Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1997-12-09
2002-12-24
Vo, Nguyen (Department: 2745)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S239100, C455S234100, C455S245100, C455S245200, C455S249100, C330S129000
Reexamination Certificate
active
06498926
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved programmable linear receiver.
II. Description of the Related Art
The design of a high performance receiver is made challenging by various design constraints. First, high performance is required for many applications. High performance can be described by the linearity of the active devices (e.g. amplifiers, mixers, etc.) and the noise figure of the receiver. Second, for some applications such as in a cellular communication system, power consumption is an important consideration because of the portable nature of the receiver. Generally, high performance and high efficiency are conflicting design considerations.
An active device has the following transfer function:
y
(
x
)=
a
1
·x+a
2
x
2
+a
3
x
3
+higher order terms, (1)
where x is the input signal, y(x) is the output signal, and a
1
, a
2
, and a
3
are coefficients which define the linearity of the active device. For simplicity, higher order terms (e.g. terms above third order) are ignored. For an ideal active device, the coefficients a
2
and a
3
are 0.0 and the output signal is simply the input signal scaled by a
1
. However, all active devices experience some amount of non-linearity which is quantified by the coefficients a
2
and a
3
. Coefficient a
2
defines the amount of, second order non-linearity and coefficient a
3
defines the amount of third order non-linearity.
Most communication systems are narrow band systems which operate on an input RF signal having a predetermined bandwidth and center frequency. The input RF signal typically comprises other spurious signals located throughout the frequency spectrum. Non-linearity within the active devices causes intermodulation of spurious signals, resulting in products which may fall into the signal band.
The effect of second order non-linearity (e.g. those caused by the x
2
term) can usually be reduced or eliminated by careful design methodology. Second order non-linearity produces products at the sum and difference frequencies. Typically, the spurious signals which can produce in-band second-order products are located far away from the signal band and can be easily filtered. However, third order non-linearity are more problematic. For third order non-linearity, spurious signals x=g
1
·cos(w
1
t)+g
2
cos(w
2
t) produce products at the frequencies (2w
1
-w
2
) and (2w
2
-w
1
). Thus, near band spurious signals (which are difficult to filter) can produce third order intermodulation products falling in-band, causing degradation in the received signal. To compound the problem, the amplitude of the third order products are scaled by g
1
·g
2
2
and g
1
2
·g
2
. Thus, every doubling of the amplitude of the spurious signals produces an eight-fold increase in the amplitude of the third order products. Viewed another way, every 1 dB increase in the input RF signal results in 1 dB increase in the output RF signal but 3 dB increase in the third order products.
The linearity of a receiver (or the active device) can be characterized by the input-referred third-order intercept point (IIP
3
). Typically, the output RF signal and the third-order intermodulation products are plotted versus the input RF signal. As the input RF signal is increased, the IIP
3
is a theoretical point where the desired output RF signal and the third-order products become equal in amplitude. The IIP
3
is an extrapolated value since the active device goes into compression before the IIP
3
point is reached.
For a receiver comprising multiple active devices connected in cascade, the IIP
3
of the receiver from the first stage of active device to the n
th
stage can be calculated as follows:
IIP
3
n
=−10·log
10
[10
−IIP3
n−1
/10
+10
(Av
n
−IIP3
dn
)/10
], (2)
where IIP
3
n
is the input-referred third-order intercept point from the first stage of active device to the n
th
stage, IIP
3
n−1
is the input-referred third-order intercept point from the first stage to the (n−
1
)
th
stage, Av
n
is the gain of the n
th
stage, IIP
3
dn
is the input-referred third-order intercept point of the n
th
stage, and all terms are given in decibel (dB). The calculation in equation (2) can be carried out in sequential order for subsequent stages within the receiver.
From equation (2), it can be observed that one way to improve the cascaded IIP
3
of the receiver is to lower the gain before the first non-linear active device. However, each active device also generates thermal noise which degrades the signal quality. Since the noise level is maintained at a constant level, the degradation increases as the gain is lowered and the signal amplitude is decreased. The amount of degradation can be measured by the noise figure (NF) of the active device which is given as follows:
NF
d
=SNR
in
−SNR
out
, (3)
where NF
d
is the noise figure of the active device, SNR
in
is the signal-to-noise ratio of the input RF signal into the active device, SNR
out
is signal-to-noise ratio of the output RF signal from the active device, and NF
d
, SNR
in
and SNR
out
are all given in decibel (dB). For a receiver comprising multiple active devices connected in cascade, the noise figure of the receiver from the first stage of active device to the n
th
stage can be calculated as follows:
NF
n
=
10
·
log
10
[
10
(
NF
n
-
1
/
10
)
+
10
(
NF
dn
/
10
)
-
1
10
(
G
n
-
1
/
10
)
]
,
(
4
)
where NF
n
is the noise figure from the first stage to the n
th
stage, NF
n−1
is the noise figure of the first stage to the (n−1)
th
stage, NF
dn
is the noise figure of the n
th
stage, and G
n−1
is the accumulated gain of the first stage through the (n−1)
th
stage in dB. As shown in equation (4), the gain of the active device can affect the noise figure of the subsequent stages. Similar to the IIP
3
calculation in equation (2), the noise figure calculation in equation (4) can be carried out in sequential order for subsequent stages of the receiver.
Receivers are employed for many communication applications, such as cellular communication systems and high definition television (HDTV). Exemplary cellular communication systems include Code Division Multiple Access (CDMA) communication systems, Time Division Multiple Access (TDMA) communication systems, and analog FM communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein. An exemplary HDTV system is disclosed in U.S. Pat. No. 5,452,104, U.S. Pat. No. 5,107,345, and U.S. Pat. No. 5,021,891, all three entitled “ADAPTIVE BLOCK SIZE IMAGE COMPRESSION METHOD AND SYSTEM”, and U.S. Pat. No. 5,576,767, entitled “INTERFRAME VIDEO ENCODING AND DECODING SYSTEM”, all four patents are assigned to the assignee of the present invention and incorporated by reference herein.
In cellular applications, it is common to have more than one communication system operating within the same geographic coverage area. Furthermore, these systems can operate at or near the same frequency band. When this occurs, the transmission from one system can cause degradation in the received signal of another system. CDMA is a spread spectrum communication system which spreads the transmit power to each user over the entire 1.2288 MHz signal bandwidth. The spectral response of an FM-based transmission can be more concentrated at the center frequency. Therefore, FM-based transmission can cause jammers to appear within the allocated CDMA band and very close to the received CDMA signal. Furthermore, the amplitude of the jamme
Ciccarelli Steven C.
Kaufman Ralph E.
Younis Saed G.
Qualcomm Incorporated
Ramos-Feliciano Eliseo
Streeter Tom
Vo Nguyen
Wadsworth Philip R.
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