Ultra linear frequency discriminator circuitry

Signals to be discriminated are supplied through a controllable attenuator and power splitter to first and second discriminators with response slopes which are equal and of opposite sign. The outputs of the discriminators are summed and a reference signal is subtracted from the sum to produce a control signal which adjusts the input attenuator to maintain the amplitude of the input signal constant. The outputs of the discriminators are also subtracted to provide an output signal which is a linear function of frequency and independent of input amplitude.

BACKGROUND OF THE INVENTION 
The present invention pertains to frequency discriminator circuitry which 
provides an output signal that is a linear function of the frequency of an 
input signal and which is independent of amplitude fluctuations of the 
input signal. Because frequency discriminators generally have a limited 
range over which they are linear, variations in the amplitude of input 
signals can produce nonlinear responses therein. To eliminate this problem 
prior art discriminators generally require the use of a limiter in the 
path of the input signal. This solution is unsatisfactory, particularly at 
microwave frequencies. AGC loops and limiters prior to the discriminator 
circuitry do not allow for variations between the AGC loop or limiter and 
the discriminator or for variations in the discriminator itself. 
SUMMARY OF THE INVENTION 
The present invention pertains to frequency discriminator circuitry wherein 
an input signal is supplied through a controllable attenuator to a power 
splitter, which splitter supplies substantially equal signals to a low 
pass filter/amplitude detector circuit and a high pass filter/amplitude 
detector circuit. The two output signals from the detectors are combined 
to produce a control signal which adjusts the attenuator so that the 
amplitude of the input signal to the power splitter is held constant and 
independent of input amplitude. The output signals of the detectors are 
also subtracted to produce an output signal which is a linear function of 
frequency and independent of the amplitude of the input signal. 
It is an object of the present invention to provide new and improved ultra 
linear frequency discriminator circuitry. 
It is a further object of the present invention to provide new and improved 
ultra linear frequency discriminator circuitry which controls the 
amplitude of the input signal applied thereto so that the output signal is 
a linear function of frequency of the input signal. 
It is a further object of the present invention to provide new and improved 
ultra linear frequency discriminator circuitry which is easily implemented 
in microwave form. 
These and other objects of this invention will become apparent to those 
skilled in the art upon consideration of the accompanying specification, 
claims and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring specifically to FIG. 1, an input terminal 10 is adapted to 
receive an input signal, designated A cos wt. The signal is applied 
through a controllable attenuator 12, which is illustrated schematically 
as a variable resistor. The attenuator 12 could be, for example, a pin 
diode attenuator utilized for microwave frequencies or any other 
adjustable type of attenuator, many varieties of which are well known on 
the market. The output of the attenuator 12 is applied to a power splitter 
14, which supplies signals of substantially equal power to first and 
second discriminator circuits 16 and 18, respectively. The power splitter 
14 and discriminators 16 and 18 are provided, in this preferred 
embodiment, as a single hybrid circuit on a substrate designated 20. By 
supplying the power splitter 14 and discriminators 16 and 18 on the single 
substrate 20, the discriminators can be matched to further reduce 
discrepancies due to amplitude changes. Discriminator 16 includes a high 
pass filter and an amplitude detector while discriminator 18 includes a 
low pass filter and an amplitude detector so that the response slopes of 
the discriminators 16 and 18 are equal and of opposite sign. The signal 
produced by discriminator 16 is defined by the equation 
EQU V.sub.1 =m(.omega.-.omega..sub.o)+b.sub.1 (1) 
where 
m =filter sensitivity 
.omega..sub.o =center frequency of interest 
b=offset amplitude 
.omega.=input frequency 
The signal provided by discriminator 18 is defined by 
EQU V.sub.2 =-m(.omega.-.omega..sub.o)+b.sub.2 (2) 
The output signals from the discriminators 16 and 18 are supplied to a 
summation circuit 22 such that the signal from the discriminator 18 is 
subtracted from the signal from the discriminator 16. The output of the 
summation circuit 22 is amplified in an amplifier 24 and the output signal 
is defined by the equation 
EQU V.sub.out =2m(.omega.-.omega..sub.o)+b.sub.1 -b.sub.2 (3) 
Since both b.sub.1 and b.sub.2 are independent of frequency, the output 
signal, V.sub.out, is a linear function of the frequency of the signal 
applied to terminal 10. 
The two output signals from the discriminators 16 and 18 are also supplied 
to a summation circuit 26. A reference voltage, V.sub.ref, is supplied to 
the summation circuit 26 so as to be subtracted from the sum of the two 
output signals from the discriminators 16 and 18. The output signal 
V.sub.s from the summation circuit 26 is defined by the equation 
EQU V.sub.s =b.sub.1 +b.sub.2 -V.sub.ref (4) 
The signal V.sub.s is supplied through an integrator 30 to the control 
input of the attenuator 12. The action of the attenuator loop is to force 
b.sub.1 +b.sub.2 to equal V.sub.ref, a constant. Thus the amplitude at the 
input to the power splitter 14 is held constant and V.sub.out becomes 
independent of input amplitude (A). 
Referring specifically to FIG. 2, a schematic diagram is illustrated of a 
power splitter and frequency discriminators which might be utilized in a 
microwave version of the apparatus illustrated in FIG. 1. An input 
terminal 35 is adapted to receive the output signal from the controllable 
attenuator and supply the signal through a 50 ohm transmission line 37 to 
a junction 39. One end of a 50 ohm resistor 43 is connected to the 
junction 39 and the other end is connected to a junction 45. One end of a 
50 ohm resistor 47 is connected to the junction 39 and the other end is 
connected to a junction 49. An open stub transmission line 53, which is 
less than one-half wavelength and preferably approximately 3/8 wavelength, 
is connected between the junction 45 and a terminal 55 adapted to have a 
source of power applied thereto. A shorted stub transmission line 57, 
which is less than 1/2 wavelength long and preferably approximately 3/8 
wavelength long, is connected between the junction 49 and a reference 
potential, such as ground 59. The anode of a semiconductor diode 63 is 
connected to the junction 45 and the cathode is connected to an output 
terminal 65 as well as through a capacitor 67 to ground. The anode of a 
semiconductor diode 69 is connected to the junction 49 and the cathode is 
connected to an output terminal 73 as well as through a capacitor 75 to 
ground. Thus, the 50 ohm line 37 and the 50 ohm resistors 43 and 47 
operate as a power splitter to divide the input signal between the two 
discriminators. The open stub transmission line 53 operates as a high pass 
filter and the shorted stub transmission line 57 operates as a low pass 
filter with the diodes 63 and 69 providing the amplitude detection. It 
will be understood by those skilled in the art that this particular 
embodiment is especially adapted for microwave frequencies and can easily 
be constructed on a single substrate to provide matched discriminators. 
Thus, ultra linear frequency discriminator circuitry is illustrated wherein 
matched frequency discriminators are utilized in a closed attenuator loop 
to eliminate input amplitude variations. The two discriminators provide 
near linear response over the band of interest with slopes of opposite 
sign. Because the discriminators are included in the attenuator loop a 
more exact control of signal amplitude is obtained. Further, the specific 
microwave circuitry illustrated provides reduced susceptibility to 
amplitude variations at microwave frequencies where interface 
discontinuities cause amplitude to vary with frequency. 
While we have shown and described a specific embodiment of this invention, 
further modifications and improvements will occur to those skilled in the 
art. We desire it to be understood, therefore, that this invention is not 
limited to the particular form shown and we intend in the appended claims 
to cover all modification which do not depart from the spirit and scope of 
this invention.