Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2000-01-24
2002-10-15
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S307000, C327S362000, C327S513000, C455S114100, C455S296000
Reexamination Certificate
active
06466084
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to radio frequency (RF) amplification. More particularly, the invention relates to a system for reducing third order intermodulation distortion in broadband CATV RF amplifiers.
Lowering distortion in RF power amplifier circuits without compromising their transient response is an omnipresent problem. High frequency amplification is widely used in communications and broadcasting and also where high-speed switching is required for use in digital instrumentation. However, high frequency amplifier applications extend linear operation into areas where parasitic effects of interelectrode capacitance, wire inductance, stored charge and even operating frequency wavelength begin to adversely affect and dominate circuit behavior.
Minimizing distortion is particularly important when a series of amplifiers is cascaded over a signal transmission path, such as a series of RF amplifiers in a CATV transmission network. Disposed throughout a CATV transmission system are RF amplifiers that periodically amplify the transmitted signals to counteract cable attenuation and attenuation caused by passive CATV components, such as signal splitters and equalizers. The RF amplifiers are also employed to maintain the desired carrier-to-noise ratio. Due to the number of RF amplifiers employed in a given CATV transmission system, each RF amplifier must provide minimum degradation to the transmitted signal.
In an ideal communication system it is preferable that the components which comprise the system are linear. However, as a practical reality, there are many nonlinearities that are typically introduced by the electronic components, such as RF amplifiers. The distortions created by an RF amplifier which are of primary concern are second (even) and third (odd) order harmonic distortions. Prior art amplifier designs have attempted to ameliorate the effects of even order distortions by employing push-pull amplifier topologies, since the maximum even order cancellation occurs when the proper 180° phase relationship is maintained over the entire bandwidth. This is achieved through equal gain in both push-pull halves by matching the operating characteristics of the active devices.
However, odd-order distortion is difficult to remedy. Odd-order distortion characteristics of an amplifier are manifest as cross modulation (X-mod) and composite triple beat (CTB) distortions on the signal being amplified. These are two types of intermodulation (IM) distortion. X-mod occurs when the modulated contents of one channel being transmitted interferes with and becomes part of an adjacent or non-adjacent channel. CTB results from the combination of three frequencies of carriers occurring in the proximity of each carrier since the carriers are typically equally spaced across the frequency bandwidth. Of the two noted distortions, CTB becomes more problematic when increasing the number of channels on a given CATV system. While X-mod distortion also increases in proportion to the number of channels, the possibility of CTB is more dramatic due to the increased number of available combinations from among the total number of transmitted channels. As the number of channels transmitted by a communication system increases, or as the channels reside closer together, the odd-order distortion becomes a limiting factor of amplifier performance.
The nonlinear properties of an RF amplifier can be described by a curve which can be expanded into a Taylor series as follows:
Uout=a
1
·U
in
+a
2
·U
in
2
+a
3
·U
in
3
+a
4
·U
in
4
+a
5
·U
in
5
. . . a
n
·U
in
n
, Equation 1
where U
in
is the input potential and Uout is the output potential and a
n
is a factor that determines the magnitude of the term. It should be noted that a
1
·U
in
is the 1st order term; a
2
·U
in
2
is the 2nd order term; a
3
·U
in
3
is the 3rd order term . . . and a
n
·U
in
n
is the nth order term. The magnitudes of the individual terms are strongly dependent on the input signal and, therefore, on the level control of the amplifier.
If we have the following:
U
in
=A
·cos &ohgr;
i
t
Equation 2
There exists for each term multiple combination possibilities of the input circulating frequencies &ohgr;
i
due to the power of the corresponding order number. Additionally, in multifrequency transmission systems, such as a CATV transmission network, the number of new circulating frequencies at the output of the network increases exponentially with the number (i) of frequencies at the input. These new circulating frequencies are referred to herein as intermodulation (IM) products and are detrimental to the accurate acquisition of CATV signals.
Using the Taylor series, it can be demonstrated that all odd order terms create products which appear at the same location as the lower-valued odd order terms. Therefore, the third order term creates a product at the base frequency, (odd order number 1), the fifth order term creates a product at the third order and one at the base frequency. If the input signal consists, for example, of the base circulating frequencies &ohgr;
1
and &ohgr;
2
(i=2) with the same amplitude A, that is expressed with:
U
in
=
A
cos
(
ω
1
t
)
+
A
cos
(
ω
2
t
)
=
0.5
A
(
ⅇ
j
ω
1
t
+
ⅇ
-
j
ω
1
t
+
ⅇ
j
ω
2
t
+
ⅇ
-
j
ω
2
t
)
,
Equation
3
then the third order term a
3
·U
in
3
of Equation 1, (which is of interest in the present application), creates the following new products: ±&ohgr;
1
; ±&ohgr;
2
; ±3&ohgr;
1
; ±3&ohgr;
2
; ±(2&ohgr;
2
±&ohgr;
1
); ±(2&ohgr;
1
±&ohgr;
2
). In this case there are 16 new circulating output frequencies due to two input circulating frequencies.
A “weakly” nonlinear transmission system can be defined such that: a) the effect of odd order terms on other lower-valued odd order terms is negligibly small; and b) higher-valued terms after the third order term are negligibly small. Accordingly, a weakly nonlinear system may be mathematically described such that the Taylor series is broken off after the third order term (a
3
·U
in
3
). Weakly nonlinear systems are characterized in that a 1 db increase in the level of the input circulating frequencies &ohgr;
i
causes an increase of 3 db in the third order IM products.
Communication systems, such as CATV systems which include broadband RF amplifiers, are further regarded as dynamically nonlinear systems whereby the amplitudes and phases of the IM products are dependent upon the input frequencies.
There are three basic ways of correcting distortion created by a non-linear device: 1) reduce the signal power level; 2) use a feed forward technique; and 3) use a predistortion or postdistortion technique. The first method reduces the signal power level such that the non-linear device is operating in its linear region. In the case of an RF amplifier this results in very high power consumption for low RF output power. Of course, the high power consumption is a disadvantage. However, this method is not an option if high output level is required on a permanent basis.
The second method is the feed forward technique. Using this technique, the input signal of the main amplification circuit is sampled and compared to the output signal to determine the difference between the signals. This difference is the distortion component which is amplified by an auxiliary amplification circuit and combined with the output of the main amplification circuit such that the two distortion components cancel each other. However, the power consumed by the auxiliary amplification circuit is comparable to that consumed by the main amplification circuit and the circuitry is also complex and expensive. At the upper frequency limit it is very difficult to maintain the magnitude and phase conditions
Callahan Timothy P.
General Instrument Corporation
Nguyen Minh
Volpe and Koenig P.C.
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