It is nothing new anymore to design a lone IC into a small-footprint radio transmitter application, just providing power and a baseband input signal on one pair of pins and a simple antenna on another. Few designers would include stock PIC microcontrollers in this category of ready-to-go, monolithic RF systems. With the new peripheral set of some newer 8-bit offerings and a little creativity, however, the humble PIC can become a self-contained—if un-optimized—analog FM transmitter.  (Passive filters tend to be very useful as well, but ignore these for now.)

In fact no extra hardware beyond a powered PIC12F1501 is needed in order to modulate an input waveform out to a rudimentary antenna. Not much programming is required, either, once the peripherals have been configured.  So rather than sketch a schematic, I’ve just labelled the pins in comments at the top of the source code. An RF bandpass filter (not shown) is highly recommended to eliminate mixer products far away from the desired carrier frequency; these are visible in the FFT plot below.

This minimalist approach is possible because a heterodyne radio transmitter requires at the bare minimum just a local oscillator (LO) and a mixer to multiply the analog input with the carrier frequency. If phase modulation instead of amplitude modulation is to be accomplished, a voltage-controlled-oscillator (VCO) usually first converts the varying-envelope modulating signal into an intermediate-frequency (IF) constant-amplitude waveform.

The 12F1501 and 16F1700-series PIC parts actually possess just enough peripheral hardware even at the lowest pincounts to bring the dream of an inexpensive monolithic hobby transmitter within reach:

  1. a factory-trimmed internal LO built into most modern low-BOM-cost microcontrollers
  2. a serviceable VCO, cobbled together from two other blocks
  3. a mixer approximated by a single core-independent logic gate
  4. a differential antenna power amplifier in the form of a complementary waveform generator (CWG)

An eight-pin PIC12F1501 can be purchased for $0.77 online from http://www.microchipdirect.com, with the pins allocated as follows:

  • 3 pins for power/reset
  • 1 pin for the Vin audio signal
  • 2 differential pins for the antenna
  • 1 pin for optional LO input (to achieve carriers other than 16MHz)

The leftover eighth pin can function as a probing point for exposing either the LO frequency divided by 4, or the single-ended RF output.

The peripherals themselves route together internally to do nearly all the heavy lifting. In fact the only thing the 8-bit digital core is ever doing is copying 10-bit analog-to-digital-converter (ADC) samples into the 16-bit increment parameter field for the numerically controlled oscillator (NCO) as they become available from the conversion.  Together these form the VCO.  The NCO is overflow-based realized by a simple counter and therefore has extremely high phase noise, but it turned out to enable a good enough VCO for this demonstration.

Perhaps most surprising is that an analog mixer to produce an FM signal after the up-conversion can be approximated by a logic gate, if the baseband signal has been pre-modulated. Since all the FM information is contained in the phase of the signals, i.e. the rising and falling edges through the DC midpoint, two digital levels are enough to represent them. Consider the AC waveforms to have passed through a high-gain sgn(x) function so that a logic Low is a -1 and a logic High is a +1. Multiplying two such continuous time waveforms together to perform mixing in the frequency domain is exactly the exclusive not-or (an inverted XOR, the “^” operator in C) function since -1 * +1 = -1 (L ^ H = H ^ L = L) and -1 * -1 = +1 * +1 = +1 (L ^ L = H ^ H = H).

Rather than hook up a square wave generator I connected the handy 1kHz calibration signal output at the front of my scope. Waveform math at a high enough sample rate produced an on-screen FFT spectrum (in red):

An oscilloscope both provides a handy 1kHz square wave output and displays an FFT showing the first three harmonics of the PIC's FM of that square wave by a 16MHz "carrier."

An oscilloscope both provides a handy 1kHz square wave output and displays an FFT showing the first three harmonics of the PIC’s FM of that square wave by a 16MHz “carrier.”

16MHz is the maximum internal LO frequency, and while trimmed enough for an taking a screenshot is not very stable across time and temperature. The next available step downward in frequency is 8MHz.  By #undef’ining the INTCLOCK symbol in the code an external precision time reference (officially 20MHz or less, presumably from a crystal oscillator’s output) can be fed into the mixer, and will incidentally run the rest of the PIC logic as well—including the ADC. So be mindful that Nyquist frequency for sampling any audio also will increase or decrease proportionally relative to the default 16MHz internally generated carrier.

Finally, let’s give it a listen on an Agilent E4402B spectrum analyzer:

1kHz square-wave frequency modulated by the PIC onto its 16MHz clock.  The spectrum analyzer is demodulating it back to a 1kHz tone on a speaker!

1kHz square-wave frequency modulated by the PIC onto its 16MHz clock. The spectrum analyzer is demodulating it back to a 1kHz tone on a speaker!

This is a design exercise only to show what is possible. Standard disclaimers about ensuring compliance with local/regional regulations on spectrum usage apply.  FM deviation (modulation index) may be reduced by scaling each ADC output before copying it into the NCO control registers, but this step will further decrease Nyquist due to the extra processing that adds onto the conversion time.