>>Is it possible to replace this signal into another of greater stability and accuracy for example generated on NI 6653?
Yes. You can find an example on page: http://zone.ni.com/devzone/cda/epd/p/id/5059%20he%20will%20install%20NI-SYNC
>> How the stability and accuracy of 10 MHz signal distributed through the PXI bus influences on the stability and accuracy of the frequency timebase and the sample clock timebase generated on NI 4461 card?
If You are not using 10 MHz signal distributed through the PXI bus for NI 4461, it has no influense - card uses it's own clock. You can always override signal on backplane with another one - more accure (for exampel from 6653) and then use it as a reference clock for 4461.
"An NI PXI-6653 or NI PXI-6652 module in Slot 2 of a PXI chassis can
replace the native PXI 10 MHz backplane frequency reference clock
(PXI_CLK10) with the more stable and accurate output of the OCXO or
TCXO. All other PXI modules in the chassis that reference the 10 MHz
backplane clock benefit from this more accurate frequency reference.
Furthermore, the DDS chip on the NI PXI-6653 or NI PXI-6652 references
its output to the backplane clock and also takes advantage of the superior
OCXO or TCXO accuracy. The OCXO or TCXO does not automatically
replace the native 10 MHz clock; this feature must be explicitly enabled in
software. The OCXO or TCXO output also can be routed out to the
CLKOUT connector"
Hope it will help
Best regards
Barbara
