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1.2GHz PLL Synthesizer



This circuit is based on the Motorola MC12210 Fractional N PLL synthesizer chip (which is unfortunately obsolete, but can still be found on eBay, etc.) It has generally been superceded by National Semiconductors LMX series, or ICs from other manufacturers like Toshiba, NEC and Infineon. However, the circuit is reproduced here because it formed part of a wireless student project and we happened to have a supply of these chips available. Of note is the use of a simple Clapp VCO based on a negative-resistance amplifier element and a microstrip transmission line resonator (whose high Q gives good phase-noise performance). Furthermore, the full frequency range (1050-1250MHz) requires 1-16V on the varactor diode, but the charge-pump on the PLL could withstand only up to 5 volts. Hence, the three-transistor voltage level shifting charge-pump at the bottom center part of the schematic was needed.






I did not have any luck digging up the performance measurements, but when I find them on some of my old backup CDs, I will post them to the site. I do remember that this circuit must be carefully shielded to eliminate any interference from power-line fields, because the stripline resonator is earthed through a 5.6Meg resistor, it is essentially “floating” at DC (well, 50-60Hz is DC at microwave frequencies!) 50Hz interference manifests itself as close-in (but outside of the loop bandwidth: about 750-1000Hz if memory serves me) phase noise where the interference appears on a spectrum analyser as frequency components separated by 50Hz.

The harmonic filter reduces the second-harmonic distortion by about 25dB and the output level of the fundamental is about 10dBm.

This circuit will likely be updated to use one of the modern PLLs for incorporation in one of the many “wireless” projects that I would like to build, when I find the time. When I rework the design, I will also post the software for the microcontroller which drives the PLL registers.