Programmable sinewave generator.

Build this programmable sinewave generator once, and you can program it forever!

A PROGRAMMABLE SINEWAVE GENerator can be a lot more accurate than an analog-based generator. When teamed with a PC or single-board computer, a programmable generator becomes an application-specific instrument offering a limitless variety of software-based "front panels" without the need to modify the hardware. Its a fun device to build and experiment with. Plus, you have to build it only once, but you can program it forever.

There are many tasks that can be performed with a programmable sinewave generator (PSG):

* Convert RS-232 signals to audio or RF frequency FSK (frequency shift keying) format for phone line or wireless modem transmitters

* Function as a signal source for bode plots

* Function as a linear, log, discrete, or programmed sweep generator

* Act as a local oscillator and carrier frequency generator for a transceiver

* Function as a general-purpose generator for industry, education, and experimenter labs

* Synthesize single-voice music compositions

* Generate telephone keypad tones

* Synthesize voice or sound

* Act as a general-purpose secondary frequency standard

* Generate frequency bursts

* Generate continuous-wave amplitude modulation

* Provide a stable, accurate voltage-controlled oscillator for amateur radios

* Generate automatic-test-equipment sweep signals

A computer's parallel port is ideal for controlling a PSG because the parallel port is easy to use, and there is one in most personal computers made today--including most laptop models. Although the serial port could be used, the required interface would be more complex, hardware-intensive, and expensive.

The heart of the PSG is an HSP45102 numerically controlled oscillator (NCO). The frequency to be generated is selected from two frequency control words. A single control pin selects the word that determines the output frequency. Switching from one frequency to another occurs in one clock cycle, with a six-clock-pulse pipeline delay from the time that the new control word is loaded until the new frequency appears at the NCO's output. Twelve-bit binary words selected from the chip's internal memory are fed to a Harris CA3338A digital-to-analog converter (DAC) that generates the synthesized sinewave output.

Once the sinewave has been synthesized, it is level-shifted to be symmetrical around zero volts and then filtered to remove the remaining components of the high-frequency NCO clock frequency (32 MHz). Finally, the sinewave is buffered and fed to the PSG's output.

How does it work? …