Noise source module for microwave test systems

Details Noise source module for microwave test systems Noise source module for microwave test systems

1999-06-04

2001-07-31

Brown, Glenn W. (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

With auxiliary means to condition stimulus/response signals

C324S601000, C324S613000, C324S614000

Reexamination Certificate

active

06268735

ABSTRACT:

This invention relates generally to automatic test equipment, and more specifically to automatic test equipment for testing and characterizing RF/microwave devices.
Automatic test equipment, commonly known as a “tester,” has traditionally been used in processes for manufacturing semiconductor devices to determine whether the manufactured devices contain defects. In this way, large volumes of devices can be tested quickly, thereby reducing both time-to-market and production costs.
More recently, testers have been used for both testing and characterizing high frequency semiconductor devices, which are designed to operate in the radio frequency (RF) and microwave ranges. Such “wireless” devices include those used in the cellular telephone industry, where high volume, low cost production is especially important.
One characteristic that is frequently measured relates to noise parameters of RF/microwave devices. For example, testers have been used to measure the noise factor of RF/microwave devices, which when expressed in decibels (dB) is commonly referred to as the noise figure of a device.
FIG. 1A
shows a partial block diagram of a conventional tester
100
used to measure the noise figure of a device under test (DUT)
118
, which is designed to operate in the RF or microwave band. The tester
100
has a tester body
102
, which includes a computerized controller
106
that can be programmed by a tester operator to perform various test and analysis operations. For example, the controller
106
may be programmed to control RF signal sources (e.g., an RF source
110
) and receivers (e.g., an RF receiver
112
). The RF source
110
and the RF receiver
112
generate and detect, respectively, test signals for the DUT
118
.
The tester
100
also includes a test head
104
, which generally routes the test signals between the tester body
102
and the DUT
118
. Accordingly, the test head
104
includes switching modules (e.g., a switching module
114
) for directing the test signals between the RF source
110
, the RF receiver
112
, and the DUT
118
.
In a typical test configuration, an external noise source
116
is coupled to the test head
104
and used for measuring the noise figure of the DUT
118
. Thus, the switching module
114
also routes signals between the noise source
116
and the DUT
118
. Further, the noise source
116
is typically attached to a device interface board (not shown), which also includes hardware for interfacing the test head
104
with the DUT
118
.
FIG. 1B
shows a simplified schematic diagram of the switching module
114
, which routes signals between the RF source
110
, the RF receiver
112
, the noise source
116
, and the DUT
118
. The switching module
114
includes a directional coupler
120
with one port connected to the RF source
110
, two ports connected to the throws of a switch
122
, and one port connected to the pole of a switch
124
.
Further, the pole of the switch
122
is coupled to the RF receiver
112
. The switch
122
can therefore be actuated for allowing the RF receiver
112
to detect signals on either the “source side” or the “DUT side” of the directional coupler
120
.
The throws of the switch
124
are connected to the DUT
118
and a throw of a switch
126
. The switch
124
can therefore be actuated to pass test signals from the RF source
110
to the DUT
118
.
The other throw of the switch
126
is also connected to the DUT
118
. Further, the pole of the switch
126
is coupled to the noise source
116
. The switch
126
can therefore be actuated to pass signals from the noise source
116
to the DUT
118
. In addition, switches
122
,
124
, and
126
can be actuated to pass signals from the noise source
116
to the RF receiver
112
.
Although the noise source
116
is primarily meant to be used for measuring the noise figure of the DUT
118
, it is well understood that noise is inherent in all electronic circuitry, including the circuitry used to implement the measurement system. It is therefore generally necessary to take into account the noise contributed by the measurement system when measuring the noise figure of the DUT
118
.
The noise figure of the DUT
118
, F
DUT
, can be calculated using the formula
F
DUT
=10 Log [
F
SYS
−(
F
RCVR
−1)/
G
DUT
],  (eq. 1)
where F
SYS
is the noise figure of the measurement system, F
RCVR
is the noise figure of the RF receiver
112
, and G
DUT
is the gain of the DUT
118
. The noise contributed by the measurement system is therefore included and accounted for in the calculation of F
DUT
using eq. 1.
The noise figure of any two-port electronic device can generally be determined using the formula
F=
10 Log {
ENR
−[(
N
1
/N
2
)−1]},   (eq. 2)
where ENR is the “excess noise ratio” of the noise source, and N
1
and N
2
are the noise power from the two-port device when the noise source is biased “on” and “off”, respectively.
In particular, ENR values for the noise source
116
can be determined experimentally. More likely, these ENR values are measured by the manufacturer of the noise source
116
and provided to the tester operator as documentation. Further, the ENR values for the noise source
116
are typically specified as a function of frequency.
Eq. 2 can therefore be used to determine values for F
SYS
and F
RCVR
in eq. 1. When F
SYS
is determined, the switch
126
is actuated to couple the noise source
116
to an input port of the DUT
118
, and the switches
122
and
124
are actuated to couple an output port of the DUT
118
to the RF receiver
112
. However, when F
RCVR
is determined, the switches
124
and
126
are actuated to by-pass the DUT
118
.
The gain of the DUT
118
, G
DUT
, can be calculated using the formula
G
DUT
=(
N
1
−N
2
)/
k
(
T
1
−T
2
)
B,
  (eq. 3)
where T
1
and T
2
are the noise temperatures supplied by the noise source
116
when the noise source
116
is biased “on” and “off”, respectively; k is the Boltzmann's constant;
and, B is the bandwidth of the DUT
118
. The product, k (T
1
−T
2
) B, is commonly called the “excess noise power” from the noise source. The noise figure of the DUT
118
can therefore be calculated after incorporating suitable values for F
SYS
, F
RCVR
, and G
DUT
into eq. 1.
We have recognized that measuring noise parameters of a device under test in the manner described above may result in measurement uncertainties, which can adversely affect the accuracy of noise figure measurements. Highly accurate noise figure measurements are important to RF/microwave device manufacturers because it allows them to provide better noise specifications for their devices. Further, RF/microwave device customers are generally willing to pay more for devices with the best noise specifications. It is therefore important that noise parameters of a device under test be made as accurately as possible.
One type of measurement uncertainty is caused by impedance mismatches, including mismatches between the noise source and the device under test, the noise source and the RF receiver, and the device under test and the RF receiver. Such impedance mismatches can cause signal reflections that affect the amount of power provided to the elements of the measurement system. Further, these impedance mismatches tend to be more prevalent in measurement systems operating in high frequency ranges.
For example, impedance mismatches between the noise source
116
and the input port of the DUT
118
can affect the noise power provided to the DUT
118
, thereby adding uncertainty to the determination of F
SYS
. Impedance mismatches between the output port of the DUT
118
and the RF receiver
112
can also add uncertainty to F
SYS
by affecting the power provided to the RF receiver
112
. Similarly, impedance mismatches between the noise source
116
and the RF receiver
112
can affect the noise power provided to the RF receiver
112
, thereby adding uncertainty to the determination of F
RCVR
. These uncertainties general

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