Wave transmission lines and networks – Automatically controlled systems
Reexamination Certificate
1999-07-14
2001-10-02
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Automatically controlled systems
C333S08100R, C455S249100
Reexamination Certificate
active
06297709
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to variable attenuators, and in particular to a method and apparatus for temperature compensating a variable attenuator for improved performance.
BACKGROUND OF THE INVENTION
In a wireless communication system, for example, a Global System for Mobile (GSM) Communication system using TDMA (Time Division Multiple Access) signaling, the signaling format includes a framed structure comprising a number of time slots. The time slots serve as channels over which mobile and base stations transmit or receive information. Each channel or time slot of a frame is assigned to a different user, with mobile-to-base station (uplink) transmissions carried on one frequency band and base-to-mobile station (downlink) transmissions carried on a separate frequency band.
Transmissions in each channel or time slot are specified to ramp up to a required power level and ramp down to a required power level in a predetermined amount of time within the time slot. GSM specifications require that the power at the start and end of a burst must be at a specified minimum level and that the transition from the minimum level to the final required level must be completed in a specified amount of time. The rates of ramping up and down are specified in order to reduce the generation of transient side bands and interference on adjacent channels.
Referring to
FIG. 1
, therein is illustrated a prior art power control loop, denoted generally as
10
. A variable attenuator
12
is coupled to the input of an amplifier chain
14
. Variable attenuator
12
and amplifier chain
14
are coupled between input
16
and output
18
. A bias signal, V
c
, is applied to the variable attenuator
12
on line
20
to control the attenuation characteristics of the variable attenuator
12
. Bias signal V
c
controls attenuation levels in variable attenuator
12
, allowing the power control loop
10
to maintain required power levels when ramping up, ramping down, and during the burst in a time slot of a TDMA signal.
Because of the limited detection range of linear detector
24
at output
18
, typically about 45 dB, the operation of power control loop
10
is divided into an open-loop mode and a closed-loop mode. Power control loop
10
may have a dynamic range of about 80 dB in open-loop mode and will run in open-loop mode until the rate of ramp-up has reached a predetermined level, referred to as the switching point. At the switching point, the output
18
can be coupled by line
22
through linear detector
24
through a feedback loop in order to implement the closed-loop mode of operation.
In closed-loop mode, reference signal source
26
supplies reference signal, V
r
, which is proportional to the required rate of ramping defined by the GSM specification. V
r
is compared in comparator
28
to the detected signal, V
d
, on line
30
. The difference, an error signal, V
e
, is applied to integrator
34
at line
32
. Integrator
34
integrates error signal, V
e
, and applies the result, bias signal, V
c
, to variable attenuator
12
at line
20
to stabilize power levels when ramping up, ramping down, and during the burst of the TDMA signal. Integrator
34
may comprise a operational amplifier
36
having a non-inverting input coupled to ground through a resistor R
1
, a inverting input coupled to comparator
28
through a resistor R
2
and an output coupled to line
20
. Integrator
34
may further comprise a capacitor C coupled between the inverting input and the output of operational amplifier
36
. V
c
may be calculated as:
V
c
=
1
RC
∫
(
V
r
-
V
d
)
ⅆ
t
Equation
1
Prior to switching from open-loop mode to closed-loop mode, amplifier chain
14
is allowed to ramp up without correction to the level of bias signal V
c
, while detected signal V
d
equals zero or a constant offset voltage. Temperature variations may affect variable attenuator
12
and produce deviations in the attenuation characteristics of variable attenuator
12
. The deviations may result in an increase or decrease in the rate of ramping. The increase or decrease in the attenuation characteristics of variable attenuator
12
may increase or decrease the output power, producing unwanted RF spectrum.
FIG. 2A
is a plot illustrating deviations in the rate of ramping in the power control loop of
FIG. 1
that are generated by changes in the attenuation characteristics of variable attenuator
12
. A switching point transient can occur at a switching point
36
when a power control loop switches from open loop to close loop mode. The switching point transient may be caused by deviations in the rate of ramping as indicated by the short, dashed lines
36
a
and
36
b
.
FIG. 2B
is a plot illustrating a switching point transient
38
a
of the power control loop of
FIG. 1
generated from an increase in the attenuation characteristics of variable attenuator
12
. The increase in the attenuation characteristics results in a decrease in the power level at output
18
.
FIG. 2C
is a plot illustrating a switching point transient
38
b
of the power control loop of
FIG. 1
generated from a decrease in the attenuation characteristics of variable attenuator
12
. The decrease in the attenuation characteristics results in an increase in the power level at output
18
.
When power control loop
10
switches from open-loop mode to closed- loop mode with negative feedback, the deviation in the rate of ramping is automatically detected, and bias signal V
c
is adjusted to correct for the drift in the attenuation characteristics as a result of the offset signal generated. This immediate correction at the switching point creates transients
38
a
and
38
b
as illustrated in
FIGS. 2B and 2C
, which produces increases or decreases in voltage at output
18
and produces an RF spectrum resulting in adjacent channel interference.
FIG. 2D
illustrates the unwanted side band harmonics generated in the output power spectrum of power control loop
10
as a result of switching point transients.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for temperature compensating a variable attenuator. The method and apparatus implements circuitry that generates at least one compensating bias signal. The at least one bias signal is generated in proportion to signal variations that are generated across the variable attenuator as a result of increases or decreases in the variable attenuator's operating temperature. The method and apparatus provides a variable attenuator that exhibits reduced deviation in attenuation characteristics.
In an embodiment, the invention comprises a variable attenuator having at least one diode affected by temperature variations that produce a temperature-offset signal across the at least one diode. A temperature compensation source is disposed between the variable attenuator and a bias source. The temperature compensation source comprises at least one diode capable of producing a temperature-offset signal approximately equivalent to the temperature-offset signal produced by the at least one diode of the variable attenuator. The compensation source temperature-offset signal is subtracted from a bias signal supplied by the bias source to produce a temperature-compensated bias signal. The temperature compensation bias signal is used to bias and offset signal variations produced across the at least one diode of the variable attenuator.
REFERENCES:
patent: 3921106 (1975-11-01), Williams
patent: 4236126 (1980-11-01), Weller et al.
patent: 4590417 (1986-05-01), Tanaami et al.
patent: 5204643 (1993-04-01), Verronen
patent: 5262741 (1993-11-01), Kitakubo
patent: 5659253 (1997-08-01), Busking
patent: 5767685 (1998-06-01), Walker
patent: 5854971 (1998-12-01), Nagoya et al.
patent: 6104919 (2000-08-01), Lyall, Jr. et al.
patent: 59-22420 (1984-02-01), None
patent: 8-274560 (1996-10-01), None
Neitiniemi Jukka-Pekka
Tran Kim A.
Wey Chia-Sam
Hayes Thomas B.
Lee Benny
Nokia Telecommunications Oy
Rivers Brian T.
Summons Barbara
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