Cheap GPS-disciplined 10 MHz oscillator- preliminary-updated

Cheap GPS-disciplined 10 MHz oscillator- updated 5/10/2014

This post is for a cheap GPS-disciplined 10 MHz oscillator. It is a work in progress, while the links are still active; life is a work in progress...

I had earlier set up a Trimble reference, but they are a bit stone-age , but work well! http://vk4zxi.blogspot.com.au/2014/02/gps-disciplined-clockoscillator.html. Modern Trimble gear is too expensive for my purposes.

 

Update 5/10/2014:

• I have the device described here now and it works well, especially the little active antenna, I can get satellites though our tile roof.
• There is a temperature controlled crystal oven version available. It can access most other GPS satellite systems as well as the USA system. ($70) TXCO version: UBLOX NEO-M8N GPS GNSS receiver board with SMA: http://www.csgshop.com/product.php?id_product=170
• The datasheet for the NEO-M8N: http://www.u-blox.com/images/downloads/Product_Docs/NEO-M8_DataSheet_(UBX-13003366).pdf accuracy of time pulse ~30 ns
• An Australian amateur has devices to support a 10 MHz GPSDO: http://www.vk3xdk.net46.net/
• A 10 MHz buffer/distribution that works with square wave: http://www.vk3xdk.net46.net/1_34_10MHz-4-way-isolated-filterered-distribution-system.html. I have ordered one of these, just what I was looking for. Should get clean 10 MHz reference (phase noise??). I may run a filter between the two devices, just to clean the signal up a bit before it is buffered and properly filtered.
• A programmable PLL to generate required frequency, with 10 MHz reference input, for use as local oscillator: http://www.vk3xdk.net46.net/1_35_Agile-PLL-V2.html.
• The software from manufacturer http://www.u-blox.com/en/evaluation-tools-a-software/u-center/u-center.html has been updated but the configuration information from https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7 is essentially the same.

Introduction

I first came across the idea from https://groups.yahoo.com/neo/groups/rtlsdr/conversations/topics/1254. I am not sure it can be accessed unless you belong to the Yahoo group. Edit: Via the sdr-radio-com group, a page on using the device: https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7

Basically it uses three components, a GPS module, a USB interface for the module and the manufacturer's software to set it up:

1. U-blox-NEO-7M-GPS-GNSS-Module: http://www.u-blox.com/en/gps-modules/pvt-modules/neo-7.html,  http://www.ebay.com/itm/Ublox-NEO-7M-GPS-GNSS-Module-Built-in-Data-Memory-Replace-NEO-6M-/301199083821?pt=LH_DefaultDomain_0&hash=item4620dd492d. Uses crystal not TXCO, but maybe ok with GPS. Board is used for positioning R/C aircraft.
2. USB interface: http://www.ebay.com/itm/1pcs-USB-2-0-to-TTL-UART-6PIN-Module-Serial-Converter-CP2102-STC-PRGMR-/141238861134?pt=LH_DefaultDomain_0&hash=item20e27dfd4e
3. Ublox software to control device: http://www.u-blox.com/en/evaluation-tools-a-software/u-center/u-center.html

The devices per the eBay seller's (goodlucksell) site (note: the four wires between the two devices are not direct and need to checked)

 

 

 

 

The software gives comprehensive GPS data and mapping as well. Useful as GPS receiver alone, with a serial NMEA stream. Interfaces with Google Earth.

 

The interface with no device connected:

 

 

GPS receiver details: http://www.u-blox.com/en/gps-modules/pvt-modules/neo-7.html

Manufacturer's software: http://www.u-blox.com/en/evaluation-tools-a-software/u-center/u-center.html

 

From the NEO-7M documentation, the device normally outputs a 1 pps pulse (at the point shown in the screen snip), but can be programmed to output a harmonic-rich 10 MHz. Presumably once programed, is standalone oscillator. 

I have ordered the bits ($33 from China)  and had a quick look at the software and data sheets. All quite neat, simple and cheap. The thing should be accurate enough for most purposes.

A lot simpler, cheaper and compact compared to an old Trimble GPSDO, per my earlier post.

 

The only trouble is antenna needs to see the sky. It may possible to use a different external antenna. The other way I was thinking to get an inside GPS signal was to use an external GPS antenna and run coax to a little dipole in the ceiling above where I wanted the signal. I haven't tried it, but it should work.

 

To be updated in a couple of weeks when the bits arrive.

 

Update (late September)

Bits still not here.

 

Spirited discussion on https://groups.yahoo.com/neo/groups/sdr-radio-com/conversations/topics/23602, mainly about merits of different frequency references.

 

A site that has already setup the board for 10 MHz reference: https://sites.google.com/site/g4zfqradio/u-blox_neo-6-7

 

There is a NEO evaluation board with the NEO-7N, with TXCO, SMA antenna connector and built-in USB port for ~$82. It has a point marked "time" in photo.

http://www.ebay.com/itm/UBLOX-NEO-7N-GPS-GNSS-receiver-board-SMA-UAV-Robots-USB-UART-I2C-/261394790627?pt=LH_DefaultDomain_0&hash=item3cdc57f8e3

 

Serendipitously, there is an article in the latest issue (Sept-Oct) of the ARRL QEX (just arrived by post in Australia!) on the "Calibration and monitoring of frequency standards-Phase method". It makes for good reading for those interested in frequency references. It does stress the difficulty in measuring accuracy of references.

 

Table 2, p14 is relevant to this discussion:

Relative Accuracy of Various Frequency References

Reference     Modification                  Accuracy Range

Crystal                                                  1 to 100 ppm

Crystal         TCXO                            ~ 0.1 ppm

Crystal         Ovenized OXCO             0.001 - 0.1 ppm

Crystal         Double Oven                   ~ 50 ppt (parts per trillion)

Crystal         GPS supervised OXCO  ~ 5 ppt

Rubidium                                            ~ 50 ppt

Rubidium     GPS supervised              ~ 5 ppt   

Cesium                                               0.01 to 0.1 ppt

Hydrogen Maser  Passive                  1 ppt

Hydrogen Maser  Active                      0.0007 ppt

ppt= parts per trillion, one part in 10(-12) = ns