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G.8266 is the specification for a frequency Grandmaster clock.  It takes in a physical layer frequency , in the form of a 2048kbit/s an E1, T1 or SyncE signal, and generates a PTP signal out, using the G.8265.1 PTP profile for frequency.

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The wander generation, tolerance and transfer specifications are all identical to that of the G.812 clock, except that the output is PTP instead of a 2048 kbit/s an E1/T1 signal.

This document concentrates on how to test wander tolerance and transfer for a G.8266 clock.

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G.812 defines the tolerance limit in three ways: as an MTIE mask (clause 9.1.1), as a TDEV mask (clause 9.1.2), and a sinusoidal wander tolerance (clause 9.1.3).  The sinusoidal wander tolerance mask in Table 13 and Figure 5 is computed from the MTIE mask defined in clause 9.1.1, and shows the relationship between wander frequencies and amplitudes required in order to meet the MTIE mask.  It can be used in place of the TDEV wander tolerance discussed in clause 9.1.2.

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Wander Transfer

Wander Clocks act as low-pass filters to any wander on their inputs, therefore wander transfer is defined in terms of a bandwidth.  For an a Type I clock (Option 1 clock (European 2048kbit/s hierarchy), the bandwidth is 1mHz3mHz.  The clock is expected to filter out the frequency components higher than the specified bandwidth.  

This can be also be verified using sinusoidal tones at different frequencies, and measuring the attenuation of those tones in the clock output.  From this, the frequency response of the filter can be plotted and compared against the expected frequency response.  If the tone frequencies and input amplitudes are carefully chosen, this can be measured simultaneously with the wander tolerance. At least two points in every decade are chosen to show the shape of the filter response.

G.8266 Tone Frequencies for Tolerance and Transfer

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This is a single test covering both wander tolerance and wander transfer.  For wander tolerance purposes, the clock must tolerate the input without causing alarms, switch reference or go into holdover.  For wander transfer purposes, the output tone amplitude must be less than the maximum amplitude indicated.  There is no minimum bandwidth defined, therefore there is no minimum amplitude either.

It is assumed that the low-pass filter in the DUT is at least a first-order with a roll-off of 20dB/decade for frequencies above the -3dB point.The lowest tone frequency specified in

Table 1: Tone Frequencies for G.8266 Type I Clocks

Type I clocks belong to the Option 1 hierarchy based on 2048kbit/s. The maximum peak-to-peak amplitude at each frequency is taken from Table 13 of G.812 is 0.000012Hz (12µHz), which has a period of over 23 hours. However, the longest observation interval specified in the MTIE mask (Figure 3) is 10000s, equivalent to a tone frequency of 0.000032Hz (32µHz).  Therefore it is sufficient to make this the lowest tone frequency in the test, reducing the overall test time from 36 hours to about 13 hours.

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.  This is a translation of the MTIE mask defined in Table 9 of G.812, using the relationship between tone frequency, f, and observation interval, τ, of  f = 1/ πτ .  

The lowest tone frequency specified below is 0.0001Hz (0.1mHz). This is 30 times lower than the bandwidth of a Type I clock.  It has a period of almost 2.8 hours, and hence requires a test time at least that long.   While Table 13 of G.812 extends as low as 0.000012Hz (12µHz), with a period of 23 hours the test time would be extremely long, and being so far into the pass-band it is very unlikely that either the wander tolerance or wander transfer would be any different.

The possible input signals for a Type I G.8266 clock are SyncE Option 1, 2048kHz, or 2048kbit/s (E1).

The parameters used to generate this table were:

• Maximum bandwidth: 3mHzMinimum bandwidth: none

• Maximum gain peaking: 0.2dB

• Filter type: Linear first order with -20dB/decade roll-off

• Noise generation allowance: ±35ns
  (half the peak-to-peak high-frequency time error defined in clause 8.2)

• At least two points per decade

The filtering characteristic expected from the clock is shown in the following diagram. While there is no minimum gain specified, the frequency response is expected to be fairly close to the maximum. Appendix II of G.812 suggests the result should be within 2% of the mask for long observation intervals.

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Tone Frequencies for G.8266 Type II and Type III Clocks

Type II and III clocks belong to the Option 2 hierarchy based on 1544kbit/s. The maximum peak-to-peak amplitude at each frequency is taken from Table 14 of G.812.  This is a translation of the MTIE mask defined in Table 10 of G.812, using the relationship between tone frequency, f, and observation interval, τ, of
f = 1/ 2.5τ .  This is slightly different to the relationship used for the Option 1 hierarchy.

The possible input signals for a Type I G.8266 clock are SyncE Option 2 or 1544kbit/s (T1).

The parameters used to generate this table were:

• Maximum bandwidth: 1mHzMinimum bandwidth: none

• Maximum gain peaking: 0.2dB

• Filter type: Linear first order with -20dB/decade roll-off

• Noise generation allowance: ±35ns
  (half the peak-to-peak high-frequency time error defined in clause 8.2)

• At least two points per decade

 The The filtering characteristic expected from the clock is shown in the following diagram. While there is no minimum gain specified, the frequency response is expected to be fairly close to the maximum. Appendix II of G.812 suggests the result should be within 2% of the mask for long observation intervals.

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