Switch circuit and method of switching radio frequency signals

Details Switch circuit and method of switching radio frequency signals Switch circuit and method of switching radio frequency signals

2002-10-08

2004-10-12

Tieu, Binh (Department: 2643)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S425000, C455S550100, C257S341000

Reexamination Certificate

active

06804502

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switches, and particularly to a switch circuit and method of switching radio frequency (RF) signals within an integrated circuit. In one embodiment, the switch circuit comprises CMOS devices implemented on a silicon-on-insulator (SOI) substrate, for use in RF applications such as wireless communications, satellites, and cable television.
2. Description of Related Art
As is well known, radio frequency (RF) switches are important building blocks in many wireless communication systems. RF switches are found in many different communications devices such as cellular telephones, wireless pagers, wireless infrastructure equipment, satellite communications equipment, and cable television equipment. As is well known, the performance of RF switches is controlled by three primary operating performance parameters: insertion loss, switch isolation, and the “1 dB compression point.” These three performance parameters are tightly coupled, and any one parameter can be emphasized in the design of RF switch components at the expense of others. A fourth performance parameter that is occasionally considered in the design of RF switches is commonly referred to as the switching time or switching speed (defined as the time required to turn one side of a switch on and turn the other side off). Other characteristics that are important in RF switch design include ease and degree (or level) of integration of the RF switch, complexity, yield, return loss and cost of manufacture.
These RF switch performance parameters can be more readily described with reference to a prior art RF switch design shown in the simplified circuit schematics of
FIGS. 1
a
-
1
c
.
FIG. 1
a
shows a simplified circuit diagram of a prior art single pole, single throw (SPST) RF switch
10
. The prior art SPST switch
10
includes a switching transistor M
1
5
and a shunting transistor M
2
7
. Referring now to
FIG. 1
a
, depending upon the state of the control voltages of the two MOSFET transistors M
1
5
and M
2
7
(i.e., depending upon the DC bias applied to the gate inputs of the MOSFET switching and shunting transistors, M
1
and M
2
, respectively), RF signals are either routed from an RF input node
1
to, an RF output node
3
, or shunted to ground through the shunting transistor M
2
7
. Actual values of the DC bias voltages depend upon the polarity and thresholds of the MOSFET transistors M
1
5
and M
2
7
. Resistor R
0
9
, in series with the RF source signal, isolates the bias from the source signal and is essential for optimal switch performance.
FIG. 1
b
shows the “on” state of the RF switch
10
of
FIG. 1
a
(i.e.,
FIG. 1
b
shows the equivalent small-signal values of the transistors M
1
and M
2
when the RF switch
10
is “on”, with switching transistor M
1
5
on, and shunting transistor M
2
7
off).
FIG. 1
c
shows the “off” state of the switch
10
of
FIG. 1
a
(i.e.,
FIG. 1
c
shows the equivalent small-signal values of the transistors M
1
and M
2
when the RF switch
10
is “off”, with switching transistor M
1
5
off, and shunting transistor M
2
7
on).
As shown in
FIG. 1
b
, when the RF switch
10
is on, the switching transistor M
1
5
is primarily resistive while the shunting transistor M
2
7
is primarily capacitive. The “insertion loss” of the RF switch
10
is determined from the difference between the maximum available power at the input node
1
and the power that is delivered to a load
11
at the output node
3
. At low frequencies, any power lost is due to the finite on resistance “r”
13
of the switching transistor M
1
5
when the switch
10
is on (see
FIG. 1
b
). The on resistance r
13
(
FIG. 1
b
) typically is much less than the source resistor R
0
9
. The insertion loss, “IL”, can therefore be characterized in accordance with Equation 1 shown below:
IL
is approximately equal to: 10
r/R
0
ln(10)=0.087
r
(in dB).  Equation 1
Thus, at low frequencies, a 3-&OHgr; value for r results in approximately 0.25 dB insertion loss. Because insertion loss depends greatly upon the on resistances of the RF switch transmitters, lowering the transistor on resistances and reducing the parasitic substrate resistances can achieve improvements in insertion loss.
In general, the input-to-output isolation (or more simply, the switch isolation) of an RF switch is determined by measuring the amount of power that “bleeds” from the input port into the output port when the transistor connecting the two ports is off. The isolation characteristic measures how well the RF switch turns off (i.e., how well the switch blocks the input signal from the output). More specifically, and referring now to the “off” state of the RF switch
10
of
FIG. 1
c
, the switching transistor M
1
5
off state acts to block the input
1
from the output
3
. The shunting transistor M
2
7
also serves to increase the input-to-output isolation of the switch
10
.
When turned off (i.e., when the RF switch
10
and the switching transistor M
1
5
are turned off), M
1
5
is primarily capacitive with “feedthrough” (i.e., passing of the RF input signal from the input node
1
to the output node
3
) of the input signal determined by the series/parallel values of the capacitors CGD off
15
(Gate-to-Drain Capacitance when the switching transistor M
1
is turned off), CGS off
17
(Gate-to-Source Capacitance when the switching transistor M
1
is turned off), and CDS
1
19
(Drain-to-Source capacitance when the transistor M
1
is turned off). Feedthrough of the input signal is undesirable and is directly related to the input-to-output isolation of the RF switch
10
. The shunting transistor M
2
7
is used to reduce the magnitude of the feedthrough and thereby increase the isolation characteristic of the RF switch.
The shunting transistor M
2
7
of
FIG. 1
c
is turned on when the switching transistor M
1
5
is turned off. In this condition, the shunting transistor M
2
7
acts primarily as a resistor having a value of r. By design, the value of r is much less than the characteristic impedance of the RF source. Consequently, r greatly reduces the voltage at the input of the switching transistor M
1
5
. When the value of r is much less than the source resistance R
0
9
and the feedthrough capacitive resistance of the shunting transistor M
2
7
, isolation is easily calculated. Switch isolation for the off state of the RF switch
10
is determined as the difference between the maximum available power at the input to the power at the output.
In addition to RF switch insertion loss and isolation, another important RF switch performance characteristic is the ability to handle large input power when the switch is turned on to ensure that insertion loss is not a function of power at a fixed frequency. Many applications require that the switch does not distort power transmitted through a “switched-on” switch. For example, if two closely spaced tones are concurrently passed through an RF switch, nonlinearities in the switch can produce inter-modulation (IM) and can thereby create a false tone in adjacent channels. If these adjacent channels are reserved, for instance, for information signals, power in these false tones must be maintained as small as possible. The switch compression, or “1 dB compression point” (“P1 dB”), is indicative of the switch's ability to handle power. The P1 dB is defined as the input power at which the insertion loss has increased by 1 dB from its low-power value. Or stated in another way, the 1 dB compression point is a measure of the amount of power that can be input to the RF switch at the input port before the output power deviates from a linear relationship with the input power by 1 dB.
Switch compression occurs in one of two ways. To understand how switch compressing occurs, operation of the MOSFET transistors shown in the RF switch
10
of
FIGS. 1
a
-
1
c
are described. As is well known in the transistor design arts, MOSFETs require a gate-to-source bias that exceeds a threshold voltage, V
t
, to turn on. Similarly

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