Precise Decade Frequency Generator  

 frequency generator assembly

Here is a circuit to generate accurate frequencies of 1 Hz, 10 Hz., 100 Hz, 1 kHz and 10 kHz based on a crystal oscillator and a PIC programmed as a decade frequency divider. A stable frequency reference has many uses around the lab, and relatively low power consumption makes battery power an option. Since it uses an ovenized reference (OCXO) it is more stable than a simple crystal oscillator. My samples show a drift rate of about 1E-9 (1 ppb) per hour.

Circuit schematic: DecadeFreqGen-schematic.pdf
Bill of materials: BOM.pdf   BOM.xls
Circuit layout (Gerber layers): DecadeFreqGen-layout.pdf
PCB layout screen image: DecadeFreqGen-layout.png  sc.png
Image of unstuffed PCB:  PCB-blank-scan.jpg
Complete KiCad project:

PIC source code (ASM): PIC16F1823-26MHz-1pps_asm.txt
PIC object code (Hex): PIC16F1823-26MHz-1pps_HEX.txt
Decade Frequency Generator PCB Layout    blank PCB

I laid out the PCB with dual 7-15 V power inputs to facilitate moving the timebase from one place to another, switching from wall power to a battery power pack without powering down the oscillator and loosing synchronization.  The small ovenized oscillator I used (Pletronics OHM40480526) draws 70 mA when running, and 6xAA batteries could run the circuit for about 24 hours. The frequency is set by comparing the 1PPS output to a GPS reciever 1PPS, and adjusting the coarse and fine trim controls until the relative drift is zero.  Initial tests suggest this part can reach short-term frequency stabilities of 1 ppb (1E-9) over one hour, although the PLE OCXO spec is 100 ppb per 24 hours. Drift is generally better after the oscillator has been powered up for some time (several days).  At the time I designed this circuit, this OCXO was available inexpensively from ebay seller petlor.  If you need better accuracy you can use for example a "Thunderbolt" GPS-disciplined oscillator (used devices online start around $100), but it is larger, requires GPS reception for long-term stability, and draws 10 watts.

The board will accept any full- or half-can DIP oscillator, although some modification may be needed depending on the particular part. The source code can be compiled for 10 MHz as well as 26 MHz clocks, and other frequencies could be accomodated by tweeking the code. The PCB provides two footprints at right angles for the 7805 regulator, to allow for laying flat and heatsinking to the PCB, or standing up and connecting an external heatsink.

The circuit uses a PIC programmed as the decade divider. The original code and basic PIC connections are from Tom Van Baak at, as modified by Richard McCorkle and myself.  If you care about edge risetimes: this PIC does about tr = 17 ns. In comparison, the ATmega328P (of Arduino fame) has about 5 ns risetimes, and a CPLD output may do 1 ns or better.  I provided the option to run the PIC at 3.3V or 5V to allow selection of the output voltage level. A possibly better way would be to run at 5V and use a buffer chip, like a 74LCX125 or 74LCX14 to provide 3.3V outputs, which would also have faster risetimes.

As of November 2011, total BOM parts cost is about $25 per board, if you build 10 units at a time, excluding assembly costs.  Less if you have the many common components in stock and purchased in larger quantites.  Note a few parts (marked "0" qty.) are on the schematic, but not used. Also, if you run the PIC at 5V instead of 3.3V, you can eliminate the 3.3V regulator, and remove R3, C8 and substitute a 0-ohm for R2.  You stuff either R4 or R6 (of course not both) to select the PIC operating voltage.

You could also use the LM7805 +5V power rail instead of the LT1021 +5V reference, at a cost in stability of course.  One sample of the PLE 26 MHz oscillator I measured has a sensitivity of about 3 ppm / volt on Vtune (U4 pin 1), although it varies over the control range (for more details see 26MHz-OCXO-tuning.xls). So a 1 mV variation on Vtune will cause a frequency shift of 3 ppb.

Frequency Offset (ppb)        Offset Drift using Vref on both OCXOs
(Click on a plot for a larger image)

Drift comparison: the plot at left shows the frequency offset between two PLE 26 MHz OCXO oscillators, using two prototype versions of this circuit. One of the prototypes did not have the +5V reference for the control voltage, using instead the +5V power rail supplying the oscillator power and the internal  heater. The two oscillators change frequency relative to each other by about 3 ppb (parts per billion, 1E-9) over the course of 32 hours. At least part of this drift is due to ambient temperature change since varying current drawn by oven changes the +5V rail.  The plot at right shows the same two oscillators, but now both using a LT1021 +5V reference to establish their control voltage. This improves the relative drift, with the plot showing a 1.5 ppb change in frequency over 72 hours.  The (indoor) ambient temperature ranged between 19.6 C and 21.8 C during the second plot period (Nov. 12-15). I believe the temperature change was similar for the first plot, but did not record it.

 Note if you adjust two of these circuits to within a few ppb of each other, with outputs connected to a third instrument (sharing a common ground) stray coupling may cause the two OCXOs to phase-lock and you will read near-zero relative drift. Of course that is not a true measure of their independent stability.

UPDATE Nov. 25 2011  26MHz 1PPS generator PCB "dd1 v.1" (OCT.30 2011)

Circuit works, after this rework:
1) J1,J5: pins 2,3 reversed. FIX: remove unused side pin on jack, and short pins 2+3 on PCB
2) RV2: footprint pins straight, should be staggered. FIX: bend pin 2 on pot to fit footprint
3) U5 pin 3 missing pullup. FIX: add 10k between U5.3 and U5.1

John Beale

last updated: Oct. 31 2011