Project #7 100 MHz PLL

From the TV synchronised 1 MHz PLL, the idea was to have a higher frequency PLL to enable locking a 100 MHz free running VCO.

The intention is to multiply an accurate 10 MHz from the narrowband 10 MHz PLL to 100 MHz to enable locking various Xtal oscillators of microwave LO’s by mixing them down to a lower more manageable frequency.

Operation is straight forward, the 10 MHz signal is applied to phase/frequency detector 74HCT9046.

The oscillator is a Minicircuit’s POS75 tuning from 37 to 75 MHz, a rather wide band tuning range.

The output of this oscillator is multiplied by 2 using a Mini-circuit’s RK3 doubler, it’s output feeding a divide by 10 and an amplifier, which in turn drives a Mini-circuit’s triple power splitter.

The divider is an oldie from the junk-box, as the Mini-circuit’s stuff are.

The PI filter around the OP27 op-amp is the PLL’s loop-filter. Due to the relatively large tuning range of the VCO, the time constant of the loop-filter is much smaller so the bandwidth of the PLL is larger. Due to this high gain the VCO’s close in noise will be reduced. Outside the loop’s bandwidth the VCO’s wideband noise will dominate.