This invention relates to frequency synthesizers of the phase-locked-

loop (PLL) type.

Background of Invention

It is known to provide such frequency synthesizers with a pulse

blanking circuit which assists in determining the output frequency of the

synthesizer. Pulse blanking is sometimes referred to as pulse swallowing,

pulse losing or pulse absorbing. Usually, such frequency synthesizers have

been developed for the production of frequencies used for communication

purposes.

Object of Invention

It is an object of the present invention to provide an adjustable

frequency synthesizer of the PLL kind utilizing the pulse blanking principle

and which can provide a range of selectable calibrated frequencies such as

those used in industrial plants, for example from 100,000 to 999,900 Hz.

Statement of Invention

The present invention provides a PLL frequency synthesizer with

adjustable frequency comprising a signal source of predetermined reference

frequency of 10, 100 or 1,000 Hz, a comparator having a first input receiving

said reference frequency signal, a second input receiving a signal of divided

- frequency, and an output producing an output signal dependent on the

difference between or equality of the reference frequency signal and the

divided frequency signal, the comparator operating to adjust the frequency of

the increased frequency signal produced by the oscillator in such a manner as

to equalize the frequencies of the divided frequency signal and the reference

frequency signal, and a voltage controlled oscillator controlled by the

comparator output signal to produce an output signal of increased frequency.

A first frequency divider divides the frequency of a signal derived from the

increased frequency output signal to produce a first divided frequency signal

at an output, a first blanking circuit operates to remove a predetermined

number of pulses per second from the increased frequency output signal to

produce the derived signal whose frequency is divided by the first frequency

divider, and a first adjustable selector adjustably selects the number of pulses

per second to be removed by the first blanking circuit. A second frequency

divider divides the frequency of a signal derived from the first divided

frequency signal to produce a second divided frequency signal, a second

blanking circuit operates to remove a predetermined number of pulses per

second from the second divided frequency signal to produce the derived signal

whose frequency is divided by the second frequency divider, and a second

adjustable selector adjustably selects the number of pulses to be removed by

the second blanking circuit, the signal of divided frequency received by the

second input of the comparator being the second divided frequency signal or

a derivative thereof.

Each blanking circuit may comprise a latch circuit operated by a signal

of predetermined latch frequency to disable the associated first or second

frequency divider a number of times per second equal to the second

predetermined latch frequency, and a counter circuit adjustably set by the

associated first or second selector to control the latch circuit and enable the

frequency divider said number of times per second after a predetermined

number of pulses of the increased frequency output signal or the first divided

frequency signal according to the setting of the associated first or second

selector, whereby the number of pulses per second removed by the first or

second blanking circuit is controlled by the setting of the first or second

selector respectively multiplied by said number of times per second the

associated latched circuit is operated.

The predetermined latch frequency may equal the predetermined reference

frequency. The predetermined frequency, the frequency of the divided

frequency signal received by the comparator and the predetermined latch

frequency may be 100 Hz, with each selector being adjustable to select any

integral number in the range of from 0 to 9.

The first divider may divide the frequency of the signal derived from

the increased frequency output signal by ten, and the second divider may

divide the frequency of the signal derived from the first derived frequency

signal also by ten, whereby the output frequency of the synthesizer can be

varied in steps of 100 Hz by adjustment of the first selector and in steps of

1,000 Hz by adjustment of the second selector to produce output frequencies

in the range of from 100,000 to 109,900 Hz.

The frequency synthesizer may also include a third frequency divider

which divides the frequency of the second derived frequency signal to

produce the signal of divided frequency received by the comparator, the third

frequency divider being a programmable divider controlled by third and

fourth adjustable selectors, the third selector being adjustable to select any

integral number in the range of from 0 to 9 and said fourth selector being

adjustable to select any integral number in the range of from 1 to 9

representing tens to enable settings of from 10 to 99 to be obtained and

thereby enable the synthesizer output frequency to be adjusted in steps of

10,000 Hz from to 100,000 to 990,000 Hz independently of the smaller steps

controlled by the first and second selectors.

The frequency synthesizer may also include a third latch circuit

operated by a signal with a frequency of 10 Hz and a third counter circuit

adjustably set by a fifth adjustable selector, the fifth selector being adjustable

to select any integral number in the range of from 0 to 9, the third latch

circuit and the third counter circuit being connected in parallel with the first

latch circuit and the first counter circuit whereby the third latch circuit

operates to disable the first frequency divider ten times per second and the

third counter circuit operates to enable the first frequency divider ten times

per second after a predetermined number of pulses as set by the fifth selector

to thereby enable the synthesizer frequency output to be adjusted in steps of

10Hz.

Description of the Drawings

Embodiments of the invention will now be described, by way of

example, with reference to the accompanying drawings, of which;

Fig. 1 is a block diagram of a PLL frequency synthesizer of fixed

frequency in accordance with the prior art,

Fig. 2 shows the frequency synthesizer of Fig. 1 with a pulse blanking

circuit added, also in accordance with the prior art,

Fig. 3 is a block circuit diagram of a frequency synthesizer with

adjustable frequency in accordance with one embodiment of the invention,

Fig. 4 is a diagrammatic view of the visual display of a frequency

selector switch which can be used with the frequency synthesizer of Fig. 3,

Fig. 5 is a block circuit diagram of a modification which can be used

with the frequency synthesizer of Fig. 3, and

Fig. 6 is a schematic view showing use of the invention to produce

pulses of different shape.

Description of Prior Art

Referring to the drawings, Fig. 1 shows a PLL frequency synthesizer

of fixed frequency in accordance with the prior art. A reference frequency

source (RFS) 10, which may comprise for example a divided-down crystal,

provides a signal with a stable reference frequency Fr which is fed to one

input of a comparator (comp) 12, the other input of which receives a signal

with frequency Fb. The output of comparator 12 is passed through a voltage

smoothing filter 14 to a voltage controlled oscillator (VCO) 16. The output

of the oscillator 16 with frequency Fo is passed to a synthesizer output 18 and

to a divider 20 which divides the frequency Fo by a factor X to produce the

signal with frequency Fb which is fed to the other comparator input as

mentioned above.

Comparator 12 causes oscillator 16 to oscillate at frequency Fo at

which frequencies Fr and Fb are equal.

Thus Fr = Fb = Fo ÷ X

ie. Fo = Fr. X

Fig. 2 shows a similar circuit also in accordance with the prior art,

with the addition of a pulse blanking circuit or blanker 22 between the Fo

output 18 and the frequency divider 20. If blanker 22 blanks out, for

example, one pulse per second, the frequency of the signal sent to divider 20

is Fo-1.

Therefore Fo - 1 = Fr.X

ie. Fo = Fr.X + 1

Thus, the synthesizer output frequency is increased by the blanking

action.

Description of Preferred Embodiments

Fig. 3 shows a frequency synthesizer with adjustable frequency in

accordance with the invention. In this embodiment, the predetermined

frequency of the reference frequency source 10 is 100Hz. The frequency

synthesizer has a first frequency divider 20 receiving a signal of frequency Fo

from the voltage controlled oscillator 16, a second frequency divider 22

receiving a signal of divided frequency from the first divider 20, and a

programmable divider 24 receiving a signal of divided frequency from the

second divider 22. The first and second dividers 20, 22 each divide the

frequency of the signal received thereby by ten. The divided signal produced

by programmable divider 24 has a frequency of 100 Hz and is fed to the

comparator 12.

A first blanking circuit associated with the first frequency divider 20

co -Tip rises a first latch circuit 26 and a first adjustable counter circuit 28, the

counter circuit 28 being adjustable by a first manually operable selector Si.

A second blanking circuit associated with the second frequency divider 22

comprises a second latch circuit 30 and a second adjustable counter circuit 32,

the counter circuit 32 being adjustable by a second manually selector S2. The

first and second selectors SI, S2 can each be set at any integral value in the

range of from 0 to 9 and are known per se in the art.

The first latch circuit 26 has a terminal R which receives a 100 Hz

trigger signal from reference frequency source 10 via a resistor-capacitor

circuit C^R _j which sharpens the trigger pulses. Resistor _{ is connected to

zero voltage. Each trigger pulse triggers the latch circuit 26 to reverse a

voltage at terminal Q from positive to negative. Terminal Q of latch circuit

26 is connected to terminal L of counter circuit 28 and to enable terminal EN

of first frequency divider 20. Terminal Q of latch circuit 26 is also connected

through resistor R ₅ to a positive voltage. Terminal S of latch circuit 26 and

terminals ZO and INH of counter circuit 28 are connected through resistor

R ₃ to a positive voltage. The signal of frequency Fo from voltage controlled

oscillator 16 received by the first frequency divider 20 is also received by

terminal CL of counter circuit 28.

Similarly, the second latch circuit 30 has a terminal R which also

receives the 100 Hz trigger signal from reference frequency source 10 via a

resistor-capacitor circuit C ₂ R ₂ which sharpens the trigger pulses. Resistor R ₂

is connected to zero voltage. Each trigger pulse triggers the latch circuit 30

to reverse the voltage at terminal Q from positive to negative. Terminal Q

of latch circuit 30 is connected to terminal L of counter circuit 32 and to

enable terminal EN of the second frequency divider 22. Terminal Q of latch

circuit 30 is also connected through resistor R ₆ to a positive voltage.

Terminal S of latch circuit 30 and terminal Z0 and INH of counter circuit 32

are connected through resistor R ₄ to a positive voltage. The signal of divided

frequency from the first divider 20 received by the second divider 22 is also

received by terminal CL of counter circuit 32.

The latch circuits 26, 30 may be provided by an IC 4013, the counter

circuits 28, 32 may each be an IC 4522, and the first and second frequency

dividers 20, 22 may be provided by an IC 4518. The programmable divider

24 may be an IC 4522. A person skilled in the art will be familiar with such

components. The programmable divider 24 is conventional and has two

manually operable selectors S4 and S3 settable in the range from 10 to 99, i.e.

selector S4 can be set at any integral number in the range of from 1 to 9

representing tens and selector S3 can be set at any integral number in the

range of from 0 to 9.

If selectors SI and S2 are each set to zero and selectors S3 and S4 are

set at their minimum, i.e. 10, the voltage controlled oscillator 16 will produce

an output signal with a frequency Fo of 100,000 Hz to balance comparator

12, since a frequency of 100,000 Hz will be sent by oscillator 16 to the first

frequency divider 20, which will then send a signal of 10,000 Hz to the

second frequency divider 22. The second divider 22 then sends a signal of

1,000 Hz to programmable divider 26, which sends a signal of 100 Hz to

comparator 12, this frequency being equ l to the reference frequency of 100

Hz from reference frequency source 10.

As previously mentioned, a sharp trigger pulse is sent to terminal R of

the first latch circuit 26 one hundred times per second. Each such pulse

causes terminal Q to become negative, with a negative signal consequently

being applied to terminal EN of first divider 20 and terminal L of counter

circuit 28. The negative signal at terminal EN of first divider 20 disables the

divider. The negative signal at terminal L of counter circuit 28 causes the

counter circuit to start counting the pulses of the Fo signal applied thereto at

terminal CL. Thus, divider 20 is prevented from effecting divider action on

such pulses. The counter continues to count the Fo pulses until a number

equal to the setting of selector SI is reached. The ZO terminal of counter

circuit 28 then becomes negative. The negative signal is applied to terminal

S of latch circuit 26 to cause the voltage at terminal Q thereof to reverse, i.e.

become positive. The positive signal is applied to terminal L of counter

circuit 28 to cause the counter circuit to stop counting and is also applied to

terminal EN of first divider 20 to enable divider 20 so that it resumes

dividing. Because the frequency of the signal applied to terminal R of latch

circuit 26 is 100 Hz, the sequence repeats itself one hundred times per second.

If for example selector SI is set to 4, four pulses are removed from the

Fo signal applied to first divider 20 one hundred times per second i.e. a total

of 400 pulses per second. Thus, the first frequency divider 20 now divides the

frequency of a signal derived from the Fo signal. In order to maintain

equality of frequency of the two signals applied to the comparator 12, the Fo

signal produced by the voltage controlled oscillator 16 becomes 100,000 Hz

plus 400 Hz, i.e. 100,400 Hz. Thus, SI controls the fourth digit of the output

frequency.

When selector SI is set at 0, the counter circuit 28 functions to

maintain a positive signal at terminal Z0 such that a positive signal is present

at terminal Q of latch circuit 26 and divider 20 is not disabled. Operation of

switch SI can therefore vary the output of the voltage controlled oscillator

16 at output 18 in steps of 100 Hz from 100,000 to 100,900 Hz.

The second blanking circuit functions in a similar manner as the first

blanking circuit except that operation of selector S2 controls the output

frequency at output 18 in steps of 1,000 Hz from 101,000 to 109,000 Hz. It

will be noted that counter circuit 32 does not receive the signal of divided

frequency from first divider 20 until the first blanking circuit has finished

blanking i.e. removing pulses. The first and second blanking circuits thus

function sequentially and do not interfere with one another.

If for example selector S2 is set to 4, four pulses are removed from the

signal of divided frequency applied to second divider 22 one hundred times

per second, i.e. a total of four hundred pulses per second. Thus, the second

frequency divider 22 now divides the frequency of a signal derived from the

signal of divided frequency from the first divider 20. In order to maintain

equality of frequency of the two signals applied to the comparator 12, the Fo

signal produced by the voltage controlled oscillator 16 becomes 100,000 Hz

plus 4,000 Hz i.e. 104,000 Hs (assuming selector SI is set at 0). Thus, selector

S2 controls the third digit of the output frequency. The selectors S4, S3 of

programmable divider 24 can vary the dividing action thereof from 10 to 99.

In other words, the minimum dividing setting for programmable divider 24

is 10. Assuming that selectors Si, S2 are set to zero, the frequency of the

output Fo from oscillator 16 when the setting of the programmable divider

24 is 10 will be 100,000 Hz. If selector S4 is then set to its minimum value

of one (representing 10), with selector S3 being set to four, the dividing factor

will be 14 and the output frequency Fo will become 140,000 Hz. Thus,

operation of selector S3 varies the output frequency from 100,000 to 190,000

Hz in steps of 10,000 Hz. If selector S4 is set at 4, with selector S3 being set

at zero, the dividing factor will be 40. The frequency Fo produced by voltage

controlled oscillator 16 will then be 400,000 Hz. Operation of selector S4

thus varies the output frequency fo from 100,000 to 900,000 Hz in steps of

100,000 Hz.

Therefore, by operation of selectors S4, S3, S2, SI, the output

frequency Fo can be varied from 100,000 to 999,900 Hz in steps of 100 Hz.

It will be noted that selectors S4, S3 control the first and second digits

respectively of the output frequency.

Fig. 4 is a diagrammatic view of a manually operable frequency selector

switch 36 with four thumb wheel or push-type selector members which from

left to right represent the selectors S4, S3, S2, SI followed by "00". The

selector 36 thus displays the complete frequency range of the synthesizer

shown in Fig. 3. Selector S5 shown in Fig. 4 will be described later with

reference to Fig. 5.

The frequency synthesizer shown in Fig. 3 can produce a range of

square waves with a range of from 100,000 to 999,900 Hz. There are 9,000

selectable frequency values in this range, all with the same accuracy as

reference source 10 (Fr = 100 Hz). The separation between each pair of

selected values is 100 Hz so the resolution is 50 Hz.

The number of selected values and the range itself may be extended by

conventional circuitry (not shown). For example, adding a four stage decade

divider at output 18 would enable a total of 45,000 values to be obtained, the

range being from 10 to 999,900 Hz. Such a synthesizer would have four

figure resolution for all selected values. The resolution in the 10 - 99 Hz

range would be 50 ÷ 10,000 ie. 0.005 Hz.

With the addition of a decade divider, it is also convenient to add five

light emitting diodes (LEDs) to the display shown in Fig. 4 to provide a

decimal point to the synthesized frequency value.

Fig. 5 is a block circuit diagram of modification which can be used

with the frequency synthesizer of Fig. 3 and comprises a third blanking

circuit which is associated with the first frequency divider 20 in parallel with

the first blanking circuit. It will be understood that the first and second

blanking circuits, the second frequency divider 22 and the programmable

divider 24 will also be present in the frequency synthesizer to be described

with reference to Fig. 5 but have been omitted for clarity.

The third blanking circuit comprises a third latch circuit 40 and a third

adjustable counter circuit 42 which are similar to the latch circuits and

counter circuits previously described. The counter circuit 42 is adjustable by

a fifth manually adjustable selector S5 which may be set at any integral value

in the range of from 0 to 9 and is known per se in the art. The third latch

circuit 40 has a terminal R which receives a 10 Hz signal derived from the

reference frequency source 10, which has a frequency of 100 Hz. The 100 Hz

signal from reference frequency source 10 is divided by ten by a divider 44

to produce a signal of 10 Hz whose phase is then inverted by inverter 46.

The phase inverted signal of 10 Hz is then applied to terminal R of latch

circuit 40 via a resistor-capacitor circuit C ₃ , R ₇ which sharpens the trigger

pulses. Each trigger pulse triggers the latch circuit 40 to reverse voltage at

terminal Q from positive to negative. Terminal Q of latch circuit 40 is

connected to terminal L of counter circuit 42 and to enable terminal EN of

the first frequency divider 20. Terminal Q of latch circuit 40 is also

connected through resistor R9 to a positive voltage. Terminal S of latch

circuit 40 and terminal Z0 and INH of counter circuit 42 are connected

through resistor R ₈ to a positive voltage. The signal of frequency fo from

voltage control oscillator 16 received by the first frequency divider 20 is also

received by terminal CL of counter circuit 42. The inverter 40 is provided

so that operation of the third blanking circuit does not overlap with

operation of the first blanking circuit.

Since a signal frequency of 10 Hz is applied to latch circuit 40, the

third blanking circuit only functions 10 times per second compared to the

first and second blanking circuits which function 100 times per second. With

selectors S4 to SI at their minimum values, operation of selector S5 will

therefore vary the output frequency F ₀ from 100,000 to 100,090 Hz in steps

of 10 Hz. Selector S5 therefore controls the fifth digit and provides a tenfold

improvement in adjustability of the output frequency.

Fig. 6 shows a square wave frequency synthesizer 50 of the kind shown

in Fig. 3 being used to provide input to a variable function generator 52,

which may be an XR 2206 type as known in the art, operable to produce

other wave shapes, for example sine, triangle or ramp wave shapes.

It should be noted that, although the frequency synthesizer of Fig. 3

uses two fixed dividers each with a blanking circuit in cascade with a

conventional two-stage programmable divider circuit, three fixed dividers each

with a blanking circuit and one programmable divider (1-9) circuit could be

used. Other combinations are of course possible.

The adjustable selectors S4, S3, S2 and Si may not necessarily be

manually operable. For example, the selectors may be timer controlled

switches which are set to produce a desired series of different frequency

bursts. Also, when manually operable, the selectors may be for example the

thumb wheel, push-button or keyboard type.

A person skilled in the art will appreciate from the foregoing

description that the present invention provides a frequency synthesizer that

is user friendly, i.e. a desired frequency is readily selectable, is relatively

inexpensive and requires no supporting frequency counter. Known frequency

counters may take several ten second periods to display the test frequency

with maximum accuracy.

Other embodiments of the invention will also be readily apparent to

a person skilled in the art, the scope of the invention being defined in the

appended claims.