3.2 Waveform Capturing Using Active Probe
SiTime OCXO is a high-speed logic output device. It is critical that the proper logic and high frequency
measurement techniques are used along with the high-quality active probe in order to ensure best
measurement results.
SiTime recommends the following minimum equipment for proper clock waveform measurement
1) 4 GHz or higher active probe with capacitance <1 pF, such as a Keysight 1134B;
2) Oscilloscope with 4 GHz bandwidth or higher such as a Keysight DSA90604A.
A passive voltage probe should not be used as it adds a high capacitive load to the part and the long
ground lead clip is not suitable for high frequency measurement applications. The inductance of the long
ground lead coupled with the input capacitance of the probe results in a resonant circuit. The
consequence of this resonance results in the distortion of the clock signal. Typical manifestations of this
distortion include ringing, overshoot, and undershoot of the clock signal.
Eliminating such distortion requires a probe with the lowest input capacitance and a low inductance
ground lead. In addition, SiTime OCXOs are typically configured for fast rise and fall times (2.5 ns or less)
with 15 pF load. It is therefore critical that the probe tip ground be as short as possible, lowest
inductance, and the return path for the ground be located as close as possible to the trace carrying the
RF logic signal.
For waveform measurement, it's recommended to remove capacitor C5. Please refer to Figure A2 for
test point locations on the SiT6731EB. If used, the soldering probe head is recommended use R11
resistor pads or solder it over it if it is needed. (Figure 1).
Figure 1: Recommended points for soldering probe head
More details on the SiTime recommendations on the oscillator’s output probing can be found in
AN10028.
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