06/30/2014 at 13:39 •
OK, I know this project page is a bit of a skeleton at this time without much content, but I'll get more content uploaded and documented soon!
You can see a photo of one prototype PCB of the RF stage (without components) I've uploaded already. The processing, amplification and filtering of the IF signal coming out of the mixer is done on a second, separate board not shown - as is the generation of the ramp or sweep signal which modulates the VCO frequency.
07/01/2014 at 08:02 •
I've just ordered some new PCBs for the RF stage, with some very minor artwork revisions from the previous iteration.
Frequency-modulated RF is generated by the Hittite HMC431 VCO at top left, at about 5.8-6.0 GHz, amplified by the HMC476 MMIC gain block at top left, and split in half by the Wilkinson divider at top center. Half the RF output from the divider is amplified again and sent to the transmit antenna connector at top right.
The local oscillator, from the other side of the splitter, is amplified again and applied to the LO port of the Hittite HMC219 mixer at bottom center. The reflected RF from the receive antenna comes in at bottom right, where it is amplified by the Hittite HMC717 6 GHz LNA and the amplified output is applied to the mixer.
A couple of pi attenuators are used, after the VCO, before the mixer LO port and after the mixer's IF output, to "tune" optimal mixer LO level and overall performance, although I haven't fixed certain values for these attenuators yet, space for them is left on the board.
IF filtering, amplification and signal processing, along with generation of the 0-10V modulation signal for the VCO, are done on a separate board.
Prototype IF amp/filter/ADC and ramp generator module. This PCB was sent to the fab house last week. For experimental prototype purposes, one of TI's USB audio codec chips is used to provide ADC / DAC capability from the PC, as a sound-card device.
Here are some earlier prototypes of the RF board.
I've fixed up the RF layout a bit to make it neater and more compact, changed the gain blocks to HMC476s, moved the IF output to a shielded SMA connector instead of one pin on the Molex header, changed the Mini-Circuits MCA/MAC-series mixers to a Hittite part for easier/cheaper availability, used quarter-wave high-impedance chokes on the PCB for the MMIC DC bias, changed most of the resistors and small capacitors to 0402 package for improved SRF / high-frequency performance, and replaced the Mini-Circuits GP2X splitter with a Wilkinson on the board - this means reduced cost, and also means the entire BOM for the board is available at Digi-Key.
Mini-Circuits makes some very fine hardware, but I wanted this design to be easily, cheaply reproducible by anyone throughout the world at minimal cost, using components that can be ordered internationally in small hobby-scale quantities (i.e. just a couple of chips at a time) without high shipping or handling costs. To be honest, despite their fantastic engineering, Mini-Circuits devices are just not available in this way through cheap, small-order "hobby-friendly" distributors without high shipping prices and/or larger minimum order quantities. This is especially true for international customers outside the USA. "I need to order 20x $20 VCOs at a time" instead of being able to just order one at a time is potentially prohibitive for non-commercial hobbyists with constrained cash flow.
However, moving over to certain RF components from Hittite means that these components are available from the major distributors such as Digi-Key at relatively low cost, in one-off or any small quantity, without excessive shipping costs, for anyone in the world. (Well, OK, if you live in North Korea or something I'm not sure that they'll export to you.) The recent announcement of Hittite's acquisition by Analog Devices is also likely to improve wider distribution.
07/01/2014 at 08:30 •
This is the previous revision (the latest assembled revision) of the RF board.
07/02/2014 at 05:20 •
Here are some quick and dirty prototype antennas for the 5.8-6 GHz spectrum. They are simple cylindrical waveguide antennas fabricated from 1.5" copper tube (about 36mm true ID) in a simple cylindrical waveguide design 69mm long. TE11 and TM01 cutoffs are at about 4.9 and 6.4 GHz respectively. Soldering the copper plate onto the end is a fun change from soldering a bunch of 0402 capacitors.
One early prototype uses a bit of semirigid coax soldered straight into the side as the feed probe, but I'm not really happy with this and will probably go for chassis-mount SMAs soldered on there in the final ones. Construction or acquisition of other antenna designs to try may come a little later, with wide-band Vivaldis for example. Tony Long's RADARduino project, which was a significant source of inspiration for my project, uses some beautiful PCB Vivaldis made and sold by Kent Britain WA5VJB.
07/02/2014 at 06:00 •
Based on my current design iteration for the RF board, the bill of materials consists of 1x HMC431LP4E VCO, 1x HMC717LP3E LNA, 1x HMC219AMS8E mixer, 3x HMC476MP86E gain blocks and 3x SMA connectors (Molex 0732511150). At single-unit quantities from Digi-Key, those will cost you about USD $60 in total. I have excluded a few parts, such as the passive capacitors and resistors, but they're quite inexpensive and this is just a very quick budget estimate. Obviously the IF signal processing components are all excluded too, but the RF parts account for the majority of the cost.
When we consider that something like the Mini-Circuits ZX05-43MH-S+ mixer specified in Greg Charvat's 2011 MIT IAP radar project (a major source of inspiration for this project) will cost over $40 alone (and the VCO, each gain block, and the LNA each cost approximately that much again) it is clear that my objective of a substantial cost reduction (compared to Charvat's design for his students' projects as the "reference design") has been achieved.
07/07/2014 at 09:46 •
It's worth reading the actual terms and conditions closely for the Hackday Prize contest.
"The project must be a connected device (i.e., transmits information through the Internet)."
"Is the entry a connected device and is that “connectedness” meaningful to the function?"
So, we're only looking for projects that are Internet-connected systems that are transmitting information over the Internet?
That's a little bit of a shame, and excludes many great projects I've seen around here.
Of course, it's possible to "loophole" just about any device or system you might build and add some Internet connectivity - but it would be kind of pointless, and not meaningful to the function and use context that the system was originally designed for - so while you could pass the "must be a connected device" criteria, you would fail the "connectivity meaningful to the function" criteria. Although the latter is just a judging factor and not a strict criterion.
I would like to see this rule be re-written, personally. However, for me personally, I don't care about the prize really, and will continue to publish this project here anyway.
I also have to laugh a little at the "participants acknowledge that commercial space travel, while currently in development, may not be available to consumers by November 14, 2017. Redemption of this prize is subject to availability" clause in the rules.
YOU WILL WIN A TRIP TO SPAAAACEEEEE!!!! (if you can actually get access to any commercial "space tourism" within the next few years, which you presently can't and probably won't be able to within the next few years, and if it is available within the prize budget, and if you're medically qualified for a spaceflight.) It would be more sensible to just advertise it as a cash prize IMO.
07/07/2014 at 13:24 •
I've just been updating the git repository with the source files, schematic, and Gerber files for the current version of the RF board. So you can go and check it out! Feel free to let me know what you think... any ideas or comments. :)
And just to flesh out this post a bit, here's a picture.
07/09/2014 at 17:34 •
Here we see the completed pair of prototype antennas, RF board, and IF amp / sweep generator board (not assembled yet)
I will have to do some serious fixing to the RF board before powering it up though... if I turn it on at the moment it will be driving about 6 dBm into the TX antenna drive amp and the mixer LO drive amp, and that'll kill them... I forgot to put the attenuators in.
07/10/2014 at 19:59 •
Here's another little board I quickly spun up and sent off to the fab tonight. It's just a really simple breakout board for the HMC431 VCO, which can just be used as a simple connectorised "building block" for rapid prototyping and quick, agile, flexible prototypes and experiments with further RF concepts I might play with in future.
I might also build similar building blocks such as MMIC amplifiers, mixer, LNA etc later. The key differentiating factor compared to existing connectorised modules and evaluation boards that you can get commercially from Hittite and Mini-Circuits etc is cost - they can be surprisingly high in price for what they are. For example the little Hittite HMC431 evaluation board costs 400 freaking dollars at Digi-Key - for a board essentially identical to the one shown below, with a $20 VCO chip and a pair of SMA connectors. And I think that huge difference in price is something valuable to address - but there is also merit in having a connectorised sub-board for each individual device, allowing re-usable prototyping of different designs, the agility to reconfigure new experiments, etc.
I'll just be using cheap, standard FR-4 instead of specialized RF substrates, but the RF loss in the dielectric isn't likely to be a huge dealbreaker.
07/11/2014 at 20:29 •
Here's a quick update for today - the ramp generator / IF amplifier / filter / ADC board. This ramp generator design is just something whipped up really quickly that is essentially the same as Greg Charvat's MIT IAP 2011 design, using an Exar XR2206 to generate the triangle wave.
However, in a future iteration I'm planning to move to a different design - probably synthesis of the modulation waveform on a small microcontroller. The XR2206 is a very old chip, obsolete now, and it's hard to get. I've still got a few in stock, but it is not really a chip to use in a new design today in terms of openness, manufacturability, low cost, reproducibility and accessibility for other people around the world to be able to easily replicate the design.
I've modified the DC bias circuit on the output of the XR2206 to bias the output at about 5V DC, compared to +2.5V on Charvat's original schematic - this is because of the different VCO I'm using. The HMC431 wants a modulation voltage of 0-10V (across its full range) compared to 0.5-5.0V for the ZX95-2536C-S+. I will be running the VCO across about 60% of the full bandwidth at most using the current modulation circuit - not quite all the bandwidth is accessible before we start to get distortion of the ramp - which we do not want. But this possible bandwidth is plenty for our purposes.
The yellow and red wires you can see tacked onto the board there are just a temporary +12V and +5V power supply, because I'm still missing a couple of power supply regulators needed to run the whole circuit off the USB port (USB connector visible at left.) This board also includes the IF amplification and processing components, but these haven't been tested yet. The IF input comes in from the RF mixer on the other board, into the SMA connector visible at right.