Low Cost USB RF Signal Generators – A Hybrid Solution Why They Should Be in Your Next Project
by David Goins, CTO, Windfreak Technologies
Low-cost Universal Serial Bus (USB) radio frequency (RF) signal generators have recently come on the scene in the wireless world. They promise to be game changers for many reasons. Until these devices came out, a prospective buyer had two choices when it came to RF signal generation solutions. The buyer could either purchase a $20K+ benchtop box with all the bells and whistles, or they could purchase a $6K narrowband “synthesizer,” either hard coded to one frequency, or with an RS-232 programming port to figure out. Both of these solutions would typically take eight weeks to deliver. They are generally build-to-order components.
USB RF signal generators are actually a hybrid approach to both of those options mentioned above—they can be much cheaper and deliver in a couple of days. A great USA-made example is Windfreak Technologies’ SynthHD Dual Channel Microwave Signal Generator for $1,299. This price is actually cheaper than some RF test cables.
The device has two independent channels that can tune in 0.1 Hz increments from 54 MHz to 13.6 GHz. The SynthHD’s RF power is adjustable in .01 dB increments from -50 dBm to +22 dBm. It also has many modulation features including FM chirps. The relative phase between the two channels can be adjusted in .01 degree increments.
It is a hybrid approach because the device can either be controlled with a PC graphical user interface (GUI) like benchtop test and measurement (T&M) equipment, or it can be programmed to function as a module inside a communication system without a PC, like the narrowband synthesizer mentioned above.
The GUI is laid out as close as possible to similar benchtop signal generators. After tuning to the spec needed in the project, those settings can then be programmed to the SynthHD signal generator’s nonvolatile memory. The device will power up that way from now on, with sweeps or modulation, or both. Another interesting feature with all of Windfreak Technologies software is that it is written in Labview, and the software source code is included with the purchase. Labview is ubiquitous for controlling test equipment in labs all around the world. This allows the end user to remove the features they don’t need, or add features they do need for ease of use.
The SynthHD signal generator is really the best of both worlds. Its RF power settings can be calibrated for accuracy. It has an external reference input or output for frequency stability, an external trigger for stepping and other functions, and a GUI for quickly adjusting parameters similar to their benchtop counterpart. The GUI, as mentioned, is open source. This means the user is allowed into the source code to look at the calibration routine and how it works. When the user sets a power or frequency, the routine goes to the calibration file and interpolates the proper 16 bit DAC setting, which drives the output power of the SynthHD signal generator. The calibration table is a text file on the PC that is easy to understand for a custom self-calibration, which could easily include other losses in the test setup. To make it even more accurate, there is an onboard temperature reading.
Module integration engineers will get much more tuneability than they would get with the old solution. This allows them to purchase one broadband device for their system instead of many narrowband variations. They don’t have to use external attenuators to perfectly set the RF power. The SynthHD signal generator also has an ultra-small form factor with mounting tabs. Similar Windfreak designs have successfully flown on fighter aircraft—suggesting sufficient ruggedness for most applications. The SynthHD signal generator is not just a continuous wave (CW) generator. It can also sweep, hop, and modulate.
The SynthHD signal generator will generate sweeps. The sweeps can be single channel or dual channel. The device can sweep up or sweep down. It can also ramp up or down amplitude while sweeping. The sweep can be controlled with an external trigger to either perform a full sweep per trigger, or perform a single step. In dual channel mode, you can program a constant frequency offset between the two channels for the sweep.
The SynthHD signal generator will generate hops. The GUI allows the user to program up to 100 arbitrary points with a frequency and amplitude in dBm. This table is stored onboard in nonvolatile memory. Like the sweep mode, the user can then have an offset on the other RF channel, hop up the list, hop down the list, and work with the trigger input.
Additionally, the SynthHD signal generator will digitally modulate FM, AM, and pulse. FM can be a typical sinusoidal signal, or it can also be a chirp. AM can also be the typical sinusoid signal or it can be ramps. The modulations can be combined. Combining a pulse and a chirp allows the user to set up a frequency modulated continuous wave (FMCW) radar signal. You can even combine this with the sweep function. As an electronic warfare example, you could sweep a pulsed waveform across a range of frequencies. Rates can be very slow, or in the case of FM, up to 5 KHz. Of course, all of these features can be saved to the device. It will begin sweeping on power up without a PC connected!
Users don’t have to use the SynthHD signal generator with the supplied software. They can also program their own application with other languages and operating systems. All of Windfreak Technologies’ products set up a virtual serial port as a Communications Device Class/Abstract Control Model (CDC/ACM) device which is an older, well-known driver technology. The drivers are included in most distributions of Linux as well as in Windows 10. For older versions of Windows, the drivers are included, but Windfreak supplies a driver file that allows Windows to know what it is. Commands to the device are simple text based commands. Serial applications such as PuTTY work well, which is available for Linux and Windows. Even free serial terminal apps from the Google Play store can communicate with these devices using an Android phone.
These features can open up opportunities for new products, or cost reduction in older ideas. The small form factor lends itself well to portability. At one time, Distributed Antenna Systems (DAS) engineers rolled around large carts full of equipment for testing their installations. Now they walk in relative freedom using a small SynthHD signal generator transmitter, generating two frequencies at once, controlled by a tablet.
Government agencies and their contractors are looking more to commercial off-the-shelf (COTS) devices for cost savings. Aviation customers are flying a much lighter, smaller device. Mobile users are putting SynthHD signal generators in the glove box instead of the trunk. Wireless labs around the world can keep a few in a drawer (with the more expensive cables) for spur of the moment experiments. You can easily set up multiple channels with different frequencies, you can also set the same frequency on each channel and adjust only the phase. This can be used in antenna beam steering experiments. Or it could also provide a quadrature local oscillator (LO) for an image reject mixer. The devices are light enough to be supported in the air by the stiff cables that are hooked to them.
Electromagnetic interference (EMI) testing is a field where these devices can really help out. EMI testing can be very expensive, so it’s a good idea to do some basic testing in your own lab first. Even if you don’t know much about RF, relatively low cost equipment and a beginner’s book from Ken Wyatt at www.emc-seminars.com could provide big savings when the device goes for actual certification.
Universities are some of the biggest customers for USB signal generators. They work well in projects like scanning electron microscopy, photonics, atomic transitions in materials, and general RF communications applications. These are just a few of the many possibilities. The price and portability allow the students to use them outside the lab. Since the students get the Labview code, they can sharpen their skills programming software for their custom applications. On top of that, the SynthHD signal generator is Arduino compatible, plus it comes with the digital schematics detailing all of the microprocessor connections. The student can completely wipe the firmware and start programming in the open source Arduino environment, with all of its benefits of its online community. There is also a 20-pin board-to-board connector which will allow analog and digital inputs and outputs to go to their custom made snap-on boards. This creates an easy environment for huge opportunities in learning and customization.
Professor Nir Bar-Gill used Windfreak Technologies devices at Harvard University starting in 2010. He continues to use the signal generators in his new role at the Hebrew University in Israel.
“I’ve used Windfreak Technologies products for basic research in quantum spin physics, mostly related to nitrogen-vacancy color centers in diamond. We use these devices for driving and manipulating various spin species, replacing much more expensive systems. We stack the Windfreak modules in order to simultaneously drive six different spin species (each at a different frequency), which allows us to create the world’s smallest quantum refrigerator,” said Bar-Gill.
Of course there are always drawbacks to any solution. One strong complaint about USB RF signal generators is the level of harmonics the devices can generate. Another fear is that phase noise will be an issue.
Low-cost USB signal generators are usually designed around an integrated phase locked loop and voltage controlled oscillator (PLL+VCO) chip. The chip will typically have a multiband, octave tuning VCO. It is multiband in order to get the MHz/volt VCO tuning gain down, which helps the phase noise. In the case of the SynthHD signal generator, the total VCO tuning range is from 3400 MHz to 6800 MHz. The rest of the range of the device is achieved by RF dividers (/2 to /64) to get to 54 MHz, and an RF doubler on the top end to get up to 13.6 GHz. All of this comes from what is effectively a digital chip generating a rough square wave which has its harmonics. From 6.8 GHz to 13.6 GHz, the SynthHD signal generator will also generate a single subharmonic at ½ the transmit frequency as well. For many applications these harmonics aren’t an issue.
Some RF mixers actually like a square wave LO input. There are cheap but good screw-on SMA (Sub-Miniature version A) RF low pass and high pass filters from companies like Crystek in the $25 range, in stock from Digikey. Until low cost, small broadband tunable filters come along, it’s up to the PLL + VCO chip designers to help knock these harmonics down.
Phase noise can be lower on narrowband devices with large high Q resonators in the VCO circuit, or in T&M devices where there is plenty of room for expensive oscillators.
Then, the question is: How much is too much? Quite a few engineers aren’t really sure, so they spec something really good when maybe it isn’t needed. At 1 GHz, the SynthHD signal generator has roughly 95 dBc/Hz phase noise at a 10 KHz offset. A radio system’s sensitivity isn’t generally set by the phase noise in its LO signals unless the LO is really bad. In general, an LO with bad phase noise will cause trickled bit errors at high received signal levels. Finding a solution that is 10 dB better than the trickle point may still have good performance. At low received signal levels, the front-end noise after maximum automatic gain control (AGC) will easily dominate phase noise when it comes to receiver sensitivity. Phase noise and spurs are probably more of a problem with adjacent channel interference or other topics outside the scope of this article
Any engineering problem can be solved in a multitude of ways. Concerns of price versus performance are the biggest balancing act an engineer faces. To tip the balance in your favor, a low-cost USB RF signal generator is a perfect tool for the engineer’s toolbox!