iDirect TAC Tips – TDM Lost

TDM Lost

What are they?
They are an aggregate count of the number of times the remotes demodulator has lost lock on the Hubs Downstream carrier since remote was reset.

Where are they measured?
For QPSK, the 8 bit I and Q soft decision data is sampled at the symbol rate after demodulation on the iNFINITI remote.

How are these calculated?
The sign bit of each signal (hard decision) is correlated separately with the known 32 symbol UW (Unique Word) sequence. When a UW sequence passes through the correlators and all bits of I and Q match, there is a perfect correlation of 64 matches. For BPSK, only the I channel is correlated so the perfect correlation is 32 matches.

Since the channel is noisy, thresholds lower than the perfect correlation are used. A high correlation is used during initial acquisition to prevent a false lock and then once frame lock is achieved, a lower threshold is used to ride out periods of low SNR.

A coincident detection and aperturing method is employed to make the circuit more robust. Since the number of symbols per frame N is known, when the first UW sequence is detected, the circuit waits exactly N symbols before checking for another correlation. This one symbol aperture prevents false detection. After four UWs are detected in a row, TDM Lock is declared. If four in a row are missed, TDM Unlocked is declared.

What are FLL DAC limits on a remote?
FLL DAC lower limit is 0 and upper limit is 4096. If FLL DAC ever gets close to its upper limit of 4096 or its lower limit of 0, this indicates that its local oscillator is not able to lock to the bit timing of the downstream carrier.  This may cause remote to loose downstream carrier lock.

iMonitor is configured by default to show FLL DAC warnings for readings lower/higher than %12.5 of min/max limits (min. warning limit= 512, and max warning limit= 3584)

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