Vector measurements with the rtl-sdr, 3

12.07.2020 18:48

After doing some scalar reflection measurements using an rtl-sdr and an RF bridge I recently started exploring the possibility of capturing phase information as well using a similar setup. I came up with a relatively simple method that does most of the signal processing in software. I already validated that the method works well enough in simulation. All I need now to try it out in practice is a simple multiplex circuit board. I need that to coherently record two RF signals with rtl-sdr's single channel analog-to-digital converter.

At the time of my last post I already had most of the circuit sketched out on paper and the basic calculations done. I've spent the last couple of days finishing up the design and transferring it from paper into the computer. This is a 3D render of the current draft of the circuit board:

3D render of the time multiplex circuit board.

It's a two-layer design on a 1.6 mm FR-4 substrate. All RF and almost all the other tracks are routed on the top layer, leaving the bottom for a mostly uninterrupted ground plane. I'm not sure yet about the surface finish and solder mask. Signal connections are using edge-mounted SMA connectors.

I had to revise the schematic a few times when I was making the PCB layout. The 50 Ω coplanar waveguides can't overlap or change layers and I wanted to have the RF part of the circuit laid out as cleanly as possible. Fortunately, the input and output ports on the MMIC switches I'm using are interchangeable. This gave me enough flexibility to come up with a combination of ports where such a layout was possible. By a lucky coincidence the exact combination I ended up using also inverted the sense of one switch. This had an added benefit that I could omit an extra quad-NOR gate from the driver circuit.

The rest of the circuit is quite straightforward. I'm using a HEF4017 Johnson counter to drive the switches in the correct sequence. I ended up going with a three-state "off-ref-dut" switching cycle to aid the clock recovery like I mentioned in one of my earlier posts. There's a selector switch that allows the sequence to be driven by an internal oscillator, a manual button or an external clock coming into the fifth SMA connector on the bottom right of the board.

I've added the manual button to aid in testing. It will allow me to lock the RF circuit in a specific configuration and perform measurements on that specific signal path. Two LEDs show the currently selected path.

The internal oscillator is a simple 100 Hz astable multivibrator using a low-voltage variant of the classic 555 timer. Its frequency will be very inaccurate, but that shouldn't be a problem. The software needs to do full clock recovery anyway over several full switching cycles and the clock only needs to be roughly in the vicinity of 100 Hz and stable for around 10 cycles. If it later turns out that I've missed something and do need a better clock source I can still bring it in from an external source.

I probably need to go over the design once more time in case I missed something, but otherwise the PCB layout seems ready to be sent out to a prototyping fab. I getting curious. For the sub-2 GHz frequency range that the rtl-sdr and my current RF bridge can handle I think the hardware should work well enough. For the full 8 GHz rating of the switches I have some more doubts, mainly due to the 10 dB attenuator made from 0603 components and loses in the FR-4 substrate.

Posted by Tomaž | Categories: Analog

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