The voltage at the output of a radio telescope is the sum of noise voltages from many independent random contributions. The central limit theorem states that the amplitude distribution of such noise is nearly Gaussian. The sampling theorem Eq. This is what the band-limited noise output voltage of a radio telescope looks like. It is convenient to describe noise power in units of temperature. The temperature equivalent to the total noise power from all sources referenced to the input of an ideal receiver connected to the output of a radio telescope is called the system noise temperature.
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We have investigated the possibility of building a singleband Dicke radiometer that is inexpensive, small-sized, stable, highly sensitive, and which consists of readily available microwave components. The selected frequency band is at 3.
Foreseen applications of the instrument are non-invasive temperature monitoring for breast cancer detection and temperature monitoring during heating. Two different Dicke radiometers have been realized: one is a conventional design with the Dicke switch at the front-end to select either the antenna or noise reference channels for amplification. The second design places a matched pair of low noise amplifiers in front of the Dicke switch to reduce system noise figure.
Numerical simulations were performed to test the design concepts before building prototype PCB front-end layouts of the radiometer.
Both designs provide an overall power gain of approximately 50 dB over a MHz bandwidth centered at 3. No stability problems were observed despite using triple-cascaded amplifier configurations to boost the thermal signals.
The prototypes were tested for sensitivity after calibration in two different water baths. Radiometer performance was also tested in a multilayered phantom during alternating heating and radiometric reading. Empirical tests showed that for the configuration with Dicke switch first, the switch had to be locked in the reference position during application of microwave heating to avoid damage to the active components amplifiers and power meter.
For the configuration with a low noise amplifier up front, damage would occur to the active components of the radiometer if used in presence of the microwave heating antenna. Nevertheless, this design showed significantly improved sensitivity of measured temperatures and merits further investigation to determine methods of protecting the radiometer for amplifier first front ends. It is well known that microwave radiometry can be used for noninvasive temperature measurements of superficial tissue in the human body.
As opposed to other active methods that stimulate the body with some kind of excitation signal, radiometry is passive and thus completely harmless. Over the past four decades, research within the field of microwave radiometry has been conducted for use in medical applications [ 1 ].
Modalities under investigation include detection of breast cancer [ 2 ], thermal monitoring of hyperthermia, and detection of inflammation [ 3 ]. In medical treatment involving microwave heating of the body, there is a need for temperature observation of the heated tissue. While conventional microwave-immune probes e. The technical details of this radiometer are not known, and are therefore difficult to evaluate.
However, the detection procedure is to make corresponding point measurements of both breasts and assess asymmetries in measured brightness temperatures. Vesicoureteral reflux VUR is an abnormal movement of urine from the bladder into the ureters and kidneys, and is a significant health problem. Younger children are more prone to VUR because of the relative shortness of the submucosal ureters. This susceptibility decreases with age as the length of the ureters increases.
Current methods of imaging reflux rely on X-ray fluoroscopy of the kidneys after injection of radioactive contrast agent into the bladder via Foley catheter. X-ray exposure and use of invasive catheters are preferably avoided in children. An alternative procedure has been proposed in which the reflux of urine can be detected by warming the bladder with microwave radiation to a fever temperature and measuring kidney temperatures with a microwave radiometer to detect temperature rise in the kidney after reflux [ 4 ].
In addition to monitoring the kidneys for reflux, a microwave radiometer could also be used to passively monitor bladder temperature during microwave heating.
This study investigates the construction of a miniature radiometer to read volume-averaged temperature of superficial tissue. Microwave radiometers can be assembled in different ways. The two most common variants are total power radiometer and Dicke radiometer [ 5 ].
A total power radiometer consists of a medium gain low noise amplifier LNA , followed by a booster amplifier and a power meter or a square law detector as well as an integrator. This radiometer is very sensitive to amplifier drift.
A Dicke radiometer uses a switch in front of the LNA to select between the sensing antenna and a known noise reference, as shown in Fig. When switching faster than the gain variations, these unwanted drift effects are mostly canceled out [ 6 ]. The goal of the present design is to determine whether it is possible to create an inexpensive and small sized front stage of a radiometer using available commercial surface mount device SMD components while meeting the design requirements of useful medical radiometry.
It has previously been shown that active antennas with LNA before the Dicke switch improve the overall thermal sensitivity of the radiometer [ 7 ]. Presently, we investigate a generalization of this concept by implementing LNAs before the Dicke switch for both the antenna input and the reference signal. In this way we get a more balanced Dicke radiometer with further improvement in accuracy of observed brightness temperature.
A microwave radiometer is an instrument that can measure temperatures inside the human body. The measuring principle is to quantify the thermally emitted power over a given frequency band.
Relating this principle to practical multi-stage radiometric systems, the theoretical noise temperature T e of a cascaded system is given by:. From Eq. This inherent property is often utilized in measurement systems through implementation of low-noise preamplifiers close to the sensor element. The theoretical sensitivity of an ideal total-power radiometer with no gain fluctuations is [ 6 ]:. For a Dicke radiometer Fig. A conventional digital Dicke radiometer front-end consists of an antenna, a noise reference, a Dicke switch, a low noise amplifier, a bandpass filter BP , and a booster amplifier.
Alternatively, a power meter can be connected to the front-end. With the Dicke switch attached permanently to the antenna, the radiometer can be reconfigured to a total power radiometer. A stable amplifier cascade configuration is important in order to avoid internal oscillations that might interfere with the extremely low radiated power received by the antenna.
The stability of a system can be evaluated using the Edward-Sinsky stability factor defined as [ 8 ]:. While the commonly used standard deviation provides a measure of overall signal variations, it does not distinguish random noise from other typical signal drift types.
Land et al. The Allan deviation seeks to quantify temporal measurement variations within a time series and is defined as [ 11 ]:. Hence, different types of noise can be distinguised by the slope of the plot in various time regions.
To detect this low power, it is crucial to have a sensitive instrument like a radiometer. From previous work [ 7 ], we have found that the frequency band around 3. This frequency range provides smaller penetration depth in the human body compared to lower frequencies.
However, the band has been shown as an adequate choice for detecting superficial breast cancer [ 12 ]. One challenge in the design phase is to find an LNA balancing the trade offs between lowest possible noise, highest possible gain, low power consumption and low cost.
The configuration gave an appropriate MHz bandwidth. The steep slopes of the frequency cutoff at the edges of the pass band filter were enhanced by the use of two pairs of consecutive filters of the same type. Another design challenge was to identify a Dicke switch with lowest possible insertion loss, high isolation, 3. The switch requires a DC block capacitor of 47 pF before and after.
Presently a capacitor from Murata Manufacturing Co. A single LNA does not provide enough gain to get the power into the required range for any power detector.
Many factors may degrade the RF stage in radiometers. This includes oscillations in amplifiers stability , system noise, temperature- and gain drift as well as electromagnetic interference EMI. An optimum radiometer front-end provides high sensitivity, low noise temperature, is unconditionally stable, has low gain drift, and yields marginal self-heating of surface mounted components.
In a testing phase, a Dicke front-end radiometer should also be possible to run as a total power radiometer for comparison with other solutions. A complete radiometer can be designed in several ways. We want to realize the Dicke concept using a powermeter and a PC with LabView as a post-detector driver of the Dicke switch. This basic design was initially simulated with two options. Both radiometers have been characterized using the following performance indices: S -parameters, system noise figures [ Eq.
Numerical simulations were performed by importing individual touchstone files for every single block in the design, and generating the overall S -parameters of the circuit in the frequency range from 1 to 6 GHz. T e for each design was derived from the theoretical noise temperature of a cascaded system given by Eq. The theoretical noise temperatures using Eq. Performance indices of radiometric designs. The measurements were undertaken with the front-end blocks as shown in Fig.
The system performance was monitored over time by logging power P m together with other system parameters. The results are listed in Table 2. The input reflection coefficient S 11 and output reflection coefficient S 22 are shown in Fig. Solid line: Simulated values. Dotted line: Measured values.
Sensitivity of the radiometer design was found by calibrating the radiometers with two different and known temperatures, T h , T c , in a water bath. Numerical values are given in Table 2. The Dicke concept was also tested for gain variations caused by temperature changes of the chassis using a Peltier element for heating and by changing the supply voltage see Fig.
Finally, the noise statistics were analyzed for the radiometer using the Allan deviation defined in Eq. From visual inspection we observe in Fig. The corresponding Allan deviation estimate is shown in Fig. Signal noise and drift variations analyzed by the Allan deviation. The radiometers were further tested with a fully automated system for interspersed heating and radiometric reading on a layered solid phantom see Fig.
The stacked phantom consisted of a 5 mm fat layer on top, a 28 mm muscle phantom in the middle and a 5 mm fatlayer at the bottom. The input power to the top antenna was approximately 20 W at MHz. Radiometric readings see Fig. The temperature within the muscle phantom was also monitored with Luxtron LumaSense Technologies fiberoptic probes.
The heating protocol was heating for 20 s with 20 W and reading for 10 s with heating turned off. After six heating cycles, the spatial temperature distribution inside the phantom was generated with an infrared camera see Fig. Radiometric reading with design 1 during protocol with interspersed 20 s heating with MHz DCC antenna at 20 Wand 10 s reading with power off.
Robert Henry Dicke — A radio receiver designed to measure weak signals in the presence of noise; also known as a Dicke receiver. The input to the receiver is rapidly switched by a Dicke switch between the antenna and a reference noise source. It is useful where accurate measurements of absolute flux are required, and has been used to measure the very weak signal from the cosmic microwave background. It is named after R.
DESIGN OF MEDICAL RADIOMETER FRONT-END FOR IMPROVED PERFORMANCE
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