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The Roos 7100A testers use a common time base, to which everything in the tester is synchronized. The accuracy of this time base is sufficient that the frequency accuracy does not need to be re-calibrated for the life of the tester. Additionally, the tester has built-in internal diagnostics that confirm the frequency accuracy with a reliability of better than 99.9995%

Time Base Description:
The time base is a 10 MHz Temperature Compensated Crystal Oscillator (TCXO), located in the RIFL master. The 10 MHz is distributed through the system as a low-level differential signal on the RIFL interface1. 10 MHz receivers on the carrier PCB assemblies buffer and re-distribute the time base to the individual functional assemblies.

All assemblies that generate or measure signals are synchronized to the time base, including all the microwave sources, the receiver, the waveform generator, the sine generator and the digitizer.

Each of the microwave sources also contains a time base that is phase-locked to the master. This is important to the self-check capability of the system, as we shall see.

The tester's time base is calibrated before it leaves the Roos factory (ref. the document Calibrating the 7100A Time Base 10 MHz Oscillator). It does not need to be re-calibrated for the life of the tester, assuming the tester life to be less than 20 years, as shown:

- The TCXO that is used as the 7100A's time base is guaranteed by its manufacturer as follows:
Aging over 10 Years: +/-2ppm inclusive
Temperature: +/-1ppm over 0 to 55C

- The specification for frequency accuracy on the tester is +/-5ppm. Referencing the TCXO aging spec, and taking into account the temperature spec, yields a re-calibration interval of 20 years.

Internal Diagnostics:
The tester uses the same process to check the oscillators that NIST and other standards agencies use for their primary frequency standards: a consensus of oscillators.

- A typical tester has 5 oscillators in it that are all phase-locked together (RIFL Master, RecLO, Source1-3). The lock range on each oscillator is approximately +/-2 ppm. Any time a tester startup is performed or 'run all' is selected for the diagnostics, these phase locks are tested. If any oscillator has changed more than 2 ppm, the self test will fail and the system will report an error.

- The only way the system could fail to report an error condition would be if all the oscillators moved out of spec at the same time, in the same direction, by the same amount. The possibility of this happening is less than 1 in 200,000 making the internal self test better than 99.9995% reliable. Note that this is the most reliable way to test time base frequency. NIST only uses 3 oscillators for consensus testing. Roos uses typically 5, and as many as 6.

Using the multiple time bases in the system enables the tester to automatically perform this ultra-reliable self-test and negates the need for a primary frequency standard at customer sites.

The RI7100A testers, by design, have a frequency accuracy that does not need to be re-calibrated for 20 years. Additionally, the frequency accuracy of the tester is automatically confirmed any time a startup is performed or full diagnostics are performed.

1 On older testers, the time base in source 2 was the standard and it was distributed in a daisy chain configuration, using BNC cables.
2 RI7100A specifications are provided for reference only. For complete specifications, refer to the RI7100A specification document.

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