Limiter circuit for servosystems

A limiter circuit is installed in a servomechanism that establishes a signal at the output terminal of a filter contained therein, in the absence of an input signal within the lock range of the servomechanism, in a manner that maintains the charge on the filter consistent with the established output signal, thus providing favorable initial conditions for rapid reacquisition and smooth reacquisition transients.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the art of servomechanisms and more 
particularly to a new and improved circuit to minimize the acquisition 
transients of such devices. 
2. Description of the Prior Art 
Applications exist in phase locked loops and in other servosystems for 
amplifiers which provide a control voltage, in the absence of an input 
signal, that is at a level near the control amplifier lock-up voltage to 
maintain the acquisition time within a desired interval. This type of 
operation is generally desired when a servo is faced with a situation 
wherein lock is broken frequently and a fast reacquisition time is a 
requirement. A communication system employing phase shift keying (PSK) 
modulation, wherein information is carried as phase discontinuities of a 
carrier signal, is an example of such a system. PSK communication systems 
employ phase locked loops to extract the carrier from the discontinuously 
phase modulated signal, and it is necessary to limit the reacquisition 
transients of the phase lock loop in order to minimize the probability of 
information error. Prior art methods of performing the necessary limiting 
have employed Zener diodes and adjustable voltage clamps which discharge 
the capacitors in the loop filter in the process of maintaining a minimum 
control voltage after lock has been lost. For the system to reacquire 
smoothly and track, starting at the minimum voltage maintained, the filter 
capacitors must maintain the charges that would have been present at that 
voltage had track not been lost. When this charge is not maintained, 
erratic initial tracking and acquisition results until the proper charge 
has been re-established. This invention provides a means by which the 
desired filter charge is maintained at the established voltage bound until 
the input signal again comes within acquisition range. 
SUMMARY OF THE INVENTION 
The subject invention provides a means for minimizing the acquisition 
transient time of a servomechanism by providing a limiter circuit that is 
coupled to the control amplifier contained within an active filter of the 
servomechanism that prevents the output voltage of the control amplifier 
from dropping below a predetermined voltage level whenever the 
servomechanism loses lock. A preferred limiter circuit according to the 
principles of this invention includes an operational amplifier and a 
diode. The non-inverting terminal of the operational amplifier is coupled 
to the output terminal of the active filter (which is the output terminal 
of the control amplifier) and the inverting terminal is coupled to an 
adjustable reference voltage source, which is adjusted to a level somewhat 
below the normal output voltage of control amplifier, while the cathode 
and anode of the diode couple to the output terminal of the operational 
amplifier and the input terminal of the control amplifier, respectively. 
When lock-up occurs, the output voltage of the operational amplifier is 
more positive than the input voltage to the control amplifier and no 
current flows through the diode in the output circuit of the limiter. When 
the servosystem loses lock, the input voltage to the control amplifier is 
increased, causing its output voltage to decrease. This decrease continues 
until the reference voltage of the limiter amplifier is reached, at which 
time sufficient current flows through the conducting diode and in the 
output circuit of the limiter to maintain the input voltage to the control 
amplifier at a level that provides an output voltage which is equal to the 
reference voltage of the limiter amplifier. Additionally, the current flow 
through the diode maintains a charge on the active filter capacitors that 
is consistent with the control amplifier output voltage. In this fashion, 
an "inner" servo loop has been established which takes over from the 
"outer" loop when the desired parameters are achieved. This inner loop 
becomes a servomechanism to hold the control amplifier in a linear 
operating mode by forcing the control amplifier output to assume a voltage 
equal to the reference voltage, thus achieving a nearly ideal limiter. 
A double valued limit action may be obtained by cascading two limiter 
circuits, each with an independent adjustable reference voltage source to 
bound the voltages of interest. Additionally, the invention may be 
utilized to observe all modes and transients of the settling process by 
simply varying the setting of the reference voltage to the limiter 
amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The operation of the invention is described with reference to a phase lock 
loop though it will be understood by those skilled in the art that the 
invention, as it pertains to the limiting of a signal at the output of a 
filter circuit is applicable to any servomechanism. Referring now to FIG. 
1, wherein is shown a phase lock loop 10 which includes: a voltage 
controlled oscillator (VCO) 11 with an output terminal 11b coupled to the 
loop output terminal 12; a phase detector (PD) 13 having a first input 
terminal 13a coupled to the phase lock loop input terminal 14, and a 
second input terminal 13b coupled to the output terminal 11b of VCO 11; an 
active filter 15 having a first terminal 15a coupled to an output terminal 
13c of PD 13, and a second terminal 15b coupled to an input terminal 11a 
of VCO 11; a limiter circuit 16 having a first terminal 16a coupled to an 
external source of voltage reference V.sub.ref (not shown), a second 
terminal 16b coupled to the second terminal 15 b of the active filter 15; 
and a diode 17, the cathode of which couples to a third terminal 16c of 
limiter 16 and the anode of which couples to a third terminal 15c of the 
active filter 15. The active filter 15 includes: a control amplifier A1, 
which may be a differential operational amplifier, with the inverting 
terminal 18 being coupled to terminal 15a via a resistor 24, and the 
non-inverting terminal 19 being coupled via resistor 23 to a positive 
external voltage source (not shown) which provides a voltage V.sub.r1 ; a 
resistor 21, one terminal of which is coupled to the inverting terminal of 
operations amplifier A1 and the other terminal of which couples to one 
terminal of a capacitor 22, the other terminal of which is coupled to the 
output terminal 20 of operational amplifier A1 which in turn is coupled to 
terminal 15b. While the limiter circuit 16 may include: a differential 
operational amplifier A2 with its inverting terminal 25 coupled to its 
output terminal 26 via resistor 27 and to terminal 16a via potentiometer 
30 and with its non-inverting terminal 29 coupled to terminal 16b; and a 
resistor 28 coupled between the terminal 16c and the output terminal 26 of 
the operational amplifier A2. It is understood by those skilled in the art 
that the phase locked loop as described above is equivalent to a 
servosystem in which an integrator replaces the VCO and an adding means 
replaces the phase detector. 
During normal operation, the voltage at terminal 13c of PD 13 servoes to 
reference voltage V.sub.r1 and a voltage V.sub.c is coupled from terminal 
15b of active filter 15 to the non-inverting terminal 29 of operational 
amplifier A2 and to the input terminal 11a of VCO 11 to maintain zero 
phase difference between the signals coupled to terminals 13a and 13b of 
PD 13. Also, a reference voltage V.sub.r2 at the inverting terminal 25 of 
operational amplifier A2 is set to a level below the normal operating 
voltage V.sub.c by adjusting potentiometer 30. By the proper selection of 
values for resistors 27 and 28, a voltage at terminal 16c is established 
that is of greater potential than V.sub.r1, the voltage at the inverting 
terminal 18 of operational amplifier A1, thus diode 17 is reverse-biased 
and no current flows through terminal 16c of the limiter 16. 
When the input signal at the input terminal 13a of PD 13 is removed or is 
outside a well defined "lock" range, the voltage at output terminal 13c 
increases and the output voltage at terminal 15b of active filter 15 
decreases until the reference voltage V.sub.r2 is matched. At this time, 
the voltage at terminal 16c of the limiter 16 is less than V.sub.r1 and 
diode 17 conducts causing current to flow from terminal 18 towards 
terminal 16c of the limiter 16 while current flows towards terminal 18 to 
terminal 13c of PD 13, thus preventing further discharge of capacitor 22 
and preventing further decrease in the voltage at the output terminal 20 
of operational amplifier A1. The voltage at output terminal 20 continues 
to servo at the level of V.sub.r2 until a signal within the lock range of 
the phase lock loop is applied to terminal 14. 
The reference voltage V.sub.r2 allows VCO 11 to produce a signal frequency 
in the absence of a signal within lock range at input terminal 14 that is 
somewhat lower than the eventual signal frequency to be applied to input 
terminal 14. Since the frequency of the output signal from VCO 11 in the 
absence of a signal at the input terminal 14 is less than the frequency of 
the anticipated applied signal the voltage at terminal 13c of PD 13, at 
the time the anticipated signal is applied, becomes less than V.sub.r1 and 
as a result current flows from terminal 18 towards terminal 13c of PD 13. 
This causes the charge on capacitor 22 and the voltage at terminal 15b to 
increase. These parameters of charge and voltage change at a faster rate 
than the output of the PD alone, because the limiter circuit now aids the 
total current flow at terminal 18 of the operational amplifier A1. When 
this voltage exceeds the reference voltage V.sub.r2 diode 17 becomes 
reverse-biased and normal phase acquisition takes place. During the period 
that the voltage at terminal 15b is equal to V.sub.r2 a charge is 
maintained on capacitor 22 that is consistent therewith. After the 
anticipated signal is applied to terminal 14, normal phase acquisition 
commences when the voltage at the output terminal 15b exceeds the 
reference voltage V.sub.r2. At this time, acquisition begins with the 
charge on capacitor 22 commensurate with a voltage V.sub.r2, thus a 
favorable initial condition exists and a smooth acquisition transient 
results. 
The circuit operation described above provides rapid acquisition and smooth 
transient response due to the favorable initial conditions that exist in 
the loop filter at the time the input signal is applied to terminal 14. 
Transient settling time for the acquisition phase is a function of the 
difference in voltage level between the voltage required at terminal 11a 
of VCO 11 to hold the loop in lock and the reference voltage V.sub.r2 at 
terminal 25 of operational amplifier A2. The minimum value for this 
voltage difference is determined by the ability of the VCO to track the 
anticipated variations of the input signal with the limited control 
voltage range imposed by the limiter. This minimum value may be determined 
experimentally by varying the setting of the potentiometer 30. 
Additionally, the setting of the potentiometer 30 may be adjusted to 
establish a frequency for the VCO 11, in the absence of an input signal at 
input terminal 14, that is a given frequency separation from the 
anticipated input signal frequency, thus providing a means with which the 
transients of the settling process for the loop may be observed for 
various frequency separations between the anticipated signal frequency and 
the VCO 11 frequency in the absence thereof. 
The system as described above provides a means for establishing a lower 
bound to the VCO control voltage of the phase lock loop. By paralleling 
two limiters as shown in FIG. 2, a double ended limit may be realized 
which provides an upper and lower bound to the VCO control voltage. In 
FIG. 2, PD 33, loop filter 34, VCO 35, diode 36, and the first limiter 37 
operate as heretofore described. The second limiter 38 possesses the same 
circuitry as the limiter 16 of FIG. 1, however a greater reference voltage 
is applied thereto. After the voltage at node 41 has reached the level of 
the reference voltage of the first limiter 37, diode 36 becomes 
non-conducting as previously described. At this instance, the voltage to 
the positive terminal of the amplifier within the second limiter 38 is 
below the reference voltage applied to the negative terminal causing a 
negative voltage to be coupled to the diode 42. Since the output voltage 
from PD 33, which is coupled to the cathode of diode 42 is positive, diode 
42 does not conduct. Thus, the control voltage at node 41 is allowed to 
linearly increase beyond the voltage level of the reference voltage of the 
first limiter 37 unitl it reaches the voltage level of the reference 
voltage of the second limiter 38, after which the anode of diode 42 
becomes more positive than the cathode and the diode 42 becomes more 
positive than the cathode and the diode 42 conducts. The conduction is in 
a direction that causes the control voltage at node 41 to decrease, thus 
establishing an upper bound to the control voltage at node 41 that is 
equal to the level of the reference voltage applied to the second limiter 
38. 
While the invention has been described in its preferred embodiments, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes may be made within the 
purview of the appended claims without departing from the true scope and 
spirit of the invention in its broader aspects.