Basic Test of Transmitter and Receiver Based on RF / Wireless System

Transmitter test basis

The transmitter of digital communication system consists of the following parts:

* codec (code / decoder)

* symbol coding

* baseband filter (FIR)

* IQ modulation

* up converter

* power amplifier

Codec uses digital signal processing (DSP) to encode sound signals for data compression. It also performs other functions, including convolutional coding and interleaving coding. Convolutional coding copies each input bit, uses these redundant bits for error checking and increases the coding gain. Interleaving coding can make the code bit error distribution more uniform, so as to improve the efficiency of error verification.

Symbol coding converts data and information into I / Q signals, and defines symbols as a specific modulation format. Baseband filter and modulation shaping filter improve the efficiency of bandwidth by trimming the steep edge of I / Q modulation signal.

The IQ modulator makes the I / Q signals orthogonal to each other (in the sense of integration), so they will not interfere with each other. The output of IQ modulator is a combination of IQ signals, which is a single IF signal. The IF signal is converted into RF signal by up converter, and then transmitted after amplification.

    Advanced digital signal processing and special application chip technology improve the integration of digital system. Now a single chip integrates most of the functions from ADC conversion to if modulation output. Therefore, the number of RF test points at module level and chip level will be greatly reduced, and the test and fault analysis at transmitter system level and antenna end will become more important.

Main test contents of transmitter

In channel test

* in channel test uses time division multiplexing or code division multiplexing to test wireless digital circuits. Multiplexing refers to frequency or spatial multiplexing, etc. In time division multiple access (TDMA) technology, a channel can be defined as a specific frequency band and time slot in a series of repeated frames, while in code division multiple access (CDMA) technology, a channel is defined as a specific code segment and frequency band. The terms in channel and out of channel refer to the frequency band we are interested in (frequency channel), rather than the time slot or code segment of the channel within the frequency bandwidth.

* the transmitter channel bandwidth is the first test, which determines the spectrum characteristics of the transmitter transmitted signal. Many design errors can be found through the shape and characteristics of the spectrum, and the error rate of the system symbol rate can be roughly calculated.

* carrier frequency test is used to test frequency errors that may cause channel interference in adjacent frequency bands or affect receiver carrier recovery. In most modulation methods, the carrier frequency should be in the center of the spectrum. The center frequency can be determined by calculating 3dB bandwidth.

* channel power test is used to test the average energy of useful signals within the frequency bandwidth. It is usually defined as the average value of useful signal energy in the signal frequency bandwidth. The actual measurement methods will vary with different standards. The wireless system must ensure that each link consumes the least energy. There are two main purposes: one is to reduce the overall interference of the system, and the other is to prolong the service life of the battery of the portable system. Therefore, the output power must be strictly controlled. In CDMA system, in order to achieve the maximum capacity, the total interference tolerance of the system also strictly limits the power of each single mobile unit. Accurate transmit power control is very important for system capacity, coverage and signal quality

* occupied bandwidth is closely related to channel power, which is defined as the spectrum covered by a given percentage of total modulated signal power.

* time test is commonly used for burst signal test in TDMA system. These tests are mainly used to evaluate whether the carrier envelope can meet the expected requirements. They include burst signal width, rise time, fall time, on time, off time, peak power, transmit power, off power and duty cycle. The time test can ensure the minimum interference between adjacent frequency channels and the time slot switching when the signal is turned on or off.

* the test of modulation quality usually involves the accurate demodulation of the transmitted signal and comparison with the ideal mathematically calculated transmitted signal or reference signal. The actual measurement will have different methods with different modulation methods and different standards.

* error vector amplitude (EVM) is the most widely used modulation quality parameter in digital communication system. It samples the output signal at the output of the transmitter to obtain the trajectory of the actual signal. Usually, the output signal is demodulated to obtain a reference signal. Vector error refers to the difference between the ideal reference signal and the actual measured signal at a certain time. It is a complex number including amplitude component and phase component. Generally, EVM will use the maximum symbol amplitude component or the square root of the average symbol power.

* I / Q offsets are caused by the DC bias of the I / Q signal, which may lead to carrier feedback.

* phase and frequency error test is used for equal amplitude modulation. By sampling the output signal of the transmitter and capturing the actual phase trajectory, an ideal reference phase trajectory is obtained after demodulation. The phase error is obtained by comparing the actual signal with the ideal reference signal, and is expressed in the form of RMS and peak. Large phase error indicates that there is a problem with the transmitter baseband or output amplifier, resulting in the decline of signal sensitivity. Frequency error refers to the error of carrier frequency. A stable small frequency error indicates that there may be some problems with the carrier being used. The unstable frequency error may be caused by the instability of the local oscillator, the use of an inappropriate filter, the amplitude modulation phase modulation conversion of the amplifier, or the modulation index of the transmitter analog frequency modulator,   Strategies and test methods.

Out of channel test

* out of channel test refers to the measurement of those frequency bands outside the system frequency.

* out of channel test is to sample the distortion or interference in the system frequency band, rather than test the transmission frequency itself

* adjacent channel power ratio (ACPR) test to ensure that the transmitter is not interfered by adjacent or interval channels. ACPR is the ratio of the average power of adjacent channels to the average power of transmission channels. Measurements are usually made between channels spaced multiple channels (between adjacent channels or spaced channels). When conducting ACPR test, it is very important to consider the statistical characteristics of the transmitted signal, because even for the same transmitter, different signal statistics will lead to different ACPR test results. For different standards, the test usually has different names and definitions.

* clutter signal is caused by different signal combinations in the transmitter. In the system frequency band, the amplitude of this signal must be less than the level specified in the standard, so as to ensure the minimum interference to other communication systems.

* harmonics are signal distortion caused by the nonlinearity of the transmitter, and the frequency of these signals is an integral multiple of the carrier frequency. The test of clutter and harmonic outside the channel is used to ensure the minimum interference of this channel to other communication systems.

Receiver Basic test

The function of the receiver is basically the reverse process of the transmitter, so their test challenges are very similar. The receiver must successfully capture the RF signal under the condition of potential interference. Therefore, there must be a front-end selection filter to filter or weaken the signal outside the system frequency band received by the antenna. The low noise amplifier (LNA) can amplify the amplitude of the target signal, but also ensure to increase the noise amplitude as little as possible. The down converter converts the RF signal into an IF signal with lower frequency by mixing with the LO signal. The output signal of the mixer passes through the IF filter to weaken the useless frequency component generated by the mixer or adjacent channels.

The digital receiver (Fig. 2) can be implemented with an I / Q demodulator or a sampling if. I / Q demodulation is realized by analog hardware, which is common in the design of digital RF receiver. Although this method is very popular, it has a potential problem: the gain on the I / Q path will be inconsistent, and the relative phase deviation is also large (greater than 90 degrees), which will lead to the problem of image suppression. Therefore, I / Q demodulation is mainly used for single channel base stations.  

Main test contents of receiver

* in channel test is used to test the minimum signal amplitude that the receiver can accept under a certain allowable bit error rate, also known as sensitivity. The ability of a receiver to correctly capture a low amplitude input signal is the sensitivity of the receiver.

* bit error rate and frame error rate are the most important performance indicators of digital receivers, just like the signal to noise harmonic ratio (SINAD) in analog receivers. They are also a measure of sensitivity. When one bit data sequence is used for modulation, the acceptable sensitivity refers to the amplitude of the minimum received signal under the condition of specified bit error rate. When measuring this parameter, it is necessary to apply the signal source to the antenna end of the receiver through the cable with known attenuation, and connect the output end of the receiver to the bit error rate detection equipment. During the test, if you do not know the approximate sensitivity, first set the amplitude of the signal to the normal level (such as - 90dbm), and then decrease the amplitude until the bit error rate reaches the specified value. At this time, the power value of the signal minus the loss of the cable is the sensitivity.

* the same channel inhibition test is similar to the sensitivity test. During the test, the distortion level of the received signal is detected after adding an interference signal to the same RF channel. The ability of the receiver to maintain the sensitivity to the required signal and suppress the interference signal is the same channel suppression ability.

* out of channel or blocking test is used to verify whether the receiver can work normally when there is an out of channel signal and the clutter response generated after the receiver is interfered under this condition. Generally, out of channel tests include:

- clutter suppression capability, which is similar to the same channel suppression, but the interference signal is the interference signal of all frequency bands, not limited to the same channel.

- Intermodulation immunity is used to test the distorted signal generated when the input of the receiver contains multiple frequency components.

- adjacent channel suppression capability is used to test the acceptance capability of the receiver when the adjacent channel has strong signal.