A Google search will give you a few billion ideas. Which is the simplest/easiest/cheapest that you know of?
Generating a square wave and then filtering out the harmonics isn't a good solution unless the filter frequency can be varied along with the square.
Making a numerically controlled oscillator (NCO) with uC + DAC is very easy. Could be a fun FPGA project. An advantage to an NCO is that you change waveforms.
I did a low frequency numerically controller oscillator Arduino sketch (see http://wiblocks.com/docs/app-notes/nb1a-nco.html). At the bottom of the webpage are a couple of references to the original articles,
wiblocks.luciani.org: 4003). Can you provide the essential part here (e.g. a schematic and the principle(s) it is based on)?
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Feb 2, 2018 at 22:45
Wien bridge with a pot to vary the frequency. I bet you could build one for less than one US dollar.
You didn't specify the frequency (100Hz or 100MHz?) or how much the frequency had to be varied (0.01% or 1000% ?) or whether the frequency had to be varied by a voltage or a physical knob. Purity of sine wave and stability matters too.
A one-transistor FET Hartley oscillator is hard to beat for cheap.
You could also PWM or otherwise DAC values out of a table to produce sine waves. Then the filtering should be easier. A very cheap MCU could probably do it up to fair frequencies.
I might second some RC + opamp design in principle. Whether the output and adjustability suits you depends on the application.
There are also some function generator IC's, ranging from classic 8038 to various complex DDS thingies. They might not come that cheap, though.
I guess there's also the option of finding an affordable second hand lab signal/function generator. It might be a long search for a cheap one, but it's all relative. Or you could take a spare AC generator and turn the shaft with variable speed. Amplify for power/impedance/voltage :)
Cheapest DIY DDS signal generators (including sine wave):
You can still do it with a square wave and filtering out the harmonics. There are a number of high order filters that can be controlled with a microcontroller easily. This one allows the user to control the corner frequency with an outside clock (second square wave from the micro). Because of the large corner frequency to clock frequency ratio you could even do it without the need of another timer/interrupt with a simple software counter...
If you want to go the direct digital synthesis route with discrete chips, capacitors, etc. the result won't be nearly as compact as what could be done with a CPLD or micro, but would be pretty reasonable, especially since a significant amount of the circuitry could be shared among the five signal outputs.
Global signal generation requirement:
Per-output requirement:
More details to follow. Given an input of 4,096,000Hz, the circuit should be able to produce square-wave outputs from 2KHz to 512Khz in multiples of 0.5Hz for signals up to 2KHz, 1Hz for signals up to 4Khz, etc. Other techniques can be used to convert a square wave thus generated into a sine wave.
Here's a circuit diagram to show the concept:
This circuit includes a configurable frequency generator (5 switches select input frequencies from 1/16 of the input up to 31/16 of the input). I also threw on a rough square-to-sine converter. Note that unlike most filtering techniques, this one maintains a reasonably consistent amplitude over the frequency range. The wave is quite rough because the above circuit only uses 4-bit counters. The MOSFETs would be replaced in practice by 4066 pass-gates (4 per chip).
