10GHz on the Cheap

SoDa Radio was originally written to provide a VHF/UHF exciter/IF strip for a 10GHz system based on the Down East Microwave 10GHz transverter, a DEMI low noise amplifier, and a surplus power amplifier. I'd spent several years building that system, and had amassed a number of pieces of test equipment, parts, tools, and widgets. Even then, building the system took a long time, lots of trial and error, and a fair amount of help from a fellow ham. The SDR portion of the system was built upon an Ettus Radio N200 with a WBX front end module. This system is pictured on the SoDa Radio web page.

Soon after SoDa Radio was released to sourceforge, I got an Ettus Radio B210 and ported SoDa Radio to it. The B210 is quite impressive: with SoDa Radio it tunes from 50MHz to 6GHz, covering all the amateur VHF/UHF and microwave bands below 10GHz.

But I really like operating 10GHz. And it occurred to me that one could build an entire 10GHz transceiver without requiring any special tools or test equipment: just a few parts that are easily available on ebay. And this is what it looks like:

Front View

Rear View

The block diagram couldn't be much simpler.

Block Diagram

Does it work? How?

Operational experience so far is limited to the antenna/radio test range in the basement of kb1vc labs -- that is, over a range of 5 feet or so. But the initial results look promising, and I'll be testing this rig out during the second weekend of the 10GHz contest.

The key feature of the B210 is that it has two transmit ports that can be independently driven. The two ports share the same front-end synthesizer, but each transmit chain has its own second IF implemented with independent digital oscillators in the B210's FPGA. (In the receive chain, the first IF is done with an analog oscillator/mixer, the second IF is implemented after the analog-to-digital conversion. In the transmit chain, the output stream is upconverted in the FPGA and then passes through the transmit oscillator/mixer stage after the digital-to-analog converter.)

So to implement a transverter, we simply turn on the second transmitter port on TX/RX port B to furnish an upconverting/downconverting LO at 5.180 GHz. An incoming signal at 10.3681 GHz (the calling frequency) will beat against the LO and produce a product at 5.1881 GHz. This is 8MHz away from the LO, far enough that many of the reference spurs from the LO oscillator are below the noise. (Though some appear, including an annoying one right on the calling frequency. But I've never actually made a contact "right on the calling frequency" as the 10GHz crowd tends to be a little mellow in that regard.) SoDa Radio then tunes the receive chain on TX/RX port A to 5.1881 GHz and up pops our incoming signal on the waterfall and through the speaker.

To transmit, SoDa Radio transmits the outbound signal on 5.1881 GHz through TX/RX port A, where it mixes with the 5.180 GHz LO from TX/RX port B and appears at the antenna at 10.3681 GHz. The difference mixer product at 8MHz is lost to heat inside the mixer.

Here's a screenshot of SoDa Radio and a B210 10GHz rig copying the transverter output (without final amplifier) from my DEMI system.

Note the relatively narrow spike on the spectrum plot. This is actually about 2kHz wide. The DEMI transverter is pretty clean, but noise in the LO and other oscillators in the B210 tends to "spread out" an incoming signal. This is phase noise and may be aggravated by the fact that I'm using the B210's internal master oscillator, rather than the much cleaner 10MHz reference oscillator that is driving the N200.

Here's a picture of the N200 display while receiving a CW signal from the B210.

The CW signal is quite broad -- about 7kHz or so -- and it sounds a little raspy. This is another hint that an external oscillator could improve things for the B210 rig.

On the other hand, it works and it is quite simple. The output power is about 2dBm when I turn the transmit power knob all the way up.

Quite a capable little radio for not a whole lot of investment.

Build One Yourself

The mixer is a Watkins Johnson M51C coaxial mixer (the grey block in the bottom center of the rear view shown above). This mixer appears from time to time on ebay for less than $100. I bought mine years ago at a swap meet and it is extremely unlikely that I spent more than $20 for it. Other mixers would work fine here. The key is that the LO and IF ports should have a range that covers 5.0 to 5.2 GHz and the RF port needs to cover 10GHz. The M51 specs are RF/LO=2-24GHz and IF=1-12GHz, more than sufficient for my needs. There's a handy table of mixer ranges and model numbers here. Many of these types appear on ebay from time to time.

The bandpass filter was salvaged from an old system. It isn't entirely necessary, as the spurious emissions from the mixer are -- for the most part -- still within the 10.0 to 10.5 GHz amateur band. Those that aren't are well below the limits set in part 15 for intentional radiators outside of the band. (The LO power is about 10dBm. The output of the mixer is less than 3dBm for the in-band part of the signal. Spurs from the B210 are well below -30dBm, and after mixer losses and other things that work to our advantage, it is unlikely that anything noxious will appear above -40dBm at the horn, even without the bandpass filter.)

The horn and waveguide transition were swap meet acquisitions from the days before ebay. These too are easily available. If you have a rudimentary set of garage tools and a few band-aids around, you could even build your own horn from the directions found in W1GHZ's Antenna Book. Paul also shows how to build a coax-to-waveguide transition here (with an erratum here).

The Code

All this is made possible by recent improvements to the SoDa Radio application. Version 3.0, which can be found here, contains a few bug fixes, but most importantly includes the new B210 transverter mode. To use this new mode, start SoDaRadio as you normally would. If you do not already have a SoDa.soda_cfg file in ~/.SoDaRadio then the default configuration will include a new band option under the "Select Band" menu called "10GHz-B210". Select this, and you are off and running.

If, on the other hand, you have been using SoDa Radio for a while, then you may need to create the new band option and add it to your configuration. First, select "Configure->Configure Band" from the menubar, and fill in the dialog box as shown here.

Then hit "OK" and select the "10GHz-B210" from the "Select Band" menu.

I've found that setting the RX gain at max ensures that you'll see lots of birdies. During the bench tests, I backed off on the gain far enough that the birdies disappeared. We'll see how that proves out in real contacts.

Let me know how it all works for you. I'll be using my new rig during the September weekend of the ARRL 10GHz and Up Cumulative Contest.