Pulse or digital communications – Repeaters
Reexamination Certificate
1998-09-25
2003-09-02
Vo, Don N. (Department: 2631)
Pulse or digital communications
Repeaters
C375S214000, C375S295000
Reexamination Certificate
active
06614837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, generally, to communication systems, devices, and processes which use radio frequency (RF) transmitters, and, in particular embodiments, to such systems, devices, and processes which minimize RF transmitter size and power requirements by producing a low noise modulated signal that eliminates the need for post-amplification transmit band filtering.
2. Description of Related Art
It has become increasingly important to minimize the size, weightand power consumption of various electronic devices, especially personal communication devices such as cellular telephones, personal pagers, cordless telephones, and the like. One way to minimize such characteristics is to minimize. the number of components and functions required in the electronic device. However, personal communication devices such as cellular telephones often require complex circuitry with a number of power-inefficient components for performing particular functions.
Upconversion and RF transmission are two such functions that typically require complex and power-inefficient circuitry. Upconversion and RF transmission are the processes of transforming a modulated information signal into an RF signal, filtering it, and amplifying it for transmission to receiving devices such as cell sites in a cellular network, and are typically performed in an RF transmitter. A conventional RF transmitter
62
is illustrated in
FIG. 1
, where a mixer
66
coupled to a modulator (not shown in
FIG. 1
) and a main synthesizer
68
receives and mixes a modulated intermediate frequency (IF) signal
64
produced by the modulator with a main synthesizer frequency
70
produced by the main synthesizer
68
. The output of the mixer
66
is an unamplified modulated carrier
72
. A transmit band small-signal filter
74
coupled to the mixer
66
receives and filters the unamplified modulated carrier
72
to produce a filtered unamplified modulated carrier
76
. A power amplifier
78
coupled to the transmit band small-signal filter
74
receives and amplifies the filtered unamplified modulated carrier
76
. The output of the power amplifier
78
is an amplified modulated carrier
80
. A transmit band large-signal filter
82
coupled to the power amplifier
78
and duplexed with a receive band filter
128
receives and filters the amplified modulated carrier
80
to produce a filtered amplified modulated carrier
100
. A wideband harmonic low pass filter (LPF)
84
coupled to the transmit band large-signal filter
82
receives and further filters the filtered amplified modulated carrier
100
to suppress harmonics of the transmit signal generated by the power amplifier
78
. The output of the harmonic LPF
84
is a transmit signal
102
. An antenna
86
coupled to the harmonic LPF
84
receives and transmits the transmit signal
102
toireceiverunits (not shown in FIG.
1
).
One requirement of any RF transmitter is to produce a very low noise output to minimize the disruption to nearby receive channels. For example, in the Global System for Mobile (GSM) communication standard, the European standard for digital cellular systems operating in the 900 MHz band, frequency bands are allocated such that a mobile subscriber unit will transmit signals over a transmit band of between 890 and 915 MHz and will receive signals over a receive band of between 935 to 960 MHz. The transmit band is broken up into 125 channels, each channel separated by 200 kHz. If, as illustrated in
FIG. 6
, a user is transmitting at the very highest channel, 915 MHz, there will be a spike
104
at the 915 MHz carrier frequency, tapering off on either side but with measurable signal even into the 935 MHz region. Signals more than 100 kHz from the 915 MHz carrier frequency represent noise, or unwanted transmitted power. Transmitted noise extending into the designated receive band above 935 MHz is called the receive band noise
106
of the transmitter, which may interfere with the reception of other nearby mobile subscriber units. It is therefore desirable for an RF transmitter to generate very low levels of receive band noise
106
to minimize the disruption to nearby receive channels. However, RF transmitters in conventional GSM cellular telephones produce a significant amount of receive band noise that must be filtered at the output of the power amplifier.
For purposes of illustration only, the following discussion will focus on an RF transmitter in a conventional GSM cellular telephone receiving a modulated signal with a 200 kHz bandwidth at an IF of 250 MHz and transmitting at the highest channel, 915 MHz, with a receive band noise rejection requirement of−164 dBc/Hz at 935 MHz, and having other intermediate component characteristics specified below. Referring again to
FIG. 1
, in a conventional GSM RF transmitter
62
, a modulated IF signal
64
at 250 MHz having a flat but high noise floor of−135 dBc/Hz (reference character
108
) as illustrated in
FIG. 7
is applied to the mixer
66
along with the main synthesizer frequency
70
(having a noise floor of−150 dBc/Hz), which is variable to facilitate tuning to different channels. Generally, the main synthesizer
68
is designed to produce a main synthesizer frequency
70
equivalent to the carrier frequency plus the IF. In the example under discussion, the channel frequency is 915 MHz and the IF is 250 MHz, so the main synthesizer
68
will produce a main synthesizer frequency
70
of 1165 MHz.
The output of the mixer
66
is the unamplified modulated carrier
72
, which retains the noise floor (reference character
108
) of the modulated IF signal
64
as illustrated in FIG.
8
. The unamplified modulated carrier
72
enters the transmit band small-signal filter
74
(having a bandwidth of 890 MHz to 915 MHz and attenuation of 20 dB at 935 MHz in this example), where the higher frequency receive band noise
106
is significantly reduced, resulting in the filtered unamplified modulated carrier
76
illustrated in FIG.
9
. However, when the filtered unamplified modulated carrier
76
passes through the power amplifier
78
(having output power of+35.5 dBm and a third-order intermodulation product IM3 of−9 dBc in this example), a significant amount of the receive band noise
106
returns in the amplified modulated carrier
80
due to the nonlinearity of the power amplifier
78
, as illustrated in FIG.
10
. Generally, if unwanted signals are present on one side of the carrier (such as the noise floor
108
in FIG.
9
), the nonlinear power amplifier
78
will create a mirror image of those unwanted signals on the other side, albeit at a lower level as determined by the third-order intermodulation product. Thus, in conventional approaches, no amount of filtering by the transmit band small-signal filter
74
on the input side of the power amplifier
78
will eliminate the need for a transmit band large-signal filter
82
on the output side.
As a result, filtering of receive band noise
106
is performed at the end of the transmit chain, after the power amplifier
78
but before the antenna
86
. In the GSM example under discussion, the transmit band large-signal filter
82
comprises a bandpass filter with a pass band of about 890 to 915 MHz and attenuation of 20 dB at 935 MHz to pass the entire transmit band, reject the entire receive band, and push down the receive band noise
106
as illustrated in FIG.
1
. However, the transmit band large-signal filter
82
has significant insertion loss, typically 2.0 dB, that may eliminate as much as half of the transmitted power. Because GSM cellular telephones can produce a selectable amount of output power, but no more than two watts, the RF transmitter
62
may therefore need to generate as much as four watts of transmit power in order to produce an actual post-filter output of two watts. In this example, significant battery power is wasted in supplying four watts of power to the power amplifier
78
to produce a two watt transmit signal
102
.
An alternative to the convent
Abdelgany MohyEldeen Fouad
Domino William John
Rozenblit Dmitriy
Skyworks Solutions Inc.
Vo Don N.
Weide & Miller, Ltd
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