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 are commercially 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 lowest 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.

Even so, the v3 design may be produced without the addition of an internal OCXO.  This moves the functionality back to that of v2 but keeps the possibility of using an external OCXO, if desired.  The incremental component cost to build v3 rather than v2 is quite low so this design may be a preferred one for many builders. The KiCad files below already DO NOT include one of the two internal OCXO options so nothing need be done to build and assemble an "OCXO-less" v3.   If an internal OCXO is desired, that would normally be ordered separately and added at final assembly time.   The ocxo131-100 option would normally be purchased from eBay while the three components for the ECOC-9775-VC option would be purchased from normal vendors such as Mouser or Digikey.     While raising the total cost of the OCXO option,  providing the design in this manner makes the most economical version automatic while allowing the user to option-in the OCXO of choice, if so desired. It's also possible to assemble the PCB with no VCXO or PLL components.  If the result is only intended to operate with either an internal or external OCXO, this may also be a very cost effective option having quite good phase noise performance.  

A careful study has not yet been made but it appears that the Si5351-C synthesizer may be the limiting component. A measurement provided below indicates that this limit may be near -115 dBC at 100 Hz offset from the carrier, when normalized to 10 MHz.

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.
The plot below shows an Isotemp OCXO in FLL mode measured over about three days time.
Blue trace is an unconnected 2nd audio channel indicating measurement system noise floor.
This next plot shows REference v3 prototype using an external Isotemp OCXOin FLL mode compared with Bodnar 'Mini at 100 MHz'. 
Blue trace is an unconnected 2nd audio channel indicating measurement system noise floor.

This indicates about -95 dBC phase noise at 100 Hz offset from the carrier,  or  -115 dBC when normalized to 10 MHz.  Measurement was made using a doubly balanced mixer as phase comparator and with a Berringer UMC202HD USB audio interface. This measurement cannot distinguish contributions from either source However phase rotation verified to be from the Bodnar GPSDO causes the display to go in and out of quadrature every few seconds. This is likely due to the comparatively small lock bandwidth of the Bodnar/GNSS system.  It is not evident when the REference is compared with high quality OCXOs.
Further measurements must be made to isolate contributions and to determine whether any of these imperfections are detectable when measuring any narrowband communications systems such as WSPR and FST4W below VHF frequencies.



Material List

What you will need to build this hardware

For better  quick  viewing of the design, download the KiCad file from the Material List  below , unzip it and drop the .sch or .pcb file onto kicanvas from a web browser.

Item Description

Provider

Notes

Approximate Material Cost

(excludes setup fees and shipping)

Assembled RE3 PCB

and Front&Rear Panels
A newer, not-yet-fabricated-and-tested version is now available as

KiCad files

Use the 'Production' sub-directory to obtain .zip (Gerber), .bom and position.csv files for JLCPCB fabrication.


US$40
2 x 20p CPU socket

ADAFruit

US$1

38x88x120mm Clam Shell Enclosure &

Front/Rear Panels

JLC


Other sources possible. Enclosure needs to accept 84mm wide PCB. US$3
& US$2

CPU
RPI Pico 2W
Arduino code 		 RPI Pico2W_with_header

ReTr4_0_0916a_fllEngine_25_v2compile.zip

 code also supports, v2, TR  options Select OCXO type in AAconfig.h
 US$8 + shipping
Auxiliary program FLL Discipline Plot (live) Stand-alone Frequency Counter Python programs to plot  FLL  OCXO performance

Final Assembly, Test & Operation

After receiving assembled PCB from fabrication and before final assembly with the CPU & socket,  first verify with an ohmmeter that the power input line and the 5V and 3V regulator outputs are not shorted. Next connect a current limited or low power source of 7-16 VDC  power the board. Without CPU verify that there is only a few mA flowing. The raw PCBs have already been pretested but doing this makes sure nothing has gone wrong during component assembly or soldering. 
Final assembly and soldering of the CPU  socket is next.  Plug a RPI Pico2W into the socket to assure alignment then mount everything on the PCB squarely. Solder corner pins on the socket making sure everything is flush and squarely aligned. Once the position is good solder all the rest of the pin connections on both the PCB and the CPU.

Program the CPU

As received from the vendor, the Pico2W will have no code or perhaps  a "Blink" sketch running.  It will need to be programmed.  This can be done by first downloading and installing the Arduino IDE.   Once this is complete,  the sketch for the reference may be downloaded from the above link, unzipped into it's own directory. 
To work with the RPI Pico2W a board manager will need to be installed. Once that is complete, the Reference sketch may be Opened from the IDE.  In the AAconfig.h tab, if an OCXO is being used it should be #defined so that suitable FLL code to support it will be included once the code is compiled.
Select Tools:"Flash Size: "4MB (Sketch 2MB; FS:2MB)" to provide space for over-the-air (OTA) code updates.  Selecting 'Compile' will then result in some errors because additional libraries will be needed.  These are satisfied one at a time by using Tools:Manage Library and installing each.  When all have been installed, the code will successfully compile and be ready to Upload into the Pico2W by way of a USB cable. The Pico2W does not need to be attached to anything else at this point.  When an Upload has successfully completed, the Pico2W may be plugged into the waiting socket on the assembled Reference PCB.

First Web Connection

At this point again apply DC power and verify that there is 3.3V output from the CPU comes alive. LEDs should light and after a few moments it should now be web accessible. Prior to associating with a local 2.4GHz WiFi AP, the unit will bring up its own AP with an address of the form  " http://re2update**.local:8078/", where "**" will indicate the last two characters of the MAC address.  In addition to being shown on the Utility page, this IP and MAC address will also be output to the USB serial port as part of startup information.   It should be recorded for use in the absence of a local WiFi AP which will be the normal access route but could change if the unit is moved to a new location.

Future Web Connections

Once there is a web connection, the Reference may be configured with a local WiFi  AP's SSID:Password  either from the serial terminal or from the Utility page and saved into EEPROM and used for all future access.  This will allow complete access from within the local network.
You can now slide the PCB into the clam shell enclosure and attach the end panels but leave the top clam shell off until later.
From here on you can use the Web page interface to preset the DAC via the utility page so that the  XO mode is nominally at the right frequency and ready to be disciplined by GNSS or Ext10 sources.

Provide either a GNSS antenna or else a 10 MHz  reference and verify that when the corresponding disciplined mode is selected that the unit shows LOCK when when using the Utility web page for reporting.
If you have compiled for use with either an internal or external OCXO,  no pretuning is necessary since the firmware will calculate and store settings once GNSS timepulse is present the first time it is run.
From this point on, all configuration and monitoring can be done by way of a web browser. This includes OTA reprogramming of the unit's FW from the Utility page, should that ever become desirable. 

OCXO Support

Selections and settings as well as OCXO monitoring are all done via the web pages.  If the OCXO option was selected and either an internal OCXO such as the Isotemp has been installed on the PCB or if an external OCXO such as the HP10811D is being used, that will  have been chosen in AAconfig.h prior to compile and upload.  This will add an additional FLL page where operation can be modified and monitored.  Once running, many OCXOs may require several days, or perhaps even a month of continuos operation to reach a steady state conditon of maximum performance.  In the case of very high quality OCXOs, it should be possible to operate for very considerable periods of time, even months or years, without any connection or use of the GNSS disciplining.  Once characterized and stable, long term drift may be so small as to be unmeasurable in the context of ionospheric communication applications.

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