Adjustable Clock Generator between 15.5 MHz and 17.4 MHz

Question

I'm looking for an inexpensive component to generate frequencies between 15.5 MHz and 17.5 MHz. My intention is a medium-run product (a few hundred units).

The issue is that I would like to change the frequency during operation and the change should not take more than 1 us. The control part will be performed by some kind of microprocessor.

It would be big advantage if the chip I'm looking for could simultaneously generate other frequencies which would be a division of the current main frequency and phase shifted.

I have thought about using an FPGA, but I don't have any experience with it, so I don't know what to look for.

Answer 1

Here is a programmable clock chip that may satisfy your requirement:
http://www.silabs.com/Support%20Documents/TechnicalDocs/Si5351-B.pdf

You can also purchase a low cost PCB module with this part already installed and ready to use with a microprocessor interface:
http://www.adafruit.com/products/2045?gclid=CIyi-r-1k8QCFYE7gQodvZ4Ang

Answer 2

Try looking up Analog Devices range of direct digital synthesizers. Here's one: -

Picture taken from here.

Features and Benefits

• Programmable frequency profile—no external components necessary
• Output frequency up to 25 Mhz
• Predefined frequency profile minimizes number of DSP/µcontroller writes
• Sinusoidal/triangular/square wave outputs
• Powerdown mode (20 µA) +2.3 V to +5.5 V power supply Extended temperature range −40°C to +125°C
• 16-pin TSSOP

Answer 3

I have thought about using an FPGA, but I don't have any experience with it, so I don't know what to look for.

Pretty good idea. A FPGA will do that, but a FPGA will be overkill for your needs.

You can use cheap CPLDs though. They are like a small FPGA, fewer pins, easier power supply and easier PCB layout. You also don't need external storage for the bit-stream (may save you a flash chip).

For programming these kind of chips I suggest that you just dive in and give it a try. Cheap FPGA and CPLD boards are available everywhere, and you should be able to go from nothing to a blinking LED within a day or two (It took me an evening the first time I did it, and most of the time was spent installing the toolchain).

Once you have this running, going from a LED blinker to a frequency synthesizer is just a few lines of HDL code.

You can even start without having a FPGA/CPLD at all by using a simulator. GHDL worked pretty well for me and has fine tutorial. http://home.gna.org/ghdl/

If you want to read about CPLD parts check the Xilinx CoolRunner2 line of chips. Super cheap dev-boards are available as well and the chip should be up for your task. If you're curious and want to dabble with FPGA I suggest either a cheap Xilinx Spartan board (I started with the XuLa200) or take a look at the Lattice parts. They sell development kits for around $25 including everything you need to get started. Oh, and they have cheap low pin-count parts as well. Something you don't find often in the FPGA world.