Modulator structure for a transmitter and a mobile station

Details Modulator structure for a transmitter and a mobile station Modulator structure for a transmitter and a mobile station

1999-10-29

2002-04-16

Kinkead, Arnold (Department: 2817)

Modulators

Phase shift keying modulator or quadrature amplitude modulator

Including discrete semiconductor device

C455S323000, C455S333000, C455S118000, C455S317000, C455S313000, C455S327000, C455S355000

Reexamination Certificate

active

06373345

ABSTRACT:

The invention relates in general to modulators. In particular the invention relates to a modulator structure which is very suitable for dual-band or triple-band mobile stations.
BACKGROUND OF THE INVENTION
In modulation a carrier frequency is modified in a certain way so that the data to be transmitted using the radio signal is carried, for example, in the changes of the phase or amplitude of the carrier frequency. There are many modulation methods, which differ in the sense which properties of the carrier wave are modulated and how they are modulated. The arrangement that performs the modification of the carrier wave is called a modulator. There are also many types of modulators. A direct conversion modulator, for example, may be used in mobile stations. In a direct conversion modulator the modulation is performed directly in the carrier frequency; there is no intermediate frequency in the modulation process.
The most important characteristics of a modulator are the linearity and the signal-to-noise ratio S/N. The signal-to-noise ratio comprises the ratio of the transmit signal to the background noise at the transmit frequency range (TX), and the ratio of the transmit signal to the noise generated at the receive frequency range (RX). It is possible to enhance the signal-to-noise ratio at the RX frequency range by filtering the modulated signal using a suitable filter. The enhancement of the signal-to-noise ratio at the TX frequency range is not feasible after the modulator.
Usually filtering at the RX frequency range is done using a low pass filter, most preferably a duplex filter after the power amplification. Table 1 presents the signal-to-noise ratio limits for the receive frequency band defined for the GSM (Global System for Mobile Communications), PCN (Personal Communication Network) and PCS (Personal Communication System) networks.
In the table N 100 kHz defines the noise level generated by the transmitter chain in the reception at a frequency band with the width of 100 kHz. The noise is measured at a power level where the noise level is at a maximum. This power level is in practice the maximum transmit power. The measured noise power level is normalized from the 100 kHz width to a width of 1 Hz, whereby the level decreases by 50 dB and is in the column N 1 Hz. Thus the measurement is performed at a wide band-width, but it is normalized to a narrower band by calculation. There are also individual noise level differences between similar transmitters, and therefore there is a margin of 3 dBm, which is subtracted from the above mentioned level limits. P
out
is the output power of the transmitter. S/N is the signal-to-noise ratio (dBc, decibel carrier) generated at the carrier frequency, which is determined by the distance between the maximum output power and the normalized noise, to which the margin is added.
TABLE 1
The signal-to-noise ratio limits for the receive frequency band
System
N 100 kHz
N 1 Hz
Margin
P
out
S/N
GSM
−79 dBm
−129 dBm
3 dB
33 dBm
165 dBc
PCN
−71 dBm
−121 dBm
3 dB
30 dBm
154 dBc
PCS
−71 dBm
−121 dBm
3 dB
30 dBm
154 dBc
It is also possible to filter the noise by placing filters between the modulator of the transmit chain and the receiver.
FIG. 1
shows the basic effect of a low-pass filter on radio signals. The original transmit signal
1
is mirrored into the frequency domain over the modulated center frequency.
FIG. 1A
shows that the final result
2
of the modulated but unfiltered transmit signal extends both into the intended transmit range TX and partly into the receive range RX.
FIG. 1B
shows that the final result
4
of the modulated transmit signal
1
, which is filtered with the filter function
3
, extends mainly into the intended transmit range TX, and only slightly into the receive range RX.
Previously there is also known a so called Gilbert cell, which is generally used in integrated multiplicator circuits of telecommunication systems, particularly in mobile stations. Multiplicator circuits are used for instance in integrated RF (Radio Frequency) and intermediate frequency sections, such as in the modulator, the mixer and the regulated amplifier.
A prior art multiplicator, such as a squaring multiplicator, is based on a mathematical formula where the difference of the squares of the sum and difference of two signals produces the product of the signals:
(x+y)
2
−(x−y)
2
=4xy  (1)
where x is the first signal and y is the second signal.
The squaring is performed for instance by a MOS (Metal Oxide Semiconductor) transistor where the drain current is proportional to the square of the gate-source voltage. A multiplicator can also be realized with bipolar transistors. The collector current of a bipolar transistor i
c
is:
i
C
=
I
S
⁢
ⅇ
v
BE
V
T
(
2
)
where the shown parameters are the bipolar transistor's saturation current I
s
, the base-emitter voltage V
BE
, and the thermal voltage V
T
. The exponent function is here approximated by the first four terms of an infinite exponential Taylor series:
i
C
=
I
S
⁢
exp
⁡
(
x
)
≈
I
S
⁢
{
1
+
1
1
!
⁢
x
+
1
2
!
⁢
x
2
+
1
3
!
⁢
x
3
}
(
3
)
where
x
=
v
BE
V
T
(
4
)
and the shown parameters are the bipolar transistor's base-emitter voltage V
BE
and the thermal voltage V
T
.
Mixers use generally a double balanced structure where the outputs of four differently phased mixers are added in order to equalize the harmonic interference. The balancing is presented mathematically as follows:
(x+y)
2
+(−x−y)
2
−(x−y)
2
−(−x+y)
2
=8xy  (5)
When the first four terms of the exponential Taylor series are substituted in the formula we still have the product of the input signals:
ⅇ
x
+
y
+
ⅇ
-
x
-
y
-
ⅇ
x
-
y
-
ⅇ
-
x
+
y
=
8
2
!
⁢
x
⁢
 
⁢
y
(
6
)
When we substitute the terms x and y for the terms representing the output signals the final mathematical formula can be written in the form:
ⅇ
V
BE
+
x
+
y
+
ⅇ
V
BE
-
x
-
y
-
ⅇ
V
BE
+
x
-
y
-
ⅇ
V
BE
-
x
+
y
=
8
2
!
⁢
x
⁢
 
⁢
y
(
7
)
FIG. 2
shows a Gilbert cell known per se, which is used for instance to realize integrated RF and intermediate frequency sections, such as a regulated amplifier and a mixer. In a Gilbert cell two input voltages recur as one output voltage, in other words the voltage difference at the outputs is the product of the voltage differences at the inputs. The first voltage difference is connected to the terminals V
X+
and V
X−
, from which the voltages are supplied to the bases of the transistors Q
1
and Q
2
and correspondingly to the bases of the transistors Q
4
and Q
5
. The other voltage difference is connected to the terminals V
Y+
and V
Y−
, from which the voltage is amplified by the transistors Q
3
and Q
6
. The transistors Q
3
and Q
6
are connected via the resistors R
E1
, and R
E2
to the field effect transistor (FET) Q
7
which is controlled by the biasing voltage V
BIAS
and connected to the negative operating voltage. The transistors Q
1
and Q
5
amplify the positive voltage difference V
X+
and V
X
which is connected to the outputs V
OUT+
and V
OUT−
. The above mentioned circuit is connected to the positive operating voltage via the resistors R
L1
and R
L2
. The transistors Q
2
and Q
4
amplify the negative voltage difference V
X+
and V
X−
which is connected to the outputs V
OUT+
and V
OUT−
.
FIG. 3
shows the circuit arrangement of the switch transistors in a direct conversion modulator. The arrangement comprising the switch transistors is often called the switching arrangement or switching block of a modulator. Noise due to the local oscillator can be to a large extent eliminated by using balanced switching arrangement, for example pairs of transistors that have very similar characteristics.
FIG. 3
shows the switch transistors Q
8
, Q
9
, Q
10
, Q
11
, Q
12
, Q
13
, Q
14
and Q
15
of a direct conversion modulator. These transi

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