Programmable fractional-N frequency synthesizer

1. A fractional divider in a frequency synthesizer comprising:

a divider means for dividing a frequency by a plurality of selectable divisors and having a divider control means for providing, in a first period, divider control signals to the divider means for selecting the divisors;

a fractional counter means for providing a first counter which counts selected synthesizer signals to establish a second period having a selected number of counts wherein the second period is longer than the first period and which provides a second period terminal count signal representative of the end of the second period, and wherein said fractional counter means is responsive to the second period terminal count signal to provide a selected number of fractional control signals to the divider control means;

wherein the divider control means is responsive to both the divider control signals and to the fractional control signals to select divisors.

2. The fractional divider of claim 1 wherein the fractional counter means comprises a second counter which provides the selected number of fractional control signals to the divider control means in response to the second period terminal count signal.

3. The fractional divider of claim 1 wherein the divider means is responsive to each fractional control signal to divide by a predetermined divisor.

4. The fractional divider of claim 3 wherein the plurality of selectable divisors comprises N and N.+-.1 and wherein the divider divides by N.+-.1 in response to fractional control signals.

5. The fractional divider of claim 1 wherein:

the divider control means provides first period terminal count signals, each of which is representative of the end of a period; and

wherein the first counter counts the first period terminal count signals from the divider control means and provides the second period terminal count signal upon counting the selected number of first period terminal count signals.

6. The fractional divider of claim 5 wherein the fractional counter means comprises a second counter which provides the selected number of fractional control signals to the divider control means in response to the second period terminal count signal.

7. The fractional divider of claim 6 wherein the divider control means counts signals from the divider means and upon reaching a selected number of divider signals, provides the first period terminal count signal.

8. The fractional divider of claim 7 wherein:

the divider control means comprises a third counter which counts said divider output signals and provides the first period terminal count signal;

the divider control means comprises a fourth counter which provides the selected number of divider control signals to the divider to cause the divider to divide by N+1 in response to each first period terminal count signal.

9. The fractional divider of claim 1 wherein the first counter is programmable for selecting the count to establish the second period.

10. The fractional divider of claim 1 wherein:

the fractional counter means comprises a second counter which provides the selected number of fractional control signals to the divider control means in response to the second period terminal count signal; and

wherein the first counter is programmable for selecting the count to establish the second period and the second counter is programmable for selecting the number of fractional control signals provided in response to each second period terminal count signal.

11. A frequency synthesizer for synthesizing a desired output frequency, the synthesizer comprising:

a reference frequency source;

an output frequency source for providing an output frequency which is higher than the reference frequency;

divider means for dividing the output frequency by a plurality of selectable divisors;

divider control means for establishing a first period, for providing a first period terminal signal representative of the end of the first period, and for selecting divisors of the divider means;

fractional control means for providing a selected number of fractional control signals to the divider control means, the fractional control means comprising a first counter which counts the first period terminal signals to establish a second period comprising multiple first periods and which provides a second period terminal count signal representing the end of the second period, the fractional control means also comprising a second counter which provides the selected number of fractional control signals to the divider control means in response to each second period terminal count signal from the first counter; and

the divider control means is responsive to the fractional control signals to select a predetermined divisor.

12. The frequency synthesizer of claim 11 wherein the plurality of selectable divisors comprises N and N.+-.1 and wherein the divider divides by N.+-.1 in response to fractional control signals.

13. The frequency synthesizer of claim 11 wherein the divider control means comprises a third counter which counts signals from the divider means and upon counting a selected number of divider signals, provides the first period terminal signal.

14. The frequency synthesizer of claim 13 wherein the divider control means comprises a fourth counter which provides the selected number of fractional control signals received from the fractional control means to the divider to cause the divider to divide by N.+-.1.

15. The frequency synthesizer of claim 11 wherein the first counter is programmable for selecting the count to establish the second period and the second counter is programmable for selecting the number of fractional control signals provided in response to each second period terminal count signal.

16. A method for dividing an output frequency, the method comprising the steps of:

dividing the output frequency by a first divisor during a first period;

providing a first period terminal signal representative of the end of each first period;

counting said first period terminal signals with a first counter;

programming the first counter to provide a second period terminal count signal upon reaching a programmed count of said first period terminal count signals, said programmed count being greater than one;

receiving said second period terminal count signal by a second counter and said second counter providing a selected number of fractional control signals in response to the second period terminal count signal; and

dividing the output frequency by a second divisor in response to the fractional control signals.

17. The method of claim 16 wherein the step of dividing the output frequency by a first divisor comprises dividing by N and the step of dividing the output frequency by a second divisor comprises dividing by N.+-.1.

18. The method of claim 16 wherein:

the step of dividing the output frequency by a first divisor comprises the step of providing divided output signals; and

the step of providing a first period terminal signal representative of the end of each first period comprises the step of counting said divided output signals from the divider means and upon reaching a selected number of divider signals, providing the first period terminal signal.

19. The method of claim 16 further comprising the step of programming the second counter to provide a programmed number of fractional control signals in response to receiving a second period terminal count signal.

20. The method of claim 19 further comprising the step of:

selecting a desired divisor of the output frequency;

wherein the step of programming the first counter comprises selecting the programmed count of said first period terminal signals in response to the selection of the desired divisor; and

wherein the step of programming the second counter comprises selecting the programmed number of fractional control signals in response to the selected divisor.