This is a 1KHz audio oscillator using a Johnson Counter. ARTA software (http://www.artalabs.hr/) reports THD+Noise of 0.3%.

The Johnson Counter is very interesting. I believe I first ran into it in Don Lancaster’s classic 1977 CMOS Cookbook. My circuit uses an opamp summing junction to make the contribution of the resistor currents more accurate.

When you start from reset it clocks in ones until it is all ones. Then it clocks in zeros until it is all zeros. You get a 16-bit sequence from an 8-bit shift register. Then it repeats endlessly.

Since the sine wave is symmetrical you can choose the proper resistors to make an accurate offset sine wave that produces 16 samples. Notice that the resistors are symmetrical.

Each resistor adds a summing current at the proper time and then removes it at the proper time.

The accuracy of the sine wave depends on the accuracy of the resistors. Here I am using 1% resistors. I then use a first order filter followed by a second order filer to match to the frequency of the sine wave. With this I have measured a THD+Noise of 0.3% at 1 KHz.

There are two things to consider.

1. In order to reduce the THD+Noise further we would need to use 0.1% resistors or do more filtering. While 1% resistors are cheap, 0.1% resistors are expensive. It would be cheaper to use a microcontroller (such as the MSP430G2402) with a 12-bit SPI DAC (MCP4921).

2. Even at 0.3% THD+Noise we would need a tracking filter in order to get a wide range of frequencies.

But if you only need a narrow range of frequencies and 0.3% is good enough, then this is a low-cost way to do it.

There is a line output controlled by trim pot R20 and a microphone level output controlled by R23. The frequency is controlled by R4.

This is designed for 1KHz. Other frequencies can be produced if you change the values in the filters.

This board runs on +12VDC that regulates it down to +9VDC and uses an ICL7660 to produce -9V. The reason I took +12V down to +9V is because I want to be able to use the very cheap +12V switching power supplies from China via eBay. Some of these power supplies have 0.5V of hash on the output. Most of them are sold as LED power supplies (LEDs shouldn’t mind the hash) but some are sold as CCTV supplies. I bought two identical supplies (12V 1A) labeled for CCTV. With a load of 120 mA one had 50mV of hash (excellent) but the other had 200mV of hash (crap).

You can operate it from a 9V transistor battery by leaving out U1 (7809) and jumpering pins 1 and 3.

I recommend using IC sockets with machine-tooled pins and not the ones with leaf springs.

If you are not already a Maker (and you want to be one) see my tutorial on Making Things at www.jmargolin.com/making/jm_making.htm

JM