Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1998-09-11
2001-02-20
Tsang, Fan (Department: 2746)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S067700, C455S063300, C455S067150, C455S254000, C455S317000, C455S318000, C455S310000, C455S258000, C455S259000
Reexamination Certificate
active
06192228
ABSTRACT:
TECHNICAL FIELD
This invention relates to electronic communication systems, and more particularly to a method and apparatus for improving the sensitivity of a radio frequency (RF) receiver at certain frequencies suffering from co-channel interference.
BACKGROUND
It is well-known that the sensitivity of a frequency modulated radio receiver is significantly degraded on channels where the received frequency is equal to a harmonic frequency of the receiver clock frequency. This problem, known as co-channel interference, is a form of “jamming”, and can be understood by reference to FIG.
1
.
FIG. 1
is a block diagram of a prior art single stage RF receiver that can exhibit the problem solved by the present invention. While a single stage RF receiver is shown, the problem of co-channel interference arises with receivers having multiple stages or direct-to-digital down conversion.
In the example shown, a modulated RF signal
100
having a particular carrier frequency, F
RFC
=N·F
CH
(where N is an integer designating, in effect, a channel), is generated in known fashion by a remote RF transmitter and received at an antenna
102
of a compatible receiver. When the audio content of the RF signal
100
is zero (that is, when an audio signal is not present in the received signal). F
RFC
is constant. Otherwise, F
RFC
is modulated in a manner which represents the audio content of the transmitted and received RF signal
100
. The instantaneous modulated frequency is F
RFC
+F
MOD
.
The received signal is typically amplified by a low noise amplifier
104
before conversion by a conventional down converter
106
to a baseband frequency (for a single stage down converter) or to an intermediate frequency (which is further down converted by one or more subsequent stages until a baseband frequency is generated).
The other input to the down converter
106
is an “injection” frequency, which is synthesized to have a nominal frequency of N·F
CH
(where N is an integer designating, in effect, a channel). In the illustrated embodiment, a processor
108
(e.g., a digital signal processor, or DSP) coupled to a clock source
109
(e.g., a crystal) outputs a control voltage having a nominal value of Vo to an oscillator
110
, which is preferably a voltage controlled crystal oscillator (VCXO) but may be a voltage controlled oscillator (VCO). Typically, the oscillator
110
outputs a reference frequency F
REF
, which equals k·Vo, where k is a constant. By way of example only, for certain cellular telephone systems (e.g., GSM), F
REF
may be 13 MHz or 19.5 MHz. This reference frequency is then divided down in a divider
111
by a value R to a channel spacing frequency, F
Ch
. The channel spacing frequency F
CH
s applied to a multiplier circuit
112
(e.g., a phase-locked loop), which outputs the injection frequency of N·FCH for a particular “channel” mapped to a multiplier value N.
The down converter
106
subtracts the injection frequency N·F
CH
from the received frequency F
RFC
+F
MOD
(which equals N·F
CH
+F
MOD
) to generate a baseband frequency F
MOD
representing the audio content of the received signal
100
. In the illustrated embodiment, the baseband frequency F
MOD
is converted to a digital frequency-modulated (FM) waveform by an analog-to-digital converter
114
. The processor
108
, which generally includes a memory circuit
116
for data and program storage, decodes the FM signal and outputs a digital audio signal. A digital-to-analog converter
118
converts the digital audio signal to an analog audio waveform, which may then be applied to an audio amplifier
120
and thereafter output as sound from a speaker
122
.
A problem arises because the output of the oscillator
110
generates harmonics that can interfere with certain received input signals
100
. These harmonic frequencies typically are coupled to and amplified by the low noise amplifier
104
, and can mask the received signal
100
. One known technique for reducing such co-channel interference is to shield the oscillator
110
, but such shielding may not achieve the desired sensitivity required for a system.
Accordingly, the inventors have determined that there is a need for an RF receiver with enhanced sensitivity and reduced co-channel interference. The present invention meets this need.
SUMMARY
The invention encompasses a method and apparatus for improving the sensitivity of a radio frequency (RF) receiver at certain frequencies suffering from co-channel interference. In the preferred embodiment of the invention, when a reference frequency, F
REF
, generates significant harmonics for a received RF signal in a selected channel, the reference frequency is varied or offset by a small amount, &Dgr;F. The harmonics of the offset reference frequency, F′
REF
=F
REF
±&Dgr;F, are thereby shifted away from the frequency of the received RF signal, thus reducing co-channel interference. The offset amount &Dgr;F is chosen such that the same injection frequency synthesized from F
REF
can still be synthesized from F
REF
±&Dgr;F by shifting N by an integer amount, preferably to N+1 or N−1. However, the invention encompasses shifting N by other integer amounts.
REFERENCES:
patent: 4653117 (1987-03-01), Heck
patent: 5101509 (1992-03-01), Lai
Agahi-Kesheh Darioush
Domino William J.
Oskowsky Mark
Rozenblit Dmitriy
Akin Gump Strauss Hauer & Feld L.L.P.
Conexant Systems Inc.
Ferguson Keith
Tsang Fan
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