Why does a 10 MHz reference multiplier PLL use a shelf filter?

[pigrew]

I'm trying to understand the design of the external timebase input of the Agilent 33120A waveform generator (perhaps to build a substitute option 001 board). It uses a PLL to multiply the 10 MHz input to become a 40 MHz system clock. I have two questions about its design:

• Why does the PLL loop filter use a shelf filter instead of a simple low-pass filter?
• The 33120A reference input is 50 ohms. However, the later models (33220A, 335xxB) all have 1kohm input impedances. In fact, I don't see a pattern for which instruments use 50 ohms and which use 1kohm. Should I prefer 50 ohm or 1kohm inputs? Is it that the 1kohm inputs allow the instruments to be connected together with BNC tee-connectors, while the 50ohm input equipment should be used with a distribution amplifier?
  

The option's schematic is in the option 001 service manual.
 The 33120A's service manual is also available.

Thanks!

EDIT: correct link for service manual.

[T3sl4co1l]

The VCO converts voltage to frequency.

Frequency to phase is an implicit (and exact!) integrator.

The loop therefore has an integral (1/s in Laplace domain) term, and corresponding 90 degree phase shift, built into it.

A pole-zero or lead-lag compensator is therefore needed.

Finite DC loop gain is not needed, but they went for R121 anyway I guess.

Tim

[Benta]

What Tim said.
A type-2 PLL system (0 deg phase difference at lock) needs an integrator in the loop filter.
This one is 100% by the book.
The relationship between the two resistors sets the damping of the loop, the relationship between cap and resistors set the natural frequency of the loop.

[Wimberleytech]

Maybe you could help us help you by linking to the proper manual.

[bson]

I guess 1k makes it less sensitive to AC vs DC, sinus vs square, voltages, and DC offsets.  Frequency standards come with so many different kind of outputs that I'm sure that 50ohm resistor would commonly need replacement.  I assume they decided 1k works just as well at 10MHz while providing greater resilience and compatibility.