An apparatus comprising: a detection circuit to detect speed and direction of a target, the detection circuit comprising: two or more magnetic field sensing elements, adapted to detect changes in a magnetic field caused by movement of the target; a first signal channel to output a first signal; a second signal channel to output a second signal, wherein the first and second signals correspond to a position of the target in relation to the one or more magnetic field sensing elements; a processor circuit to receive the output signals and generate a signal representing speed, a signal representing direction, or both; and a first test circuit coupled to the first and second signal channels to determine whether the first and second output signals are operating at the same frequency, and to assert an error signal if the output signals are not operating at the same frequency, wherein the first test circuit comprises a counter circuit coupled to increment a count in response to the first signal and to decrement the count in response to the second signal, wherein the first test circuit includes circuitry to assert an error condition if a count of the counter exceeds a predetermined threshold, wherein the predetermined threshold includes high and low thresholds, and the circuitry is arranged to assert an error condition if the count is greater than the high threshold or less than the low threshold. a detection circuit to detect speed and direction of a target, the detection circuit comprising: two or more magnetic field sensing elements, adapted to detect changes in a magnetic field caused by movement of the target; a first signal channel to output a first signal; a second signal channel to output a second signal, wherein the first and second signals correspond to a position of the target in relation to the one or more magnetic field sensing elements; a processor circuit to receive the output signals and generate a signal representing speed, a signal representing direction, or both; and two or more magnetic field sensing elements, adapted to detect changes in a magnetic field caused by movement of the target; a first signal channel to output a first signal; a second signal channel to output a second signal, wherein the first and second signals correspond to a position of the target in relation to the one or more magnetic field sensing elements; a processor circuit to receive the output signals and generate a signal representing speed, a signal representing direction, or both; and a first test circuit coupled to the first and second signal channels to determine whether the first and second output signals are operating at the same frequency, and to assert an error signal if the output signals are not operating at the same frequency, wherein the first test circuit comprises a counter circuit coupled to increment a count in response to the first signal and to decrement the count in response to the second signal, wherein the first test circuit includes circuitry to assert an error condition if a count of the counter exceeds a predetermined threshold, wherein the predetermined threshold includes high and low thresholds, and the circuitry is arranged to assert an error condition if the count is greater than the high threshold or less than the low threshold.