AU620110B2

AU620110B2 – Frequency synthesizer with spur compensation
– Google Patents

AU620110B2 – Frequency synthesizer with spur compensation
– Google Patents
Frequency synthesizer with spur compensation

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Publication number
AU620110B2

AU620110B2
AU37410/89A
AU3741089A
AU620110B2
AU 620110 B2
AU620110 B2
AU 620110B2
AU 37410/89 A
AU37410/89 A
AU 37410/89A
AU 3741089 A
AU3741089 A
AU 3741089A
AU 620110 B2
AU620110 B2
AU 620110B2
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Australia
Prior art keywords
output
divider
providing
accumulator
input
Prior art date
1988-06-03
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AU37410/89A
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AU3741089A
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Inventor
Frederick Lee Martin
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Motorola Solutions Inc

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Motorola Inc
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1988-06-03
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1989-05-11
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1992-02-13

1989-05-11
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Motorola Inc

1990-01-05
Publication of AU3741089A
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patent/AU3741089A/en

1992-02-13
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1992-02-13
Publication of AU620110B2
publication
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patent/AU620110B2/en

2009-05-11
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Ceased
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Classifications

H—ELECTRICITY

H03—ELECTRONIC CIRCUITRY

H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES

H03L7/00—Automatic control of frequency or phase; Synchronisation

H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop

H03L7/16—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop

H03L7/18—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop

H—ELECTRICITY

H03—ELECTRONIC CIRCUITRY

H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES

H03L7/00—Automatic control of frequency or phase; Synchronisation

H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop

H03L7/16—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop

H03L7/18—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop

H03L7/197—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division

H03L7/1974—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division for fractional frequency division

H03L7/1976—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a time difference being used for locking the loop, the counter counting between numbers which are variable in time or the frequency divider dividing by a factor variable in time, e.g. for obtaining fractional frequency division for fractional frequency division using a phase accumulator for controlling the counter or frequency divider

Description

OPI DATE 05/01/90 APPLN. ID 37410 89 6 P DATE 01/02/90 PCT NUMBER PCT/US89/02040 NAT NAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: WO 89/12362 HO3L 7/18 Al (43) International Publication Date: 14 December 1989 (14.12.89) (21) International Application Number: PCT/US89/02040 (81) Designated States: AU, BR, DK, FI, HU, JP, KR, NO, SU.
(22) International Filing Date: 11 May 1989 (11.05.89) Published With international search report.
Priority data: With amended claims.
202,065 3 June 1988 (03.06.88) US (71)Applicant: MOTOROLA, INC. [US/US]; 1303 East Algonquin Road, Schaumburg, IL 60196 (US).
(72) Inventor: MARTIN, Frederick, Lee 1419 Avon Lane, .115C, North Lauderdale, FL 33068 (US).
(74) Agents: PARMELEE, Steven, G. et al.; Motorola, Inc., Intellectual Property Department, 1303 East Algonquin Road, Schaumburg, IL 60196 (US).
(54) Title: FREQUENCY SYNTHESIZER WITH SPUR COMPENSATION (57) Abstract A synthesizer circuit with spur compensation utilizes fractional division (17) in the synthesizer loop. Two accumulators (24, 25) are utilized for determining the divisor value N.
The capacity of the two accumulators is selectable (19, 20, 22).
An offset value is selectively introduced (19, 20, 22, 23, 26) into the accumulators in order to produce a waveform having an acceptable spurious content.
tl WO 89/12362 PCT/US89/02040 1 FREQUENCY SYNTHESIZER WITH SPUR COMPENSATION 9 A CV.G RO’J, “i OF TeL ‘V;TICL I This invention relates to frequency synthesizers in general, and more particular, to a fractional-N-frequency synthesizer in which selectable frequency outputs are produced while reducing unwanted spurious outputs. Frequency divider circuits are used in frequency synthesizer circuits such as in a phase lock loop (PLL). In a fractional-N-synthesis PLL circuit, the output frequency fo of a voltage controlled oscillator (VCO) is first divided and then applied to a phase detector which operates in a conventional manner comparing the phase of the divided output signal with a reference frequency fr from a reference oscillator, in order to control the VCO output frequency fo. The output frequency fo is related to the reference frequency of the reference frequency source by the relationship fo x fr. N.F is the effective divisor by which the output frequency is divided before it is compared with the reference frequency. N.F is produced by a divider control circuit and consists of an integer part N and a fractional part F. The fractional part F k/D where k and D are both integers.
Since a divider operates with integer values, fractional division is simulated by switching between different integer values of divisors. However, this switching of the divisors results in WO 89/12362 PCT/US89/02040 2 spurious sidebands in the synthesized output frequency signal fo.
The goal in designing a synthesizer is to keep the amplitudes of these sub-harmonic spurs below some maximum acceptable limit.
An approach, illustrated in U.S. Patent No. 4,204,174, to cancel unwanted spurious signals utilizes two accumulators to simulate the fractional division and a digital-to-analog converter to generate a correction signal to back-off the resultant spurious sidebands. U.S. Patent No. 4,694,475 also illustrates the use of two accumulators for a frequency divider circuit. Basically, both methods utilize a first accumulator to correct for phase error and a second accumulator to which the instantaneous contents of the first accumulator is summed at each cycle of the divider output.
For each clock cycle in which the second accumulators fixed capacity D is reached, the divisor is increased by one from its programmed value. On each succeeding clock cycle, the divisor N is decreased by one from its programmed value. The net effect on the average divisor is zero since counts are always added and subtracted in pairs. Such two accumulator approaches provide a single unique waveform and associated spurious response for each value of numerator k for the fractional part of the divider and capacity D of the accumulators for a synthesizer of a predetermined loop bandwidth.
The one unique waveform can result in unacceptable spurious signals for a desired output frequency fo. In some applications, spurious signals with 20 kHz of a desired frequency fo must be 60 dB below the carrier fo frequency signal while spurious signals further than 20 kHz from the carrier frequency must be 90 dB below the carrier level. With the waveform provided by prior art two accumulator approaches, the spurious signals can exceed the desired limits. Fig. 6 illustrates such a situation. In this illustration, spur 134 is within desired limits, however, spur 132 exceeds desired limits.
In applications such as two-way radios, minimizing hardware along with eliminating the effects of spurs is of utmost importance.
r ,r, 9** 0*B* 4e *4
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S
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S
The frequency synthesizer of the present invention with spur compensation may provide means for changing the spurious output of a synthesizer for any output frequency f thereby providing different spurious responses. A 0, waveform having acceptable spurs may be utilized to produce the desired output frequency.
The synthesizer may include a loop having a programmable divider. A divider control means may provide divider values to the programmable divider and may include means providing varying values to the divider for fractional division to produce a desired output frequency. The divider control means may include first S and second accumulator means. The first accumulator means may include an input for receiving data, a first output 15 for varying the divide value and a second output for providing data to the second accumulator means. The second accumulator means may include an input connect to Sthe second output of the first accumulator means and an output for varying the divide value.
In one aspect of the invention the first and second accumulator means may each have a variable capacity. In S another aspect of the invention means may provide an Soffset value to the first or second accumulator means.
According to one aspect of the present invention .25 there is provided a frequency synthesizer for providing a synthesized output frequency f comprising: 0: 0 a synthesizer loop including a programmable divider; a divider control means providing divider values to the programmable divider, the divider control means ?0 providing varying values to the programmable divider for fractional division to produce a desired output frequency f and including first and second accumulator means, 0, the first accumulator means including an input for receiving data, a first output for varying the divider value, and a second output for providing data to the second accumulator means, and the second accumulator means including an input connected to the second output of the first accumulator 39 means and an output for varying the divider value; 4
OS
S
0S*I
S
S..
S
-3the first and second accumulator means each having a variable capacity as herein defined; and means for providing an offset value to at least one of the first and second accumulator means.
According to a further aspect of the present invention there is provided a frequency synthesizer for providing a synthesized output frequency f 0 comprising: a synthesizer loop including a programmable divider; a divider control means providing divider values to the programmable divider, the divider control means providing varying values to the programmable divider for fractional division to produce a desired output frequency fo, and including first and second accumulator means.
the first accumulator means including an input for e 1 5 receiving data, a first output for varying the divider 0*SS value, and a second output for providing data to the second accumulator means, and the second accumulator means including an input
S
connected to the second output of the first accumulator means and an output for varying the divider value; means for providing an offset value to the first and second accumulator means.
According to a still further aspect of the present S invention there is provided a frequency synthesizer for providing a synthesized output frequency f comprising: 0 a synthesizer loop including a programmable divider; memory means for providing information for said synthesizer loop; a divider control means providing divider values to said programmable divider, said divider control means providing varying values to said programmable divider for fractional division to produce a desired output frequency fo, and including at least a first and second 0, accumulator means; said first accumulator means including an irput for receiving data, a first output for varying said divider value, and a second output for providing data to said second accumulator means, and 39 said second accumulator means including an input -3a- <$1 WnT :A connected to said second output of said first accumulator means and an output for varying said divider value; means for providing an offset value to at least one of said first and second accumulator means. According to a still further aspect of the present invention there is provided a method of providing a synthesized output frequency, comprising the steps of: a) receiving data for providing a divider value to a programmable divider to produce a desired output frequency fo; b) offsetting said data to provide an offset data; c) accumulating said offset data to provide an se*. accumulated signal and a first control signal; d) accumulating said accumulated signal to provide 0.666. 1 5 a second control signal; 9Ss@ e) delaying and inverting said second control signal to provide a third control signal; and f) varying said divider value in response to said first, second, and third control signals. According to a still further aspect of the present invention there is provided a method of providing a synthesized output frequency, comprising the steps of: 0 9 a) receiving data for providing a divider value to g a programmable divider to produce a desired output 55 frequency fo; b) accumulating said data to provide an accumulated 0*SSSS S signal and a first control signal; 0 c) offsetting said accumulated signal to provide an offset accumulated signal; d) accumulating said offset accumulated signal to provide a second control signal; e) delaying and inverting said second control signal to provide a third control signal; and f) varying said divider value in response to said first, second, and third control signals. A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: 39 FIG. 1 is a block diagram of a frequency synthesizer VS MJ;P46 -3bwith spur compensation in accordance with the present invention. FIG. 2 is a block diagram of the divider control circuit of the frequency synthesizer of FIG. 1. FIG. 3 is a block diagram of the offset control of FIG. 2. FIG. 4 is a block diagram of the control logic of FIG. 2. FIG. 5 is a block diagram of an accumulator of FIG. 2. FIG. 6 illustrates an example of the frequency response of a two accumulator synthesizer in accordance with prior art approaches. o FIG. 7 illustrates the frequency response of the frequency synthesizer of the present invention for a selected waveform. 0 o C 0 C -3c- L WO 89/12362 PCT/US89/02040 4 EQ66C6pinTc, QT11 GE'r, nE PAPERA MBQPC- IMNT Referring now by characters of reference to-the drawings and first to FIG. 1, it will be understood that a frequency synthesizer 10, in accordance with the present invention, includes a reference oscillator 11. The output of reference oscillator 11 fr is applied to a phase detector 12 that has its output coupled, via a low pass filter 13, to a voltage controlled oscillator (VCO) 14. The output of VCO 14 is connected to the output 15 of the frequency synthesizer 10 and to a programmable divide by N divider 16. VCO 14 provides the synthesizer output signal fo. The output of divider 16 provides a divided signal fd to the phase detector 12 in a conventional manner and to a divider control circuit 17. Divider control circuit 17 is connected to the programmable divider 16 and provides the divide or information used by the divider. In a fractional N frequency synthesizer, the desired output frequency fo cannot be obtained utilizing a single divisor for the programmable divide by N divider 16. It is necessary to periodically adjust the value N in a manner such that the average output frequency is equal to the desired output frequency. The divider control circuit 17, as is shown in further detail in FIG. 2, is designed to provide the required N values to the programmable divider 16 while minimizing spurious signals. A memory 19, constituting memory means, which can include a programmable read only memory as well as ROM and RAM is utilized to contain data for use by the divider control circuit 17 for obtaining the value N for application to the programmable divider 16. A microprocessor controller 20 is used to read the data from the memory 19 and supplies the data, via a data bus, to a data register 22 which serves also as a latch. A frequency selector 21 is coupled to the microprocessor controller 20 for choosing the synthesizer output frequency fo. In applications such as two-way radios, the frequency selector corresponds to the channel switch. The data register 22 provides the various data outputs which have been labeled as numerator or is the k value, offset, WO 89/12362 pCT/US89/02040 denominator or is the D value, and Nnom which is the nominal value for the N divide value. The numerator and offset data lines Sare connected to A and B inputs respectively of a multiplexer 23. Output data lines of multiplexer 23 are connected to the input of a first accumulator 24 constituting first accumulator means. Its output which is labeled contents, is connected to the input of a second accumulator 25 constituting second accumulator mea's. Each of the accumulators 24 and 25 has a capacity input connected to the denominator output of data register 22. Carry outputs are provided from both accumulators 24 and 25 and are connected to two inputs of a control logic circuit 27. The output of control logic circuit 27 is connected to the programmable divider 16. The Nnom data line of data register 22 is also connected to the control logic circuit 27. The microprocessor controller 20 provides an output that is applied to the trigger inputs of data register 22 and an offset control circuit 26. The offset control circuit 26 has a select output that is connected to a select input of multiplexer 23, and a reset output that is connected to reset inputs of the accumulators 24 and 25. Clock inputs of offset control 26, control logic 27 and accumulators 24 and 25 are provided with the fd output of programmable divider 16. Alternatively, these clock signals could be provided directly by the reference oscillator 11 as fd and fr are in phase lock. Referring now to FIG. 3, the offset control circuit 26 is illustrated in further detail. The clock output from the programmable divider 16 is coupled to a series circuit comprised of inverters 31 and 34 and a delay element 33. The output of inverter 31 is connected to the input of inverter 34 which has its output coupled via the delay element 33 to the clock input of a flipflop 36. The output of inverter 31 is also coupled to a clock input of a flip-flop 35. A flip-flop 37 has a D input coupled to VDD for maintaining the input high. Its clock input is the TRIGGER input of offset control 26. The Q output of flip-flop 37 is coupled to the D input of flip-flop 35. The RESET output of offset control 26 is WO 89/12362 pT/US89/02040 6 proviued by the Q output of flip-flop 35 which is also connected to the D input of flip-flop 36. The SELECT output of offset control 26 is provided by the Q output of flip-flop 36. The Q bar outputs of flip-flops 35 and 36 are connected to inputs of a NOR gate 38 which has its output coupled to the reset input of flip- flop 37. Referring now to FIG. 4, a description of the control logic 27 will be given. The carry output of the accumulator 24 is fed to an input A of one bit adder 41, while, the carry output of the second accumulator 25 is fed to an input B of the adder 41 and to a D 1 0 input of a flip-flop 42. The clock input of flip-flop 42 is connected to the output of programmable divider 16. The Q bar output of flipflop 42 is coupled to input C of adder 41. The sum and carry outputs of adder 41 are applied to the least two significant bit positions, respectively, of the word B input of an adder 43. The Nnom data stored in data register 22 is coupled to the word A input of adder 43. The sum output of adder 43 is the N value used as the divisor of the programmable divider 16. Referring now to Fig. 5, an accumulator of the type used for accumulators 24 and 25 of Fig. 2, is illustrated in detail. Two adders 45 and 46, 2-to-1 multiplexer 47 and latch 48 are connected serially through their respective inputs and outputs. The RESET output of offset control circuit 26 is coupled to the reset input of the latch 48 for initializing the output of the latch. The adder 45 sums the value at its input A, which is the accumulator input, with the output of latch 48 and applies the result to input A of the second adder 46, and also to the INo input of 2-to-1 multiplexer 47. A value corresponding to the two's compliment of the capacity is applied at input B of adder 46, which is the accumulators CAPACITY input. The capacity is defined as being the minimum value which causes the accumulator to generate a carry signal. The sum from adder 46 is applied at the IN1 input of the multiplexer 47. The carry outputs of the adders and 46 are applied to inputs of an OR gate 49. The output of the OR gate 49 is brought out as the CARRY output of the accumulator. The output of OR gate 49 is coupled to the select WO 89/12362 PCT/US89/02040 7 input of multiplexer 47, to determine whether IN 0 or IN 1 of multiplexer 47 will be fed into the input of latch 48. The output of multiplexer 47 is the CONTENTS output of the accumulator. The clock input to latch 48, which is accumulator clock input, is pulsed to transfer the value from the input to the output of the latch. Basically, in operation, if the accumulator capacity has been reached by adding any two numbers, the carry output from one of the adders 45 or 46 will be high. This will cause the output of OR gate 49 to go high which selects the IN 1 input of multiplexer 47 as the contents of the accumulator. This, in effect, subtracts the capacity from the original sum. If the sum of the two numbers does not exceed capacity, the carry outputs from the adders and 46 will be low and the resultant low from OR gate 49 will select the sum INo input of multiplexer 47 as the contents of the accumulator. It is thought that the advantages of the frequency synthesizer have become fully apparent from the foregoing description of parts, but for completeness of disclosure a brief description of the operation and use of the circuit will be given. The divider control circuit 17 of the preferred embodiment utilizes a multiplexer and offset control to introduce an offset value in accumulators, for improved fractional N-synthesis. Various other circuit implementations could be utilized to obtain this desired control of the N divider including implementing the accumulators in a microprocessor. The capacity of the accumulators 24 and 25 is a variable. Capacity information is stored with the other frequency information in the memory 19. The actual stored value is the 2's complement of the D value which is ultimately applied to the capacity inputs of accumulators 24 and 25. The value of D is derived from the equation D fr/channel spacing. The input to accumulator 24 and, hence, the relationship between the two accumulators 24 and 25, is determined by which of two input words latched in the data register 22 is selected by the offset control 26 as the output of multiplexer 23 to be fed into WO 89/12362 PCT/US89/02040 8 the input of the first accumulator 24. The two input words are the numerator k for steady state conditions and the offset value which provides a predetermined starting value for the accumulators. The offset value for each desired frequency fo is stored in a table in memory 19 along with the other frequency information, namely numerator, denominator and Nnom values which are loaded into data register 22. The offset value varies with k, D and the required application and can be found by trial and error in actual field tests and/or preliminarily by computer simulation. To provide an offset the value can not equal zero, the numerator or the denominator. If one of these values is utilized there would be no offset. The offset control 26 determines when a particular input word will be selected. Upon initializing the synthesizer (i.e. selecting a new output frequency fo) the microprocessor controller provides a trigger signal to strobe data into the data register 22 and clock flip-flop 37 to transfer a high Q output from its D input into D input of flip-flop 35. When the inverted clock signal from inverter 31 clocks flip- flop 35, its high D input will be transferred to its Q output and the D input of flip-flop 36 and as a high reset signal to be applied at the reset inputs of accumulators 24 and This causes the contents of both accumulators to asynchronously reset to the value applied at the input of accumulator 24 and inhibits their clock inputs. The reset is returned low. As a result of the high value at the D input of flip-flop 36, upon clocking from the delayed clock signal of delay element 33, the Q output of flip- flop 36 of the offset control circuit 26 toggles high to select the B input of the multiplexer 23. This causes the offset value to appear as the contents value of accumulator 24. At the same time, the low 0-bar outputs of flip-flops 35 and 36 reset the 0 output of flip-flop 37 to low via the NOR gate 38. On the next clock cycle, the inverted clock signal clocks flip-flop 35 to return the reset signal back to low, allowing the accumulators to increment in response to the clock signals. On the next low to high transition of the clock signal, the value at the first accumulator 24 input is stored in L r WO 89/12362 PCT/US89/02040 9 accumulator 24. After a certain delay set by the delay element 33 clocking flip-flop 36, the low D input of flip-flop 36 is transferred as a low SELECT output. This low SELECT signal causes the input A value containing the numerator to be transferred to the output of the multiplexer 23 for steady state operation. Whenever the frequency selector 21 is actuated to select a new output frequency fo the microprocessor controller 20 reads the data from memory 19 for the selected frequency causing the data to clocked into data register 22. The microprocessor controller 20 triggers the data register and offset control to cause the offset value to be applied to the first and second accumulators 24 and 25. The multiplexer 23 is then switched to provide the numerator value to the input of accumulator 24 where it is summed with the previously loaded offset value. For each clock 1 5 pulse from the fd signal the numerator value is again summed with the contents of accumulator 24. Similarly, the output of accumulator 1 is summed in accumulator The first accumulator 24 has a capacity of D as does the second accumulator 25. For each clock cycle, an input is added to the contents of the first accumulator 24. Contents from the first accumulator 24 are added to the contents of the second accumulator 25. For each clock cycle the accumulator capacity D is reached, that particular accumulator overflows and a carry value of one is generated. Otherwise, a carry value of zero is generated. For each reference clock cycle fd, the control logic 27 generates an instantaneous divisor output N to the programmable divider 16 based on the inputs to the control logic from the radio memory register's programmed N value, the two instantaneous (i) carry outputs from the first and second accumulators, Cli, C2i, respectively and the previously stored carry output of the second accumulator C2(I-1) where N Nnom Cli C2i The net effect over D cycles of the reference clock, is that k carry pulses are produced by the first accumulator 24. Accumulator has no effect on the average value of N since the counts are l WO 89/12362 PCT/US89/02040 always added and subtracted in pairs by the second accumulator The average value of the programmable divisor then has a whole part equal to the programmed value N and a fractional part equal to k/D. In this way, a non-integer value for the loop divider is created to obtain the desired output frequency fo from the multiplication of the reference frequency fr by the non-integer loop divider where fo fr (N While the circuit of the preferred embodiment utilizes the multiplexer 23 to load the offset into the first accumulator 24, other variations, such as loading the offset into the second accumulator 25 or loading the offset value directly to the input of the first accumulator 24 or second accumulator for one or more clock cycles are possible. For any particular output frequency fo it may be necessary to experiment for different values for the offset. Once an offset value has been determined which has an acceptable spurious response, that value is stored with the numerator, denominator and Nnom in the memory 19 and is selected whenever that particular frequency is desired. For frequencies where an offset is not necessary zero or the numerator value can be stored in the memory 19 as the offset value. For a given frequency or channel spacing a single denominator or D-value can be utilized. For a particular frequency fo it is also possible to vary both the N and D values and still obtain the same frequency output. When the variation of the offset value alone does not provide an acceptable spurious output level, selection of other N and D values for the frequency in conjunction with selection of an offset value can be utilized. The use of variable capacity accumulators 24 and permits the channel spacing of the synthesizer 10 to be easily changed. For example, to permit either 5 or 6 1/4 kHz channel spacing, the accumulators need only have sufficient capacity (i.e. length or number of bits) to support 5 kHz spacing. If fixed length accumulators were used, they would have to support 1 1/4 kHz spacing to synthesize both 5 and 6 1/4 kHz channels. This would WO 89/12362 PCT/US89/02040 11 require much larger accumulators than two programmable accumulators 24 and I claim as my invention: Claims (10) 2. A frequency synthesizer as defined in claim i, wherein the means for providing an offset value comprises S means for providing an offset value to the first or second 25 accumulator means. 3. A frequency synthesizer as defined in claim 2, in Swhich: .:Goo: said means for providing an offset value includes a S memory means, the memory means having offset information 0 for each desired output frequency f 0. 4. A frequency synthesizer as defined in claim 3, in which: said memory :.eans includes capacity information for determining the capacity of said first and second accumulator means. A frequency synthesizer as defined in claim i, in which: each of the accumulator means includes a first adder 39 having first and second inputs, a sum output, and a carry -12- output, a second adder having first and second inputs, a sum output, and a carry output, the sum output of the first adder being operatively connected to the first input of the second adder, a multiplexer having first and second inputs operatively connected to the sum outputs of the first and second adders respectively and having an output, and a latch having an input operatively connected to the output of the multiplexer and an output operatively connected to the second input of the first adder, the first input of the first adder comprises an input of the accumulator means, the second input of the 6000 •g*e second adder comprises a capacity input of the accumulator oo••o S 5 means, the output of the multiplexer comprises an output goes of the accumulator means, and the carry outputs of the. first and second adders cooperatively comprise a carry output of the accumulator means. 6. A frequency synthesizer for providing a synthesized output frequency fo comprising: a synthesizer loop including a programmable divider; a divider control means providing divider values to 0. the programmable divider, the divider control means providing varying values to the programmable divider for fractional division to produce a desired output frequency f and including first and second accumulator means. 0,o S" the first accumulator means including an input for ~receiving data, a first output for varying the divider Svalue, and a second output for providing data to the 40 second accumulator means, and the second accumulator means including an input connected to the second output of the first accumulator means and an output for varying the divider value; means for providing an offset value to the first and second accumulator means. 7. A frequency synthesizer for providing a synthesized output frequency fo comprising: a synthesizer loop including a programmable divider; 9 memory means for providing information for said -13- 2 j synthesizer loop; a divider control means providing divider values to said programmable divider, said divider control means providing varying values to said programmable divider for fractional division to produce a desired output frequency f and including at least a first and second o, accumulator means; said first accumulator means including an input for receiving data, a first output for varying said divider value, and a second output for providing data to said second accumulator means, and said second accumulator means including an input Sconnected to said second output of said first accumulator see* 'e means and an output for varying said divider value; 5 means for providing an offset value to at least one of said first and second accumulator means. 8. The frequency synthesizer of claim 7 wherein said means for providing said offset value comprises: a data register means, said data register means including an input from said memory means to provide said data and said offset value, a first output for providing said data and a second output for providing said offset 0 value for the desired output frequency f o; an offset control means for selecting said second S 2.5 output of said data register means; and a multiplexer coupled to said first and second **ego: Soutputs of said data register means for providing said offset valie to at least one of said first and second *goes: Saccumulator means in response to said offset control means. 30 9. The frequency synthesizer of claim 8 wherein said offset control means comprises inverting and delaying means coupled to a series of at least three flip-flops. The frequency synthesizer of claim 9 wherein said offset control means comprises: a series circuit including at least a first and a second inverter and a delay element, said output of said series circuit is connected to a clock input of said third flip-flop; 39 said second flip-flop includes a clock input for -14- coupling to an output of said first inverter; and a NOR gate including inputs connected to a Q bar output of said second and of said third flip-flop and an output coupled to a reset input of said first flip-flop. 11. A method of pr:oviding a synthesized output frequency, comprising the steps of: a) receiving dati for providing a divider value to a programmable divider to produce a desired output frequency fo; b) offsetting said data to provide an offset data; c) accumulating said offset data to provide an accumulated signal and a first control signal; d) accumulating said accumulated signal to provide a second control signal; e) delaying and inverting said second control signal to provide a third control signal; and f) varying said divider value in response to said tg first, second, and third control signals. 12. A method of providing a synthesized output frequency, comprising the steps of: a) receiving data for providing a divider value to a programmable divider to produce a desired output frequency fo; b) accumulating said data to provide an accumulated signal and a first control signal; c) offsetting said accumulated signal to provide an offset accumulated signal; d) accumulating said offset accumulated signal to provide a second control signal; .30 e) delaying and inverting said second control signal to provide a third control signal; and f) varying said divider value in response to said first, second, and third control signals. 13. A frequency synthesizer as defined in claim 1, 6 or 7 substantially as herein described with reference to the accompanying drawings. 39 M:P mi 14. A m,.xhod of providing a synthesized output frequency as defined in claim 11 or 12 substantially as herein described with reference to the accompanying drawings. DATED: 20 November, 1991. 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