System and method for calibrating a high-speed cable
An HDMI cable carries high speed encoded data which are transmitted differentially over data channels, along with a clock. High-frequency loss and differential skew within a differential signal may be compensated by analog circuits embedded in the cable. These embedded circuits are tuned at production for best performance by observing the quality of the recovered analog signal. The embedded circuits are powered by a combination of power sources, both carried within the cable, and harvested from the high-speed signals themselves. Corresponding method and system for calibrating the cable are also provided.
1. A method for calibrating a cable for transmitting differential signals, the cable including a boost device for equalizing the differential signals, the boost device having adjustable parameters and a parameter memory, the method comprising the steps of:
sending a differential data signal into the cable;
sending a trigger to the boost device;
performing a training run in the boost device, including performing at least two evaluation runs with different settings of the adjustable parameters and evaluating the results with each of the at least two settings, retaining the best settings; and
storing the best settings in the parameter memory;
wherein each evaluation run comprises:
processing the differential data signal into an equalized signal; and
generating a preprocessed signal, which is a digital representation of the equalized signal; and
wherein the step of evaluating the results includes:
determining a run length of contiguous “1” or “0” samples in the digital representation of the equalized signal within a window of at least one bit period;
counting numbers of occurrences of selected run lengths during an observation period of “N” bits;
storing the counted numbers of occurrences in counters according to the selected run lengths; and
processing outputs of the counters into a Quality Number indicating the quality of the equalized signal.
2. The method as described in claim 1 , wherein the processing the differential data signal into the equalized signal comprises:
processing the differential data signal into a deskewed signal; and
processing the deskewed signal into the equalized signal.
3. A system for calibrating a cable for transmitting differential signals, the cable including a boost device for equalizing the differential signals, the boost device having adjustable parameters and a parameter memory, the system comprising:
means for sending a differential data signal into the cable;
means for sending a trigger to the boost device;
means for performing a training run in the boost device, including evaluation means for performing at least two evaluation runs with different settings of the adjustable parameters, evaluating the results with each of the at least two settings and retaining the best settings; and
means for storing the best settings in the parameter memory;
wherein the evaluation means comprises:
means for processing the differential data signal into an equalized signal; and
means for generating a preprocessed signal, which is a digital representation of the equalized signal; and
wherein the evaluation means comprises:
means for determining a run length of contiguous “1” or “0” samples in the digital representation of the equalized signal within a window of at least one bit period;
means for counting numbers of occurrences of selected run lengths during an observation period of “N” bits;
means for storing the counted numbers of occurrences in counters according to the selected run lengths; and
means for processing outputs of the counters into a Quality Number indicating the quality of the equalized signal.
4. The system as described in claim 3 , wherein the evaluation means comprises:
means for processing the differential data signal into a deskewed signal; and
means for processing the deskewed signal into an equalized signal.
5. A system for calibrating a cable for transmitting differential signals, including a boost device having adjustable parameters and a parameter memory, the system comprising:
network analyzer capable of sending at least two signals into a cable input and measuring the response at a cable output;
a computer connected to the network analyzer and to the parameter memory of the cable, the computer having a computer memory; and
a computer program code stored in the computer memory for causing the computer to perform a calibration of the cable by changing the adjustable parameters and storing the results in the parameter memory of the cable;
wherein the computer program code causes the computer to perform the calibration of the cable by performing a training run in the boost device, including performing at least two evaluation runs with different settings of the adjustable parameters and evaluating performance of the cable for each of the settings, and retaining the best settings;
wherein each evaluation run includes:
processing the differential data signal into an equalized signal; and
generating a preprocessed signal, which is a digital representation of the equalized signal;
wherein the computer program code causes the computer to evaluate the performance of the cable for each of the settings by:
determining a run length of contiguous “1” or “0” samples in the digital representation of the equalized signal within a window of at least one bit period;
counting numbers of occurrences of selected run lengths during an observation period of “N” bits;
storing the counted numbers of occurrences in counters according to the selected run lengths; and
processing outputs of the counters into a Quality Number indicating the quality of the equalized signal.
6. The system as described in claim 5 , wherein the processing the differential data signal into the equalized signal comprises:
processing the differential data signal into a deskewed signal; and
processing the deskewed signal into an equalized signal.
7. The system as described in claim 5 , further comprising the cable.
8. A method for operating a system for calibrating a cable for transmitting differential signals, including a boost device having adjustable parameters and a parameter memory, the method comprising:
(a) measuring attenuation at a predetermined frequency;
(b) changing the adjustable parameters and repeating the step (a) provided the attenuation is outside a predetermined attenuation range at said predetermined frequency;
(c) storing the adjustable parameters in the parameter memory;
(d) setting the predetermined attenuation range to a value close to 0 db, and less than a predetermined limit at said predetermined frequency; and
(e) setting the predetermined frequency to approximately a frequency of the differential signals for which the cable is intended.
9. The method as described in claim 8 , further comprising:
(a1) measuring differential skew of the differential signals at an output of the cable;
(b1) changing the adjustable parameters and repeating the step (a1) provided the differential skew is higher than a predetermined skew threshold; the steps (a1) and (b1) being performed before the step (a); and
(e1) setting the predetermined skew threshold to the minimal value observed within a predetermined number of repeats of the step (a1); the step (e1) being performed after the step (c).