What happens when you are developing your own radio and you have RF testing needs that aren't quite met by your current bench setup? You make your own tool of course. While the engineers at Othernet were developing their Dreamcatcher Data Radio Kit, they found that they needed a quick and easy way to up-convert and down-convert output signals from other radios they were using as input signals to their Dreamcatcher radio. As a result, they created a tool that could be used as an add-on to the input or output of an RF device to mix RF signals and generate continuous wave RF signals.
Housed in an aluminum enclosure to provide proper shielding to help prevent spurious emissions along with its strategic RF design, the moRFeus is pocket sized with its dimensions of 3.5 x 2.7 x 1.5 inches and 7.4-ounce weight.
The moRFeus has two function modes: frequency mixer and continuous wave (CW) signal generator. As a frequency mixer, moRFeus can take an input signal within the range of 30MHz to 6GHz and mix it will a specified CW tone within the range of its onboard local oscillator (LO). The LO's range is from 85MHz to 5.4GHz with a step size of 1-2Hz for the lower side of the frequency range and 2-3Hz for the higher end of the frequency range. Since the LO's range is from 85MHz to 5.4GHz, this means that when the moRFeus is in signal generator mode, its output signal is constrained to this same slightly reduced range. This tiny little test instrument provides functionality across three frequency bands (VHF, UHF, and SHF) which gives hobbyists and even industry engineers plenty of room to work. The signal generator functionality is extremely impressive on this little gadget as it can generate a stable +/-2.5 ppm CW signal. With a current price tag of $125 on Crowd Supply, it's a super valuable addition to your work bench.
The moRFeus' user interface consists of 5 buttons and an LCD display to navigate through six different menu options to select parameters such as frequency, functionality, mixer current, enabling/disabling the bias tee, turning the LCD backlight on/off, and activating the USB boatload functionality for the MCU. There is also open source GUIs available for it that can be installed on Windows, Mac, Linux and even a Raspberry Pi.
The key behind moRFeus' design is its use of a fractional N synthesizer for its LO. To generate a sinusoid, a phase-locked loop (PLL) is used which consists of a negative feedback loop where the phase of a generated signal is forced to follow that of a reference signal. A PLL consists of a reference source, phase frequency detector, loop filter, and voltage-controlled oscillator (VCO). The output of the VCO is phase-compared with the reference at the phase frequency detector (PFD). In a normal PLL, the output frequency is an integer multiple of the phase detector frequency, this means that changing this division ratio between the two frequencies by one, changes the output frequency by an amount equal to reference source frequency. The downside to a normal PLL is that to achieve smaller frequency steps that will actually result in a distinguishably different output frequency, a large division ratio is needed which consumes more resources and comes with a set of performance issues. A fractional N synthesizer overcomes this downfall by using a fractional division ratio rather than an integer one. This fractional division ratio is achieved by the synthesizer quickly alternating between two division ratios, N and N+1. The proportion between these two smaller division ratios is what determines the output frequency.
If you're getting started with any sort of SDR hardware I'd highly recommend the moRFeus to generate a know signal for you to analyze at first to get to know your new SDR. It will grow with you from there as a handy tool in your RF kit.