Design of Amplitude and Phase Calibration Test System for Multi-channel Receivers

In order to obtain good performance and detect targets in a strong clutter environment, modern radar systems usually down-convert the received RF echo signal to an intermediate frequency, and then decompose it into I and Q signals through an orthogonal demodulator. However, due to the asymmetry of the circuit, the imperfection of the components selected for each branch, and the changes in the radar operating frequency and ambient temperature, the amplitude and phase characteristics of the I/Q branches of each channel are unbalanced and the amplitude and phase between channels are inconsistent, which causes false alarms in the system or increases the measurement error of the system. Therefore, the amplitude and phase consistency index of the I component and Q component of each channel and the amplitude and phase consistency index between channels are one of the main factors affecting the performance of the receiving system.

2. Calibration principle of amplitude and phase calibration test system

The radar receiving unit adopts a 5-channel working system, and the working frequency range covers 0.05GHz~20GHz. In order to solve the problem of amplitude and phase consistency under broadband receiving conditions, the receiving unit adopts the method of inserting a phase equalization network and an amplitude adjustment network in the channel to perform amplitude and phase compensation to achieve the amplitude and phase consistency performance between the channels. Figure 1 is a simple processing flow from the RF signal to the intermediate frequency signal inside the receiving unit. Each branch of the 5-way linear channel is composed of a receiving front end, a filter and a switch circuit, a channel intermediate frequency processing circuit, an AGC control circuit, an orthogonal phase detection circuit, a phase equalization network and an amplitude adjustment network. When designing the receiving channel, the reserved amount of the intermediate frequency receiver gain is increased, and a programmable attenuator is used for pre-attenuation. In actual operation, it is used as an adjustment network to realize the amplitude adjustment function.

3. Composition of amplitude and phase calibration test system

3.1 Hardware composition of amplitude and phase calibration test system

The amplitude and phase calibration test system is centered on the measurement and control computer, including the RF signal source, digital oscilloscope, programmable multi-way switch and GPIB interface card for connecting intelligent instruments. The system block diagram is shown in Figure 2. The measurement and control computer uses Advantech IPC610 industrial computer, and the RF signal source is HP83732B, which can provide RF signal output in the frequency range of 10MHz~20GHz. The digital oscilloscope uses Agilent 54845, which has 4 channels, 1GS sampling, 500M bandwidth, and supports phase comparison function. The NI GPIB-USB-A interface card is used in the system to realize the conversion from USB to GPIB bus. The card can be directly inserted into the USB interface of the computer, and the oscilloscope, signal source and other instruments can be easily connected to the computer via the GPIB bus.

3.2 Composition of Amplitude and Phase Calibration Software

The amplitude and phase calibration software adopts a modular design and is mainly divided into four functional modules: instrument configuration, test control, data analysis, and data communication, as shown in Figure 3.

3.3 Workflow of Amplitude and Phase Calibration Test Software

(1) Program initialization

Includes network initialization, instrument configuration and instrument response testing to confirm that the test system is working properly.

(2) Receiving unit status setting

Before the radar works, an amplitude and phase consistency test is required. At this time, the measurement and control computer, as the main control device, sends control commands to the receiving unit to make necessary control settings for the extension's frequency, channel, status, and RF channel attenuation, so that it is in the working state required for the test.

(3) Channel amplitude and phase parameter measurement and recording

At the same time, the measurement and control computer controls the signal source and outputs the RF calibration signal, which is injected into the microwave receiving front end of the five channels of the receiver through the power divider. After a series of frequency conversion amplification and processing such as the RF channel, intermediate frequency circuit, and signal processing, the I/Q video output after orthogonal demodulation of each channel is obtained and sent to the digital oscilloscope.

The measurement and control computer uses operating frequency, gain, bandwidth and other parameters as variables to test the amplitude and phase output of the phase detection signal of each receiving branch of the receiving unit under different parameters, and summarizes the measurement results. During the test, one branch of the receiver can be arbitrarily selected as a benchmark to test the amplitude and phase error relationship of other branches to this branch, and a measurement result record table of a database of frequency, gain, bandwidth, channel number, amplitude measurement value and phase measurement value is established.

(4) Amplitude and phase correction data analysis and calculation

According to the compensation algorithm of amplitude and phase, the corresponding amplitude and phase correction factors can be calculated and stored in the compensation result table of the database. The compensation result table can be imported into the controller of the receiving unit so that when it works normally, it can retrieve data from the compensation result table to compensate the receiving channel.

4. Conclusion

An amplitude and phase consistency calibration system composed of intelligent instruments is used in a radar simulation system to automatically test the broadband receiving unit, solving the problems of many test parameters and large test volume, and realizing amplitude and phase consistency test and compensation in the frequency range of 0GHz~20GHz, so that the amplitude consistency of the system channel I/Q branch reaches ±0.3dB, and the phase consistency reaches ±2°; the amplitude inconsistency error between channels is less than 0.5dB, and the phase inconsistency error is less than 10°, which meets the system's amplitude and phase consistency requirements for multi-channel receivers.