Extremely simple 8bit, 2MS/s direct digital synthesis board: Basically it's a microcontroller conntected to a DA converter. Includes an output amplifier and an optional USB connection.

Read more...

A digital noise source produces seemingly random low/high pulses. Applications include testing of transmission lines and generation of analog noise using an additional filter stage.
Features:

• Build-in clock with 20MHz (100ns basic pulse width); external clock available.
• Clock prescaler by 1, 10 and 100.
• Selectable period in 8 steps from 15 to 2^31-1.
• 5V, can drive into 50 Ohm.

Circuit and additional info...

A tiny variable clock generator: No precision thingy, compared to crystal stabilized oscillators but reasonable for enough "quickly need a square wave" applications.
Features:

• Frequency range 1kHz to 30MHz
• 50% duty cycle (±1% <2MHz, ±5% <20MHz)
• 5V, low power, logic compatible

Circuit and additional info...

When testing digital circuits, especially microcontroller-based ones, the above wave generator is not the device of choice. For reliable clock cycle counting (to see how many clock cycles some instructions take), you need a very stable and accurate clock frequency, preferrably 100kHz, 1Mhz or so. For my Atmel microcontroller works, I needed some clock generator with the following properties (most of which the above device does not fulfill):

• Up to 10MHz (which is the highest frequency for the ATMega161)
• 50% duty cycle at every frequency
• Precision oscillator at exactly defined frequencies, e.g. 100kHz, 1MHz, 10MHz
• Freqency selection without the need to use an Oscilloscope or frequency counter
• Switching frequencies without dead/undefined gaps (as with analogous switches) to allow changing the clock speed while running

There was another critical aspect: I needed the device now, quickly, preferrably with those parts available at home. After one day for the design and the manual PCB layout, I came up with this precision clock generator which I actually made to work using one additional day.

Intersil's ICL8038 is a simple VCO (voltage controlled oscillator) chip which is "not recommended for new designs" but still available and relatively cheap. Due to its wide spread application it is easy to obtain and most likely still available in the future. It is capable of producing sine, triangle and square wave output with a frequency up to several hundred kHz.

I used the ICL8038 to build a simple oscillator which should fulfill the following requirements:

• Frequency range <1Hz to >100kHz
• High output power (>1A) for the sine output (e.g. to drive inductivities) with some protection against destruction
• CMOS-compatible complementary square wave output for digital circuit clocking (can drive into 50 Ohm)
• Single-supply operation (well, sometimes I regret this one)
• Affordable in prize and availability of the parts.

After some experiments, I built an ICL8028-based Oscillator (follow the link for more info; currently this is the 3rd revision dated 09/2004).