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High-frequency components

Figure 1: Assembly with high-frequency components.

Figure 1: Assembly with high-frequency components.

High-frequency components

Transmitter

The transmission frequency is generated in a separate plug-in module. The transmission frequency is generated in a free oscillating voltage controlled oscillator (VCO). Depending on the type selected, this has an output power of between 5 and 9 dBm, which corresponds to approximately 3 to 8 mW.

Caution! Do not operate this component without a connected load at the output. Otherwise, it is destroyed immediately after applying the operating voltage!

Depending on the type, the operating voltage of the VCO is between 6 and 10 volts, which is why voltage stabilization with a 7806 or 7810 controller is recommended.

The output power of the VCO is divided in two ways in a -3dB power divider. One way is amplified as a transmission power output. The amplifier must be selected so that it has both the required frequency response and sufficient gain. However, it should not be overdriven: this means that the output power must not exceed a certain value.

Here in the example shown with the type, this would be a maximum of 24 dBm at a gain of 15.3 dB, which cannot be achieved by the input power. However, this output power was still much too high for a close-range application and had to be drastically attenuated in order not to drive the receiver into its limits by crosstalk between the antennas. This output power in the 20 dBm range corresponds to 100 mW and is sufficient for a range of up to 50 km[1] in maritime navigation radars with separate transmit/receive antennas!

A second output is attenuated and forwarded to the receiver. This power component is needed there to mix the echo signal down into the baseband.

This module also contains a small circuit board with a programmable signal generator based on the AD9833. This board can be programmed with a serial data connection and generates either a sinusoidal voltage, a triangle voltage or a square wave voltage in the range from 1 Hz to 20 MHz. This signal can be applied directly to the VCO as a tuning voltage. This extends the transmitter to an FMCW radar.

Figure 2: AD8347 - Evaluation board with the recommended circuit of the manufacturer.

Figure 2: AD8347 - Evaluation board with the recommended circuit of the manufacturer.

Receiver

The receiver is an integrated component with the I&Q demodulator (AD8347). The power of the oscillator voltage should be between -8 and 0 dBm (therefore the attenuator in the transmitter). An analog/digital converter from the same manufacturer can be connected directly to the outputs. For test purposes, the cheap USB oscilloscope was simply connected to one of the outputs. However, both bipolar buffer amplifiers and an analog/digital converter with two channels for I&Q signal processing should be used.

Due to the excessively high transmission 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 transmission 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 remove the amplifier used in the transmitter and use it in the receiver as a preamplifier. In this case, only the 5.2 dBm is transmitted directly from the power splitter. This should be sufficient for a maximum range of up to 50 meters.