MP20 - PLL Circuit
The MultiPIG PLL is modeled after the PLL used in the Elecraft K2.
The K2 used a serially programmed PLL chip while the MP used the
parallel input version.
The purpose of the PLL is to generate a MIXING frequency for the
MP, enabling the MP to transceiver from 0 thru 30 MHz.
The PLL frequency is determined by two controls; The reference
frequency generated by crystals X1 and X2 and also the
divider switches, set by S1 and S2.
The Reference frequency is either 8.192 or 10.24 MHz and divided
by 2048 within the PLL chip to give us a base frequence of 5 KHz.
Also, the voltage controlled oscillator (Q2) is fed to the PLL
and frequency divided to generate a 4 or 5 KHz signal. These two
frequencies are compared within the PLL and then the PLL generates
an output that represents an error signal. This error signal is
integrated in op-amp U2 to provide an error correction voltage,
which is applied to the varactors (D3-6) in the VCO.
The VCO tuning circuit is comprised of the varactors and L4.
The output of the VCO is amplified and buffered by U4, a nifty
video amplifier. It's output is then delivered to the Freq
counter, one of the transceivers's mixers, and the PLL itself.
The output is also connected to Q3 and Q4 which act as an AGC
circuit to stabilize the VCO output voltage at different freqs.
Switched S1 and S2 are cheap DIP switches. These could be
replaced with octal or hex switches if available. I know of
a source that sells decimal rotary switches that could be used
as octal switches and at 25 cents each, the price is right.
Output of the PLL/VFO is about 10-20 milliWatts, enough to be
a qrp transmitter in it's own right.
An important note about this circuit concerns C25, the large
variable capacitor. This cap is susceptible to microphonics.
The new versions of the MP will replace C25 with switched fixed
capacitors paralleled with a trimmer cap.
The fine tuning is done with the reference oscillator,
controlled by X1 and X2 crystals. These crystal frequencies
up "pulled" so as to provide a 5 MHz to 40 KHz range per
S1/S2 switch setting. In other words, at trhe low end of the
spectrum, you have a 5 KHz tune range and at 10 meters the tune
range is about 40 KHz. Not bad for a crystal reference.
PLL Schematic - Updated 03/11/01
NOTE: Pins 5 and 6 do NOT need to be connected to a switch. Also replace D9 with a 10k resistor.
