HeIMDALL R2 receiver
In this page the description of an RTL-SDR based receiver system can be found. The receivers system is planed to operate with a total of seven RTL-SDRs. One receiver channel is used to obtain the reference signal and the remaining six channels are surveillance channels. The multiple surveillance channels are used for the direction of arrival estimation of the detected targets. The detailed architecture is depicted in the next figure.
The signals coming from the antenna output are first received by the RF Front unit. In this stage the signal is first amplified by the LNAs to grant low overall noise figure performance. In the next stage the RF BPF (Band Pass Filter) filters the out of band signals. As the used IOPs (Illuminator of Opportunity) are FM transmitters for this passive radar demonstrator the filter passes signal from 88 MHz to 108 MHz. The next stage on the reference channel is a power splitter while on the surveillance channel it is a power combiner. The main function of these parts is described in the A-DPIS section bellow.
The signals after these stages are connected directly to the RTL-SDR receivers. A figure from the realized PCB can be seen bellow.
Of course the coherent operation of these dongles has great importance in our case. As suggested by many authors, Juha Vierinen, Piotr Krysik, YO3IIU and recently on https://coherent-receiver.com the synchronization is feasibly by sharing the 28.8 MHz reference clock between the receivers. The CLK distribution unit accomplish this by utilizing a 28.8 MHz TCXO and the CDCLVC1310 clock distribution circuit.It can be seen on the next picture.
After reception with the RTL-SDRs the digitalized samples are transfered to a raspberry PI. The receiver system is accessible through the Ethernet interface of this P,I from which the samples can be downloaded to perform the further signal processing steps. The realized hardware can be seen on the picture bellow. At the current state only two channel have been prepared to perform some initial measurements and to test the A-DPIS.
A-DPIS (Analog - Direct Path Interference Suppression):
In passive radar scenario the receiver system must able to handle both the high power direct path signal and the extremely weak reflected signal. This means that the ADC of the applied receiver should has enough dynamic range to receive both signals without signal to noise ratio degradation. As the RTL-SDR receiver has only 8-bit ADC, some trick must be done to overcome this weakness. A-DPIS is a method which is able compress the dynamic range required for surveillance channel by canceling the high power direct path signal prior to the ADC. To perform this operation the reference channel signal is splitted into two way. One of the signal path is connected directly to the reference channel RTL-SDR, while the other is used for the cancellation. The cancellation is done by adding this signal out of phase to the surveillance channel signal, but to do that, the proper amplitude scaling and phase shifting must be done before. This operation is realized by an IQ modulator in the RF Front unit. A picture from the PCB of this unit is shown in the next picture.
If you are interested in this method in more detail you can read our paper presented in the Radioelektronika conference:
Tamás Pető, Levente Dudás, Rudolf Seller : Analog Direct Path Interference Suppression for FM Based Passive Radars, 28th International Conference on Radioelektronika, Prague, Czech Republic, April 19 – 20, 2018