Injection-locked voltage controlled oscillators

An improved frequency memory system of the type using a voltage controlled oscillator is described. Incoming RF signals are power divided with a first power divided portion applied to a frequency discriminator where a voltage level signal indicative of frequency is derived. This voltage level signal is shaped and applied to the volage controlled oscillator wherein the oscillator is broadly tuned to the incoming frequency. A remaining portion of the power divided signal is applied directly to the RF circuit portion of the VCO oscillator to injection lock the VCO oscillator to the incoming signal.

BACKGROUND OF INVENTION 
This invention relates to voltage controlled oscillator (VCO) systems and 
more particularly to VCO systems used in frequency memory systems. 
There are presently in use several types of electronic systems whose 
function is to provide a continuous wave output at the same frequency as 
the received signal where the received signal is a short duration RF burst 
such as from a radar. These systems sometimes referred to as frequency 
memory systems must be capable of tuning to a given frequency within a 
fairly broad range of frequencies rapidly and accurately. One commonly 
used system employs a microwave delay line and a broadband amplifier, such 
as a traveling wave tube, in which the incoming RF burst signal is 
recirculated many times to produce a practically continuous output. 
Another approach is the use of a set-on voltage controlled oscillator 
(VCO) in a system where a discriminator produces a voltage related to the 
frequency of the incoming signal, a sample and hold circuit is used to 
store this voltage (memory), and a VCO is tuned by this stored voltage. 
The accuracy of this system is determined by the discriminator and the VCO 
transfer characteristics and the memory time is limited by the 
sample-and-hold circuit storage time and the VCO drift. In the usual VCO 
system, the discriminator output is amplified and shaped so as to match 
the discriminator to that of the VCO tuning curve and to tune the VCO to 
the same frequency as the incoming signal. The accuracy of the set-on 
voltage to tune the VCO is limited by the calibration of the discriminator 
and the VCO and the changes in the operating characteristics of the 
discriminator and the VCO caused by variation in ambient temperatures and 
power supplies. 
In conventional set-on VCO memory systems, the incoming signal is 
discriminated and the resulting output voltage is stored in the memory 
such as the sample and hold circuit. Because the discriminator calibration 
is limited in accuracy and resolution, the stored voltage is inaccurate to 
some degree. The problem is compounded by the fact that the incoming 
signal will be varying in amplitude unless major limiting and leveling 
accessories are added to the system. Furthermore, additional error is 
produced by drift or instability of the VCO after the stored output 
voltage is applied. 
BRIEF DESCRIPTION OF INVENTION 
Briefly, a fast and accurate system is described for generating an output 
signal approximately the same frequency as a received input frequency. The 
system includes a voltage controlled oscillator (VCO) and means including 
a discriminator responsive to the frequency of the incoming signal for 
providing a voltage to the VCO to tune the oscillator to the incoming 
signal frequency. The improvement in the VCO system is that the incoming 
signal is power divided with only a first power divided portion of the 
signal used to generate the voltage that tunes the VCO in a conventional 
way and the remaining portion of the power divided signal being coupled 
directly to the RF portion of the VCO oscillator to injection lock the VCO 
oscillator.

Referring to the FIGURE, radio frequency (RF) burst signals at microwave 
frequencies for example are picked up at antenna 11 and are coupled to 
power divider 13. The power divider 13 may be a 3 db hybrid coupler. One 
half of the power is coupled via lead 14 to discriminator 15 and one-half 
of the power is coupled to lead 16. At discrimintor 15, the RF signals are 
discriminated and detected to provide an output voltage dependent on 
frequency of the incoming RF signal. The voltage may be stored in a memory 
such as a sample and hold circuit 17. The voltage signal from the sample 
and hold circuit 17 is coupled via terminal 20 of switch 18 to an 
amplifier and shaping circuit 19. The output of amplifier and shaping 
circuit 19 is coupled to voltage controlled oscillator (VCO) 21. The 
amplifier and shaping circuit 17 provides a voltage which matches the VCO 
oscillator 21 tuning curve to tune the VCO oscillator 21 to approximately 
the same frequency as the incoming signal. The output from VCO oscillator 
21 is coupled via an RF transmission line 23 to port 27 of a three port 
microwave circulator 25. The output from the circulator 25 is coupled via 
port 29 and power divider 41 to antenna 40. The power divided signal on 
lead 16 of divider 13 is coupled to port 31 of circulator 25. This RF 
signal at port 31 is coupled to the RF portion of the VCO oscillator 21 
via port 27 and the RF output transmission line 23. This RF signal coupled 
to the RF portion of the voltage controlled oscillator 21 causes injection 
locking of the VCO signal to the incoming RF signal. The VCO oscillator 21 
is locked exactly on the same frequency as the incoming signal when the 
tuning voltage causes the VCO to be within the locking range. 
The sample and hold circuit 17 can continue to provide the proper VCO 
tuning voltage after cessation of the input RF burst signal by providing 
this voltage from a memory in the sample and hold circuit 17. A signal 
from system command 60 can either energize or deenergize this memory. 
A portion of the output signal to the antenna 40 is coupled via power 
divider 41 to a second discriminator 43. Power divider 41 may be a coupler 
as described by A. Schwarzmann in U.S. Pat. No. 3,742,392. At the second 
discriminator 43, the RF signals are discriminated and detected to provide 
an output voltage that is a function of frequency. This voltage matches 
the tuning curve of VCO oscillator 21. This output voltage from the 
discriminator is coupled over a first path via a second sample and hold 
circuit 47 to terminal 51 of comparator 50 and over a second path 49 to 
terminal 52 of comparator 50. At comparator 50 the real time output of the 
second discriminator 43 at terminal 52 is compared with the reference 
voltage from the sample and hold circuit 47 at terminal 51 and the 
resultant error voltage is applied thru error amplifier 53 to terminal 22 
of switch 18. When the system command 60 determines that the input pulse 
burst has been received long enough (100 nanoseconds for example) for the 
VCO to be accurately locked to the proper frequency or that the burst has 
ended, a signal from system command 60 causes switch 18 to change state 
and to disconnect sample and hold circuit 17 at terminal 20 and to connect 
the output from error amplifier 53 via terminal 22 to amplifier and 
shaping circuit 19. Also, this signal from system command 60 de-energizes 
sample and hold circuit 17. Error amplifier 53 amplifies the error voltage 
in the proper polarity such that the voltage applied through the amplifier 
and shaping circuit 19 is such as to cause the voltage controlled 
oscillator 21 to change frequency in a direction such that the error 
voltage is adjusted towards zero. For example, if a positive increase in 
voltage level causes a rise in frequency at the VCO and the error voltage 
indicated a downward movement in frequency, a positive signal is coupled 
to VCO 21. System 60 could include a timer responsive to the detected 
presence of a signal at discriminator 15 for providing switching signals 
100 nanoseconds after the detection of a signal at discriminator 15 and 
de-energizing signals a fixed time period after that. 
This feed system described above eliminates discriminator error, the VCO 
tuning calibration and drift errors and the effect of incident power level 
variations on the accuracy of the memory.