# Patent

## Title
Multi-channel radiometer imaging system

## Abstract

A radiometer system includes a housing and an RF board contained within the housing. Balanced radiometer channels receive an unknown signal and known reference. Each balanced radiometer channel includes at least one quadrature hybrid and amplifier circuit to provide distributed gain and an amplified output signal. A detection circuit receives and detects the amplified output signal forming a detected signal. A controller board is contained within the housing and has an integration circuit and microcontroller that receives the detected signal and performs video signal digitization and conditioning and real-time corrections on any radiometer channels to account for changes in temperature or gain.

## Background
RELATED APPLICATIONS
This application is a continuation of application Ser. No. 10/847,892 filed May 18, 2004, now issued as U.S. Pat. No. 7,034,516, which claims a benefit of a provisional application Ser. No. 60/504,182 filed Sep. 18, 2003.


FIELD OF THE INVENTION
The present invention relates to the field of focal plane radiometers, and more particularly, the present invention relates to a multi-channel radiometer applicable for use at millimeter wave (MMW) frequencies.
BACKGROUND OF THE INVENTION
Since radio waves may be considered infrared radiation of long wave, a hot body would be expected to radiate microwave energy thermally. In order to be a good radiator of microwave energy, a body must be a good absorber. The best thermal radiator is a “black body.” The amount of radiation emitted in the MMW range is 108 times smaller than the amount emitted in the infrared range. Current MMW receivers, however, have at least 105 times better noise performance than infrared detectors, and with some temperature contrast, the remaining 103 may be recovered. This makes passive MMW imaging comparable in performance with current infrared systems. This unique characteristic makes MMW radiometers a popular choice for sensing thermal radiation. MMW radiometers have been used in many different applications such as remote terrestrial and extra-terrestrial sensing, medical diagnostics and defense applications. MMW electromagnetic radiation windows occur at 35 GHz, 94 GHz, 140 GHz and 220 GHz. The choice of frequency depends on specific applications.
Focal plane arrays are used to form images from radiation received by a reflector antenna. Millimeter wave (MMW) focal plane array radiometers also have been used in many applications to form images based on thermal sensing of radiated microwave energy. The sensitivity of existing radiometer designs, however, has been limited to about 1 deg K, resulting in poor images.
The principle of operation of the radiometric technique is fully described in the literature. The design of a typical radiometer is based on the technique of comparing the level of electromagnetic noise emitted by an unknown source to a reference or stable noise source. This technique and devices were initially proposed by Dicke [R. H. Dicke, “The Measurement of Thermal Radiation at Microwave Frequencies,” The Review of Scientific Instruments, Vol. 17, No. 7, July 1946].
In a Dicke radiometer circuit, the signals from an antenna are sampled and compared with signals from a reference source maintained at a known constant temperature. This overcomes some of the problems of amplifier instability, but in general does not alter effects resulting from imperfect components and thermal gradients.
While other types of radiometric devices have been used with some success, the Dicke (or comparison) type of radiometer has been the most widely used for the study of relatively low level noise-like MMW signals, especially where the noise signals to be examined are often small in comparison to the internally generated noise level within the radiometer receiver. While there are several types of comparison radiometers, one popular type of radiometer for use in the microwave/millimeter wave frequency bands is that in which an incoming signal to be measured and a standard or calibrated reference noise signal are compared. This type of radiometer consists essentially of the comparison of the amplitude of an unknown noise signal coming from the source to be examined with a known amplitude of a noise signal from a calibration source. This method has been found useful in measuring with considerable accuracy the effective temperature of an unknown source.
In the Dicke or comparison type radiometer, the receiver input is switched between the antenna and a local reference signal noise generator. The detected and amplified receiver output is coupled to a phase sensing detector operated in synchronism with the input switching. The output signal from such a radiometer receiver is proportionate to the difference between the temperature of the reference signal source and the temperature of the source viewed by the antenna inasmuch as the phase-sensing detector acts to subtract the background or internal noise of the receiver.
A Dicke radiometer uses an RF switch coupled between an antenna and a radiometer receiver, allowing the receiver to alternate between the antenna and a known reference load termination. The receiver output is connected to a synchronous detector that produces an output voltage proportional to a difference between the antenna and the reference temperature. Null balance operation for the Dicke radiometer has been achieved by coupling in noise from a hot noise diode to the antenna port of the RF switch thereby enabling matching the temperature from standard reference loads.
The sensitivity of radiometer measurements are also often limited by random gain fluctuations in the RF front end, low frequency noise (1/f), and bias in the detector circuits. Over the last decades many special techniques, including Dicke switching, have been implemented to reduce measurement errors. Many of these proposals do not yield a true solution that will allow MMW radiometers to be commercially viable. In addition, the high cost of MMW RF receivers has limited the number of channels in the radiometer to a low number, resulting in a requirement to scan both azimuth and elevation to create an image.
SUMMARY OF THE INVENTION
The present invention eliminates the need for a Dicke switch and does not use a synchronizing circuit because it uses the source and reference all the time, and runs the source and reference signal through the amplifiers. The present invention uses a balanced channel approach using preferred MMIC chips. Thus, a radiometer channel can be implemented by the use of either a single millimeter wave monolithic integrated circuit (MMIC) or through discrete implementation using printed hybrids and multiple MMIC low noise amplifiers (LNA's).
The radiometer module of the present invention has at least six times higher sensitivity than current radiometer sensitivity, such as a Dicke radiometer sensitivity. The present invention is advantageous and provides a radiometer with the sensitivity of less than 1° K. The radiometer module (sensor) is at least ten times smaller than other radiometers currently in use, which typically measure more than three cubic feet even with a small number of channels. The radiometer of the present invention also is at least ten times lighter than existing radiometers, which weigh no less than 20 pounds even with a small number of channels. The radiometer of the present invention is less than about three pounds.
The present invention also is self-correcting for temperature and gain variations. It uses a balanced pair of diodes for detection and the chopper operational amplifiers to eliminate any bias and reduce 1/F noise. The microcontroller can monitor temperature changes between the antenna and the reference by reading any temperature sensors located on the antenna and near the reference. This can be based on temperatures to adjust the correction factor. The gain can be continuously monitored and the bias adjusted of the low noise amplifier (LNA) to maintain the constant gain. Real-time corrections can be performed on all video channels to account for any changes in temperature or gain.
The radiometer of the present invention also has self-healing capability because of the distributed gain approach. Failure of one or more LNA's in each channel will not result in failure of the channel The microcontroller can compensate for the drop of any amplifiers in the chain.
The compact radiometer of the present invention can fit directly in the antenna focal plane. A quadrature hybrid network is used in the front end to distribute RF input signals and reference signals to a balanced amplifier chain, thereby reducing gain variations and improving radiometer sensitivity. A balanced detector diode circuit, for example, a pair of diodes in one nonlimiting example, eliminates drift errors introduced by a detector diode as a function of temperature.
A video signal chopper amplifier circuit, also referred to by some as a auto zero amplifier, eliminates bias introduced by the video amplifier. A near perfect channel-to-channel matching exists through the use of quadrature hybrid network or through digital signal processing corrections.
The radiometer system of the present invention includes a housing and an RF board contained within the housing. Balanced radiometer channels receive an unknown signal and known reference. Each balanced radiometer channel includes at least one quadrature hybrid and amplifier circuit to provide distributed gain and an amplified output signal. A detection circuit receives and detects the amplified output signal forming a detected signal. A controller board is contained within the housing and has an integration circuit and microcontroller that receives the detected signal and performs video signal digitization and conditioning and real-time corrections on any radiometer channels to account for changes in temperature or gain.
In accordance with another aspect of the present invention, a quadrature hybrid and amplifier circuit comprises a MMIC chip. It can also be formed as discrete printed hybrids and low noise amplifiers. The RF board is preferably formed from a soft board or ceramic material. A radiometer system of the present invention is sized and of such weight to fit directly into an antenna focal plane. It includes a cover having RF launch openings that are typically sized to allow for one wavelength spacing between any sensing radiators.
A video signal chopper amplifier circuit can be contained on the controller board to receive the detected signal and aid in eliminating bias and reducing 1/f noise. The detection circuit can include at least one pair of balanced diodes. The controller board is also adapted to interface with an external display system. A plurality of sensing radiators are formed on the RF board. The analog-to-digital conversion circuit is positioned on the controller board and digitizes detected signals received from the integration circuit for processing by the microcontroller. Isolation vias can be formed on the RF board and isolate the quadrature hybrid and amplifier circuits.



BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention will become apparent from the detailed description of the invention which follows, when considered in light of the accompanying drawings in which:
FIG. 1 is a fragmentary environmental view of a typical radiometer antenna system for a focal plane array.
FIG. 2 is a block diagram showing a receiver front end in a typical radiometer system.
FIG. 2A is a block diagram showing how radiometer modules are typically connected to the antenna with a waveguide manifold in current art.
FIG. 3 is a block diagram illustrating the basic functional components of the radiometer of the present invention.
FIG. 3A is a block diagram of the quadrature hybrid used in the radiometer of FIG. 3 showing how inputs A, B are divided equally in the first hybrid, then reconstructed in the second hybrid.
FIG. 3B is a block diagram showing a two-stage MMIC LNA chip of the present invention as a representative example.
FIG. 3C is a block diagram showing a three-stage MMIC LNA chip of the present invention.
FIG. 3D is a block diagram illustrating the basic functional components of the radiometer of the present invention using the MMIC chip of FIG. 3B.
FIG. 4 is a block diagram showing functional components of another example of a multi-channel radiometer of the present invention.
FIG. 5 is a block diagram showing the layout for the RF front end in the radiometer of the present invention.
FIG. 6 is a plan view showing a multi-channel radiometer layout on a single RF board.
FIG. 7 is an exploded isometric view of a compact multi-channel radiometer module showing a base housing, RF board, controller board and top cover.
FIG. 8 is an isometric view of the assembled multi-channel radiometer module of the present invention.
FIG. 9 is a top plan view of the multi-channel millimeter wave radiometer module shown in FIG. 8.
FIG. 10 is a chart showing Dicke radiometer sensitivity of the type shown in FIG. 2.
FIG. 11 is a chart showing the radiometer sensitivity of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.
The present invention overcomes many exiting shortcomings of current radiometers, including gain variation of the amplifiers, low frequency noise, detector bias, low sensitivity and high cost. The present invention reduces cost and size of a radiometer by a least a factor of ten and provides a commercial advantage over many current radiometers.
The low cost MMW radiometer of the present invention includes a housing section, a multi-channel RF board, and a controller board. The housing section is preferably made up of a base metal housing and a metallized plastic or metal cover that includes the RF launch opening, typically filled with a dielectric material, for example, a plastic material to allow for less than one wavelength spacing between the sensing radiators. The RF board preferably is formed from a single soft board or ceramic material. All MMW microstrip circuits, for example, 50 Ohm lines, filters, 90° hybrids and RF radiators, are printed on this board. MMIC amplifiers can be either attached directly to the board or, through cut-outs, on a carrier plate underneath to the RF board. A simple MMIC chip can also be used. A controller board, which uses a low cost microcontroller, performs any necessary video signal amplification, digitization and conditioning, automatic RF amplifier bias adjustment, and DC power regulation. The controller board interfaces with an external display system.
The compact radiometer of the present invention can fit directly in the antenna focal plane. A quadrature hybrid network is used in the front end to distribute RF input signals and reference signals to a balanced amplifier chain, thereby reducing gain variations and improving radiometer sensitivity. A balanced detector diode circuit, for example, a pair of diodes in one non-limiting example, eliminates drift errors introduced by a detector diode as a function of temperature.
A video signal chopper amplifier circuit, also referred to by some as a auto zero amplifier, eliminates bias introduced by the video amplifier. A near perfect channel-to-channel matching exists through the use of quadrature hybrid network or through digital signal processing corrections.
FIG. 1 shows a typical radiometer antenna system 20. The main antenna 22 collects temperature data or other pertinent data to be analyzed. The data is focused in the middle of the antenna at the focal plane array 24 using a sub-reflector 26.
FIG. 2 shows a common prior art “Dicke” type radiometer system 30, including a receiver front end. In a Dicke radiometer, generally a receiving circuit detects weak signals in noise and modulates these signals at an input. The circuit demodulates the signals and compares the output with a reference from the modulator. Coincidence indicates a signal presence. For example, microwave noise power can be measured by comparing it with the noise from a standard source in a waveguide.
In this illustrated example of a Dicke radiometer, the antenna 32 senses target temperature, which is proportional to the radiated target energy. The energy passes through a Dicke switch 34 of the type known to those skilled in the art and into a series of MMIC amplifiers 36a, 36b, 36c. A band pass filter 38 sets the receiver bandwidth. A square law detector 40 detects the signal and passes it to an integrator 42, which sums the signal over an observation period. A data acquisition and processing circuit 44 receives the integrated signal, where it is digitized, compensated for gain variation, and processed for display on a video or for further processing. To cancel the effects of gain variation, the Dicke switch 34 samples a reference source 46. Gain variations in the receiver are cancelled using the measured reference gain.
Radiometer sensitivity is important. The precision in estimating the measured temperature is often referred to as the radiometer sensitivity, ΔT. This parameter is a key quantity characterizing the performance of a MMW radiometer. In radiometer terminology, this is the smallest change in temperature that can be detected by the radiometer. The equation, which derives the sensitivity of the system 30 shown in FIG. 2 is:

Psys=PA+Prec 

where Psys=total input power
PA=Noise power at the antenna=kTAB
Prec=Noise power generated in the receiver=kTrecB
K=Boltzmann's constant
B=receiver bandwidth
Assuming a square law detector, the radiometer output voltage is an average value of the radiometer output noise power. The square law detector can have an output proportional to the square of the applied voltage, e.g., the output is proportional to the square of the input amplitude. A radiometer output voltage is:

Vout=Psys×Gsys 

where Gsys is the receiver gain.

Assuming that Gsys, and Trec are constant, the radiometer sensitivity is:

ΔTideal=(1/√{square root over (Bτ)})Tsys 

where τ is the integration time.

In most applications, however, Gsys, and Trec are not constant, and their variations cause degradation of the radiometer sensitivity as follows:
Gain variations effects:

ΔTG=(TA−Tref)×(ΔGsys/Gsys)

Assuming a five degree difference between the antenna temperature and the reference temperature, a +/−3 dB gain variation (over the 3 LNA's 36a, 36b, 36c), and a 40 dB total system gain, the radiometer sensitivity will vary by about 5%.
Temperature variation effects can be shown:

ΔTant=(TA+Trec)/(√{square root over (Bτ/2))}=√{square root over (2)}(TA+Trec)/(√{square root over (Bτ))}

ΔTref=(Tref+Trec)/(√{square root over (Bτ/2))}=√{square root over (2)}(Tref+Trec)/(√{square root over (Bτ))}

Assuming statistical independence, the temperature variation can be shown:





  
    Δ
    ⁢
    
      
    
    ⁢
    T
  
  =
  
    
      
        [
        
          
            
              (
              
                Δ
                ⁢
                
                  
                
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                  T
                  G
                
              
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                Δ
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            2
          
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              (
              
                Δ
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                  T
                  ref
                
              
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        ⁢
        
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        2
      
    
    =
    
      
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                2
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                    (
                    
                      
                        T
                        A
                      
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                        rec
                      
                    
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                  2
                
              
              +
              
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                        T
                        ref
                      
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                    ref
                  
                
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Assuming a balanced Dicke radiometer (i.e. TA=Tref) the above equation can be simplified to:





  
    Δ
    ⁢
    
      
    
    ⁢
    T
  
  =
  
    
      2
      ⁢
      
        
          (
          
            
              T
              A
            
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            B
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      Δ
      ⁢
      
        
      
      ⁢
      
        T
        ideal
      
    
  




Therefore, the Dicke radiometer sensitivity is twice that of an ideal total power radiometer. The factor of two (2) comes about because the Dicke switch alternates between the reference and the antenna such that TA is observed for only half of the time.
FIG. 2A shows how the radiometer channels 48, indicated as channels 1 . . . N, as part of RF modules, are typically connected to the antenna 48a. Because of the large size of the radiometer RF modules, which cannot fit directly in the antenna focal plane, a waveguide manifold 48b is used to connect the modules to the focal plane. The waveguide manifold 48b increases the front end losses by at least 2 dB, resulting in reduced radiometer sensitivity. The channels 48 connect to data acquisition and processing circuit 48c. 
FIG. 3 is a block diagram of the radiometer 50 of the present invention. This radiometer design does not use a Dicke switch, yet it still delivers superior sensitivity and can be readily manufactured.
A radiator 52 provides a first signal input A while a reference 54 provides a second signal input B. The radiator 52 could be many types of radiator elements used in radiometers, including an antenna. Microstrip quadrature hybrid circuits 56 are operable with low noise amplifier circuits 58. The hybrid circuits can be 90° hybrids. Bandpass filter circuits 60a, 60b receive the signals represented at A and B, which are output to detector circuits 62a, 62b. These components are typically mounted on an RF board indicated by the dashed lines at 64. The RF board is typically formed from a single soft board or ceramic material. All MMW microstrip circuits, for example, 50 ohm lines, filters, hybrids and RF radiators, are printed on this board. Any MMIC amplifiers can be attached directly to the board, or through cut-outs, on a carrier plate underneath to the RF board.
The signals (A and B) are output to a controller board indicated by dashed lines at 70. On this board, any necessary video signal amplification, digitization and conditioning, automatic RF amplifier bias adjustment, and DC power regulation occurs. This board can interface directly with a video display system. The signal is received at two chopper amplifier circuits 72a, 72b. After amplification, the signals are integrated at integrator circuits 74a, 74b, and digitized at analog/digital (A/D) circuits 76a, 76b. A microcontroller circuit 78 provides digital video processing and receives an antenna temperature signal 80, amplifier control signal 82, and reference temperature signal 84. The output from the microcontroller circuit 78 is sent to a display or other external sensors 86.
The radiometer 50 of the present invention uses microstrip quadrature hybrids 56 to distribute the signal and reference powers to the balanced amplifier chain as illustrated. The pairs of low noise amplifiers (LNA's) 58 are cross-coupled to each other, similar to a conventional balanced amplifier configuration.
The quadrature hybrid shown in FIG. 3A is a well known four-port device that splits the energy into equal parts at the output, but with a 90 degree phase difference. For example, the signal A at the input port P1 of the hybrid is divided up to two parts at the output ports P3 and P4. The same is true for the input signal B at the input port P2 of the hybrid, which is also divided equally at the output ports P3 and P4. When the output of the first hybrid is used as input into a second hybrid, the signals A and B are restored at the output of the second hybrid (of course with some losses due to the hybrids). The two inputs A and B, which can represent the antenna port and the reference port, or represent two antenna ports representing two different polarizations, are divided equally among the amplifiers and reconstructed at the output, as shown in FIG. 3A. One other unique feature of this hybrid design is that failure of one or more of the LNA's 58 in the chain does not result in failure of the channel itself. Because of the distributed gain approach, the gain of the channel will drop by a small amount, which can be accounted for in the microcontroller 78. This is different from the traditional radiometer shown in FIG. 2, where failure of one LNA will result in total failure of the element.
Because each signal passes through each amplifier in the chain, any fluctuation in the gain of any of the amplifiers is applied equally to both signals (TA& TRef). Assuming that the hybrid circuits 56 are well balanced by using good design practices, this radiometer design guarantees that the gain in each channel is substantially the same. In addition, because the gain in each channel is essentially the average of that of all the amplifiers in the chain, the overall gain fluctuation is effectively reduced by a factor of the square root of N, where N is the number of amplifiers.




  
    Δ
    ⁢
    
      
    
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      sys
    
  
  =
  
    
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            N
          
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              =
              1
            
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          ⁢
          
            
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                ⁢
                
                  
                
                ⁢
                
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      1
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      2
    
  




Assuming the same amount of the LNA's gain variation (+/−3 dB) used for the Dicke radiometer as shown in FIG. 2, the radiometer system gain variation of the present invention will be only about +/−0.7 dB. Therefore, it is evident that the radiometer of the present invention provides the inherent benefits of receiver gain fluctuation reduction and guarantees equal gain for both the antenna power and the reference signal. This feature provides the same benefit as the Dicke switch without the added losses and the complex switching circuitry. Also, the absence the Dicke switch in the present invention allows continuous observation of the antenna temperature, thereby achieving the sensitivity of a total power radiometer.

ΔTideal=(1/√{square root over (Bτ))}Tsys 

Using commercially available W-band LNA's with over 20 GHz bandwidth, such as an ALH394 circuit made by Velocium of Redondo Beach, Calif., and assuming an integration time of 20 msec and 1200 K total system temperature, this radiometer sensitivity is less than 0.1 degree. The ALH394 is a broadband, three-stage, low noise monolithic HEMT amplifier. It has a small die size and is passivated. Bond pad and backside metallization can be Ti/Au and compatible with conventional die attach, thermocompression and thermosonic wire bonding assembly. It can have a usable radio frequency of 76 to about 96 GHz, linear gain of about 17 dB, and a noise figure of about 5 dB depending on applications. It can use DC power of about 2 volts at 34 mA. Bond pads can include VG1, VG2 and VG3, VD1, VD2, VD3, with an RF in and RF out pad.
The RF signals at the output of the band pass filter 60a, 60b are detected using the square law detector 62a, 62b. In order to eliminate any detector variation over temperature, a pair of balanced diodes 62a, 62b, such as a DBES105a diode manufactured by United Monolithic Semiconductors, can be used. This dual Schottky diode is based on a low cost 1 μm stepper process with bump technology and reduced parasitic conductances and having a high operating frequency. It can be a flip-chip dual diode with high cut-off frequencies of about 3 THz and a breakdown voltage of less than −5 volts at 20 uA. It has a substantially adequate ideality factor of about 1.2.
The diodes output an equal amount of power, but with opposite polarity. This method effectively cancels any bias or drift caused by the diodes. The very small DC voltages at the output of the diodes are typically very difficult to amplify accurately. DC offsets introduced by the op-amps are usually a cause of the problem, aggravated often by low frequency noise (1/f). The radiometer 50 of the present invention uses chopping op-amp circuits 72a, 72b, also known as auto zero amplifiers, such as the AD8628 amplifier manufactured by Analog Devices. This amplifier circuit eliminates DC offset and low frequency (1/f) noise.
The AD8628 amplifier has ultra-low offset, drift and bias current. It is a wide bandwidth auto-zero amplifier featuring rail-to-rail input and output swings and low noise. Operation is specified from 2.7 to 5 volts single supply (1.35V to 2.5V dual supply). It has low cost with high accuracy and low noise and external capacitors are not required. It reduces the digital switching noise found in most chopper stabilized amplifiers, and has an offset voltage of 1 μV, a drift less than 0.005 μV/° C., and noise of 0.5 uV P—P (0 Hz to 10 Hz). This amplifier is available in a tiny SOT23 and 8-pin narrow SOIC plastic packages.
An offset voltage of less than 1 μV allows this amplifier to be configured for high gains without risk of excessive output voltage errors. The small temperature drift of 2 nV/° C. ensures a minimum of offset voltage error over its entire temperature range of −40° C. to +125° C. It has high precision through auto-zeroing and chopping. This amplifier uses both auto-zeroing and chopping in a ping-pong arrangement to obtain lower low frequency noise and lower energy at the chopping and auto-zeroing frequencies. This maximizes the signal-to-noise radio (SNR) without additional filtering. The clock frequency of 15 kHz simplifies filter requirements for a wide, useful, noise-free bandwidth. The amplifier is preferably packaged in a 5-lead TSOT-23 package.
1/f noise, also known as pink noise, is a major contributor of errors in decoupled measurements. This 1/f noise error term can be in the range of several μv or more, and when amplified with the closed-loop gain of the circuit, can show up as a large output offset. 1/f noise is eliminated internally. 1/f noise appears as a slowly varying offset to inputs. Auto-zeroing corrects any DC or low frequency offset, thus the 1/f noise component is essentially removed leaving the amplifier free of 1/f noise.
The output of the integrator circuits 74a, 74b for both the antenna signal and the reference signals are digitized using highly linear A/D circuits 76a, 76b and are sent to the microcontroller 78, where the reference signal is subtracted from the antenna signal to obtain the actual target temperature. The microcontroller 78 can monitor the temperature of the antenna through a sensor attached to the antenna. Any differences between the antenna and the reference are accounted for and corrections are applied appropriately in software. The microcontroller 78 also controls the LNA bias and monitors the amount of current drawn by each amplifier and adjusts the amplifier gain.
FIG. 3B shows a two-stage MMIC chip 112 that can be used in the present invention to replace the discrete implementation of the hybrid and cascade LNA's shown in FIG. 3. This MMIC LNA chip receives a signal from the antenna 114 or reference load 116 that enters through signal inputs A and B into the hybrid circuit 118 and into amplifiers 120a. 120b through amplifiers 124a, 124b, through hybrid 122 to be output as signals A and B amplified.
FIG. 3C is a block diagram showing a three stage MMIC LNA chip implementation 125 with respective amplifier circuits 128a and 128b. 
Thus, the balanced channel approach of the present invention can use MMIC chips and the implementation of a radiometer channel can occur either by the use of a single millimeter wave monolithic integrated circuit (MMIC) or through discrete implementation using printed hybrids and multiple MMIC LNA's.
FIG. 3D is a block diagram of the radiometer 50 of the present invention. This radiometer design uses the MMIC chip 112 shown in FIG. 3B to replace the printed hybrids and individual LNA chips. This figure shows yet another example of how this invention can be used to build radiometer channels. Only one detector 62 as a pair of diodes is used. One chopper 72 amplifier and any integrator 74 and A/D circuit 76.
The radiometer of the present invention can also be manufactured in an arrangement having a larger number of channels, such as shown in FIG. 4. Prime notation is used to show the various radiators 52′, hybrid circuits 56′, and low noise amplifiers 58′. As illustrated, signals A, B and C are generated from radiators 52′ and a reference signal is generated from the reference 54′. Two parallel hybrid circuits 56′ are illustrated at the front end and input into four parallel, low noise amplifier circuits 58′ instead of two as shown in FIG. 3. This follows by other parallel hybrid circuits 56′ and low noise amplifier circuits 58′. Four bandpass filters 60′ are illustrated with detector circuit 62′ forming a three-element radio frequency module.
FIG. 5 shows an example of a layout for the RF front end used in the radiometer 50 of the present invention, forming a radiometer cell 90. The radiator elements 52, the quadrature hybrids 56, 50 ohm microstrip lines 59 and the filters 60a, 60b are all printed on a soft board or a ceramic board. Isolation vias 100 are used to isolate the amplifiers 58 and reduces the likelihood of oscillations.
FIG. 6 shows a multi-channel radiometer layout on a single RF board. A plurality of radiometer cells 90 are illustrated, forming a N element array 102 with channels 110. Radiators 52 are also illustrated. This design approach allows for low cost implementation of a large number of channels. The radiator elements 52 can be spaced half a wavelength (λ/2) apart for lower cross coupling, lower sidelobes and overall improved operations. The channels 110 are stacked on both sides of the board in order to achieve two rows 110a, 110b of radiometer cells 90 in a very small amount of space. For dual polarization applications, one row 110a may be vertically polarized while the second row 110b could be horizontally polarized. The radiators, for example as antenna elements, can be alternated between vertical and horizontal polarization in the same row. For example, a 32×2 array can easily fit a 3×4 inch RF board. This board can become part of a radiometer module of the present invention.
FIG. 7 shows an exploded view of a compact, multi-channel radiometer module 130 of the present invention. A base housing 131, typically made-up of aluminum, is used to receive the RF board 64, which can be attached to a CTE matched carrier 132. The controller board 70, which supplies all the DC voltages and control signals, makes contact with the RF board 64 through the use of DC contact connectors. The top cover 134, which can be made from a plastic material, is metallized everywhere except where the radiator areas 136 correspond to the location of the radiators. These unmetallized radiator areas 136 provide a dielectric media for the RF energy to travel through. Thus, RF launch openings are formed. A slot 70a in the controller board provides access to the antenna elements. The entire unit is assembled using fasteners, such as screws received in fastener apertures 138a. 
FIG. 8 shows fully assembled multi-channel radiometer module of the present invention forming a radiometer module or “sensor package” as illustrated.
FIG. 9 is a top plan view of the multi-channel millimeter wave radiometer module shown in FIG. 8 and showing the radiator areas 136 and radiators. FIGS. 10 and 11 show various components of a Dicke radiometer (FIG. 10) and showing the Dicke radiometer sensitivity as compared to the radiometer sensitivity of the present invention as shown in FIG. 11. The different components of the radiometers and the relative sensitivity and operating or reference values are shown under the specific elements as illustrated.
The radiometer module of the present invention has at least six times higher sensitivity than more current radiometer sensitivity, such as the Dicke radiometer sensitivity explained above with reference to FIG. 2 and shown in FIG. 10. The present invention is advantageous and provides a radiometer with the sensitivity of less than 1° K. The radiometer module (sensor) is at least ten times smaller than other radiometers currently in use. The radiometer of the present invention also is at least ten times lighter in weight than any other radiometer in existence, which typically weighs no less than 20 pounds. The radiometer of the present invention is typically less than about three pounds.
The present invention also is self-correcting for temperature and gain variations. It uses the balanced pair of diodes for detection and the chopper operational amplifiers to eliminate any bias and reduce 1/F noise. The microcontroller can monitor temperature changes between the antenna and the reference by reading any temperature sensors located on the antenna and near the reference. This can be based on temperatures to adjust the correction factor. The gain can be continuously monitored and the bias adjusted of the low noise amplifier (LNA) to maintain the constant gain. Real-time corrections can be performed on all video channels to account for any changes in temperature or gain.
The radiometer of the present invention also has self-healing capability because of the distributed gain approach. Failure of one or more LNA's in each channel will not result in failure of the channel. The microcontroller can compensate for the drop of any amplifiers in the chain.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiment disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.

## Claims

1. A radiometer system, comprising:
a plurality of balanced radiometer channels that receive an unknown signal and known reference, each balanced radiometer channel comprising a MMIC comprising
a first quadrature hybrid as an input having at least one radio frequency (RF) input and parallel signal outputs;
at least one amplifier connected to each signal path output of said quadrature hybrid; and
a second quadrature hybrid as an output connected to each at least one amplifier and having parallel RF outputs, wherein the at least one amplifier provides an equalized amplifier gain;
a detector circuit the receives and detects the amplified output signal forming a detected signal; and

a circuit that receives the detected signal and performs video signal digitization and conditioning and real-time corrections on any radiometer channels to account for changes in temperature or gain.

2. The radiometer system according to claim 1, and further comprising dual RF inputs at the first quadrature hybrid as an input.

3. The radiometer system according to claim 2, wherein said dual RF inputs represent two different polarizations that are divided equally among amplifiers.

4. The radiometer according to claim 1, and further comprising an RF input and reference input at the first quadrature hybrid as an input.

5. The radiometer according to claim 1, and further comprising two amplifiers connected in series in each signal path.

6. The radiometer according to claim 1, and further comprising three amplifiers connected in series in each signal path.

7. The radiometer according to claim 1, wherein the at least one amplifier connected in each signal path comprises a Low Noise Amplifier (LNA).