137.7kHz oscillator

This article describes a simple oscillator for the DX portion of the 2200m amateur band.

Design criteria

The 1kHz reference oscillator is a handy utility for calibration and frequency measurement by beating with an SSB receiver.

Design

The circuit uses a common NTSC colour burst crystal at 3.579545MHz to clock a Atmel AVR, ATTiny25, which divides the clock by 26 for a symmetric square wave output at nominally 137.675Hz.

Implementation

Circuit

The heart of the reference is an Atmel AVR MCU, an ATTiny25. It uses a crystal oscillator for the clock, and divides the clock down to 137.7kHz. The /KEY line can be used to key the 1137.7kHz output, tie it to ground for continuous output.

The circuit is so simple as to be suited to building on a piece of Veroboard in just minutes.

Frequency can be adjusted slightly by padding the crystal with a trimmer capacitor.

Programming

Table 1: Parts list
Part	Value	Comment
C1	0.001µF
C2	15pF
C3	15pF
C4	0.1µF
C5	10µF
D1	1n751
IC1	ATTiny25 or equivalent (45, 85)	programmed as below
R1	470
X1	3.579545MHz crystal

set PRG=usbasp
set PORT=usb
set OPTS=-B 10

rem program flash
avrdude %OPTS% -c %PRG% -P %PORT% -p t25 -U flash:w:137700.hex

sleep 1

rem program fuses
avrdude %OPTS% -c %PRG% -P %PORT% -p t25 -U hfuse:w:0xDF:m -U lfuse:w:0xFD:m

Above is a batch file for programming the flash and fuses using AVRDUDE. The code should also run in ATTiny45 and ATTiny85. After the fuses have been set correctly, comment out the last lines.

Warning

Make sure that the fuse bits are correct

Programming fuses can have unintended outcomes. The ATTiny25 MUST have a working clock oscillator to program it in ISP mode. If fuse changes prevent the clock oscillator running, it may be recoverable with a High Voltage Serial Programmer (such as an AVRDragon), but may not be recoverable with the more common ISP programmers. Be careful!

Once programmed, the fuse bits do not need to be reprogrammed, they are not affected by flash erasure during the normal flash reprogramming cycle. It is better not to keep reprogramming the fuses, it only risks a mistake!