https://teknokoodiradio.vuodatus.net/lue/2020/08/cc1120-generate-custom-frequency-modulation

Uncomment this in setup code

 // Burst address increment disable
 // writeExtAddrSPI(EXT_CTRL, EXT_CTRL_SETTTING);

  // Burst address increment disable
  writeExtAddrSPI(EXT_CTRL, EXT_CTRL_SETTTING);

modify in setup code

 // add burst variable in globals
uint8_t chipStatus  = 0;
volatile int8_t t   = 0;
volatile bool burst = true;

  v = readExtAddrSPI(MARCSTATE);
  Serial.print(F("MARCSTATE "));
  Serial.println(v, BIN);
  // add flush
  Serial.flush();

  // add digitalWrite(SS_PIN, LOW)
  digitalWrite(SS_PIN, LOW);
  attachInterrupt(digitalPinToInterrupt(CFM_TX_DATA_CLK_PIN), isr, RISING);
  // Terminate extended register burst access by setting SS_PIN HIGH

modify isr routine code

void isr() {
  // Foffset = (Fdev x value) / 64 [Hz]
  if (burst) {
    SPI.transfer(W_BURST_BIT | EXT_ADDR);
    SPI.transfer(CFM_TX_DATA_IN);
    burst = false;
  }
  SPI.transfer(t);
  t++;
  if (t > 39) {
    t = 0;
  }
}