What is it? This is a GPS disciplined high precision temperature compensated or oven controlled crystal oscillator. The TCXO version has a short-term Allan deviation specification of < 1E-9 @ 𝜏 1s,...Read More…
This is a GPS disciplined high precision temperature compensated or oven controlled crystal oscillator. The TCXO version has a short-term Allan deviation specification of < 1E-9 @ 𝜏 1s, which means that most of the time the frequency will be within 1 ppb. The OCXO version has a short-term Allan deviation specification of < 1e-11 @ 𝜏 1s, which means that most of the time the frequency will be within 0.1 ppb. GPS is used to continuously discipline the frequency to maintain accuracy.
The unit comes with a 5 VDC power supply (90-240 VAC 50/60 Hz with a North American plug - a passive prong adapter is all that would be required for worldwide use). The TCXO version requires 250 mA and the OCXO version requires 1A. The output frequency of 10 MHz is provided as either a 5 volt square wave or as a 17 dBm sine wave. The two independent output channels are supplied on BNC jacks. Each channel comes from a separate channel of a fanout buffer, so each should remain independent and stable. The output picture in the gallery shows the TTL waveform delivered through 50Ω coax into a 50Ω load (1.5v P-P, 5 ns rise/fall time to 90%).
The board has three LEDs, labeled FIX, 0 and 1. The FIX LED comes directly from the GPS module and is its indication of a GPS fix. The 0 and 1 LEDs indicate the operating mode of the firmware. If they're blinking back and forth, then there is no GPS fix. If they're not, then they represent a binary value 0-3, which mean coarse (0 - both LEDs out), fine (1 - 0 on, 1 off) and run (2 - 0 off, 1 on) mode. For the OCXO variant, the value 3 is the run mode and 1 is "medium," 2 is "fine."
The first time it is powered up, it should be able to obtain a complete lock in a few hours (assuming good GPS reception). In the case of the OCXO variant, it will take several days before the unit will completely stabilize and the phase range will settle, but the frequency will be well disciplined in the meantime. Restarts after that should be able to obtain a lock faster (depending on how long it's been powered off). It is recommended that the unit be (more or less) permanently powered, as the stability of the crystal will improve with age and use. The TCXO variant, in general, achieves its maximal stability right away.
There is an SMA connector for an external GPS antenna (required and not included). The oscillator will hold-over when GPS is unavailable (however, it will not attempt to "guess" at the aging over time while holding over. Good, constant GPS reception is required for maximal accuracy). There is also a mini-DIN 6 diagnostic connector that supplies a separately buffered PPS output, diagnostic serial output from the controller and serial NMEA I/O from the GPS module.
The board has a 6 pin ISP header for firmware updates. Using it requires an AVR programmer with a 6 pin cable and a pogo pin programming adapter. The firmware requires only standard AVR GCC and AVR libc to build, and is open source and available on GitHub.
For the OCXO variant, there are two oscillator choices - the Connor-Winfield OH300 or the ECS ECOC-2522. The ECS oscillator costs a bit more, but offers slightly better stability out to 𝜏 200s or so. Check the ADEV graph in the pictures that shows the green line for the ECS oscillator.
The design is intended to produce an Allan variance of 1E-9 or less for the TCXO or 1E-10 or less for the OCXO (assuming good GPS reception and good mechanical and temperature stability). The free-running DOT050V oscillator has a short term spec of no worse than 1E-9 @ 𝜏 10^0, but in actual fact usually achieves around 7E-11. The free-running OH300 oscillator has a short-term spec of no worse than 1E-11 @ 𝜏 10^0, but usually achieves around 8E-12. The blue graphs in the gallery are comparisons between a unit equipped with a DOT050V oscillator running against a Thunderbolt GPSDO over a 24 hour period, and are reasonably characteristic. The pink graphs are comparisons between a unit equipped with an OH300 oscillator running against the Thunderbolt after several days of warming up. Again, they are reasonably characteristic. The graph that has blue, pink and green shows the ADEV of the ECS oscillator after several days. The ECS frequency plot is indistinguishable from the OH300.
I had a project where I needed to perform particularly accurate frequency measurements and wanted those measurements traceable to some respected standard. This was the cheapest way I could find to do it. It is not intended to compete with commercial GPSDOs (which are far costlier, but you get what you pay for), but rather offer an entry level frequency standard for a reasonable price.
In terms of accuracy and value, this is the best unit I know how to make. And for most hobbyist and light commercial uses, it should be more than adequate.
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Leslie | Aug. 17, 2017
Paul | Nov. 9, 2016
Randal | Sept. 2, 2016
Stephane | Dec. 6, 2015
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