Electronic – MCU clock drift and radio frequency drift – are they the same

oscillatorpllRF

Let's assume that I have two sensor nodes, one with crystal oscillator running at 24 MHz frequency, the other at 24 MHz + 10 ppm frequency.

To my understanding, in system-on-chip (for example, Texas instruments CC2650) based sensor nodes this single high-frequency crystal (MHz range) powers both the MCU and the radio (where GHz range is needed). I mean, the "local oscillator" component in RF diagrams that generates the 2.4 GHz sine wave is calibrated by using the MHz oscillator as the source (through PLL).

Assume that on both nodes the radio is configured to use the 802.15.4 channel 11, which has 2405 MHz center frequency. Is it the case that one node will communicate using 2405 MHz and the other 2405 MHz + 10 ppm?

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Is it the case that one node will communicate using 2405 MHz and the other 2405 MHz + 10 ppm?

Yes, one will transmit at 2405 MHz and the other will use 2405.02405 MHz plus there will be jitter caused by the PLL (in both systems) and this may be in the region of +/- 100 ppm at a variable frequency in the hundreds of Hz to low kHz range (with some randomness too). This dwarfs the static 10 ppm error.

When it comes to receiving, it is likely that the "misaligned" receiver will lock-onto the precise transmission frequency irrespective of its own slightly misaligned local clock. This can also be done using PLL techniques.

On more complex transmissions where the actual centre frequency can be missing (such as in phase modulation or other suppressed carrier types), a special PLL (called a Costas loop) is employed.

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Electronic – Generate a 4.25 GHz using 50 MHz crystal/oscillator and PLL

So a PLL uses 2 oscillators. One, the reference, is generally lower frequency, and the other needs to be able to operate at the output frequency. In this case, you'll need a 4.25 GHz VCO as well. Using a PLL IC, the 50 MHz Crystal, and the 4.25 GHz VCO, you'll be able to do what you've described you need. You should download Analog Devices SimPLL application, that will walk you through the calculations. There's no frequncy multiplication taking place in a PLL, only frequency division

National Semiconductor has an Excellent application Note http://www.national.com/en/clock_timing/pll_designbook.html that covers the details of PLLs.

The Prescaler details (64/65) for example are called "Dual Modulus" prescalers, and they work in conjunction with another divider to provide the correct divide ratio.

Electronic – How to compute Timer1 period

Two things to recognize:

the timer is an upcounter that interrupts on overflows at which point you should reload the start of the upcounter.
the frequency is the output of the PLL if you have one configured

= (f_osc / 4)^-1 * prescaler * (65535 - TMR1)
= (16MHz / 4)^-1 * 1 * (1000)
= 250us

Where timer is being used as a 16-bit timer and TMR1 is the value written to TMR1. I like to specify the preload as a negative number because it's more intuitive than seeing something as less than UINT16_MAX:

#define TIMER1_TIME  (-1000)

void InitTimer1(void) {
  T1CONbits.T1CKPS = 0b00;              // Prescaler is divide-by-1
  T1CONbits.TMR1CS = 0b00;              // Clock source is (f_osc / 4)
  TMR1H = TIMER1_TIME >> 8;             // Load the start of the upcounter
  TMR1L = TIMER1_TIME & 0xFF;
  IPR1bits.TMR1IP = 1;                  // Set to high priority
  PIR1bits.TMR1IF = 0;                  // Clear interrupt flag
  PIE1bits.TMR1IE = 1;                  // Enable interrupt
  T1CONbits.TMR1ON = 1;                 // Turn on the the timer
}

void MyIsr(void) {
  // Reload the start of the upcounter
  TMR1H = TIMER1_TIME >> 8;
  TMR1L = TIMER1_TIME & 0xFF;
}

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Electronic – Generate a 4.25 GHz using 50 MHz crystal/oscillator and PLL

Electronic – How to compute Timer1 period

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