Precision frequency monitor

1. A precision frequency monitor for providing a precision frequency monitor (PFM) output value indicative of the precision of the frequency or period of an input reference signal, comprising:

an averaging module responsive to an average signal T x representing an average frequency or period of said input reference signal during successive predetermined time periods defining operational cycles, said averaging module being operative to average said average signal T x over N operational cycles, where N is an integer, and provide an updated PFM value every N operational cycles;

an infinite impulse response (IIR) filter operative to filter said average signal T x to generate interim PFM values;

a modulo N counter generating a reset signal every N operational cycles to prompt said averaging module to output said updated PFM value;

said IIR filter including an output switch operative to select said interim PFM values within each sequence of N operational cycles as said PFM output value, and said updated PFM value in response to said reset signal; and

said IIR filter further receiving as a feedback input said PFM output value.

2. A precision frequency monitor as claimed in claim 1 , wherein the IIR filter comprises a one pole filter.

3. A precision frequency monitor as claimed in claim 1 , wherein the averaging module comprises an adder with a unit delay register in a feedback loop followed by a divider-by-N module.

4. A precision frequency monitor as claimed in claim 3 , wherein said modulo N counter resets said unit delay register every N cycles.

5. A precision frequency monitor as claimed in claim 4 , further comprising a further switch responsive to said reset signal to send the output of the said adder to the divider-by-N module.

6. A precision frequency monitor as claimed in claim 5 , wherein said output switch receives a first input from the IIR filter and a second input from the divider-by-N, said output switch being configured to normally output the PFM value from the IIR filter and to output the PFM value from the divider-by-N in response to reset said signal.

7. A precision frequency monitor as claimed in claim 6 , wherein the IIR filter comprises a unit delay register in a feedback loop including a multiplier by a constant value α.

8. A precision frequency monitor as claimed in claim 1 , further comprising a disabling module for preventing said reset signal from being applied to said output switch when the input reference signal meets a predetermined stability criterion.

9. A precision frequency monitor as claimed in claim 8 , wherein the disabling module comprises a comparator for comparing the updated PFM values with the interim PFM values, and a threshold detector for determining when the difference between PFM values is less than a predetermined threshold.

10. A precision frequency monitor as claimed in claim 9 , wherein said disabling module further comprises an AND gate receiving as a first input the output of said threshold detector and as a second input said reset signal.

11. A method of monitoring the precision in frequency of an input reference signal in a precision frequency monitor (PFM) comprising: an averaging module responsive to an average signal T x representing an average frequency or period of said input reference signal during successive predetermined time periods defining operational cycles; an infinite impulse response (IIR) filter including an output switch operative to select alternative inputs as PFM output values; and a modulo N counter generating a reset signal every N operational cycles; said method comprising:

averaging said average signal T x over N operational cycles, where N is an integer, to find an updated precision frequency monitor value (PFM) every N operational cycles;

generating said updated PFM value every N operational cycles;

filtering said average signal T x with said infinite impulse response (IIR) filter to generate interim PFM values within each sequence of N operational cycles;

operating said output switch to select to select as said PFM output values, said interim PFM values within each sequence of N operational cycles and said updated PFM value in response to said reset signal; and

feeding back said output PFM values to said IIR filter.

12. A method as claimed in claim 11 , wherein said average signal T x is filtered with a one pole IIR filter.

13. A method as claimed in claim 11 , wherein said averaging said average signal T x is performed by an adder with a unit delay register in a feedback loop followed by a divider-by-N.

14. A method as claimed in claim 13 , further wherein said reset signal resets said unit delay register every N cycles.

15. A method as claimed in claim 14 , wherein the output of the adder is sent to the divider-by-N every N clock cycles.

16. A method as claimed in claim 15 , wherein said output switch couples the output of said divider-by-N to an output of said precision frequency monitor in response to said reset signal.

17. A method as claimed in claim 11 , wherein said reset signal is inhibited from reaching said output switch when the input reference signal meets a predetermined stability criterion.

18. A method as claimed in claim 17 , wherein the updated PFM values are compared with the interim PFM values, and said reset signal is inhibited from reaching said output switch when the difference between updated PFM values and said interim PFM values is less than a predetermined threshold.