"10 Mhz GPS Frequency Standard"

Last edited -- 03/14/14 10:15:25 PM  

How To Build W7CQ Version Of 10 MHZ / GPS Frequency Standard 


Web Warranty :  PLEASE READ


Background for this web page is the article by Brooks Shera, W5OJMSK "A GPS-Based Frequency Standard" appearing in July 1998 QST pg 37.

More information is available at  http://www.rt66.com/~shera/index_fs.htm  along with a pdf of the QST article.


Brooks Shera  W5OJM SK   1935 to 2013

Brooks passed away at the age of 78 after a long illness.  His excellent work on GPS controlled Frequency Standards has made this home made, highly accurate, lab quality, 10Mhz standard possible for the average ham.   He is greatly missed.    Jimmy W7CQ


What follows are potential sources to contact and ordering information for the major components of my 10Mhz GPS Frequency Standard.  Major Components Include:

Frequency Standard Controller Board

PIC Controller chip  (PIC16C73A)  ( new version uses a PIC16F876A)

DAC chip  (PCM-61) out of  production but available surplus on e-bay

GPS Receiver and TAPR GPS receiver interface board

GPS ANTENNA

TAC32 Software (really nice but not necessary)

10mHz OCXO

Buffer Amplifier


Frequency Standard Controller Board  by Brooks Shera   W5OJM SK

Contact A&A Engineering and ask for Stas, W6UCM.

Phone # 1-714-952-2114
Fax #      1- 714-952-3280

Stock no. 217-PCB $19.95 + 1.50 shipping

Web page: http://a-aengineering.com/ 


 Programmed PIC 16F876A  - No longer Available

Shera 1.402NE HEX file for programming the PIC 16F876A now available.

Download a text file of the commands for Shera 1.402NE.txt

This is the latest  HEX file (Shera 1.402ne) to program the 16F876A has been released by the Shera family to interested hams for Non Commercial Use.  I have the HEX file available.  I do not provide programmed PIC's.

 Please contact me via the e-mail located at the bottom of this page on how to get this HEX file.


GPS Receiver

You need a GPS engine that has 1PPS signal output and can be placed in Position Hold to obtain its most accurate stability.  A GPS receiver without position hold will work but at reduced accuracy.   NMEA output is nice ( and easy to find software to control the GPS) but this does not guarantee 1PPS output.  Read the specs before you buy.  You must have 1PPS output and all 1PPS outputs are not of the same accuracy due to jitter in the 1PPS.   The smaller the jitter of the 1PPS,  the better accuracy.  A few extra bucks here can improve the accuracy of your frequency standard.  There are some real poor GPS engines out that advertise 1PPS.   Again - Check it out and compare specs before you buy.

 I used the Motorola Oncore UT+ (out of production)  but does show up as new surplus from time to time on eBay.   

   The Motorola Oncore UT+ uses binary input and outputs  and DOES NOT have the NMEA outputs you normally see with most off the shelf GPS receivers.  This unit is built for extreme accuracy on the 1PPS signal.  This receiver, set in Position Hold, provides a 1PPS that is extremely accurate.  The jitter of the 1PPS for UT+ is below <50 nsec.  So unless you are a computer programmer you better get some software like TAC32  (described below) to control this receiver. 

You will need to provide power for our GPS unit and ASCII output from your gps to your computer so you can set up your GPS unit.  if you use the UT+  the TAPR T-32 Software described below does a great job with the UT+.   The UT+ and most GPS units provide +5 volts on the antenna connector to power an active GPS Antenna.

I used the TAPR interface Card for the UT+ (now out of production)  board to control my UT+ with the T-32 Software.  This card was  the same size as the Oncore UT+ and made for a very neat compact unit with the Oncore UT+. The Oncore receiver plugged right into the board.  It provided power for the receiver and ASCII output to your computer but alas neither are available any more.


GPS Antenna

    Almost any amplified GPS antenna will work.  The Oncore UT+ provides a positive 5 volts at up to 70ma on the antenna terminal to power an amplified GPS antenna.   Just check to see the current draw of your chosen GPS antenna is less than 70 ma at 5vdc before you buy.


 Tac-32 Software

This software is available from TAPR and can control the Motorola Binary serial data used by the Oncore UT+ GPS receiver and is available to MEMBERS of TAPR for $55 for personal non commercial Ham use only. This software will setup your UT+ and will also keep your computers internal clock right on time.  TAC32 will also handle  data from other GPS engines.   I  use run this software continuously to keep my home computer network time base very accurate.  You are getting two systems for the price of one. The TAC-32 software can be purchased from TAPR and there are links to downloaded it on a 30day free trail basis from TAPR: http://www.tapr.org/gps_tac32.html  check it out to see if it is what you want in software control of your GPS engine.

There is also a free DOS program available to control the Motorola Oncore UT+ binary coding and  allows you to control the Motorola Oncore UT+ receiver and get the basic 1PPS signal output but no graphic output to the computer.  I have a copy.

Screen shots of TAC32 from my computer:

 

 


10mHz OCXO

This section covers information about HP HP 10811A 10 MHz Crystal Oscillator (or HP 10544A) series OCXO that I have used.

The HP10811 or the HP 10544A  series OCXO are available on e-Bay under TEST EQUIPMENT at different times. Search for HP 10544 or HP 10811 with the HP 10811 series being the newer version of the OCXO's.

    There are two basic internal differences in the design of the HP 10544 series and the HP 10811 series OCXO.  The major change  is how the heat is generated in the oven heater and in the cut of the 10mHz crystal.   In our application these changes will not make noticeable differences in operation.   Buy the one you find at the best price you deem right for you.

The crystal oven heater in a HP 10544 uses a heater control circuit to drive a Darlington transistor Q4 that controls the current flow to a 47 ohm Wire Heater Resistor (a film wire heater wrapped around the crystal aluminum casting) and this provides a variable heat source for the crystal oven. 

In the HP 10811 series, the Wire Heater Resistor has been replaced with two Darlington transistors Q7 and Q8 on a heat sink made of aluminum.  These transistors do not control current flow to a heater resistor.  They are the oven heaters.  The heater control circuit drives these two transistors (Q7 & Q8) and the heat generated by these transistors is dissipated into a large aluminum heat sink surrounding  the 10Mhz crystal.  The oven heater control circuit maintains the oven temperature at approximately 85C (185F).  This can burn your fingers.

    Note sent to me from Peter, NI6E: "The 10544A oscillators use a switch mode oven controller. An LC filter using a series 10 mH, 0.75A, inductor and a shunt 200 uF capacitor is recommended to isolate the oven controller switching transients from the oscillator when the oscillator and oven controller employ the same 10.6-11.7V power supply."  This  will come into effect if you share the same power supply for the Oven Temperature Control with the Oscillator supply.   I decided to build with all separate supplies for each for this very reason.

The other change involves the crystal cut.  The HP 10811A/B, using an SC-cut resonator, is very similar physically to the HP 10544 that used a AT-cut resonator.  The SC-cut crystal has several advantages compared to the AT cut.  It has a much smaller temperature coefficient,  the resonator can be operated at a higher drive level, which improves the signal-to-noise ratio and short-term frequency stability without degrading the aging rate.   It has faster warm up with less frequency overshoot. The SC cut is a doubly rotated resonator and requires much tighter angular tolerances when the crystal is cut from the quartz bar. The SC-cut resonator came into commercial production around 1980.  The use of SC-cut resonators in oscillators immediately improved their performance of the stand alone HP 10811 series OCXO installed in their test equipment.  In our GPS controlled frequency standard  we are able to improve on the original HP frequency stability specs  by a factor of 100 to 1000 (depending on conditions) when compared to a stand alone HP 10811or HP 10544.

    Note from NI6E:"The nominal 10544A crystal current is about 240 uA rms, somewhat less than the 1mA in the 10811 with its SC cut crystal."

    Note from WB5KYE:"Jim discovered that BT-cut crystals were also used in some versions of the HP 10544. This add another variable in the warmup drift of the OCXO".  I was not  aware of  this but Jim was having problems stabilizing his OCXO and discovered this. 

This said, in our use,  the temperature control features of the HP series OCXO and EFC GPS Lock we use make these differences very minor.  In a stand alone oscillator the above SC-cut  features are very desirable but are effectively controlled by the way we are using the OCXO.  If price is no object the HP 10811 is newer and of a slightly better design. The model you find will be the best one for you.  It is really a small difference and hardly measurable.

HP 10811 A/B Manual :    I have just run across a Vintage HP Site that has a downloadable HP10811A/B Service manual in PDF form. (8.22Meg)   http://www.hparchive.com/   This is a new site and is getting more equipment manuals all the time.  Looks interesting.

    Also another site new to me has HP/ Agilent information with good information about the HP10544 and a Manual for the HP10811A/B also. 

Check it out.  http://www.ko4bb.com/cgi-bin/manuals.pl?dir=HP_Agilent

I also have a pdf file of the schematic of a HP10544 available.  Write me if you need it.

Wanted HP 10544 Service Manual:

    If anyone has a Service Manual or a copy of a service manual for the HP 10544, I am interested in one.  Contact me at the e-mail below.  It would be greatly appreciated.   I have the two page HP product brochures.   I am looking for the multi-page Service Manual if one exists --- Jimmy


CONSTRUCTION of the Oscillator Section

HP 10811A 10 MHz Crystal Oscillator (or HP 10544A)

    The homemade aluminum box is  5 1/2" high x 10" wide x 6" deep with a aluminum divider to separate the the heat of the crystal oven from the the electronics of the gps receiver and the Shera controller board.  The power supply in mounted on a 2" high x 4" wide x 6" deep sub chassis mounted to the right side of the receiver/controller section. 

    The HP oscillator was placed in a 5 1/2"high x 6" wide x 6" deep that was made by adding a divider to the above aluminum box and is lined inside with Styrofoam about 1 inch thick on all sides.  (top foam removed for photo)  This helps lower the frequency swings of the oscillator caused by room temperature and improves overall stability.   I think if I had to do it again I would place even more insulation around the oven.  I run 24 volts power to my oven all the time so it never has to warm up.  The switch on the rear power supply total turns of the power to the unit.  The front panel GPS switch is the on-off for the GPS and the Crystal Oscillator in the HP 10544A but leaves the OVEN Running.

Click on to EnlargeHP 10455A in Foam Lined Box.  Click to Enlarge

 

 

 

 

( the close-up picture of the oven on the right is inverted )

    The HP 10811A is a later version of the HP 10544A.  Both units have been available surplus on e-Bay at various times for between $75 and $200 depending on your luck. Either unit is fine and both are easy to use for this project.

    The 10544A requires an extra jumper to +12v on the bottom connector (see below) that is not needed on the later 10811A. They both require 20 to 30 volts for the oven heater and 11 to 13.5 volts for the crystal oscillator and both output terminals for EFC (electronic frequency control) and 10 Mhz at about .5 volts rms to a 50 ohm load.

    The 24 volt oven supply draws about 500 ma on startup and in about 5 minutes drops to around 120 ma after the oven warms and stays around there for ever. The 12 volt oscillator draws about 60 ma or so and so does the Shera PCBoard. 

IMPORTANT :  HP 10544  -   It is necessary to apply  +12 volts control voltage to pin #8 of the HP10544 edge connector to turn on the oven and draw current from the 24v oven supply.  With no control voltage on pin #8 the oven does not warm and does not draw current from the 24 volt supply.  The oven acts like it is defective and unable to heat.  Pin #8 is pin not used on the HP10811.  Ground pin # 9 for the return. 

    The EFC allows the HP oscillator to be tuned over a 1 Hz range (1X10-7) by applying from 5 to +5 volts to the EFC input on pin 6. The oscillator must be manually adjusted to 10,000,000hz before the EFC can really hold the oscillator exactly on frequency. This is probably the hardest part of the setup. More later in the PCBoard section.  On HP crystal ovens the EFC runs inverted.  That is to say as EFC control voltage increases the frequency of the HP oscillators go down and as the control voltage decreases the frequency of the  HP oscillators go up.

    The HP 10544A/ HP 10811A 10MHz  oscillator output is not very stiff and is designed to put out  1.0V rms  into 1000 ohms from oscillators ac coupled (.01uF) emitter-follower.  This is sufficient to drive the Shera board by itself but MUST be run into a distribution amplifier to buffer and isolate the oscillators output from other pieces of test equipment.  You can run it straight into the Shera board during testing and setup but in the final layout IT MUST BE BUFFERED with a distribution amplifier described further down this page. 

     I use the buffered output of the frequency standard to lock, the time bases for my HP 5328A counter, a HP 5342A counter, a HP 8656B signal generator, a PTS160 LO Signal Generator, a  Tek 494AP Spectrum Analyzer, a DSP-10,  and lately to lock a 30 mhz reference signal for my ICOM 756 so the 765 is now accurate to 1Hz have spare 10mHz outputs to use on other equipment  that can use a common and very accurate 10Meg reference source.  I have everything locked to this 10MHz standard in my ham shack.


HP 10544A or  HP 10811A  Crystal Ovens

Pin outs for the 15 pin printed circuit connector: HP 10544A bottom view. Pin  #1 at top.

Hold oven like the picture for correct pin numbering top to bottom.

  1. 10mhz output
  2. Gnd 10mhz
  3. +12 volt oscillator supply
  4. Gnd 12 volts
  5. EFC return (Gnd)
  6. EFC
  7. Nc
  8. +12 volts Oven control (used on 10544 only)
  9. Gnd Oven control (used on 10544 only)
  10. Nc
  11. Oven monitor (not used)
  12. Nc
  13. Nc
  14. +24 volts Oven Heater supply
  15. Gnd Oven Heater supply

All Grounds (Gnd) can be tied together or be tied in any combination as needed by your power supply.  Nc = No Connection.

IMPORTANT: The Oven on the HP10544 will not turn on without the +12v to pin #8  (just a couple of mA) and your unit will not warm up.  It acts just like a broken oven.


CONSTRUCTION of the GPS Section

The Motorola Oncore UT+ and the TAPR interface Card that I used are now out of production.

    The Motorola Oncore UT+ mounted on a TAPR GPS  interface card. My UT+ has an onboard battery so the receiver doesn't have to be reprogrammed on each startup.   This card had a highly efficient regulator to provide 5vdc @ 250 ma from a 7- 30 vdc source. This was about  twice what the receiver needs. The receiver plugged into a 10 pin connector on the interface card and has 4 standoffs (one in each corner) to hold it about 3/4 of an inch above the interface card.  Made for a very nice solid compact unit.  To bad it is not around any more.

    The UT+ has a powered (+5 vdc) OCX antenna connector (these are really small, they make a SMA connector look big) so buy or build the short cable to connect the OCX connector to a BNC chassis connector.  I use BNC connectors and BNC interconnecting cables in my ham shack for my equipment interconnections.   The receiver has a jumper to provide +5v power to the GPS antenna via the OCX connector and its output  is good to about 70 ma and is more than enough for most active GPS antennas.  I leave the GPS antenna choice up to you.  Just check to see the current draw is less than 70 ma at 5vdc before you buy.

    I wired a short cable from the interface card to a female DB9 chassis mount connector to give you computer access to the receiver through a computer serial port.  A jumper on my interface card from R3 provides 1PPS (Pulse Per Second) to pin 1 of the DB9  for the computer serial port and is also sends 1PPS  to the Frequency Standard Controller Board  by Brooks Shera that controls the EFC on the crystal oven. 

    I used TAC-32 software to control the GPS receiver and use 1PPS from the serial port to run and control the computer internal clocks with TAC-32 on my ham shack computer (a real slow 486DX66).  Getting this program for the GPS receiver is like getting two systems for the price of one.  TAC32 program is also used to update the time format on all the computers on my home computer network in real time on an adjustable regular basis when the computer clock drifts too far from the REAL time so all computers are exactly on time.  Great for satellite tracking programs or EME scheduling and JT65 use.  The TAC-32 also allows a multitude of setup options for the GPS  receiver and is very easy to set up.  Down load the TAC-32 from TAPR for a 30 free trial to see if you like it. http://www.tapr.org/gps_tac32.html

    The Motorola Oncore UT+ uses binary input and outputs  and DOES NOT have the NMEA outputs you normally see with most off the shelf GPS receivers.  This unit is built for extreme accuracy on the 1PPS signal.  This receiver, set in Position Hold, provides a 1PPS that is extremely accurate.  So unless you are a computer programmer you better get some software to control this receiver. 

    You need to find a GPS engine with 1PPS output for this project.  A GPS with NMEA output is fine for engine control is OK but does not necessarily mean you have 1PPS output.  Check the specs before you buy.


CONSTRUCTION of the GPS Controller Board

designed by Brooks Shera

     The basic board was assembled according to the layout from the information provided by A&A Engineering with the PC Board purchase. 

 

Click on to Enlarge to High Res Picture

Click on to see High Resolution Picture! (252k) opens another window

     For a 10Mhz oscillator, a total division of 32 on the output of U2, as described in the Setup Notes (page 5, paragraph 7) was desired.  So I stacked (Piggy back style) two  74HCT4520s in U2's spot on the board.  One 74HCT4520 provides division by 16 and this stacked arrangement adds an additional divide by two bot a total division of 32.  This was done to lower the output of U2A down around 312 kHz.  This probably is only necessary on oscillators of 10Mhz or higher.  Attached Stacked 74HCT4520.pdf

    In the above picture the Mode controller switches (blue dip switch package upper right) S1, S2, S3 of U11 are set for operation in Mode 7 with S4 set to cause the frequency of the VCXO to decrease with increasing DAC voltage and S5 set to allow the controller to change the DAC voltage.  S1, S2, S3 are open circuit in this setting.  The switches of U11 are left in this position so the front panel switches, shown in the picture below, can control the Modes.  This is done by bringing out the resistor side of the U11 switches  S1, S2, and S3 and and a ground to the three front panel switches.   By grounding various combinations of the front panel switches S1, S2, S3 the modes can be set  externally without opening up the unit.

     I added the ICL7662 as described by Bob Larkin  W7PUA in 5th paragraph of the Miscellaneous section of the Setup Notes on Brooks web site.    http://www.rt66.com/~shera/setup_notes.html   Adding the ICL7662 is a real easy way to supply the - 12vdc needed for the board from the +12 vdc supply and saving a much more complicated main power supply.  There is a blank section on the pcb layout for the the small 8 pin dip ic (ICL7662)  and filter cap to fit in real nicely.  Check pcb picture above to see the location.  It is located on right side of the board just below the +5v TO220 voltage regulator and just next to the red and green wires.  Mine is mounted on a 8 pin socket.  To the left of the socket is a 47uf 16v electrolytic that is needed on the  7662. 

Pinouts used on ICL7662: (five pins used)

Pin#8 (v+) to PCB Pin 7 P6:   Pin #5 (v out) to PCB pin#2 P6:  Pin#3 (gnd) to PCB Pin #8 P6:   between Pin#2 and Pin#4  connect a 47uf 16v electrolytic cap with the + lead to Pin #2. 

    The perf board in the picture (HAS BEEN MODIFIED from the above Picture) and now holds a 7805 regulator,  additional Caps and the disc ceramic are gone and replaced with LOW EFC caps in the +5 volt reference for the EFC of the HP 10811A crystal oven and Ra and Rb.   (See the EFC Supply  for a current schematic.)  R3 are set to provide a reference voltage of  +1.000 v for the EFC.  This was only chosen to center the EFC supply in the 0 to 1.999voltage DVM that I mounted in the front panel.  With a possible swing of  3 volts by the DAC the the EFC to the HP 10811A can swing from + 4 volts to - 2 volts and well in its range of 5 volts.  (The full swing of 5 volts on the EFC  will  swing the HP 10811A only one hertz at 10 mhz.) The EFC line of the HP 0811A is inverted.  That means as the EFC voltage goes up the frequency goes down and as the EFC voltage drops the frequency goes up.  This involves the setting of switch # 4 of the dip switches to the ON or right position, as seen in the photograph, for inverted DAC operation.

    My calculations of my HP 10811-06111 OCXO Show a S value of 2.10E-08 for a EFC voltage change of 1.000v and was calibrated against a Rubidium Standard.   This means my HP 10811 moves 0.21Hz at 10Mhz with a change of 1 volt  in the EFC circuit.  The S value theory is from reference #8 on Page 44 of Brooks Shera QST article.

    After calculations, R5 was set a 470 ohms (R5 goes to ground) and R6 was set to 580 ohms on both units we built using the HP 10811A.  (for R6 I kept measuring 560 ohms resistors from the junk box until I found one close)   

    I have written a very short Excel program that will calculate values for R5 &  R6,  and helps you calculate the "S" value for your particular VCXO.  Contact me for a copy.  It is very basic but works!  Nothing fancy.

    Ra is a combination of the top half of R3 a 5k ohms 15 turn pot and a 10K ohm resistor in series to the +5v regulator. This was done to spread out the adjustment of Ra and make it smoother.  A .22 Poly cap to ground was added between the 10K resistor and the top of the 5K pot  forming a nice filter to the 5 volt line.  Rb is bottom half of the 5k pot and a 470 ohms going to the junction of R5 and R6 on the Shera Board..  As stated earlier I set the arm of R3 to supply close to +1.000volt (within 100mv or so) to the EFC pin #6 of the HP10811A before doing the manual frequency adjustments.  

    For my schematic of  the EFC setup for the HP OCXO and my R5, R6 values click on  EFC Supply Schematic.  I had problems with phase noise being added to the EFC line so several special LOW ESR (Effective Series Resistance) caps and some .22 Polypropylene caps added to the circuit to suppress any noise from the 7805 IC.   Also a filter of a 10 K resistor and .1 Polypropylene cap was added to pin 6 of the HP 10811A. My phase noise id gone now.

    Setup on GPS Control Frequency Standards is a SLOW process and so take your time.   If you rush it you will just have to go over it again.  Done slowly it will lock on and be very stable but it takes time.  

    With the EFC at 1.000 volt and Dip Switches in Mode   (N=0) you can now manually set the HP10811A to exactly 10,000,000 hz with the manual frequency adjustment located through a hole on the top of the oven.  Insert an INSULATED non Metallic Screwdriver in the hole and adjust SLOWLY making real small adjustments.  The oven must be warm before you start this.  By warm,  I mean on and up to temperature for several hours. THIS IS A MINIMUM TIME!  I let mine warm for 24 hours in the insulated box for all to come to normal operating temperature.   If you start your adjustments too soon they will drift away.  The oven starts low in frequency and drifts up as it warms up and will finally settle and remain very steady. (Very steady by non GPS Controlled Standards) 

    Bob Larkin W7PUA has come up with a nice software modification that Brooks has incorporated into the PIC software (look in the Setup  Notes) that helps in setting the frequency to 10,000,000.0hz.  Connect an ANALOG METER to pin #2 of the PIC (P4 #6 on the A&A PCB)  (DO NOT USE A DIGITAL METER, AS THE READINGS WILL DRIVE YOU CRAZY). 

    Set-up an analog meter connected to Pin #2 of the PIC to read about  2 volts full scale.   This voltage reading, will sort of stair step up or down depending on which side of 10,000,000 hz you are at.  When the oscillator is on frequency the voltage will still move around the 1 volt area as the phase angle changes from reading to reading,  but not too much and the stair stepping will stop.  You do get new readings every second, but be patient, this is a very slow process until you get it right. The meter MUST sit very near 1 volt for 30 seconds (longer than 30 seconds is better) before switching to Mode 1 (N=1).

      Left is a view of the front panel.  (The  -.151 was the DAC voltage that is controlling the oven at that time).  The three spst switches below the Mode -7 label were added to allow front panel control of the Mode Settings of the PCBoard, and are wired to the 4 pin connector I added to the PCBoard. (Labeled jumpers to front switches  in the pc board picture above). These were added to make changing MODES (N=1 to 7)much easier without having to open the chassis. They are wired in parallel with the DIP Switches 1,2,3, on the Shera board and make setting the mode much easier.  The switches are set in Mode #4 in the picture.

     The spdt switch below the Digital Meter switches the meter between monitoring  DAC voltage and or in Setup position I can check the EFC voltage on the HP10544A pin #6.  Normal position is on the DAC.  The switch is not necessary but is a nice way to check on the operation of the unit.  It is real easy to notice day to day changes in the ham shack room temperature by just watching the DAC voltage.  My unit has run for five years now (starting March 2001) and the DAC control today is  +0.457v  with the EFC voltage now close to  1.277v up about .277 volts from the approximate setting of 1.000v during the initial setup and a readjustment made 3 years ago.  This is still well within the DAC control range and is caused by the long term aging of the crystal.  The unit will now hold lock in MODE 5 (N=5) most of the time.. but for long term convince I run in Mode #4 and never lose lock.   For use with the DSP-10  Mode 4 (N=4) is more than accurate enough so we have lots to spare.  I can lock in Mode 6 but due to oscillator drift and temperature changes in the shack we will lose it after a few hours and not be able to reestablish lock without dropping down a mode a step or two.

     The 3 leds to the right of the DVM, monitor Pins 2, 3, &4 of the PIC on the PCBoard. The 1PPS Green is the heartbeat pulse from the GPS and lets you know all is working.  The led pulse is on for one second and off for one second from the Shera board.  In Mode the High and Low leds help in manually adjusting the oven to 10,000,000 hz by indicating if you are high or low but they only update every 30 seconds so it take time to adjust.  Refer to the Setup Notes from Brooks to understand how the leds  work in the Modes -7.  Remember, in Mode the DAC is shut down and the PCBoard does not control the EFC voltage.  The DAC does not quite fall to Zero Volts (in Mode ) but rests at only a few millivolts offset depending on your DAC.  Zero is not necessary.  So, in my case, in Mode the EFC voltage on Pin #6 is approximately +1.000volt  and if the oven has been manually adjusted to 10,000,000.0Hz, the High and Low leds will be out. (easier said than done)


Buffer Amplifier                       Distribution Amplifier

A distribution amplifier is used to isolate and amplify the HP OCXO output from all the devices you connect to it.  It must provide isolation, be noise free and distortion free as to not cause problems with equipment interaction (like ground loops and phase noise).

The 10MHz signal from the HP oscillator is not too stiff ( this means the signal level drops under additional loads) and needs some help.  Since I wanted to use this 10MHz frequency source for several different functions in the shack I decided to buffer and amplify the output with a distribution amplifier.

I have a Mini-Circuits ZFSC-2-1 Power Splitter connected to the output of the Hp oscillator.  See EFC Supply file. This provides about 55db Isolation between the Shera Board and the input of a distribution amp I built.  This isolation helps keep the digital signals from the Shera Board from being superimposed on the OCXO output as it is feed out to my equipment.   Basically this ZFSC-2-1 Power Splitter is a single section ( a bifilar wound toroid) of the 50 ohm 8 way splitter in the next paragraph and provides two outputs from the OCXO in a neat little aluminum box.  One to drive the Shera board and the other one to drive a buffered distribution amplifier described below.

    My distribution amplifier includes a 2N5109 transistor buffer amplifier connected  to the input of a  50 ohm 8 way toroid power splitter.  The Splitter uses a 6dB matching padded attenuator on its input and is built with 10 small toroids provides 8,  50ohm isolated outputs. There are simpler splitters but they do not provide the isolation on the outputs that this circuit does. 


CONSTRUCTION of the Power Supply

Don't skimp on your power supply!

        I underestimated the power supply requirements for my unit (mainly the cooling) and this caused us more headaches than anything else in the construction of the frequency standard.  The power supply must be overbuilt to almost commercial standards as this unit is designed to run all the time. 

    By using a heavy transformer the power supply will be run running warm but running NOT HOT.  I found a old surplus transformer to handle the load and the two outputs while KO7N used two separate transformers to handle the the two outputs.  

    The transformer must provide + 24 volts at 550ma for the oven (drops to around 160ma after about 6 minutes) and + 12 volts at 670ma.  ( the crystal oscillator draws 60ma:  the Shera board draws 60ma:  the GPS receiver, interface board and gps antenna, all together about 180 ma:  the buffer/amplifier board 150ma,  the Digital Led readout about 200 ma (depending on how many segments are on):  and the EFC power supply on the perf board, draws just a few ma from the 12 volt supply for a total around 670ma). 

     Use large seat sinks on the regulators in your power supply to keep them under thermal control.  And FUSE EVERYTHING since this unit is designed to for continuous operation.  Don't skimp on your power supply!  A HOT power supply will eventually fail.



 Good Luck on your GPS Controlled Frequency Standard! 

        I wish to thank Brooks Shera,  W5OJM, for all the hard work in the original design. He has done a outstanding job of making it possible for the average Ham to build a state of the art, stable frequency standard at a reasonable cost. 

      I hope this construction page helps you with your version of this project.

     Drop me a line via e-mail if you have any questions about this unit and I will try and answer them. 

Jimmy Oldaker  W7CQ
27640 Gibraltar Loop
Eugene, Oregon 97405
541- 686- 8396

 E-Mail  W7CQ

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Copyright 2006 - Jimmy Oldaker    Last edited:  14 March 2014 10:15 PM

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