After reading some of the interesting projects published by Greg, along with the great little MIT short course, I became interested in having a go at my own little FMCW build.
I also came across the "RadarDuino" project by Tony KC6QHP, and this was a substantial source of inspiration. (You can read his thread in this very forum!)
I wanted to dive in and learn more about microwave design and layout, as this is something I have little practical experience with, and I wanted to develop my skills a bit more even if that means making a couple of boards that don't work at first. So I wanted to go with something a little more ambitious than just screwing SMA connectors onto connectorised off-the-shelf building blocks.
Also, I wanted to keep the cost down as much as possible, and use components that were easy for me to source - and also create design materials that were suitable for hobbyists or students to re-use around the world at relatively low cost, using components that were accessible in small volumes. In my experience, here in Australia, it is difficult to access most components from Mini-Circuits for example, in small volumes compatible with low-budget hobbyist experiments.
So, here are some photos of the early work I have come up with so far. It's a work in progress.
Following Tony's lead, ~5.8 GHz was chosen as the frequency of operation, and the Hittite HMC431 VCO and the Hittite HMC717 LNA were chosen. Both these components are stocked at Digi-Key for relatively low-cost easy access in small quantities of just a few units. I haven't been able to find any other frequency where both a suitable LNA and VCO for that frequency are stocked at one of the big component distributors.
I'm going to be using FR-4, sacrificing a little bit of RF loss at microwave frequencies to gain a substantial advantage in the price and availability of PCB manufacturing capacity, compared to the alternative of using something specialized like PTFE or alumina board.
At what sort of frequency does using FR-4 really become untenable? I don't know... this is a bit of an experimental learning exercise for me. But obviously FR-4 is very cheap and every cheap PCB fab in the world stocks the stuff, so there is an obvious advantage there over the more exotic RF substrates if you can accept a bit of loss.
Can anyone tell me, at what frequency does FR-4 really become impractical?
After the VCO, before the splitter, there is a gain block, and there is also another gain block before the transmit antenna. Hittite HMC474s are used, again, because they're cheap, and stocked by DigiKey which is convenient since I'm ordering from them all the time anyway.
100pF coupling capacitors are used to block DC on both the input and output of the gain blocks, as well as at the receive antenna before the LNA.
However, I'm struggling to find capacitors with an appropriately rated series resonant frequency >= 6 GHz. Anybody have any guidance or recommendations on choosing these? I might have to redesign the board to use 0402 caps (at the moment it's all 0603s) and bring down the capacitance a bit to keep the resonant frequency appropriately high.
High-ish frequency 0603 chip inductors (3.3nH, Kemet L0603C3N3SRMST) were used in the DC bias path to the gain blocks, along with an appropriate resistor and some caps. (Are those inductors suitable at this frequency?)
In one of the next prototype board revisions, pictured, I replaced the choke in the DC path with a thin meandered l/4 line in place of the 0603 chip inductor. This is really just an experiment so I can see how it behaves, does it work? Making distributed passive elements on the PCB is kind of cool, so I just wanted to try it out one component at a time.
On this board, all the solder mask is deliberately left off along the RF signal path between the VCO, transmit and receive antennas, and mixer. This makes it easier to show the signal path and explain how a radar works to an audience.
The original layout uses a Mini-Circuits MCA1-85 mixer (although originally labelled as a MAC-60, I wasn't able to get any of those), and a Mini-Circuits GP2X splitter (not yet placed on the board). However, it is difficult for me to get any stock of the GP2X here, so I have tried replacing it with a Wilkinson divider. 100 ohm resistors are much easier and cheaper to source!
Because the idea of being able to specify a BOM that most people anywhere in the world can easily order in one-off quantities from somewhere like Digi-Key is appealing, I'm tempted to consider replacing the Mini-Circuits mixer with something like the Hittite HMC218/9. However they have just announced that the HMC218 is EOL next year, so I'll go with the 219. (Annoyingly the pinout is different, so a board change is required.)
The SOT-23 devices on the board are just a 3.0V LDO that powers the VCO, and a couple of FETs allowing me to completely power-down the VCO when it is desired. This board basically only consists of the RF stage - the filtering and processing of the IF output from the mixer, as well as the generation of the modulation voltage input to the VCO, is done on another external board which isn't finished yet.
Anyway, that's where things are at so far. I haven't quite gotten around to testing anything in a complete state, and it probably won't work on the first board. Or the first few boards. But it's an interesting educational project.
Your feedback and comments and recommendations would be appreciated!