Constant frequency signal generator circuit and method

An improved apparatus for generating a stable frequency signal is disclosed. A master oscillator provides an output signal having a preselected frequency value X. A main feedback loop, a first feedback loop and a second feedback loop are provided. Deviation of the output signal from the preselected frequency value X causes the second feedback loop to be responsive at twice the frequency rate as that of the first feedback loop. The frequency difference between the values of the first feedback loop and the second feedback loop is used to produce a control signal for the main feedback loop used to control the frequency value X of the output signal. The circuit is self-correcting and exhibits high order short and long term stability. Two embodiments are disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to signal generators. More particularly, 
this invention relates to signal generators which generate a signal of a 
frequency which is essentially constant over time. 
2. Related Art 
There are many technical applications in which constant frequency signal 
generation is required. Such applications include all forms of measuring 
and testing equipment, as well as clocks, computer and communications 
systems, navigational systems, and all forms of devices in which accurate 
measurement of periods of time is required. 
Accurate measurement of time has been a technological goal for literally 
hundreds of years. In the recent past, substantial improvements have been 
made through the use of crystal oscillators. Even more recently, atomic 
clocks using the rate of electron transitions in atoms of selected 
elements, such as cesium, rubidium and hydrogen, have been increasingly 
common as sources of constant frequency signals. However, both quartz 
oscillators and atomic clocks suffer some substantial instability caused 
by, for example, temperature, magnetic influences, component aging, 
humidity, mechanical shock, etc. 
There are a wide variety of uses for accurate oscillators, that is, signal 
generators which provide time signals which are constant. It has been said 
that nearly every electronic instrument contains an oscillator of some 
sort, and that oscillators are as important to such instruments as are 
stable sources of supply voltage. See generally Horowitz and Hill, The Art 
of Electronics (1980), generally at chapters 4 and 14, and particularly at 
.sctn.14.09, incorporated herein by reference. Note that the various 
oscillators described therein range greatly in accuracy and price. 
For example, temperature-compensated crystal oscillators cost about $50 and 
are stable to several parts per million. Ovenized crystal oscillators 
range in price from several hundred to more than one thousand dollars, and 
have stability rates of parts per million, but are noted for their 
tendency to degrade with age, such that the frequency varies over time. 
They are large in size and are costly to calibrate and maintain. 
Next on the scale of complexity and price are the atomic standards: 
rubidium, cesium and hydrogen. These are stated to have stability of up to 
one part in 10.sup.15, but are priced on the order of tens of thousands of 
dollars ($20,000 to $30,000 is typical). These also show degradation with 
time. 
a. Representative Crystal Oscillator 
FIG. 1 shows a schematic of a conventional ovenized crystal oscillator 
circuit, as described in U.S. Pat. No. 4,479,096 to Fowks, which is 
incorporated by reference herein. It illustrates the complexity of such 
crystal oscillator circuits. 
The oscillator circuit may be considered to comprise three main sections, 
namely, a crystal oscillator section 150, a buffer/amplifier section 152, 
and a temperature control section 154. Note that a 10 MHz crystal 156 
falls between sections 150 and 154 by virtue of its electrical connection 
to the circuit of 150 and its physical interaction with the teperature 
control section 154. 
In operation, a DC error signal derived from a synchronous phase detector 
(not shown) is applied to an RC network 158 in the feedback loop of 
oscillator 150, which is of the Colpitts type. Network 158 includes a gain 
leveling diode 160 and a variable capacitor 161, the latter being 
responsive to the magnitude of the DC error signal to control the fine 
variation in feedback capacitance and thus the fine frequency of 
oscillator 150. The active element of oscillator 150 is a field effect 
transistor (FET) 162, to which RC network 158 is connected at the gate 
terminal thereof. 
The source of FET 162 is connected to the base of emitter follower 
configured transistor 164, which provides power gain to the output of FET 
162 without changing its phase angle signal. This power gain without phase 
angle variation permits the use of greater feedback capacitance. 
Oscillator 150, in effect, "swamps out" temperature sensitive transistor 
junction capacitance variations. Also connected to the gate terminal of 
FET 162 is capacitor 165 in series connection with crystal 156 and 
capacitor network 166 which also form part of the feedback path of the 
oscillator. 
An FET oscillator is used with a gain leveling diode as an AGC device and a 
bipolar emitter follower for high signal level. Frequency adjustment is 
obtained by way of varicap 161 and crystals having a relatively high 
effective series resistance can be tolerated. No tuned circuits are 
required. The oscillator output is derived through crystal 156 and network 
166. 
The oscillator output signal (OSC) is applied to buffer amplifier section 
152 to increase the output level of the crystal oscillator to a useable 
value for driving the frequency multiplier and synthesizer (not shown), 
and for providing the output signal to the end user of the crystal 
oscillator. This has to be accomplished without permitting outside load 
variations to cause any significant level changes to the multiplier 
circuit or pulling of the crystal oscillator which cause loop control 
frequency perturbations. 
The crystal oscillator has an output signal, OSC, at a low voltage level 
(approximately 200 millivolts peak to peak) and at a low output impedance. 
It is applied to a grounded gate FET 170 and a 100 ohm source bias 
resistor 168. Resistor 168 is in parallel with the equivalent source 
impedance of FET 170 and provides approximately a 50 ohm termination 
impedance to the oscillator signal. The gate of FET 170 is internally 
connected to the case of the transistor, thereby isolating the drain 
circuit. The amplified output of the oscillator signal OSC at the drain 
terminal of FET 170 is applied both to a coupling transformer 172 and to a 
high input impedance buffer/amplifier 174. The secondary winding of 
coupling transformer 172 provides the 10 MHz output signal to the 
multiplier and to the synthesizer (not shown). 
The other 10 MHz output available externally to the user is capacitively 
coupled from the drain terminal of FET 170 to buffer/amplifier 174, the 
output of which is current limited by a resistor 175 and then applied to 
the primary winding of a coupling transformer 176. The secondary winding 
of transformer 176 provides the 10 MHz output signal to an externally 
accessible plug 178. In addition to providing isolation to variation in 
the load impedance to which plug 178 is connected, coupling transformer 
176 also inhibits potential ground loops. 
Temperature control section 154 includes an oven 180 in which 10 MHz 
crystal 156 is located for temperature control, preferably at the crystal 
tuning point for optimum stability. Heating is achieved by passing current 
through a 27 ohm, 3 watt wire wound resistor 182, which is in the emitter 
circuit of transistor 184. The temperature is monitored by thermistor 186, 
which is connected to a positive input terminal to amplifier 188, the 
output of which is connected to the base terminal of transistor 184. Thus, 
heating resistor 182, transistor 184, thermistor 186 and amplifier 188 
form a feedback control loop for controlling the temperature within oven 
180 for precise stability of oscillator 122. 
The technical complexity needed to achieve relatively stable output from a 
crystal oscillator can now be appreciated. Obviously, such a complex 
circuit is difficult and expensive to build and maintain, and is of a 
large size and has considerable power requirements. 
b. Representative Atomic Clock 
FIG. 2 shows a block diagram of a conventional atomic clock as described in 
U.S. Pat. No. 4,290,030 to Wang, incorporated by reference herein. This 
illustrates dramatically the complexity of atomic clocks. 
A voltage controlled crystal oscillator (VCXO) 200 having a nominal output 
frequency of 5 megahertz has output terminals 200a, 200b and a voltage 
control input 200c. The voltage present on the voltage control input 200c 
controls the frequency of the output produced on outputs 200a, 200b in a 
manner well known to those skilled in the art. 
A frequency synthesizer 205 is provided and has its input 205a connected to 
VCXO output 200b. The synthesizer 205 multiplies the nominal 5 megahertz 
output of the VCXO 200 to a nominal value of 1.4 gigahertz. The 
synthesizer 205 outputs the 1.4 gigahertz signal at its outputs 205b, 
205c. A frequency synthesizer 210 has its input 210a connected to VCXO 
output 200b. The frequency synthesizer 210 generates a 20.405751 megahertz 
signal phase locked to the VCXO 200. 
The signal from the frequency synthesizer 210 passes through output 210b 
through a normally open electronic gate 215. A mixer 220 receives the 1.4 
gigahertz signal from the frequency 205 through its input 220a, and 
receives the 20.4 megahertz signal from the gate 215 through its input 
220b. The frequency sum of the 1.4 gigahertz signal and the 20.4 megahertz 
signal appears on output 220c of the mixer 220. Therefore, the signal 
appearing on the output 220c of the mixer 220 contains a nominal frequency 
of 1.420405751 gigahertz. 
The maser cavity 225 includes an atom storage bulb 230, shown in simplified 
schematic form only. The storage bulb 230 contains hydrogen gas. An atomic 
beam source 245, well known in the conventional art, supplies the hydrogen 
gas to the storage bulb 230 through atomic beam entrance 250. The atomic 
beam source 245 preferably includes a static magnetic field source 
surrounding the atomic beam which selects hydrogen atoms having a 
particular magnetic spin orientation in a manner well known in the art. 
The maser cavity 225 is defined by a conductive enclosure, typically a 
copper coating. The maser cavity 225 has an input 225a connected to the 
output 220c of the mixer 220. The 1.420405751 gigahertz signal from the 
mixer output 220c passes through the maser input 225a and is introduced 
into the maser cavity. The maser cavity 225 also has an output 225b 
through which the signal from the maser can be coupled out. 
Another mixer 235 has its input 235a connected to the maser output 225b. 
Input 235b to the mixer 235 is connected to output 205b of frequency 
synthesizer 205. The mixer 235 mixes the signal from maser output 225b 
with the 1.4 gigahertz signal from frequency synthesizer output 205b. An 
amplifier 240 having a tuned output receives the resulting mixed signal 
from mixer 235. The frequency difference between the 1.4 gigahertz signal 
of frequency synthesizer output 205b and the maser signal from maser 
output 205b is outputted by the tuned amplifier at output 240a. The tuned 
amplifer 240a is tuned to pass the frequency difference signal in the 
mixer 235. 
The signal from tuned amplifier output 240a passes through a normally 
closed electronic gate 255 into mixer input 260a of mixer 260. A frequency 
synthesizer 265 is provided, which has its input 265a connected to VCXO 
output 200b. The frequency synthesizer 265 multiplies the nominal 5 
megahertz output signal of the VCXO 200 to 20.4 megahertz, and outputs 
this signal on frequency multiplier output 265b. Mixer 260 receives the 
20.4 megahertz signal from synthesizer output 265b through mixer input 
260b. The signal corresponding to the difference frequency of the inputs 
at 260a and 260b of the mixer 260 is fed to the tuned amplifier 270. The 
output signal R at tuned amplifier output 270a represents the frequency 
difference between the signal from the maser output 225b and a nominal 
frequency of 1.4204 Ghz. 
Signal R, on tuned amplifier output 270a, is utilized to generate a 
corrective feedback signal sent to voltage control input 200c as follows. 
A presettable up-down counter 275, which has its clock timing input 275b 
connected to a frequency synthesizer 280, is provided. The frequency 
synthesizer 280 is connected to VCO output 200b and multiplies the nominal 
5 megahertz output of the VCO to 100 megahertz. The 100 megahertz signal 
from frequency synthesizer 280 enters the timing control input 275b as a 
time base clock signal for the up-down counter 275. 
The up-down counter 275 operates to count the number of 100 megahertz 
pulses received from the frequency synthesizer 280 between a preselected 
number of zero crossings 281 of the difference signal from tuned amplifier 
output 270a. The residue accumulated in up-down counter 275 is sent as 
digital data to digital-to-analog converter 290. The digital-to-analog 
converter 290 preferably has a high resolution, such as 20 bits, and an 
analog output range to match the voltage tuning range of the VCXO, 
typically between 0 and 10 volts. The digital-to-analog converter 290 
converts each accumulated count into a corresponding voltage level. This 
voltage level is then furnished to VCO voltage control input 200c. It is 
now apparent that the system illustrated in FIG. 2 includes a closed 
feedback control loop comprising the voltage control oscillator 200, the 
synthesizer 205, the maser 225, the mixer 235, the mixer 260, and the 
synthesizer 280 generating the time base clock signal for the up-down 
counter 275. 
Logic control circuitry is provided which temporarily deactivates operation 
of this feedback control loop whenever the maser 225 is externally 
stimulated, in accordance with the free induction technique. A pulse 
generator 300 generates a clock signal controlling electronic gates 215 
and 255. Pulse generator 300 also transmits the clock signal to input 305a 
of control logic device 305. The control logic device 305 also receives 
the output difference signal of from tuned amplifier output 270a through 
input 305b. 
Initially, the VCXO 200 outputs a 5 megahertz signal at output 200b. 
Frequency synthesizer 205 multiplies this signal to 1.4 gigahertz and 
introduces it into mixer 220. The electronic gate 215 is normally open, 
and therefore the mixer 220 normally receives only a 1.4 gigahertz input 
signal. This signal is not transmitted by the maser cavity 225 since it is 
far from the cavity resonance frequency. In any case, the 1.4 gigahertz 
signal is not sufficiently close to f.sub.0, the ground state hyperfine 
transition frequency of 1.42040575168 gigahertz, to stimulate the 
hyperfine transition in the hydrogen gas. Consequently, there is no 
emission from the hydrogen gas inside maser cavity 225. 
Later, the clock signal is pulsed positively. Therefore, electronic gate 
215 is then closed, thereby allowing the 20.405751 megahertz signal from 
frequency synthesizer 210 to enter mixer input 220b. Also, at time 
T.sub.1, gate 255 is opened, which isolates the feedback loop. Mixer 220 
causes the frequency sum of the 1.4 gigahertz signal from frequency 
synthesizer 205 and the 20.405751 megahertz signal from frequency 
synthesizer 210 to enter the maser cavity 225 through maser input 225a as 
a nominal 1.420405751 gigahertz signal. 
The resulting 1.420405751 gigahertz stimulation signal has a frequency 
bandwidth determined by the pulse width and, therefore, contains a 
frequency component at the exact frequency f.sub.0 of the hyperfine 
transition of the hydrogen gas. Therefore, the hydrogen atoms inside the 
storage bulb 230 start to radiate as a result of the introduction of the 
stimulating signal returning to zero amplitude; the electronic gate 215 
opens while gate 255 closes, thereby permitting the output of the maser to 
reach the feedback loop. 
Therefore, the frequency of the output signal on mixer output 220c 
introduced into maser input 225a returns to the nominal 1.4 gigahertz 
frequency, which as discussed above, is not sufficiently close to the 
transition frequency f.sub.0 of the hydrogen gas inside the storage bulb 
230. The hydrogen gas therefore is no longer excited by the signal from 
mixer output 220c. However, the stimulating pulse succeeded in creating a 
population of radiating hydrogen atoms in the storage bulb 230, and these 
atoms radiate energy at the hyperfine transition frequency. The radiation 
from the hydrogen atoms in the storage bulb 230 resonates inside the 
cavity 225 so that the signal at maser output 225b has a frequency equal 
to the hyperfine transition frequency f.sub.0. The resulting exponential 
decay of the maser output causes exponential decay of the difference 
output. The resonance of the stimulation signal in the cavity 225 
decreases so rapidly that it has a decay envelope which is not 
significant. 
The 1.4 gigahertz signal from synthesizer 205 is fed into mixer 235, where 
it is substracted from the maser output signal frequency f.sub.0, and the 
difference frequency then appears on tuned amplifier output 240a. The 
output from mixer 235 is subtracted in mixer 260 from the 20.4 megahertz 
signal generated by synthesizer 265, the difference of which appears on 
tuned amplifier 270a. If it is assumed that the VCXO 200 is generating a 
signal at the precise frequency of 5 megahertz, it is seen that the signal 
on maser output 225b, which has the frequency of 1.42040575168 gigahertz, 
is subtracted from the 1.4 gigahertz signal in mixer 235 and the resulting 
signal is then subtracted from the 20.4 megahertz signal in mixer 260. The 
resulting response appears at tuned amplifier output 270a and has a 
relatively low frequency of 5.75168 kilohertz. 
The resonance signal decays exponentially as soon as the stimulative signal 
is turned off. Processing of the resonance signal occurs while there is no 
external stimulation. After the signal has decayed to zero, a new 
stimulation pulse occurs, and the entire foregoing sequence is repeated. 
The stimulation pulses are repeated at selected intervals throughout the 
entire operation of the invention, and each stimulation signal is followed 
by a new response signal which in turn must be processed by the control 
logic 305. 
Processing of each response signal from tuned amplifier output 270 will now 
be described. The counter control logic 305 receives the clock pulse 
signal from pulse generator 300. Counter control logic 305 receives the 
response signal from tuned amplifier output 270 through the input 305b. 
The control logic 305 senses each zero crossing 281 of the response 
signal. As soon as the first zero crossing 281 is sensed, the control 
logic 305 enables the up down counter 275, and therefore, the up down 
counter 275 begins to accumulate counts of 100 megahertz pulses received 
from the synthesizer 280. The control logic 305 may be preset to a certain 
value, which, for example, may be equal to 1,000, so that as soon as the 
control logic 303 has sensed 1,000 zero crossings 281 in the response 
signal, the control logic 305 disables up down counter 275 so that the 
counter 275 stops counting. 
Thus, it is seen that the operation of the control logic 305 causes the 
updown counter 275 to count the number of 100 megahertz pulses from 
synthesizer 280 which occurs during an interval defined by 1,000 zero 
crossings of the response signal. It is apparent that if the oscillator 
200 is precisely adjusted such that the frequency of the response signal 
is precisely 575.1680 hertz, it will take precisely 0.17386225 seconds for 
the 1,000 zero crossings to occur. Therefore, in this exemplary situation, 
counter 275 will be enabled for exactly 0.17386225 seconds, and will count 
precisely 17,386,225 pulses from 100 megahertz synthesizer 280. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide an improved constant 
frequency signal generator. 
It is another object of the present invention to provide a constant 
frequency signal generator in which no elaborate compensation arrangements 
are required due to the inherent self-regulating nature of the signal 
generation circuit. 
It is a further object of the present invention to provide a constant 
frequency signal generator which is technically simpler, less expensive, 
and more reliable and accurate than those of conventional technology. 
It is a further object of the present invention to provide a source of 
frequency that does not significantly degrade in stability over time. 
Those and other objects are achieved by the present invention discussed 
below. 
SUMMARY OF THE INVENTION 
The present invention satisfies many needs of the art and the objects of 
the invention described above by provision of a self-regulating signal 
generator circuit having a constant frequency output signal. 
The basic concept of the present invention is of providing an output signal 
having a preselected frequency value X, a main feedback loop, a first 
feedback loop, and a second feedback loop. Deviation of the ouput signal 
from the preselected frequency value X causes the second feedback loop to 
be responsive at twice the frequency rate as that of the first feedback 
loop. The frequency difference between the values of the first feedback 
loop and the second feedback loop (that is, the frequency difference 
between the offset component of the first divided signal and the offset 
component of the second divided signal) is used to produce a control 
signal for the main feedback loop used to control the frequency value X of 
the output signal. In this way, the circuit of the present invention is 
made self-correcting. Elaborate temperature compensation or the like is 
not required to ensure a stable frequency source. Both short and long term 
stability of a high order is obtained. 
In a first embodiment, the first feedback loop comprises a first mixer 
which mixes the master oscillator X signal with a X+N signal generated by 
a first local oscillator. The difference N signal is supplied to the first 
input of a first comparator. The X+N signal from the first local 
oscillator is divided by a value A to produce a first divided (X+N)/A 
signal which is supplied to the second input of the first comparator. The 
output signal from the first comparator (which has a zero value if the 
frequency X+N of the first local oscillator is correctly related to the 
master X signal) is fed back to the first local oscillator to control its 
frequency X+N. 
The second feedback loop of the first embodiment includes a multiplier 
which multiplies the X signal from the master oscillator by a value B. The 
XB signal thus produced is supplied to a second mixer which mixes it with 
a XB-D signal generated by a second local oscillator. The difference D 
signal from the second mixer is supplied to the first input of a second 
comparator. The XB-D signal is divided by a value E to produce a second 
divided (XB-D)/E signal, which is supplied to the second input of the 
second comparator. The output signal from the second comparator (which has 
a zero value if the XB-D frequency of the second local oscillator is 
correctly related to the master X signal) is fed back to the second local 
oscillator to control its frequency XB-D. 
The master loop of the first embodiment includes a master or third 
comparator having a first input provided with the first divided (X+N)/A 
signal from the first feedback loop, and having a second input provided 
with the second divided (XB-D)/E signal from the second feedback loop. 
Note that the second feedback loop is responsive at twice the frequency as 
that of the first feedback loop. The control signal at the output of the 
master comparator, which has a value indicative of the frequency 
difference between the first divided (X+N)/A signal and the second divided 
(XB-D)/E signal, is used to control the frequency value X of the output 
signal from the master oscillator. 
Turning now to the second embodiment of the present invention, a master 
oscillator generates an output signal of a preselected frequency X, which 
is supplied to a first input of a balanced modulator. The X signal is also 
supplied to a master divider which divides it by D to produce a N signal. 
The N signal is supplied to the second input of the balanced modulator, 
which produces a sum X+N signal and a difference X-N signal. 
The first feedback loop of the second embodiment includes a first mixer 
which mixes the X-N signal with a X' signal generated by a first local 
oscillator. The difference N signal from the first mixer is supplied to 
the first input of a first comparator. The X' signal also is divided by a 
value M to produce a first divided X'/M signal which is supplied to the 
second input of the first comparator. The output signal from the first 
comparator (which has a zero value if the frequency X' of the first local 
oscillator is correctly related to the master X signal) is fed back to the 
first local oscillator to control its frequency X'. 
The second feedback loop of the second embodiment includes a second mixer 
which mixes the X+N signal with a X" signal generated by a second local 
oscillator. The difference N signal from the second local oscillator is 
divided by a value M to produce a second divided X"/M signal which is 
supplied to the second input of the second comparator. The output signal 
of the second comparator (which has a zero value if the frequency X" of 
the second local oscillator is correctly related to the master X signal) 
is fed back to the second local oscillator to control its frequency X". 
The master loop of the second embodiment includes a master or third 
comparator having a first input provided with the first divided X'/M 
signal from the first feedback loop, and having a second input provided 
with the second divided X"/M signal from the second feedback loop. Note 
that the second feedback loop is responsive at twice the frequency as that 
of the first feedback loop. The control signal at the output of the master 
comparator, which has a value indicative of the frequency difference 
between the first divided X'/M signal and the second divided X"/M signal, 
is used to control the frequency value X of the output signal from the 
master oscillator. 
Consequently, the output signal from the master oscillator is of a very 
precise frequency that does not vary over time, temperature, or humidity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The conventional art discussed above illustrates that there exists a 
significant need for an oscillator which is simple, accurate and 
inexpensive. The present invention satisfies these goals by provision of 
an oscillator which does not require expedients such as temperature 
compensation or the like to ensure accuracy, but which instead relies on 
its inherent design to provide a very stable source of oscillation. Note 
that throughout this section of the application exemplary values for 
frequency are given. However, this is merely for purposes of illustration, 
and these frequencies and the ratios between them should not be construed 
to limit the present invention. 
Speaking broadly, the constant frequency oscillator of the present 
invention is realized by a circuit including a controllable master 
oscillator which outputs a signal precisely at the desired frequency. 
1. First Embodiment 
A block diagram of a first embodiment of the present invention is shown in 
FIG. 4. It gives values for the frequencies of the various signals in the 
circuit in algebraic form. The user can cause the master oscillator 410 to 
generate an output signal of a selected frequency X by using the values 
specified by the algebraic equations in the three loops of the present 
invention. The output X of the master oscillator 410 is locked down to the 
desired output frequency even though the local oscillators 414 and 424 of 
themselves are not that stable. FIG. 5, discussed below, gives examples of 
these frequencies for a specific example of this embodiment of the present 
invention. FIGS. 3A to 3D illustrate the operation of this representative 
example of the first embodiment of this representative example of the 
first embodiment of the present invention. 
In the circuit of FIG. 4, master oscillator 410 generates the output signal 
of very precise frequency X for the circuit of the present invention. This 
output signal has a frequency of X, which is selected by the user. In a 
mixer 412, the signal of frequency X from master oscillator 410 is mixed 
with a signal of frequency X+N generated by a local oscillator 414. The 
output of mixer 412, which is a signal of difference frequency N, is 
supplied to a first input of a comparator 416. A second input of 
comparator 416 is supplied with the signal at the output of a divider 
network 418, which divides the signal of frequency X+N at the output 
signal of local oscillator 414 by an interger number A, such that 
(X+N)/A=N when the nominal frequency X+N of local oscillator 414 is 
correct. The output of comparator 416 is zero if the frequency of local 
oscillator 414 is correctly related to the signal of frequency X. The 
output of comparator 416, indicative of the frequency error between the 
output of master oscillator 410 and local oscillator 414, is fed back to 
local oscillator 414 for frequency control, thus completing this first 
phase-locked-loop circuit. 
Turning now to the second phase lock loop circuit, the signal of frequency 
X outputted by master oscillator 410 is supplied to a multiplying network 
420, where it is frequency multiplied by a quantity B, which will be an 
integer. The signal of frequency XB from multiplying network 420 is 
supplied to a mixer 422. A local oscillator 424 generates a signal having 
a nominal frequency of XB-D as is defined below, which is supplied to 
mixer 422. The difference frequency signal of value D at the output of 
mixer 422 is supplied to a first input of a comparator 428. The other 
input to comparator 428 is supplied with a signal of a frequency (XB-D)/E, 
which is obtained after division in a divider network 426 by a quantity E 
chosen such that (XB-D)/E=D when the frequency of the signal XB-D is 
correct. The output of comparator 428 is zero if the frequency of local 
oscillator 424 is correctly related to the signal of frequency XB. The 
output of comparator 428, indicative of the effective error between the 
multiplied output of master oscillator 410 and the local oscillator 424, 
is fed back to local oscillator 424 for frequency control, thus completing 
this second phase-locked-loop circuit. 
The first input of a comparator 430 is supplied with the signal of 
frequency (X+N)/A present at the output of divider 418 of loop 1. As 
illustrated in FIG. 3A, this signal (X+N)/A is equal to the frequency 
difference between the output of local oscillator 414 and the master 
oscillator 410. Another way to look at this is that this signal (X+N)/A is 
half of the numerical difference between signal X and signal XB. 
The second input of comparator 430 is supplied with the signal of frequency 
(XB-D)/E present at the output of divider 426. As shown in FIG. 3A, this 
signal (XB-D)/E is equal to the frequency difference between the output of 
frequency multiplier 420 and local oscillator 424. Another way to look at 
this is that this signal (XB-D)/E is half the numerical difference between 
signal X and signal XB. 
The output of comparator 430 is used to control the frequency X generated 
by master oscillator 410. When signal (X+N)/A is equal to signal (XB-D)/E, 
the output of comparator 430 is zero, indicating that the value of signal 
X is correct. This state of operation is shown in FIG. 3A. The operation 
of comparator 430 used to correct signal X is now described with reference 
to FIG. 5 and FIGS. 3B to 3D. 
Referring now to FIG. 5 which shows representative values for the variables 
of the embodiment of FIG. 4, master oscillator 410, which may comprise a 
voltage-controlled oscillator (VCO) or a voltage-controlled crystal 
oscillator (VXCO), provides an output signal X at a nominal frequency of 5 
MHz, which signal X is also provided to mixer 412. A first loop oscillator 
414 provides a signal of nominal frequency of X+N=7.5 MHZ to the mixer 
412. The mixer 412 frequency subtracts the 5 MHz signal X from the 7.5 MHz 
signal X+N to produce the signal N=2.5 MHz. 
The output N of mixer 412 therefore is a 2.5 MHz signal, which is supplied 
to comparator 416. The 7.5 MHz signal X+N from oscillator 414 is also 
supplied to a divide-by-3 network 418 where A=3 such that it also outputs 
a 2.5 MHz signal (X+N)/A. This 2.5 MHz signal (X+N)/A is compared in 
comparator 416 to the 2.5 MHz signal N generated by the mixer 412. The 
difference, termed the "control signal", is supplied to the oscillator 
414, such that the desired 7.5 MHz signal X+N is frequency controlled to 
be correct. This control signal thus provides a phase-locked loop defining 
a 2.5 MHz frequency (X+N)/A with exactitude which in turn defines the 7.5 
MHz signal X+N accurately. 
This proper operation of the first loop is shown in FIG. 3A. As 
illustrated, the value of (X+N)/(A)=2.5 MHz, and the value of X+B=7.5 MHz. 
The value of X=5.0 MHz. Note that X+B=7.5 MHz is exactly halfway between 
X=5.0 MHz and XB=10.0 MHz. Referring now to loop 2, the signal X from the 
master oscillator 410 also is supplied to a frequency multiplier 420, 
which multiplies this signal by an integer B, where B=2. In the case where 
B=2, a preferred embodiment for frequency multiplier 420 is shown in U.S. 
Pat. No. 3,681,705 to Spence, which is incorporated by reference herein in 
its entirety. Note that in the present example, B=2. 
The multiplier 420 therefore provides a 10 MHz output signal XB, which is 
multiplied in mixer 422 with a 7.5 MHz signal BX-D provided by oscillator 
424. A divide-by-3 network 426 divides the 7.5 MHz signal by 3, producing 
an (XB-D)/E signal of 2.5 MHz. This 2.5 MHz signal is supplied to 
comparator 428. Comparator 428 provides a "control signal" to oscillator 
424 to maintain the (XB-D) value of 7.5 MHz illustrated by the diagram for 
loop 2 of FIG. 3A. 
The two 2.5 MHz signals outputted by the loops 1 and 2 are supplied to 
comparator 430, which provides the control signal to the master oscillator 
410, effectively defining a third or master phase locked loop. This master 
loop locks the value of X generated by master oscillator 410 to 5.000,000 
MHz, as illustrated in FIG. 3A. 
Referring now to FIG. 3B, assume that the frequency value of X moves 1 
Hertz upward so that X=5.000,001 MHz. This change in X causes the value of 
X+N generated by oscillator 414 of loop 1 to become 7.500,001 MHz. In 
addition, this change in X causes the value of XB-D generated by 
oscillator 424 of loop 2 to become 7.500,002 MHz. Note that the value for 
loop 2 is twice the frequency change of loop 1. The frequency difference 
between XB-D and X+N is 1 Hertz, which is also the change of the value in 
X for which the present invention will compensate. 
Referring now to FIG. 3C, the master oscillator 410 value X has been moved 
down 0.5 Hertz from its 5.000,000 MHz value so that it now has a X value 
of 5.000,000,5 MHz. This change in X causes the value of X+N generated by 
oscillator 414 of loop 1 to become 7.5000005 MHz. In addition, this change 
in X causes the value of XB-D generated by oscillator 424 of loop 2 to 
become 7.500,001 MHz. Note that the value for loop 2 is twice the 
frequency change of loop 1. The frequency difference between XB-D and X+N 
is 0.5 Hertz, which is also the change in the value in X that has not yet 
been compensation. 
FIG. 3A shows the completion of the correction process that has been 
occuring in FIGS. 3B and 3C. Note that the value of X+N now equals to 
value of XB-D. This is the "locked up" state of the present invention. 
Since loop 2 moves twice as fast frequency wise as loop 1, the present 
invention can compensate for unwanted changes in X and maintain X at a 
very precise frequency value both with respect to short and long term time 
periods. Note that long term stability is indicated by the mathematics, 
while short term stability is determined by the slew rate of the three 
loops. Integration (not shown) can be added to each of the three loops to 
effectively control the amount of short term instability. 
FIG. 3D shows the condition when the value of X from the master oscillator 
goes 1 Hertz below the desired value such that X=4.999,999 MHz. Note that 
the value for XB-D has moved twice as fast as the value for X+N so that 
XB-D=7.499,998 MHz and X+N=7.499,999 MHz, a value which is now greater 
than XB-D as opposed to being less as was the case in Figures 3B and 3C. 
The correction process back to X=5.000,000 MHz is the same as described 
above with loop 2 moving twice as fast frequency wise as loop 1 towards 
the desired "locked in" condition. 
2. The Second Embodiment 
A second embodiment of the present invention is shown in block diagram form 
in FIG. 6. FIG. 6 also includes FIG. 6A, which shows in greater detail an 
embodiment of one of the two phase locked loop circuits 555,557 shown in 
FIG. 6. The principle of operation of the embodiment of FIG. 6 is somewhat 
similar to that of FIG. 4. 
Referring now to FIG. 6, a master oscillator 640 outputs a signal having a 
nominal frequency of X. This signal X is supplied to a balanced modulator 
644, where it is summed and differenced with a mixing signal N from a 
divider 642. Divider 642 receives the signal X from master oscillator 640 
and divides it by D to produce the mixing signal N. 
The outputs of the balanced modulator 644, which are sum and difference 
signals having frequencies X+N and X-N, respectively, are supplied to 
upper bandpass filter 646A and lower bandpass filter 646B, respectively, 
of bandpass filter state 646. The sum signal X+N at the output of bandpass 
filter 646A is supplied to phase locked loop 2. The difference signal of 
frequency X-N at the output of bandpass filter 646B is supplied to phase 
locked loop 1. 
Referring now to loop 2 and to the embodiment shown in FIG. 6A, signal X+N 
is mixed with signal X' generated by a voltage controlled oscillator (VCO) 
or voltage controlled crystal oscillator 556. When the frequency of 
X'=X--which means that master oscillator 640 is "locked" to the desired 
value of X--the difference signal from mixer 650 has a value of N. The 
output X' of local oscillator 653 is also supplied to a divider 657, which 
frequency divides the signal by M to produce a signal of X'/M. The signal 
N is frequency compared with the signal X'/M to produce a control signal 
supplied to VCO 653 to control the value of X'. The value of the control 
signal is zero when N=X'/M. 
Referring now to loop 1, signal X-N is mixed with a signal X" generated by 
a local oscillator (not shown). The operation of loop 1 is similar to that 
of loop 2, except that loop 2 moves at a frequency rate that is twice the 
rate of loop 1. Note that loop 1 can utilize the approach shown in the 
embodiment of FIG. 6A. Loop 1 provides a signal X"/M'. 
The signal X'/M from loop 2 is supplied to a first input of a comparator 
662. The signal X"/M' from loop 1 is supplied to a second input of 
comparator 652. The value at the control output of comparator 662 is 
indicative of the frequency amount that X is from its desired value. This 
control signal is fed back to the control input of master oscillator 640. 
Referring now to FIGS. 7A-7C, the operation of a specific implementation of 
the second embodiment of the present invention is illustrated. Assume for 
purposes of discussion that X=5.000,000 MHz, N=1.000,000 MHz, 
X+N=6.000,000 MHz, X-N=4.000,000 MHz, X'/M=1.000,000 MHz, and 
X"/M'=1.000,000 MHz. This "locked in" condition is shown in FIG. 7A, 
causing a control signal on the master loop for controlling master 
oscillator 640 to be indicative of no frequency change. 
Suppose for purposes of explanation that the value of X from master 
oscillator 640 changes by 1 Hz so that X=5.000,001 MHz. As shown in FIG. 
7B, this causes the signal X+N=6.000,001,2 MHz, and the signal 
X-N=4.000,008 MHz. Loop 2 thus produces a value for X'/M=1.000,002 MHz. 
Similarly, loop 1 produces a value for X"/M'=1.000001 MHz. The value of 
the control signal from comparator 662 is 0.1 Hz, which causes the 
frequency value of X to start moving back to the desired X=5.000,000 MHz. 
FIG. 7C illustrates the correction process half way between the deviation 
of FIG. 7B and the desired "locked in" condition of FIG. 7A. As shown, the 
frequency value of the output of master oscillator 640 is X=5.000,005 MHz, 
thus producing a control signal having a value of 0.05 Hz. The correction 
continues until the locked in condition of FIG. 7A is again attained. 
Note that loop 2 moves at twice the frequency rate of loop 1. Also note 
that the value of X'/M&lt;X"/M' when X is less than the desired "locked in" 
value. This means that the embodiment exhibits a very high degree of 
control over the value of X from master oscillator 640 both with respect 
to short and long term time periods. Note that long term stability is 
indicated by the mathematics, while short term stability is determined by 
the slew rate of the three loops. Integration (not shown) can be added to 
each of the three loops to effectively control the amount of short term 
instability. 
2. Conclusion 
It will be appreciated that there has been described a novel constant 
frequency generator which may be implemented in a number of different 
ways. The basic concept of the present invention is of providing a nominal 
frequency, a main feedback loop, and first and second feedback loops. 
Deviation from the nominal frequency causes the second feedback loop to be 
responsive at twice the frequency rate as that of the first feedback loop. 
The frequency difference between the values of the first feedback loop and 
the second feedback loop is used to produce a control signal for the main 
feedback loop used to control the nominal frequency. In this way, the 
circuit of the present invention is made self-correcting; elaborate 
temperature compensation or the like is not required to ensure a stable 
frequency source. Both short and long term stability of a high order is 
obtained. 
Two preferred embodiments of the present invention have been described. The 
present invention should not be limited thereby, but only by the following 
claims.