[Premium-Rx] use of precision time base

[Ed Tanton]

While I'm hardly an expert, I have a Rhubidium Standard that was one of
those AT&T retired several years ago when they went to satellite-based
standards. I have a simple GPS setup too-though not actually set up. You can
get and set up similar GPS-based systems relatively inexpensively (or
expensively as well) if you look for them. Found mine on ebay.

The largest drawback to Rhubidium is that, oddly enough they must be
calibrated-normally over a period of days-against WWVB. I have a Tracor VLF
receiver designed for performing that calibration. I have the data on some
of that somewhere, but-perhaps obviously-it has been several years since I
paid this any attention.

As I recall, you can achieve at least the same incredible accuracy using
GPS-and I also THINK I recall you can surpass Rhubidium-but I'm just not
sure. 

Others have likely fooled with this more recently than I have, but hopefully
this will be some help. Basically, what you use is a "Disciplined" OXCO
phase-locked to a GPS signal. Here are the OXCO notes from the guy I bought
mine from:

This is for one new Isotemp model OCXO134-10 10.000000MHz OCXO. These were
used by a major 
 manufacturer in their GPS disciplined frequency standards. This is a great
OCXO for building a GPS 
 disciplined frequency standards using a Rockwell Jupiter GPS receiver which
has a 10Khz output. 
 Only a few other components are needed including a 74HCT4046 and 3 decade
counters (divide by 
 1000). Specifications for this OCXO are: Frequency: 10.000000MHz Output
voltage: 2 Volts p-p Sine-
 Wave +-10% into 50 ohms. Harmonics: < -25dBc. Spurious: < -60dBc. Ambient
temp. stability: <5 x 10-
 9th from -30C to +60C. Aging: <1 x 10 -9th after 30 days. Short term
stability: <1 x 10-10th. Phase 
 noise@ 10Hz: <-105dBc/Hz. Phase noise@ 100Hz: <-125dBc/Hz. Phase noise@
1000Hz: <-145dBc/Hz. 
 Internal reference voltage: +8.0v +-5%. Internal reference voltage
Temperature stability: +-10MV 
 total from -30 to +60C. Electronic frequency control range: > +-.45PPM.
Electronic frequency 
 control ( VCO) positive slope input: 0 to +8v input centered at +4V. Input
automatically floats to 
 4v center voltage if left open. A 10K ohm POT can also be used between 8V
ref. and ground with the 
 wiper to EFC pin. Supply voltage is +11 to +15Vdc. Supply current is less
than 200Ma after warm-up. 
 Oven monitor logic output is >3.5v with oven stabilized and <1v not
stabilized. Dimensions are 2 
 inches x 2.9 inches x 1.4 inches. Weight is 8.5 ounces.

There a BUNCH of other related Notes and schematics, etc. but you can likely
see where they are coming from/headed towards. Once you have this very
accurate 10MHz, you can then carefully distribute it to your receiver(s). I
say carefully because there is some care required to maintain accuracy due
to cable and active buffer variations that I did not fully understand.

By the way, the 3rd option is a cesium beam standard. They do not require
any sort of calibration like the Rhubidium standards do; are at least an
order of magnitude more accurate than Rhubidium, but they cost a lot more;
and the cesium beam tubes actually burn out with usage. Used, working, they
used to go for about $800 - up.


73  Ed Tanton N4XY <n4xy at earthlink.net> 
 
website: http://www.n4xy.com   
 
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