NF is What is T E for F=2dB? 170K 75K 288K
6/6/2014 Antenna, TL and Rx Receiver T E2 Transmission Line, T E1
Superheterodyne Receivers Rx in which the RF amplifier is followed by a mixer that multiplies the RF signal by a sine wave of frequency LO generated by a local oscillator (LO). The product of two sine waves contains the sum and difference frequency components The difference frequency is called the intermediate frequency (IF). The advantages of superheterodyne receivers include doing most of the amplification at lower frequencies (since IF<RF), which is usually easier, and precise control of the RF range covered via tuning only the local oscillator so that back-end devices following the un-tuned IF amplifier, multichannel filter banks or digital spectrometers for example, can operate over fixed frequency ranges. 6/6/2014
RF amp G rf,F rf,T rf Superheterodyne receiver Mixer L M,F M,T M IF amp G if,F if,T if LO P ni P no G=30dB F=2.3dB G=23dB F=7.5dB G=30dB F=3.2dB Example: T rf =290(10.32 -1)=638K T m =1,340K T if =203K T REC =?
6/6/2014 Equivalent System noise power at antenna terminals Taking into consideration the losses at the antenna and T.L. with a physical temperature of T p : Receiver Transmis sion Line
6/6/2014 Equivalent System noise power at antenna terminals Then the total noise for the system is: Receiver Transmis sion Line For radiometer, P sys = P rec For Radar, S/N= P r /P sys
6/6/2014 Summary Antenna Antenna + losses Receiver Receiver + T.L. All of the above
6/6/2014 Measurement Accuracy and Precision Accuracy (certeza) – how well are the values of calibration noise temperature known in the calibration curve of output corresponding to T A. (absolute cal.) Precision (precisión)– smallest change in T A that can be detected by the radiometer output.(sensitivity) T
6/6/2014UPR, Mayagüez Campus Total Power Radiometer Super-heterodyne receiver: uses a mixer, L.O. and IF to down- convert RF signal. Usually B RF >B IF
6/6/2014UPR, Mayagüez Campus Detection- power spectra @: From Ulaby, Moore & Fung, 1986
6/6/2014 Noise voltage after detector, V d represents the average value or dc, and d represents the rms value of the ac component or the uncertainty of the measurement. IF x2x2 square-law detector VeVe VdVd
6/6/2014 Noise voltage after Integrator Integrator (low pass filter) averages the signal over an interval of time. Integration of a signal with bandwidth B during that time, reduces the variance by a factor N=B where B is the IF bandwidth. x2x2 integrator Low-pass, g LF V out VdVd VeVe
6/6/2014 Radiometric Resolution, T The output voltage of the integrator is related to the average input power, P sys x2x2 integrator Low-pass, g LF V out VdVd VeVe
Noise averaging By averaging a large number N of independent noise samples, an ideal radiometer can determine the average noise power and detect a faint source that increases the antenna temperature by a tiny fraction of the total noise power. http://www.cv.nrao.edu/course/astr534/Radiometers.html http://www.millitech.com/pdfs/Radiometer.pdf 6/6/2014
Receiver Gain variations Noise-caused uncertainty Gain-fluctuations uncertainty Total rms uncertainty Example p.368 T Rec =600K T A =300K B=100MHz =0.01sec Find the radiometric resolution, T
Gain Variations and Dicke radiometer As you can see gain variations in practical radiometers, fluctuations in atmospheric emission, and confusion by unresolved radio sources may significantly degrade the actual sensitivity compared with the sensitivity predicted by the ideal radiometer equation. One way to minimize the effects of fluctuations in both receiver gain and atmospheric emission is to make a differential measurement by comparing signals from two adjacent feeds. The method of switching rapidly between beams or loads is called Dicke switching after Robert Dicke, its inventor. [Using a double throw switch.] 6/6/2014
Dickie Radiometer Noise-caused uncertainty Gain-fluctuations uncertainty Total rms uncertainty Quiz T Rec =500K T A =150K B=100MHz =1 msec Find the radiometric resolution, T
6/6/2014 Dicke Radiometer Dicke Switch Synchronous Demodulator Noise-Free Pre-detection Section Gain = G Bandwidth = B Switching rate, f s = 1/ s
6/6/2014 Dicke Radiometer The output voltage of the low pass filter in a Dicke radiometer looks at reference and antenna at equal periods of time with the minus sign for half the period it looks at the reference load (synchronous detector), so The receiver noise temperature cancels out and the total uncertainty in T due to gain variations is
6/6/2014 Dicke radiometer The uncertainty in T due to noise when looking at the antenna or reference (half the integration time) Unbalanced Dicke radiometer resolution Give example: B=100MHz, =1s, T rec = 700K, G/G=.01, T ref =300K for T A =0K and 300K, for Total P radiometer and Dicke radiometer
6/6/2014 Balanced Dicke A balanced Dicke radiometer is designed so that T A= T ref at all times. In this case,
6/6/2014 Balancing Techniques Reference Channel Control Antenna Noise Injection Pulse Noise Injection Gain Modulation Automatic Gain Control
6/6/2014 Reference Channel Control V out Synchronous Demodulator T ref Pre-detection G, B, T REC Feedback and Control circuit Switch driver and Square-wave generator, f S Integrator L Variable Attenuator at ambient temperature T o VcVc TNTN Noise Source T A Force T A = T ref
6/6/2014 Reference Channel Control T N and T o have to cover the range of values that are expected to be measured, T A If 50k<T A< 300K Use T o = 300K and need cryogenic cooling to achieve T N =50K. But L cannot be really unity, so need T N < 50K. To have this cold reference load, one can use cryogenic cooled loads (liquid nitrogen submerged passive matched load) active cold sources (COLDFET); backward terminated LNA can provide active cold source.
6/6/2014 Cryogenic-cooled Noise Source When a passive (doesnt require power to work) noise source such as a matched load, is kept at a physical temperature T p, it delivers an average noise power equal to kT p B Liquid N 2 boiling point = 77.36°K Used on ground based radiometers, but not convenient for satellites and airborne systems.
6/6/2014 Active cold or hot sources http://www.maurymw.com/ http://sbir.gsfc.nasa.gov /SBIR/successes/ss/5- 049text.html http://sbir.gsfc.nasa.gov /SBIR/successes/ss/5- 049text.html
6/6/2014 Ideal radiometer Real radiometer Usually we want T=1K, so we need B=100MHz and =10msec B, G radiometer TATA P n =k B G T A B, G radiometer T A =200K P n =k B G (T A + T N ) T N =800K
6/6/2014 Active noise source: FET The power delivered by a noise source is characterized using the ENR=excess noise ratio where T N is the noise temperature of the source and T o is its physical temperature. Example for 9,460K: ENR= 15 dB
6/6/2014 Antenna Noise Injection Variable Attenuator V out Synchronous Demodulator T ref Coupler Pre-detection G, B, T rec Feedback and Control circuit Switch driver and Square-wave generator, f S Integrator L VcVc TNTN Noise Source T A T N Force T A = T ref = T o F c = Coupling factor of the directional coupler *Measures v c
6/6/2014 Antenna Noise Injection Combining the equations and solving for L from this equation, we see that T o should be > T A If the control voltage is scaled so that V c =1/L, then V c will be proportional to the measured temperature,
6/6/2014 Example: Antenna Noise Injection If 50K 300K, say T o =310K If F c =100(20dB) and T n =50,000K Find L variation needed:
6/6/2014 Antenna Noise Injection For expected measured values between 50K and 300K, T ref is chosen to be T o =310K, so Since the noise temperature seen by the input switch is always T o, the resolution is
6/6/2014 Pulse Noise Injection V out Synchronous Demodulator T ref Coupler Pre-detection G, B, T rec Feedback and Control circuit Switch driver and Square-wave generator, f S Integrator Pulse- Attenuation Diode switch f r TNTN Noise Source T A T N *Measures f r
6/6/2014 Pulse Noise Injection R p Pulse repetition frequency = f R = 1/ R Pulse width is constant = p Square-wave modulator frequency f S < f R /2 Switch ON – minimum attenuation Switch Off – Maximum attenuation Example: For L on = 2, L off = 100, T o = 300K and T N = 1000K, We obtain T on = 650K, T off = 307K Diode switch TNTN T N T on T off
6/6/2014 Pulse Noise Injection Reference T is controlled by the frequency of a pulse The repetition frequency is given by For T off = T o, is proportional to T A
6/6/2014 Gain-Modulation V out Synchronous Demodulator Pre-detection G, B, T rec Control circuit Switch driver and Square-wave generator, f S Integrator v c T ref T A *Measures v c Fixed attenuator L o Variable attenuator L v Drawback: slow variations of receiver noise temperature, yields error in reading.
6/6/2014 Automatic-Gain-Control AGC Feedback is used to stabilize Receiver Gain Use sample-AGC not continuous-AGC since this would eliminate all variations including those from signal, T A. Sample-AGC: V out is monitored only during half-cycles of the Dicke switch period when it looks at the reference load. Hach in 1968 extended this to a two-reference- temperature AGC radiometer, which provides continuous calibration. This was used in RadScat on board of Skylab satellite in 1973.
6/6/2014 Automatic Gain-Control (AGC) V agc Synchronous Demodulator 2fs Pre-detection G, B, T rec Feedback amplifier Switch driver and Square-wave generator, f S Integrator Gv Reference Switch 2f s T2T2 T1T1 gvgv Synchronous Demodulator fs Hach radiometer: insensitive to variations from G, and T rec.
6/6/2014 Dicke Switch Two types Semiconductor diode switch, PIN Ferrite circulator Switching rate, f S, High enough so that G S remains constant over one cycle. To satisfy sampling theorem, f S >2B LF (Same as saying that Integration time is =1/2B LF ) http://envisat.esa.int/instruments/mwr/descr/charact.html
6/6/2014 Dicke Input Switch Important properties to consider Insertion loss Isolation Switching time Temperature stability http://www.erac.wegalink.com/members/DaleHug hes/MyEracSite.htm
6/6/2014 Radiometer Receiver Calibration Most are linear systems Hach-radiometer is connected to two known loads, one cold (usually liquid N 2 ), one hot. Solve for a and b. Cold load :satellites use outer space ~2.7K
6/6/2014 Imaging Considerations Scanning configurations Electronic (beam steering) Phase-array (uses PIN diode or ferrite phase- shifters, are expensive, lossy) Frequency controlled Mechanical (antenna rotation or feed rotation) Cross-track scanning Conical scanning (push-broom) has less variation in the angle of incidence than cross-track
6/6/2014 Uncertainty Principle for radiometers For a given integration time,, there is a trade-off between spectral resolution, B and radiometric resolution, T For a stationary radiometer, make larger. For a moving radiometer, is limited since it will also affect the spatial resolution. (next) M= figure of merit
6/6/2014 Airborne scanning Consider a platform at height h, moving at speed u, antenna scanning from angles s and – s, with beamwidth, along-track resolution, x The time it takes to travel one beamwidth in forward direction is The angular scanning rate is The time it takes to scan through one beamwidth in the transverse direction is the dwell time
6/6/2014 Dwell time Is defined as the time that a point on the ground is observed by the antenna beamwidth. Using For better spatial resolution, small For better radiometric resolution, need large As a compromise, choose
6/6/2014 Radiometer Uncertainty Eq. Equating, we obtain; Radiometric resolution Spatial resolution Spectral resolution This equation applies for this specific scanning configuration.
6/6/2014 Problem 6.6 A 1GHz balanced Dicke radiometer with a 100 MHz bandwidth is to be flown on a satellite at an altitude of 600 km with average speed of 7.5 km/s. The radiometer uses a 10-m diameter antenna, and the receiver is characterized by T rec =1000K and T ref =300K. Take antenna efficiency k=1.5 [ k /l] The radiometer integration time is chosen to be equal to 0.1 of the dwell time of the antenna beam for a point on the ground. If the antenna is fixed so that its main beam is always pointed in the nadir direction, What will T be? = 0.1678 K