Wayne Maxwell KD4YGU
It seemed like every time I needed a good stable low frequency signal, for instance, to use as a PLL reference, out came the Protoboard to wire up a temporary circuit to get the job done. No more. Epson makes a neat crystal oscillator / programmable divider in a 16 pin DIP that can generate squarewaves from megahertz to minutes. Skeptical of the datasheet claim of 5ppm stability, due mostly to experience with many other crystal oscillator DIPs (they output near the stated frequency, but never at that frequency), I decided to try one. I was pleasantly surprised when my frequency counter read 100000 hertz when I applied power to the IC, but was thoroughly surprised when it was still reading 100000 the following day, after running all night. At 5ppm, this oscillator should have been within ½ hertz of 100khz, and it was. Then it was just a matter of putting together a nice compact assembly so as not to take up too much space on the workbench. As can be seen from the photos, it doesn’t take up much space.
The generator is strictly ‘no frills’, simply two switches for frequency selection, an output jack and power input jack. The output frequency is selected by placing each switch, A and B, in one of eight positions as determined from the front panel chart. The range of this unit is from 100khz to 20 minutes in various steps. The generator outputs a 5 volt squarewave for use with TTL or CMOS. If small size is not overly important, an adjustable output could be added to allow it’s use in higher voltage CMOS circuits.
I decided to go with a power-pack style DC power supply to save space, and salvaged one from an old, non-working portable phone, along with the PC mount mating connector. The connector is not necessary, but I used it because I had it. The supply is rated 12 volts at 200ma, and is overkill as the IC operates at 500ua. I included a 1k ohm resistor in the design just to give the regulator something to regulate. Pretty much any DC power-pack between about 7 volts and 24 volts should work. The 24 volt limit is because 35 volts is the maximum input for the 7805 voltage regulator used in the circuit, and most all power-packs output higher than the stated voltage with no load, and 500ua is for all practical purposes, no load.
The project box is from Radio Shack and measures 2” by 4” by 1” deep. The switches are Grayhill series 94 octal rotary DIPs from Digi-Key. They are the ‘extended actuator, right angle mount’ style switches with the ‘open window’ style knobs. Of course, switches of many other styles will work just fine, to match any type of construction. I was going for small, and this type switch fit the blueprint. The oscillator IC is an Epson SPG8651B, also available from Digi-Key. Three other oscillator types are offered that operate at different frequencies. I have only used the 100khz IC that is used here, and cannot comment on the other three types, other than saying that the datasheet specs are probably accurate. The 1mhz oscillator is stated to have 100ppm accuracy, and that is just a little too loose for my intended application. If you have ever tried to lock a 2mhz PLL with a slightly drifty 100hz reference, you know what I mean.
As can be seen from the photos, printed circuit board construction was used. Two boards are used, one to hold the switches, and another for everything else. Because of space limitations, the filter and bypass caps are mounted on the foil side of the board. The project box has vertical guides to support the board with the switches, and the second board is soldered to this at a 90 degree angle. The other end of the second board is soldered to the RCA phono jack, which provides support for the board as well as being the output port of the unit. No hardware is used in mounting either board. I made a third board to support the power jack for the power-pack plug. No etching was involved with this board and its only function is to hold the jack in place. I didn’t include photos of this board because it is unlikely that this power jack is an off-the-shelf item. The jack is not a necessity, although it does make the overall project cleaner. Perhaps a chassis mount, thru-hole type jack could be found.
The front panel frequency chart was done in Microsoft Word, with data from the Epson datasheet. It is printed on 8½ by 11 adhesive-backed paper and cut to size. Clear, adhesive acetate may be used to cover and protect the paper. That’s about all there is to it. Whichever method of construction is chosen, should produce a long lasting versatile piece of test equipment for your bench.
www.digi-key.com/ Parts supplier
www.eea.epson.com/ SPG8651B datasheet
www.grayhill.com/ Series 94 Rotary DIP switch datasheet
You can contact me at firstname.lastname@example.org