The system design of communication uses the advantage of the differentiating signal
One of the main challenges of system design of communication is how to succeed in catching the high fidelity signal. In order to avoid strong jamming effect, signal distortion and decrease of sensitivity, the cellular communication system must meet the canonial strict requirement of the honeycomb, for example have high dynamic range, linear scale and low-noise code division multiple access (CDMA) of high input And broadband CDMA (W-CDMA) .
In the past, some practicality questions often cause the characteristic advantage sheet end signal chain of the complete differentiating signal chain to conceal, but construct the constant development of the module with the circuit engineering and radio frequency of the high-performance differentiating of integrated radio frequency, the differentiating framework can already be applied to the high-performance receiver design nowadays. This text will discuss differentiating signal chain characteristic and advantage in 3G and 4G wireless application.
Receiver signal chain
Fig. 1 is the topological structure of the traditional superheterodyne receiver, it well describes the advantage of the relative single-end signal chain of signal chain of the differentiating. No matter what is adopted topological, our goal is to succeed in sending the design signal to ADC end and transforming figures. The signal path is made up by the module of radio frequency of following several: Aerial, electric-wave filter, low-noise amplifier (LNA) , mixer, ADC driving amplifier and ADC.
Fig. 1: The receiver is being developed constantly, the increasingly many receiver will use the differentiating component. The trend begin with ADC, and chain upstream to move to signal gradually. The advanced circuit engineering and radio frequency of the differentiating of integrated radio frequency construct the expansion of the module to allow the differentiating framework to be applied to the high-performance receiver and designed.
LNA is the first module after the aerial, are used for amplifying the signal above the thermal noise. The noise in this grade of circuit is very important, because it determines system sensitivity, amplifying can guarantee the subsequent mixer summing amplifier will not increase the prominent noise. Along signal path being a band-pass filter later on, are used for inhibiting the out-of-band signal, reduce the noise caused by other circuit level and distorted.
Follow after the LNA, the intersection of mixer and signal that frequency conversion interested in, high-frequency radio frequency signal from down-conversion to the intersection of frequency and lower, easier intermediate frequency signal (IF) of management . ADC driving amplifier and resisting and mixing the electric-wave filter (AAF) Carry on the preconditioning to the digitized signal. The driver offers the gain to noise temperature ratio, AAF inhibits the first signal that endured outside the Kui ’s, including sending to noise and out-of-band stray weight of ADC. Imitating the end of signal path, is finished the digital conversion of the information of base band by ADC.
Under the ideal situation, there are only signals of interest (the left-handed blue figure of Fig. 1) Will be conveyed to the digital domain. Need to deal with this target signal that may be very little with a robust system, may inhibit the greater undesired signal at the same time. The design of the robust system, need having highly sensitive and inputting the linearity, selectivity and resisting the noise performancely. According to concrete application and framework, the performance index may be changed to some extent, but in most communication systems, key element that will usually consider such as distorted, background noise and dynamic range. Input the Third order and intercept the point (IP3) And 1dB point of compression (P1dB) It must be high. Other factors that need to consider also include low cost, low power consumption and small dimension.
Differentiating advantage
Basic difference between comparative single-end signal and differentiating signal of Fig. 2. Having used a common gain module here, but the identical concept can be applied to mixer and other devices in the signal chain. In comparative single-end and differentiating signal, should estimate the standard of the system level characteristic * to keep firmly in mind on the heart, in order to realize the good overall receiver is designed.
Fig. 2: The differentiating signal is inherent can resist the noise and interfere with, offer the negative function of the even harmonic to offset the advantage at the same time.
According to defining, do not one kind weigh signals, and fix the difference between the points of reference to weigh through the signal of interest in single-end signal. This point of reference is usually, used as the return path of the signal. If there are error sources that are introduced into the signal path, will produce the question. Because influence that consult not to be been on injectinged into error,so forward convey passing signal error. If does not use extremely complicated offsetting technology, any incoming signal is very difficult to change to dispel in single-end is disposed. So, the single-end signal is very easy the noise and influence that the electromagnetic coupling interferes with.
On the other hand, the differentiating signal is made up of paired equilibrium signal, these signals are with the centre of the point of reference, the range is the same, the phase place is opposite. The difference with negative balance signal is correspondent to compounding the differentiating signal. If the error is introduced into the systematic route of the differentiating, it will increase on two balanced signals at the same time with the identical range. Because the return path is not a stationary point of reference, the error will be offsetted in the differentiating signal. So the signal chain of the differentiating is difficult to influence by noise and interference. Such inherent error can also offer better common-mode rejection ratio (CMRR) to offset the function Compared with the situation that the power is inhibited (PSSR) .
The signal chain of the differentiating has advantage that a single-end signal chain does not possess, namely can reach twice of the single-end excursion to compound signal swing under the same mains voltage, thus has increased the signal to noise ratio. In other words, have increased the amplifier excess under the same mains voltage, has reduced the distortion; Or can offer identical signal swing with lower mains voltage, thus reduce the power consumption.
Fig. 2 has revealed the inherent even harmonic is offsetted in the difference subsystem. Nonlinear device, such as single-end and difference amplifier in the copies of example, can expand transfer function, come, describe with given the intersection of sine and power series at the input signal. In the single-end scheme, weight of each frequency multiplication outputted has a constant, including even-order and strange second-class frequency. In the module of differentiating, even-order is non-linear and is offsetted in compounding output response. Though the real device can not realize perfect offsetting the function, they can really be benefited because of reducing in even harmonic.
Fig. 3 has shown the harmonic distortion situations of 8 to 16 ADC and ultralow distorted, low-noise difference amplifier optimized to driving high speed. Have revealed the quadratic sum third harmonic when being topological that ADC device is configured in the picture to the single-end and differentiating. Though the intersection of single-end and mode distorted and very low, HD2 at the 100MHz 82dBc in value, it adopts even-order of differentiating when the operation to be characteristic kind, lower than 100dBc in the same frequency point HD2 value. So under the same power rail condition, P1dB and IP3 of the whole signal chain adopting the differentiating to be topological are expected to improve by about 6dB.
Fig. 3: Though the distorted characteristic in the single-end mode is very low, that the differentiating is operated really there are obvious advantages for even-order characteristic. Under the same power rail condition, output 1 dB point of compression and IP3 with topological differentiating are expected to improve by about 6 dB.
Signal chain of the differentiating
With the development of receiver, the differentiating components and parts have got increasingly extensive use, they can offer more high characteristic grade. The gradual progress begin with ADC at first, and chain upstream to develop to signal gradually.
In the past, it caused people to only choose single-end or some differentiating signal chains that the application problem of the signal and radio frequency of the definite differentiating constructed the module. An example of some differentiating signal chains is to save the differentiating ADC driver, drive ADC instead of the single-end device summing amplifier. Though this is a kind of simple solution, topological that the constant pursuit to the characteristic requires more upstream circuit to adopt the differentiating. Besides consuming more power consumption, it is distorted that the single-end driving amplifier usually has worse even-order, CMR and PSR.
,The commonly used framework of the receiver is input by the radio frequency of the single-end and the differentiating is exported. The separatrix between single-end and differentiating operation seems to be in the mixer, the radio frequency component such as LNA is still a single-end component. Most SAW electric-wave filters and mixer kernels are the inherent difference channel, but according to employing the purpose to be converted to single-end way.
For many years, it is used in the apparatus of the honeycomb extensively because of its high linear scale that the double balanced mixer is topological. Unfortunately, are used for in coupling the signal to frequency mixing the nuclear traditional voltage transformer network has taken up the area of the significant circuit boarded, give and design increasing very large cost. Newer radio frequency component, such as mixer series ADL535x, integrated balun and voltage transformer, offer single-end of bringing radio frequency input and differentiating whom intermediate frequency output simple and easy to spend the module of radio frequency.
Fig. 4 indicates to be all differentiating structure within all three mixer ports. For convenience act on, radio frequency and local oscillation port use the voltage transformer to connect to the outside, so allow the interface of single-end. By contrast, intermediate frequency output port includes a driving amplifier with 200 output impedence, and adopt differentiating way in order to connect with differentiating SAW electric-wave filter. Integrating the operating frequency of limiting the mixer of local oscillation and radio frequency balun, so demand to use the device series of the special work on the frequency domain of honeycomb.
Fig. 4: The technical latest development of the integrated radio-frequency circuit allows the designer to use and have the radio frequency of single-end to input the mixer exported to the intermediate frequency of the differentiating conveniently. All three internal mixer ports can give full play to the differentiating advantage, link with external world more conveniently at the same time. (Author: The intersection of radio frequency and the intersection of department and radio frequency, Company of Carlos Calvo ADI, employ engineers)
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