Figure 1: AD8347 - test board with the manufacturer's recommended circuit.
The echo signal from the receiving antenna and the reference frequency for downmixing from the transmitter are the input signals of the receiver circuit. The simplest version is the AD 8347 circuit (I&Q demodulator) from Analog Devices, for which a test board is also available (Fig. 1).
The power of the oscillator voltage should be between −8 and 0 dBm (hence the attenuator at the output in the transmitter). An analog/digital converter of the same manufacturer can be connected directly to the outputs of the demodulator. (Again, the recommended test board with the circuit recommended by the manufacturer is used). The necessary amplification of the demodulated echo signals is already done internally in the demodulator circuit. For test purposes, the cheap USB oscilloscope was simply connected to one of the outputs. The differential output of the test board (the desired signal is connected between two SMA sockets) has to be connected to a ground-referenced level by an external circuit to enable the USB oscilloscope to evaluate the signal. However, both bipolar buffer amplifiers and a two-channel analog-to-digital converter should be used for I&Q signal processing.
Because of the excessive transmit power in this test circuit, the mixer board could be connected directly to the receiving antenna without a low-noise preamplifier. If a much lower transmit power is assumed (in the range of less than 10 dBm), a preamplifier with a gain of up to about 15 dB is recommended. In principle, it would be sufficient to use the amplifier used in the transmitter in the receiver and to transmit only with the 5.2 dBm directly from the power divider. This should be sufficient for a maximum distance of up to 50 m.
In the circuit board shown in figure 1 the soldered bridges (e.g.: J8) are designed for automatic gain control (similar to a radio). This is unfavorable for a radar: these bridges have to be removed and instead bridge J7 for a voltage controlled gain control (lowest SMA socket: “VIN”) has to be connected. The manufacturer's datasheet offers help in this regard.
There is also a separate receiver board under development, which contains a powerful USB 2.0 processor. This board also contains an attempt to provide the control voltage for the amplification digitally. The schematic designed so far has not yet been tested, so it may still contain errors and is only intended for an overview.