Small frequency offset in PXIE 5650 RF Signal Generator

[BobAtSky]

Using the soft panel controls for a 5650 RF signal generator, did a simple configuration:

RF OUT:  50 MHz at 0 dBm

Ref Clock Source: Board clock

Ref Clock Export:  REF OUT

Modulation: none

Connected the RF OUT and REF OUT to an oscilloscope, triggering off of the 10 MHz REF OUT.  I expected the RF OUT (at 5x the 10 MHz frequency) to be solidly in sync with the 10 MHz, it's not.  It's slowly walking across the screen.  On a Keysight 53230A the ratio is 4.999 999 996.  Very close, but not the exact 5.0 that it should be.  There are no front panel alarms on the 5650.

Question:  Is this expected? I would expect the internal PLL with a simple integer relationship would be spot on.

Bob

[Henrik_Volkers]

You need to configure the 'right' clock source. I assume the 5650 is using it's internal clock and ist not locked to the PXI 10/100 MHz clock.

Without looking into the spec/documentation, yes, the default configration choose the internal clock. (more information in the horizontal timing chapter in the help to your device)

I'm not shure about the the REF-OUT .. butto my understanding the PXI backplane clock distribution is dedicated to one specific slot (marked) to be able to replace it. 

[BobAtSky]

Henrik:

Thank you for you suggestion.  It doesn't matter if the card is set to its internal oscillator or the PXI clock, relative to REF_OUT the offset is a few ppb for a 50 MHz RFOUT.  Also verified by triggering a scope with the REF_OUT and watching the RFOUT slowly walk across the scope.

Given the simple integer relationship (RF OUT = 5 * REF_OUT), with a simple PLL the output would be perfect. Obviously, this isn't a simple PLL.

Bob