Creating a large number of crystal-stabilized pulse waves at precise frequencies

Question

I intend to generate 22 separate 50% duty cycle pulse or square waves. The frequencies range from about 5 Hz to 350000 Hz and are all irrational numbers, so I would like as much precision as possible.

Is there some multi-channel microcontroller setup or frequency synthesizer IC which can take crystal clock sources at commonly available frequencies and use complex phase-locked-loops (PLLs) to make a large number of arbitrary frequencies?

Another post discusses using a PIC32MX1xx or 2xx series to generate a precise frequency based on complex PLLs and it seems like it would work well. But how many PLLs can one of these microcontrollers conduct simultaneously? Ideally I would like to avoid using 22 separate microcontrollers, but I suppose I could since a PIC32MX110F016B-I/SP is just $2.93. My budget is about $100.

Answer 1

Not a problem. Ultimately, those "pulse" outputs are bandwidth limited, so in most general sense, for each channel you need a DAC with enough bandwidth to capture those requirements, and then the frequency generated can be arbitrary, up to the limits imposed by the SNR of the DAC.

Of course, there's a question of where you'll get a reasonably priced chip with a "bazillion" of integrated DACs and/or PLLs or FLLs that would work for your application.

I believe that either a small FPGA running at 100MHz and updating the binary outputs at that rate (10ns p-p jitter, much better long-term resolution), or a Parallax Propeller II would work.

The latter has a dedicated 8-bit DAC on every GPIO pin, and can update those DACs as fast as the cores can generate the values (update rates > 100MHz per DAC). It takes one instruction cycle to load 4 samples into a DAC.

In essence, Prop II could generate those pulse waveforms exactly to the bandwidth you need, with timing resolution likely much better than 1ns for each transition. That is generally considered a hard thing to do without fast DACs. The chip costs on the order of $20, and is dead easy to develop for - in fact, you could implement the whole thing just from the FORTH interpreter available in the boot ROM, without any PC-based dev environment if you really wanted an 80s-like workflow using a serial terminal :)

If you asked me to implement it, there'd be a table of pre-computed transition "snippets", say 8 samples long, with 100ps spacing. Assuming 100MHz DAC update rate, that's 800 samples, or about 256 32-bit HUB words to generate transitions with 100ps timing resolution.

Another approach - quite likely entirely sufficient for your needs - would be to use the high-resolution PWM frequency generators, also one per GPIO pin.

The chip has 64 GPIO pins.

Prop-II has enough "oomph" on board to serve as the transmit chain of a 100BASE-T PHY using nothing but onboard DACs, and with external ADC it can implement several full 100BASE-T PHYs in software. If you can afford the development costs for a high-volume low-cost product, it's quite a versatile part.