RE3  Disciplined Frequency Reference with OCXO Support

 

From the original RE to the latest version, these disciplined REferences  have had the primary objective to be very low cost,  accurate and high value devices for synchronizing amateur radio communications equipment.  LF through VHF communications by way of the ionosphere has been a particular target.   Design choices have consistently favored low cost above numerical performance  as long as it  exceeds that which is useful for these radio applications.   These designs have proven themselves to meet these objectives well.

It has been suggested that v1 and v2  "under-perform" because phase noise is higher than other,  more expensive,  disciplined references that arecommercially available. Because v2 does meet the original goals of the project to be very economical and work "well enough" for any foreseeable amateur radio LF-VHF uses involving the ionosphere,  a 'better' version has not previously been offered.  The total parts cost for that entire PCB, enclosure, and all other parts meet the goals  well.  Without US tariff penalties, projected total cost is  < US$50  for small quantities and is deemed part of its "performance".

  

Fundamentally, beyond the disciplining PLL 's bandwidth,  a disciplined reference is no better than the local reference oscillator being disciplined. In v2 that was a VCXO - currently costing only US$1.54. Using a low-cost GNSS module as the disciplining source, the possible bandwidth is quite limited. This is because internal  algorithmic phase corrections performed by the module's code inject small phase steps in the disciplining time pulse as that module's local TCXO is kept synchronous with GPS time. "Time keeping" versions of GNSS modules which avoid these phase corrections are available but are either more expensive or require proprietary code.  However, as described for forseeable and known  LF-VHF communications applications there should be no discernible difference with this design compared to more costly solutions, except for its much lower cost and extended features.

v3 is an increment of the v2 design.  It does NOT meet the low cost, high value goals of the original v1 and v2 projects.  Instead it allows for internal or external OCXOs.  This  can easily double the total component cost.   One improvement it can offer is stand-alone operation - operation not requiring a GPS antenna.  While the most precise OCXO mode is disciplined by way of a frequency locked loop (FLL) to potentially provide sub-ppb accuracy, that too uses the GNSS constellation and module for discipline.  However if a high quality (expensive) OCXO  such as the HP10811D is used, once calibrated, it is possible to operate v3 for extended periods of time without GNSS required.   For some amateur applications this could be a benefit worth the significant added component cost.

Features & Improvements

•     May use either an internal or external OCXO to provide improved phase noise and long term stability
•     Can operate apart from any GNSS connection - continuous antenna connection may not be necessary
•     Provides up to 7 configurable user outputs,  master clock for HF transceiver or SDR    
•     Implements Continuous Fractions algorithm for essentially perfect frequency accuracy
•     GNSS satellite or local  10 MHz frequency reference
•     Flexible supply 8-30 VDC
•     Typically better than .1 ppb, .00000001% long-term accuracy and stability
•     WiFi web Interface - works with Windows, Apple or Linux systems.
•     Common SDR and HF transceiver Frequency presets

•  Connections:

  • SMA: GNSS Antenna
  • External 10 MHz reference
  • Seven SMA:User Selectable outputs  4 kHz to 225* MHz
  • 2.1mm power barrel connector

• Four-layer,  through-hole plated, silk-screen printed PCBs
• Aluminum enclosure

Example Web Interface

OCXO Monitor and Control page, generally after the fashion of VE2ZAZ which it is patterned after.

HP10811D OCXO with FLL discipline. After warmup and settling this is showing ~20 parts-per-trillion (ppt) accuracy.

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