Active antenna system with fault detection

1. An active antenna power interface circuit for coupling a supply voltage from a radio receiver to an antenna feed, comprising:

a DC power input;

a power switching element connected in series between said DC power input and said antenna feed, said power switching element having a control input for selecting a conducting state or a nonconducting state of said power switching element;

a high side voltage sensor sensing a first voltage proportional to a voltage at said DC power input;

a low side voltage sensor sensing a second voltage proportional to a voltage at said antenna feed; and

a controller coupled to said control input and to said voltage sensors, said controller placing said power switching element into said conducting state, sampling said first and second voltages, calculating a voltage difference in response to said first and second voltages, comparing said voltage difference to an upper limit and a lower limit, if said voltage difference is greater than said upper limit then signaling a short-circuit fault, and if said voltage difference is less than said lower limit then signaling an open-circuit fault.

2. The circuit of claim 1 wherein if said short-circuit fault is signaled, then said controller placing said power switching element into said nonconducting state.

3. The circuit of claim 2 wherein after placing said power switching element into said nonconducting state, said controller waits for a predetermined delay and then places said power switching element back into said conducting state and repeats said sampling, calculating, and comparing functions to detect whether said short-circuit fault continues to be present.

4. The circuit of claim 1 wherein said controller repeatedly samples said first and second voltages, averages respective pluralities of said samples, and calculates said voltage difference in response to a difference between an average of said first voltages and an average of said second voltages.

5. The circuit of claim 4 wherein each of said averages is comprised of a moving average including a predetermined number of samples.

6. The circuit of claim 1 wherein said controller is comprised of a microcontroller, and wherein said circuit further comprises analog-to-digital (A/D) converters for sampling said first and second voltages as respective digital values.

7. The circuit of claim 1 wherein said power switching element is comprised of a first transistor having a base terminal, an emitter terminal, and a collector terminal, and wherein said interface circuit further comprises a current limiter coupled to said first transistor.

8. The circuit of claim 7 wherein said current limiter comprises:

a current sense resistance in series with said first transistor; and

a second transistor coupled between said base terminal and said emitter terminal of said first transistor and driven by a voltage across said current sense resistance in order to reduce said conducting state of said first transistor in proportion to a current through said current sense resistance.

9. The circuit of claim 1 wherein said high side and low side voltage sensors include respective storage capacitors in parallel with said A/D converters.

10. The circuit of claim 1 further comprising a lowpass filter connected between said power switching element and said antenna feed.

11. A method of powering a remote active antenna from a radio receiver via an antenna signal line, said method comprising the steps of:

coupling a DC power supply voltage to said antenna signal line through a power switch transistor;

sampling a first voltage proportional to said DC power supply voltage;

sampling a second voltage at a point between said power switch transistor and said antenna signal line;

calculating a voltage difference between said first and second voltages;

comparing said voltage difference to an upper limit and a lower limit;

if said voltage difference is greater than said upper limit then signaling a short-circuit fault; and

if said voltage difference is less than said lower limit then signaling an open circuit fault.

12. The method of claim 11 wherein if said short-circuit fault is signaled, then decoupling said DC power supply from said antenna signal line.

13. The method of claim 12 wherein after decoupling said DC power supply from said antenna signal line, waiting for a predetermined delay and then repeating said coupling, sampling, calculating, and comparing steps to detect whether said short-circuit fault continues to be present.

14. The method of claim 11 further comprising repeating said sampling steps, averaging respective pluralities of samples of said first and second voltages, respectively, and calculating said voltage difference in response to a difference between an average of said first voltages and an average of said second voltages.

15. The method of claim 14 wherein each of said averages is comprised of a moving average including a predetermined number of samples.

16. The method of claim 11 further comprising the steps of:

sensing a magnitude of current flowing through said power switch transistor; and

limiting conduction of said power switch transistor in response to said current magnitude.