Sunday, 7 September 2014

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:

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:
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:
 
 

 
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.
 
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
 
 
 
 
 

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