To completely remove any doubt that you cannot use an external 10 MHz Standard Frequency to increase the accuracy of the Parametric Measurements in the 2465B, I spend several hours going through the pages of "Theory Of Operation" and looking at schematic diagrams of the area around the 10 MHz xtal oscillator.
It was soon apparent that using the Parametric Measurements to measure Frequency or Time, do not depend on the 10 MHz clock signal. What determines those readings is the information that is stored in the battery backed up NVRAM that is calibrated with the procedure in the CAL 09 - Parametric Measurements procedure. Once calibrated, that data provides the standards for measurement of signal frequency and time. It is just that simple!
To prove that to myself, once and for all I did a small experiment!
First I severed the tiny circuit board link between Pin 2 & Pin 3 of U2440A & B of the microprocessor clock circuit. I then carefully ran two cables out to a small switch that was wired to either reconnect pins 2 & 3, or allow the injection of my 10 MHz standard into Pin 3 to drive the Microprocessor Frequency Divider at 5 MHz. That 10 MHz signal out of Pin 4 of U2440B also feeds other parts of the scope with it's 10 MHz signal.
Using the Parametric Measuring to measure frequency of my standard 10.000,000 signal showed 9.967 MHz with a Period of 100.3ns. Flipping the switch to inject the 10 MHz Standard in place of the internal 10 MHz Xtal, gave EXACTLY the same readings! Proving once and for all that it is not possible to use an external standard to get more accurate readings out of the TEK 2465B.
Picture of area of interest attached.
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