Ein eigenes Radargerät
Figure 1: Transmitter/Exciter module for generating and modulating the RF frequency.
RF generation
The required high-frequency oscillation is most easily generated by a voltage controlled oscillator (VCO). This can provide a continuous oscillation over the entire permitted frequency band, the frequency of which is dependent on the tuning voltage. Later (as a so-called “gold-plated solution”) this VCO can even be integrated into a phase-locked loop (PLL). This allows a much more precise frequency constancy. But within the start, the simple free oscillation is sufficient.
The generated power of this VCO is divided into two parts by a power divider. One part is used as transmit power and the second part is available to the receiver as a reference for downconverting the echo signals. Therefore this module has two outputs on the front panel.
A buffer amplifier must be connected between the VCO and the load so that the possible fluctuations in the load from the antennas do not affect the frequency constancy. However, this should not be overdriven immediately, so an attenuator reduces the power produced by the VCO. The mechanical arrangement is such that this attenuator can easily be replaced by another one without disassembling the module. The circuit diagram for this board is shown in Figure 2:
Figure 2: Schematic diagram of the transmitter or exciter
modulation
Figure 2: Schematic diagram of the transmitter (or exciter)
(interactive picture)
In the first expansion stages, this module functions directly as a transmitter. If further modules such as a pulse modulator and power amplifiers are used, then this module is called exciter.
Figure 3: Internal circuit of a ring demodulator
Bi-phase modulation
For an extension of the possibilities of the do-it-yourself radar, a bi-phase modulation can be optionally provided. Optional means: for the beginning, this component can be left out. This bi-phase modulation is made with the help of a standard ring mixer and is switched into the feed line to the antenna. The mixer is used for different than its original purpose.
The function is simply explained in the circuit diagram. Due to the symmetrical design, the RF frequency can flow either from L (local-oscillator) to R (receiver input) or vice versa. A DC voltage is applied to the jack I (intermediate frequency). The diodes then work as simple switching diodes. Depending on the polarity of the switching (bias) voltage, either D1 and D3 or D2 and D4 are conductive. Thus, depending on the polarity of the switching voltage, the current flow of the high-frequency oscillation at the output coil is switched in the opposite direction. This means that its phase is turned by 180° depending on the switching voltage.
With this circuit, switch-overs can be used in a cycle of down to 10 ns. This corresponds to a clock frequency of 50 MHz and theoretically allows a distance resolution of about 1.5 m. This is a very high clock frequency for logic circuits and therefore requires a matching of the supply line with a terminating resistor at the mixer input jack.
If this function is not used, however, at least one of these voltages must be permanently applied to mixer input I.