High frequencies with small changes possible?

[peterbmarks]

Thanks for this library. I'm interested in generating radio frequencies such as 7Mhz. I find that the available frequencies generated with your library are in quite big steps.

My application is to generate WSPR which consists of an RF frequency that slowly moves between frequencies in 1.4648 Hz increments.

Is the accuracy of the frequency a limitation of the PIO on the 2040? or is there a way I could use the library to achieve this?

[khoih-prog]

Hi @peterbmarks

Is the accuracy of the frequency a limitation of the PIO on the 2040?

That sounds interesting project, but your concern, IMHO, is more accurate than what the RP2040 can provide.

The TOP register to control PWM is only 16-bit, giving 64K resolution can't provide the 1.4648 Hz increments in MHz range (nearly 32-bit resolution)

I suggest you use the hardware solution, such as Si570 programmable crystal oscillator, as described in Standalone Multiband WSPR
beacon transmitter (MEPT)

The following is a design of a standalone MEPT beacon used for WSPR broadcasts [2]. It is
based on a Si570 programmable crystal oscillator (silabs.com) which is controlled by means of an
AVR attiny45 microcontroller. The Si570 device is FSK modulated by the AVR via a sequence
of I2C control commands that are sent to slightly change the frequency for every transmitted
WSPR symbol. The Si570 device outputs a power of 10mW which is sufficient for WSPR
operation (i.e. an antenna with some LPF can be directly attached). For increased power up to
500mW, a small BS170 based power amplifier may be added. As the beacon alternately changes
frequencies, the filters should be made switchable. Optionally this filter switching could be
controlled by the microcontroller via I2C and a PCF8574 device (not supported by supplied
firmware).

Just use the RP2040 to replace the old AVR attiny45

[peterbmarks]

Thanks @khoih-prog for getting back to me. Yes, I can see the issue. I have built these transmitters in the past using the Si5351 clock generator but was hoping to reduce the parts count.
Thanks again for your contributions.
Peter