I am designing an FSK transceiver that operates from roughly 902-928 MHz with at least 18 possible channels. Using the 5-bit DAC in my microcontroller, I plan to control a Crystek VCO (CVCO25CL-0902-0928). The voltage vs. frequency equation is as follows:
$$\\(0.5V,902 \text{ MHz})\\(3.5V,928 \text{ MHz})\\ \therefore f_\text{MHz}(v)=\frac{26}{3}v + \frac{2693}{3}$$
The 5-bit DAC at Vdd=5V will provide 18 unique frequency channels, however, I am uncertain of how my microcontroller will identify each of them. Below is a receiver block diagram:
900 MHz is far too high for the ADC of the MCU to sample but is there some sort of frequency divider to bring it down? Some FSK receivers have two band-pass filters for the mark/space frequencies but I plan to have 18 with a 1.355 kHz separation – that would require 18 tuned band-pass filters. My hope is to avoid using a DSP as well.
In other words:
- How can the MCU infer each individual channel with 1.355 MHz spacing?
- Is 1.355 kHz spacing to little (i.e. should I reduce the number of channels)?
EDIT
@Andy aka: The reason I have only 18 unique channels is because of the DAC resolution. A 5-bit DAC provides the following table of voltages. Plugging them into $$f_\text{MHz}(v)$$ gives:
The SAW filter I was thinking of using has a center frequency of 915 MHz (AFS14A26-915.00-T3). Haven't thought about interference as of yet but I plan to use a unique communication protocol depending on how many channels are used. If just two channels are used (2FSK) then I could have an address byte to identify the slave/master (somewhat like I2C). Alternatively, the PIC could switch to different channels that are less noisy. If you have more thoughts on this I would be interested!
Best Answer
Standard practise to use a mixer (multiplier), with a local oscillator to drive it. Don't forget you'll end up with two side bands when you do that, you might want to then filter one out, but you should have an antialiasing filter on the input to the DAC anyway.