Here is a basic example which puts 122 Hz pwm from Timer 2 on pin 11. I have added code so that if you jumper pin 11 to pin 2, you can read the pwm frequency with an interrupt.
There is a good tutorial on timers here Gammon Forum : Electronics : Microprocessors : Timers and counters
When you start using timers, you are entering “data sheet territory” and you may wish to stay with the default PWM frequency of 490 hz on pins 3,11 (Timer 2) or pins 9,10(Timer 1). Timer 0 has pwm output on pins 5 and 6 at 980Hz. The data sheet for the RCD-24-1.20 indicates that 490 hz should work but may be non-linear. Wawa has alot of hardware experience and he indicates it should work with default PWM if you don’t want to get into changing Timers.
volatile unsigned long count = 0;
unsigned long copyCount = 0;
unsigned long lastRead = 0;
unsigned long interval = 1000;
attachInterrupt(0, isrCount, RISING); //interrupt on pin 2
TCCR2B = 0;//initialize register
TCCR2B |= 1 << CS21 | 1 << CS22; // set Timer2 prescaler 256
pinMode(11, OUTPUT); // pwm pin 11 -> jumper to pin 2
analogWrite(11, 127);//50% duty cycle
if (millis() - lastRead >= interval) //read interrupt count every second
lastRead += interval;
copyCount = count;
count = 0;