Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-11-02
2001-05-01
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C332S144000, C327S246000
Reexamination Certificate
active
06225864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the art of RF amplifiers and, more particularly, to an RF amplifier having a dual slope phase modulator for varying the phase of an RF signal.
2. Description of the Prior Art
It has been observed in an AM transmitter that the RF carrier signal may become shifted in phase because of variations in the level of audio modulation. This may result in distortions particularly in digital radio broadcasting. It is an objective to provide RF phase predistortion to compensate for any phase error caused by an amplitude modulation system. This should be accomplished without changing the duty cycle (mark to space ratio) of the output RF signal as compared with the input RF signal.
It has been known in the prior art to phase shift an RF signal. One example of a phase shifting circuit is illustrated in
FIG. 1
to which attention is now directed. In this version, an RF signal made up of a train of pulses
10
is obtained from a suitable RF source
12
. This RF signal is supplied by way of a phase shifting circuit to an RF transmitter, including a power amplifier
20
, and, thence, to an antenna
22
. The phase shifting circuit includes a potentiometer including a resistor portion
30
connected between ground and a B+ voltage supply source and an adjustable wiper arm
32
which applies a control level signal, such as V
c
, by way of an inductor
34
to a junction point
36
. This junction point is fed with the RF signal by way of a capacitor
40
and a resistor R
1
. A tuning diode CR
1
is connected between ground and the junction point
36
. The junction point
36
is connected to the power amplifier
20
by way of a capacitor
42
. The capacitor
42
is connected by a resistor
44
to ground. In this version of the prior art, the slopes of the input RF signal
10
are modulated to create a small amount of phase shift. The modified RF output signal is digitized and transformed back to a logic level. Consequently, a simulated phase shift is created.
The tuning diode CR
1
is the heart of the circuit and its body capacitance changes proportionally to the voltage applied across it. A variable low pass filter comprised of resistor R
1
and diode CR
1
is created by varying the control signal V
c
. This variable low pass filter modifies the slope of the incoming RF signal to create a virtual phase shift by tilting the slopes of the rising and falling edges to obtain the RF output signal as shown at
50
. The control is limited because the control level (V
c
) is filtered through a low pass filter which creates group delay and limited bandwidth.
Reference is now made to
FIG. 2
which illustrates another prior art circuit for use in shifting the phase of an RF signal. This circuit is similar to that of FIG.
1
and to simplify the description herein similar components are identified in both figures with the same character references and only the differences will be discussed below.
In
FIG. 2
, the phase control shifting circuit includes a differential amplifier
60
including a pair of NPN transistors
62
and
64
having their emitters connected together in common and, thence, through a resistor
66
to the wiper arm
32
of the potentiometer. The base of transistor
62
is connected to the junction of a resistor
70
and a capacitor C
1
connected together in series between ground and the collector of the transistor. The base of transistor
64
is connected to ground by a resistor
76
and the collector is connected to a V+ voltage source. The collector of the transistor
62
is connected to resistor R
1
and by way of a capacitor
42
it is also connected to the input of the power amplifier
20
. The capacitor
42
is connected to ground by way of resistor
74
. The control voltage V
c
obtained from the potentiometer is used to control the current gain of the transistors and, in turn, the current flow through the capacitor C
1
. By varying the current through capacitor C
1
, a variable low pass filter is created comprised of resistor R
1
and capacitor C
1
. This variable low pass filter modifies the slope of the incoming RF signal and creates a virtual phase shift by tilting the slopes of the rising and falling edges of the RF output signal
50
.
The output duty cycle (mark to space ratio) in the circuits of
FIGS. 1 and 2
is not directly proportional to the input signal because the RF output signal is AC-coupled and this removes the DC information on the original RF input signal causing a level shift of the RF output signal.
Also, the circuits shown in
FIGS. 1 and 2
have very limited dynamic range of adjustments because while the slope of the signal is gradually changed, the envelop of the signal is also gradually changed. The acceptable range of phase modulation is on the order of one tenth of an RF wavelength.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide means for shifting the phase of an RF signal to obtain a phase shifted output signal while maintaining the duty cycle of the RF input signal.
In accordance with the present invention, apparatus and method are provided for varying the phase of an RF signal. An RF source provides an RF signal made up of a first train of pulses exhibiting a fixed frequency and fixed duty pulse cycle and wherein each pulse is of a fixed amplitude and duration. The modulator includes an integrator for converting each pulse cycle of the RF signal into a dual slope symmetrical ramp signal which varies in magnitude from a minimum level to a maximum level and then back to the minimum level for each pulse cycle. A first level signal and a second level signal are provided and are equally spaced from a reference level which is midway between the minimum and maximum levels of the ramp signal. A dual slope comparator compares the ramp signal with the first and second level signals and provides a first pulse signal for a time duration corresponding with the time duration that the ramp signal exceeds the first level signal and a second pulse signal for a time duration corresponding with the time duration that the second level signal exceeds the ramp signal. First and second pulse generators respectively receive the first and second pulse signals and provide first and second trigger pulses. A bistable circuit responds to the first and second trigger pulses to be in respective first and a second states and provides an output RF signal made up of a second train of pulses shifted in phase from the first train by an amount related to the magnitude of the first level signal or the second level signal.
REFERENCES:
patent: 4415863 (1983-11-01), Tokumo
patent: 4968948 (1990-11-01), Tokumo et al.
patent: 5014016 (1991-05-01), Anderson
patent: 5389829 (1995-02-01), Milazzo
patent: 6130528 (2000-10-01), Aoyama
Choe Henry
Harris Corporation
Pascal Robert
Tarolli, Sundheim, Covell Tummino & Szabo L.L.P.
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