MVR GPSDO

Description

Low Cost GPSDO
The MVR (“Minimum Viable Reference”) GPSDO is a low-cost GPS disciplined oscillator intended primarily for HF radio applications. It has a 10 MHz output as well as three additional outputs that are programmable to frequencies between about 200 kHz and 200 MHz (operation outside these ranges is possible, but not guaranteed). Thus it can serve as an external clock source for SDRs such as the RX888, WEB-888, or KiwiSDR.

The MVR is powered from a USB-C connector which is also used to access the configuration menu. Once the unit is configured, it can operate from a standard USB-C power supply; the computer does not need to be connected except when changing configuration. The antenna connector provides 3.3 volts to drive a GPS antenna. The outputs are square waves at about +10 dBm into 50 ohms.

The MVR’s long term accuracy approaches that of the GNSS satellite constellation, but its low-cost design goal means that its short-term stability is not as good as some other GPSDOs. To be more specific, its 10 MHz output wiggles from second to second by about 9 milliHertz. For applications below 30 MHz, this is usually lost in the noise. But frequency and phase noise scale with frequency, so at VHF or UHF frequencies, the short-term noise may become an issue. Other instruments are a better choice for such applications.

Another limitation of the MVR is that it does not have a “holdover” capability – if GNSS signal is lost for more than a second or so, the unit will lose lock and the frequency will shift significantly. However, with multiple satellite constellations available today, and the GNSS module supporting most of them, loss of lock should be very rare unless the antenna is severely compromised, or there is active GNSS jamming in the area – in which case you probably have bigger problems to deal with!

The initial firmware configuration is that the first clock (Clk0, because us computer people always start counting with 0…) is set for 27 MHz, and Clk1 and Clk2 are disabled. This can be changed from a serial console over the USB cable. Please see the manual for configuration details.

MANUAL
The MVR GPSDO Assembly and Operation Manual is available here.

Firmware and GitHub repository
https://github.com/TAPR/mvr_firmware

We recommend that you may want to use a ‘HF Small Signal Isolation Transformer SMA’ on each clock output to fully DC isolate each RX888 from the other. The MVR GPSDO has capacitor coupling on each output, so there is no DC bias present. Yet adding a transformer, using the suggested isolation device, will eliminate a potential ground loop.
https://tapr.org/product/hf-small-signal-isolation-transformer-sma/