The multiple channel radio frequency receiver measures the new method (1) of the noise coefficient
While the multichannel receiver receives, signal to noise ratio and sensitivity of the joining qualifier signal of the noise, because the signal that the radio frequency receiver receives is comparatively faint, its noise characteristic seems particularly important. On the other hand in the course of imaging of multiple channel, the receive paths of different passways are probably different, even the receive path is the same, the noise characteristic of every passway can not be totally the same either. So the precision measure of the noise coefficients at all levels of each passway of the receiver is very important. The traditional noise coefficient measurement method can’t meet the request of measuring the big noise coefficient. And in the actual multiple channel is measured, need making the passway in receiving state examined, other passway out-of-operations guarantee metric reliability by reducing the noise interference of the interchannel. The ones that considered and received radio frequency signal are faint, the forward end of the radio frequency receiver usually has a pre-amplifier stage realized with low-noise amplifier, this text proceeds from receiver, this kind of characteristic, take MRI radio frequency receiver as examples and put forward a multiple channel to switch over the method to measure noise coefficient, eight selects the radio frequency switch of one to make the test of the noise coefficient switch over in different passways through designing one, and utilize to control the direct current power supply make-and-break of leading low-noise amplifier to make it in amplifying and not amplifying the state. Under enlarging the state, the noise is enlarged in order to offer the hot noise source, offer the cold noise source on the contrary. This kind of amplifier switch measurement method has optimized Y factor and measured the law of noise coefficient, make measurement comparatively accurate and easy.
A principle of measuring the noise coefficient optimally
The noise of the radio frequency receiver mainly includes the thermal noise of the resistance and shot noise of PN junction, belong to the category of the white noise. The white noise excludes any dispersed composition, its level accords with gauss distribution. The power spectrum is the uniform distribution on certain frequency domain. The noise coefficient is attribute linear two-port network or two port converter system noises and a characteristic important parameter. Its standard is defined as: The ratio of signal and noise power ratio to signal and noise power ratio of Ausgang of input end of the receiver. According to Nyquist’s theorem, are in standard noise temperature T0(290 K) Input end produce noises available power to be kT0 B; K is Boltzmann’s constant (1.38* 10-23J/K) ; B is the equivalent bandwidth. The available noise gain of setting up the network is G, available noise gain is equal to the power gain of the available signal to linear network, the available noise power of output produced only by input end is GkT0B, signal and noise power which sets up Input/Output of port are Psi respectively, Pni, Pso, Pno, therefore can get the noise coefficient (F) 2 kinds of definitions each other equivalent:
Because the receiver examined is not the work in the linearity region, the signal source law needs knowing the equivalent noise bandwidth of examining the network, determine accurately the equivalent noise bandwidth is very difficult, so the signal source law tests the error is relatively great, need to adopt the law of noise source while testing actually. The commonly used adoption noise source law measurement noise coefficient method includes: The law of gain to noise temperature ratio, Y coefficient law and noise coefficient appearance law. It is the most direct method to measure noise coefficient to use the noise figure meter. In most cases it is most accurate too. And can measure the noise coefficient within the range of particular frequency, the analysis meter can reveal gain to noise temperature ratio and noise coefficient help measurement at the same time. But as the noise coefficient exceeds 10 dB, the measuring result is very inaccurate. To radio frequency receiver of MRI, the method can metric range of noise coefficient too little, obviously not applicable. And gain to noise temperature ratio law and the intersection of Y and the intersection of coefficient and law utilize spectrum analyzer come measurement, a different one the intersection of gain to noise temperature ratio and law need, know, examine into available gain to noise temperature ratio of components and parts in advance, and is limited by base of noise of the spectrum analyzer. Y coefficient law is a typical method to measure the noise coefficient. While measuring, when the input end of the network is in 2 pieces of different available power that examined (for example: Hot state T and cool T) of the noise generator ,Ausgang can get 2 corresponding the intersection of available power and PNO, PNO, usually to act as Y writing two power these, set up this and carry the oral network (or carry the oral components and parts two times) two times The equivalent noise temperature is Te, the gain to noise temperature ratio is G, and F the noise coefficient which examines the network, can have:
It measures the noise coefficient to need cold noise source and hot noise source in order to realize different noise power input in the input end to utilize Y factor, it is usually through adding the voltage and not adding the voltage to realize to the noise source of the solid condition, namely when the noise generator is exerted the direct-current volts, the noise generator produced the formation heat noise source of noise output, while not exerting the voltage, the surplus noise produced in hot perturbation within the noise generator of the existence forms the cold noise source. Alive method suitable for, measure minor noise coefficient only, as examine into noise coefficient of network when being larger, need to obtain higher Y factor to reduce the error in emasurement, so need higher direct current power supply to obtain the hot noise source, this is different to realize in practice, namely the traditional Y factor measures the method error bigger, so need to optimize the noise source. Because receiver leading low-noise amplifier, noise coefficient of it other magnitude in receiver very light, can measure with the appearance of the noise coefficient directly. The gain to noise temperature ratio of low-noise amplifiers used is 30 dB in the receiver, so can control amplifier make it amplify terms, offer hot noise source to the next grade promptly in work, offer the cold noise source under not enlarging the condition, can receive the greater Y factor in this way, reduce the error while measuring the big noise coefficient. And does not need extra noise source and direct current power supply, has simplified and designed.
2 multiway channels are switched over (RF SWITCH) Realization
The receiver that the experiment uses has 8 passways, measure the noise coefficient to need to measure each passway with the spectrum analyzer alone actually, namely there is only a passway job in eight passways, other 7 passways are in broken circuit state, in the radio frequency receiver, there is not passway input of the received signal that need covering with the resistance of 50 . Design one and 8 passways to select the radio frequency switch in any passway according to the needs of the above-mentioned analysis, and idle other passway output ends present 50 impedance.
This kind of characteristic usable PIN switch is designed. PIN switch utilizes the intrinsic barrier diode with biasing and making the radio frequency semiconductor made of the electric property and accusing of the device. It has fine switching characteristic: Presented and similar to the short-circuit condition the radio frequency signal when the forward direction direct current of the intrinsic barrier diode is biased; Presented and similar to the state of opening a way the radio frequency signal as the reverse bias of intrinsic barrier diode. Characteristic that PIN diode switch is controlled fast, lossed small, the power is great in capacity.
As shown in Fig. 1, put the switch of radio frequency of single throw of a short-hilted broadsword in every No. passway, each switch has a control lines that controls its make-and-break. Can realize eight switch which select any way to turn on through setting up and choosing only No. 1 to the control lines of No. 8.
Measure the frequency domain, insertion loss and isolation of switch of this radio frequency with the network appearance R&S ZVB4 of the vector, the result is shown as in Fig. 2:
The radio frequency switch that this text designs is 63.6 MHz in center frequency, the bandwidth is a characteristic under 120 MHz, Fig. 2 is S21 curve when the switch is turned on. The Fig. 2 one top curve is for S21 when switch end,below curve, for S22 (reflection characteristic to reflect Ausgang) when not ending . Know this switch of insertion losses under the conducting state are only – 0.259 dB from pursuing; The transmission loss near center frequency under isolating the state is – 32.205 dB, and the reflection factor of the Ausgang is – 34.568 dB. Explain this switch and have good feed-through and close at the characteristic in the broadband range taking operating frequency of the receiver as center frequency, and the Ausgang is matched well under the cutoff state. Because the receiver only works in the narrowband range near center frequency, the switch designs the index to fulfil requirements, and the characteristic is more superior than to design the index.
