[Elecraft] Unique synthesizer LO ?

[Don Wilhelm]

Peter,

Let's see if I can help.  I think of the PLL and VCO in two separate pieces.
The VCO and the PLL chip (U4) lock the VCO in 5 KHz steps.  The PLL
reference oscillator provides the fundemantal clocking and frequency control
for the 5 KHz steps.  The VFO range for each band is chosen by switching in
the proper capacitor shown in the schematic area labeled VFO Range
Selection, and frequency excursions (in the 5 kHz increments) is done by
controlling the voltage on the varactor diodes.

The frequency increments between these 5 kHz steps is done by 'pulling' the
PLL Reference Oscillator crystal, so the VCO output shifts in smaller than 5
kHz increments because of the change in the reference frequency while the
PLL synthesizer chip and the VCO loop are still locked onto the same
division ratio.

This may be a simplified viewpoint of the operation, but it has been
sufficient for my troubleshooting purposes - if the problem is gross
frequency errors, the source can be found in the VCO and synthesizer loop,
and small variations in frequency are usually found in the PLL Reference
oscillator.

Of course the control is all done from the microprocessor.  It uses values
stored in the EEPROM during CAL PLL along with the frequency that the
operator dials in to apply the proper values to the DAC which in turn
produce a specific voltage that is to be applied to the varactors to create
the correct frequency output.

It works well and produces relatively low phase noise.  Phase lock at 5 kHz
increments is easier to maintain than attempting to lock at a 10 Hz
increment.  I believe the prime contributor to phase noise should be the PLL
reference oscillator phase noise, although there will be some noise
contribution from the VCO lock control.  I have not analyzed the phase noise
aspects of the K2 in detail, so this is just speculation on my part, if
anyone else has done phase noise analysis, perhaps they will provide a more
definitive answer.

73,
Don W3FPR


> -----Original Message-----

> Maybe not the cleverest question of the year but I cannot resolve this one
> myself and I'm eager to understand.
>
> It is said that the synthesized LO of the K2 is of a unique design.
> Though, when I compare the LO phase noise specs of the K2 and the AOR7030,
> the latter seems to show much better specs.
>
> What makes the K2 synthesizer so special ?
> I'd appreciate any comment.
>
> Peter, PE1E
>