G.8262 / G.8262.1 Noise Transfer: How are the gain limit values for CAT masks derived?
The values in the CAT masks have different gain limits than the ITU-T standard appears to allow, why?
ITU‑T Recommendations define a “clean” or “ideal” filter, specifying the performance of the internal filter function in isolation. In practice, however, filter performance is measured at a device timing output, which itself has an allowance for generated noise.
Consequently, the device output has an independent noise generation performance specification that must be accounted for when making a noise transfer measurement at that output.
The gain limits in the CAT masks include both the internal filter performance requirements and an allowance derived from the noise generation requirement. The next section shows an example of how this works in practice.
Example: G.8262 Option 1 Wander Transfer
This example focuses on a single specific sinusoidal waveform used to test ITU-T G.8262 EEC Option 1 clocks, the same principle applies to all waveforms used when using the sinusoidal waveform method to test noise transfer for all SyncE clock types.
The G.8262 specification for noise transfer is:
In the passband, the phase gain of the equipment clock should be smaller than 0.2 dB (2.3%).
The minimum bandwidth requirement for an Option 1 equipment clock is 1 Hz. The maximum bandwidth requirement for an Option 1 equipment clock is 10 Hz.
More information regarding noise transfer can be found: What does Noise Transfer Mean?
The requirement is tested by applying sinusoidal waveforms at specific frequencies and comparing the measured gain against a limit mask. The mask is defined by the maximum (upper) and minimum (lower) passband gain requirements below 1 Hz, and by an upper limit beyond the passband above 10 Hz, limits are based on a first-order low-pass response with a 20 dB/decade roll-off. More information about this can be found: What Frequencies should be used for testing SyncE Wander Transfer?
An example CAT test result for a G.8262 EEC Option 1 clock is below, note that the mask is plotted using values to allow convenient and accurate analysis for the specific frequencies tested, other frequencies are not represented in the mask.
The below calculation uses the example of Point 3 to illustrate how the gain limits are calculated
The MTIE limit for an EEC Option 1 defined in G.8262 clause 8.1.1 is 40 ns (peak-to-peak). For use in the filter gain limit calculation, this value is converted to an equivalent bounded timing noise of ±20 ns, derived directly from the peak-to-peak limit.
The calculation for the Upper Limit CAT mask for this tone (frequency/amplitude) is below, displayed values are rounded to 1ns.
Input pk-pk wander amplitude = 250 ns
Clean filter maximum gain = 0.2 dB
DUT noise generation allowance = 20 ns
Ideal filter max amplitude = 250*10^(0.2/20) = 256 ns
DUT max amplitude = 256 + 20 = 276 ns
DUT Gain limit = 20*LOG10(276/250) = 0.854 dB
Lower gain limits
Where a lower gain limit is specified for a frequency (in the passband), the ideal filter is assumed to have the conventional minimum gain (maximum attenuation) requirement of −3 dB. The noise generation allowance is then subtracted from this value to derive the resulting lower limit in dB.