Differential signal squelch detection circuit and method
An electrical circuit detects a “squelch,” or “out-of-band,” state of a differential signal pair having a positive (“p”) signal line and a negative (“n”) signal line. In one embodiment of the invention, a first and a second comparator each have positive inputs driven by the positive and negative signal lines, respectively. The negative inputs of the comparators are driven by the outputs of separate digital-to-analog converters (DACs) which are set to a lower squelch threshold voltage. The outputs of the comparators then drive the inputs of a logical AND gate, the output of which indicates the current squelch state of the differential signal pair.
1. An electrical circuit for detecting a squelch state of a differential signal pair, the differential signal pair having a positive signal line and a negative signal line, the circuit comprising:
a first digital-to-analog converter having an analog output;
a second digital-to-analog converter having an analog output;
a first comparator having a negative input, a positive input, and an output, the analog output of the first digital-to-analog converter driving the negative input of the first comparator, the positive signal line of the differential pair driving the positive input of the first comparator;
a second comparator having a negative input, a positive input, and an output, the analog output of the second digital-to-analog converter driving the negative input of the second comparator, the negative signal line of the differential pair driving the positive input of the second comparator; and
a logical AND gate having a first and second inputs and an output, the output of the first comparator driving the first input of the logical AND gate, the output of the second comparator driving the second input of the logical AND gate, the logical state of the output of the logical AND gate indicating the squelch state of the differential signal pair when the voltage of the analog outputs of the first and second digital-to-analog converters is a lower squelch threshold voltage.
2. The electrical circuit of claim 1 , further comprising a capacitor coupled to the output of the logical AND gate.
3. The electrical circuit of claim 1 , wherein the first and second digital-to-analog converters are driven by a microprocessor configured to calibrate the analog output of both the first and second digital-to-analog converters.
4. A method for calibrating the analog output of the first digital-to-analog converter of claim 3 to the lower squelch threshold voltage, comprising:
allowing the positive inputs of the first comparator and the second comparator to be undriven;
driving the second digital-to-analog converter so that the output of the second comparator maintains a logic HIGH;
determining a midpoint digital value for the first digital-to-analog converter that most closely results in the output of the logical AND gate exhibiting equal proportions of logic LOW and logic HIGH values; and
subtracting from the midpoint digital value a predetermined digital offset, resulting in the voltage of the analog output of the first digital-to-analog converter being the lower squelch threshold voltage.
5. A method for calibrating the analog output of the second digital-to-analog converter of claim 3 to the lower squelch threshold voltage, comprising:
allowing the positive inputs of the first comparator and the second comparator to be undriven;
driving the first digital-to-analog converter so that the output of the first comparator maintains a logic HIGH;
determining a midpoint digital value for the second digital-to-analog converter that most closely results in the output of the logical AND gate exhibiting equal proportions of logic LOW and logic HIGH values; and
subtracting from the midpoint digital value a predetermined digital offset, resulting in the voltage of the analog output of the second digital-to-analog converter being the lower squelch threshold voltage.
6. An electrical circuit for detecting a squelch state of a differential signal pair, the differential signal pair having a positive signal line and a negative signal line, the circuit comprising:
a first digital-to-analog converter having an analog output;
a second digital-to-analog converter having an analog output;
a first comparator having a negative input, a positive input, and an output, the analog output of the first digital-to-analog converter driving the positive input of the first comparator, the positive signal line of the differential pair driving the negative input of the first comparator;
a second comparator having a negative input, a positive input, and an output, the analog output of the second digital-to-analog converter driving the positive input of the second comparator, the negative signal line of the differential pair driving the negative input of the second comparator; and
a logical AND gate having a first and second inputs and an output, the output of the first comparator driving the first input of the logical AND gate, the output of the second comparator driving the second input of the logical AND gate, the logical state of the output of the logical AND gate indicating the squelch state of the differential signal pair when the voltage of the analog outputs of the first and second digital-to-analog converters is an upper squelch threshold voltage.
7. The electrical circuit of claim 6 , further comprising a capacitor coupled to the output of the logical AND gate.
8. The electrical circuit of claim 6 , wherein the first and second digital-to-analog converters are driven by a microprocessor configured to calibrate the analog output of both the first and second digital-to-analog converters.
9. A method for calibrating the analog output of the first digital-to-analog converter of claim 8 to the upper squelch threshold voltage, the method comprising:
allowing the negative inputs of the first comparator and the second comparator to be undriven;
driving the second digital-to-analog converter so that the output of the second comparator maintains a logic HIGH;
determining a midpoint digital value for the first digital-to-analog converter that most closely results in the output of the logical AND gate exhibiting equal proportions of logic LOW and logic HIGH values; and
adding to the midpoint digital value a predetermined digital offset, resulting in the voltage of the analog output of the first digital-to-analog converter being the upper squelch threshold voltage.
10. A method for calibrating the analog output of the second digital-to-analog converter of claim 8 to the upper squelch threshold voltage, comprising:
allowing the negative inputs of the first comparator and the second comparator to be undriven;
driving the first digital-to-analog converter so that the output of the first comparator maintains a logic HIGH;
determining a midpoint digital value for the second digital-to-analog converter that most closely results in the output of the logical AND gate exhibiting equal proportions of logic LOW and logic HIGH values; and
adding to the midpoint digital value a predetermined digital offset, resulting in the voltage of the analog output of the second digital-to-analog converter being the upper squelch threshold voltage.
11. A method for detecting a squelch state of a differential signal pair, the differential signal pair having a positive signal line and a negative signal line, the method comprising:
comparing the voltage of the positive signal line to a lower squelch threshold voltage;
comparing the voltage of the negative signal line to the lower squelch threshold voltage; and
indicating the presence of the squelch state if the voltage of the positive signal line and the voltage of the negative signal line are both higher than the lower squelch threshold voltage.
12. A method for detecting a squelch state of a differential signal pair, the differential signal pair having a positive signal line and a negative signal line, the method comprising:
comparing the voltage of the positive signal line to an upper squelch threshold voltage;
comparing the voltage of the negative signal line to the upper squelch threshold voltage; and
indicating the presence of the squelch state if the voltage of the positive signal line and the voltage of the negative signal line are both lower than the upper squelch threshold voltage.