Characteristics and considerations for HOTLink jitter

Question: What are the characteristics and considerations for HOTLink jitter?



The phase-locked loops (PLLs) in the HOTLink Transmitter and Receiver act like low-pass filters to jitter that is embedded in the data or clock signal source. For the transmitter, the signal source is the CKW input. Any jitter that appears at CKW is passed unattenuated if it has frequency components below the natural frequency of the PLL filter (approximately 500 kHz). Spectral components above the natural frequency are attenuated at about 6 dB/octave. Frequency components that fall very near the natural frequency of the filter are slightly amplified (approximately 0.5 dB). These are the normal characteristics of a Type-2, second-order PLL filter. When the transmitter is fed by a low jitter clock source, typical output jitter will be less than 20 ps RMS and 200 ps peak-to-peak. It is possible to measure significantly more jitter than that which is actually present if the complete system is not well understood. A few hundred millivolts of Vcc noise, while insignificant to the logic of a normal system board, will add imaginary jitter to the measured output. This imaginary jitter appears because a single ended oscilloscope sees the waveform as if it were measured against a fixed threshold, while the differential serial interface sees Vcc noise as a common mode signal to be ignored (e.g. 100 mV of Vcc noise could create 100-200 ps of imaginary jitter). Likewise, the normal method of measuring peak-to-peak jitter, an infinite persistence scope trace, will show larger jitter than that contributed by the HOTLink Transmitter. Low frequency jitter (wander) in the oscillator, scope trigger, temperature, and voltage related delay variations will all contribute to the width of the stored scope trace. Delay variations include TTL threshold variations that cause apparent delay variation (e.g. 100 mV of TTL threshold change can cause 100-200 ps of apparent jitter).

The signal source for the receiver is the serial data stream and, like the transmitter, it passes the spectral components of received jitter that fall below the natural frequency of its filter (approximately 300 kHz to 1000 kHz depending on actual data transition density being received). Frequency components above the natural frequency are attenuated and there is minor jitter peaking at about the natural frequency of the PLL. Since the characteristics of the input jitter determine the jitter content on the receiver CKR output (the only place to directly measure Rx-PLL jitter) it is somewhat difficult to predict the output jitter. Maximum CKR output jitter is less than 200 ps (peak-to-peak) when the receiver is tracking normal data (BIST data is typical) that exhibits maximum tolerable peak-to-peak jitter. Jitter from normal data is wide-bandwidth, has significant high-frequency content, and can have peak-to-peak amplitude of up to about 90% of a bit time. If the serial data contains a significant low frequency jitter component (typical in crystal oscillators and some pulse generators) the output jitter measured on the CKR pin could be much higher. Jitter measurements at the receiver output can be more misleading than those associated with the transmitter serial outputs, since all measurements are made on TTL outputs. The jitter characteristics mentioned here affect system performance in the following ways. Any low-frequency jitter (below the bandwidth of either transmitter or receiver PLL) is treated as wander. For purposes of the PLLs, wander (usually caused by low-frequency power supply variations or temperature fluctuations within the timing ICs) does not reduce the system timing margins and does not contribute to bit-error-rate. Wander can affect system timing at interfaces where the transmitter clock source is used to clock information received from a receiver tracking data from another clock source. The variation in clock frequencies may violate set-up and hold times, the exact problems usually solved by FIFO memories in typical communication systems. High-frequency jitter (at or above the natural frequency of the PLL filters) may contribute to BER. High-frequency jitter can be caused by the clock source, media transfer characteristics, or external noise. The recovered internal bit-rate clock does not track high-frequency jitter above the PLL natural frequency. High-frequency jitter, therefore, may cause a bit edge to move into the receiver sampling window causing the bit to be erroneously sampled (a bit error).

A suitable clock source should be selected with the above effects in mind. The only clock source guaranteed to offer the required stability and high-frequency specifications is a crystal oscillator. High-frequency jitter is minimal, and low-frequency wander is usually small and very low frequency. Frequency accuracy is easily guaranteed by mechanical means, and high accuracy devices are relatively low cost. Free-running resistor-capacitor (RC) oscillators, logic gate ring oscillators, or inductor-capacitor (LC) oscillators include too much high-frequency jitter, experience wide frequency variation as a function of process and environmental conditions and thus are unsuitable for this application. See the "HOTLink Jitter Characteristics" application note for more information.

Useful Link:
HOTLink Transmitter/Receiver

HOTLink Jitter Characteristics-AN1161