Total posts : 45366
It’s not easy.
The GPS receiver module originally referenced, like many others that come and go on the market, has a “1 pulse per second” output pin that is typically “TTL” logic compatible.
The simplest way to sync multiple transmitters would be to use a PLL/counter circuit to multiply the 1 Hz reference frequency from the GPS up to the operating frequency. In reality this might involve one PLL/counter circuit to multiply 1 Hz to 10 kHz which would then serve as the reference for a second PLL/counter that multiplies the 10 kHz reference to the operating frequency (in increments of 10 kHz). Multiple transmitters will then be phase locked to the 1 Hz GPS reference, resulting in essentially identical frequency and phase at each transmitter.
This simple approach has a MAJOR drawback: loss of the integrity of the 1 Hz signal will cause the transmitter’s frequency synthesizer to go into la la land. The transmitter’s frequency will be unpredictable.
So, the safe, but more complicated, way to do it is to use the 1 Hz GPS signal through a PLL/counter frequency multiplier to produce the same frequency as the transmitter’s crystal oscillator. The phase difference between the multiplied GPS reference and the crystal oscillator is then used to “pull” the oscillator frequency until the frequency and phase are identical. The oscillator needs to be a “VCXO” (Voltage Controlled Crystal Oscillator). A varactor diode is used to vary the capacitance across the crystal to adjust the frequency in a range of a few hundred Hz. A phase comparator circuit must be used to detect the phase difference between the GPS-based reference frequency and the crystal oscillator frequency. The phase-difference voltage is applied to the varactor diode resulting in locking the crystal oscillator to the GPS-based reference.
The crucial advantage here is that loss of the GPS-based reference will only result in the transmitter frequency being off by a few hundred kHz.