# Clock Signal Probing Options

Subtopics

- Single-Ended Signals
- Differential Signals
- Probes

The following table describes the clock source probing options for phase noise analysis. Note that two copies of a signal are required in order to use the two-channel cross-correlation technique to lower the oscilloscope's noise floor.

Clock signal type | Connection method (into #input channels) | Phase noise source 1 (for example) | Phase noise source 2 (for example) | Uses 2-channel cross-correlation technique | Notes
--- | --- | --- | --- | --- | ---
Single-ended | Direct (1) | Channel 1 | None | No |
Single-ended* | Split (2) | Channel 1 | Channel 2 | Yes | See Single-Ended Signals.
Differential | Converted into a single-ended difference signal using a balun transformer or limiting amp (1) | Channel 1 | None | No |
Differential | Converted into a single-ended difference signal using a balun transformer or limiting amp, then split (2) | Channel 1 | Channel 2 | Yes |
Differential | DiffP, DiffN (2) | Channel 1 | Channel 2, inverted | Yes | Does not yield the correct result if there is common-mode noise on the differential clock or if there is uncorrelated noise on one of the polarities of the differential signal.
Differential | Single-probed using one differential probe (1) | Channel 1 | None | No | Not recommended because a probe's attenuation of the SUT reduces signal-to-noise ratio (SNR).
Differential | Double-probed using two differential probes (2) | Channel 1 | Channel 2 | Yes | Two-channel cross-correlation removes probe and input channel noise. However, double-probing is more difficult to attach and gives twice the loading. See Probes.
Differential* | DiffP split, DiffN split (4, for example, DiffP1 into ch1, DiffN1 into ch3, DiffP2 into ch2, DiffN2 into ch4) | Channel 1 - 3 | Channel 2 - 4 | Yes | Best signal-to-noise ratio. See Differential Signals.
* = recommended for this signal type

## See Also

- Two-Channel Cross-Correlation Lowers the Oscilloscope Noise Floor