a project report on
TRANSCRIPT
A PROJECT REPORT ON
STUDY OF SWR METER
&
IT’S CALIBRATION FOR POWER MEASUREMENT
UNDER THE GUIDANCE OF: PREPARED BY:
SHRI O.P.N CALLA (FIE) KAPIL NEGI
& CO-GUIDANCE OF: (MEMBERSHIP NO: ST-433082-5)
SHRI DINESH BOHRA.
OBJECTIVES OF THE STUDY: -
The prime objective of calibrating SWR meter for measuring power is to design a
low cost power meter. Moreover, SWR has a number of implications that are directly applicable
to microwave use, thus by studying about SWR meter and related parameters, we can achieve the
following objectives: -
To gain knowledge about operating principle of SWR meter.
Understanding various microwave parameters such as standing waves, voltage standing
wave ratio, reflection co-efficient, return loss, and mismatch loss.
Methods of measuring VSWR and mismatch impedance.
Study of importance of SWR meter in transmission technology and its utility.
Relation between various parameters and their importance in microwave.
Relation between VSWR and transmitted power.
Calibration of SWR meter for measuring power.
Study of various implications of SWR meter.
METHODOLOGY OF THE STUDY:-
Before calibrating SWR meter for measuring power, first of all we will
understand the basic working principle of SWR meter, methods of taking measurements by
meter, depth knowledge of various transmission parameters such as voltage standing wave ratio,
reflection co-efficient, return loss, mismatch loss, relation among various parameters and
implications of SWR meter in microwave technology.
SWR: - In telecommunications, standing wave ratio is the ratio of the amplitude of a
partial standing wave at an antinode (maximum) to the amplitude at an adjacent node
(minimum).
The SWR is usually defined as a voltage ratio called the VSWR. It is
also possible to define SWR in terms of current resulting in the ISWR, which has the
same numerical value. The power standing wave ratio (PSWR) is defined as the square of
the SWR.
VOLTAGE STANDING WAVE RATIO: - is the ratio of voltage at the highest and
lowest point of standing wave. It is also called as ratio of cable impedance and load
impedance.
VMAX
VSWR = -------------- VMIN
Ei + Er OR VSWR = ------------------- Ei - Er
Where, VMAX = Maximum voltage on the standing wave.
VMIN = Minimum voltage on the standing wave.
EI = Incident voltage wave amplitude.
ER=Reflected voltage wave amplitude.
REFLECTION CO-EFFICIENT: - is the ratio of reflected voltage to the incident
voltage. It is always less than unity because reflected voltage cannot be greater than the
incident voltage.
VREFLECTED
Reflection co-efficient () = --------------------- VINCIDENT
VSWR - 1 Or = ------------------------ VSWR + 1
RETURN LOSS: -is a measure in dB of the ratio of power in incident wave to that in the
reflected wave and always have a positive value. For example if a load has a return loss
of 10dB, then 1/10 of the incident power is reflected. The higher the return loss, the less
power is actually lost.
Return loss= 10 log Pi Pr = -20log Er Ei = -20log (vswr – 1) (vswr + 1)
Where Pi = Incident power.
Pr = Reflected power.
VSWR= Voltage standing wave ratio.
INTRODUCTION OF SWR METER: -
SWR meter (Model VS-411DX) is a high gain tuned amplifier operating at a
fixed frequency of 1 KHz. It is designed primarily for use in making standing wave
measurements in conjunction with a suitable detector and slotted line or wave-guide section. This
may also be used for impedance measurements, relative power level measurements, as a null
detector in bridge circuits and in other applications requiring a sensitive fixed frequency
indicator. It is calibrated directly to indicate SWR directly or dB. When used with square law
devices such as crystal diodes and baratters.
Input circuit of SWR meter is arranged to match various external signal sources,
such as crystal diode, baratter or relatively high impedance devices.
Model VS-411DX has a provision of expanded scale for accurate measurement of
small variations in power levels. Coarse and fine controls are provided for fine adjustment of
amplifier gain to desired convenient value on the meter scale. Output of SWR meter can also be
recorded through a connector provided at the back panel of the instrument.
Model VS-411DX operates on 220vAC/50Hz mains supply.
SPECIFICATIONS: -
INPUT POWER SUPPLY 220VAC, +/- 10%, 50Hz
INPUT CONNECTOR BNC (F)
AMPLIFIER TYPE HIGH GAIN TUNED AT 1000Hz
BAND-WIDTH (3dB) 50Hz
INPUT SELECTOR XTAL HIGH (200K ohm) IMPEDANCE
XTAL LOW (200 ohm)
BOLO 200ohm, 4.5mA BIAS
BOLO 200ohm, 8.7mA BIAS
ACCURACY +/-0.2dB PER 10dB STEP
SENSITIVITY <1MICRO VOLT FOR FULL SCALE
AT XTAL LOW/BOLO POSITION
RANGE OVER 70dB
METER SCALES SWR 1-4, SWR 3-10, EXPANDED
SWR 1-1.13, dB 1-10.
MODE NORMAL/EXPANDED
GAIN CONTROL COARSE ADJUSTMENT 10dB
FINE ADJUSTMENT 2dB
RECORDER OUTPUT THROUGH BNC (F) CONNECTOR
RECORDER OUTPUT LEVEL 1V FOR FULL SCALE DEFLECTION
OF METER.
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DESCRIPTION OF BLOCK DIAGRAM: -
1 KHz signal from detector is fed to the SWR meter through a BNC female connector.
A four position input selector band switch selects the type of input coupling desired to
match various external signal sources such as a crystal diode, baratter, or a relatively high
impedance device to the input of SWR meter.
In Xtal high impedance position, the input signal is directly fed to 60dB step attenuator
stage via a coupling capacitor with reflected impedance of 200Kohms.
In Xtal low impedance & bolo 4.3ma/8.7ma positions, input is fed to attenuator stage
through a coupling transformer tuned at 1khz. This transformer provides a reflected
impedance of 200ohms to the input signal. It also provides over 30db of gain.
In bolo position a dc bias current of 4.3ma or 8.7ma is provided to the detected connected
at the input bnc connector.
The 0-60db gain control band switch controls the 60db step attenuator through a set of
eight relays. The input signal is amplified & band pass filtered in four amplifier cum filter
stages.
The overall gain of the system is also controlled by the coarse and fine gain
potentiometers provided on the front panel.
The output signal from amplifier stages is amplitude detected and fed to meter via output
meter circuit. An auxiliary output (through a bnc connector on the back panel of SWR
meter) is also provided for recording/monitoring the output of SWR meter on a
recorder/oscilloscope.
Normal/expand switch when set to the expand position, applies a dc bucking voltage to
the meter circuit forcing the meter needle to go backwards. The amplifier sensitivity must
then be increased to obtain an upscale reading, which can then be read on the expanded
meter scale.
Note: - It is important to note that 10db change shown on the meter scale actually
corresponds to a 20db change in the input. This has been done to obtain a “square law meter
calibration” on the meter.
In the expand position, a 2db change in meter scale corresponds to 4db change in
the input. The expand mode is provided to make the meter more sensitive to detect minute
changes in the input signal, i.e. While reading very low SWR readings. In this mode full
scale deflection of meter occurs with a step change of 4db in the input (corresponding to 2db
change on the meter reading); while in the normal mode full scale deflection of meter
corresponds to 20db change in the input (corresponding to 10db change on the meter
reading).
The low voltage regulated power supply is generated with the help of a step down
mains transformer and +/- 12 volts regulator circuit.
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SLOTTED SECTION: - The slotted section should cover the desired frequency and be
equipped with an accurate scale or indicator.
DETECTOR: - The detector should be a square law (output proportional to input RF
power) device such as a barratter or a crystal diode operated at low signal level. A
barratter is reasonable square-law detector when used at low signal levels but in general
this is not true in all cases with crystal diodes. However, the sensitivity of crystal
detectors is considerably better than that of barraters. For this reason crystal diodes
detectors are widely used for SWR measurements.
PRECAUTIONS WHEN USING CRYSTAL DETECTORS: - Whenever a crystal
detector with a matched load resistor is used, the input selector switch must be set at the
xtal-200kohm position to obtain an accurate square-law response. With an unloaded
crystal, select the input impedance, which gives maximum sensitivity. Usually the xtal-
200ohm position will give the best sensitivity. However, some crystal diodes may give
higher output in the xtal-200kohms position. Maximum sensitivity is desirable so that the
probe penetration in the slotted line can be kept to a minimum.
Crystal diodes exhibit a departure from the square-law response for which the
instrument is calibrated. This departure tends to occur when the RF power level exceeds a
few microwatts. This corresponds to a reading of approx, full-scale deflection on the 30db
range of the instrument with gain controls set to maximum.
PRINCIPLE OF OPERATION: -
Basically the measurement of SWR consists of setting the probe carriage at a
voltage maximum position and setting the gain of the SWR meter to obtain a reading of 1.0
marking on the SWR scale.
OPERATING PROCEDURE: -
The operating procedures for SWR meter are divided into two classifications: -
a) LOW SWR MEASUREMETNS (10 OR BELOW).
b) HIGH SWR MEASUREMENTS (ABOVE 10).
The step-by-step procedure for making these measurements are as given below: -
a) LOW SWR MEASUREMENTS (10 OR BELOW): -
1. Turn on the instrument; allot a few minutes warm up time for maximum stability.
2. Set the input selector switch for the type of detector that is to be used.
3. Connect the detector cable to the input.
4. Set gain control potentiometers at 12’o clock position.
5. Set the range switch at 30 or 40db position. Adjust the probe penetration to obtain
an up scale reading on the meter.
6. Peak the meter by adjusting the modulation frequency of the signal source. Reduce
probe penetration to keep the meter on the scale.
7. Peak the meter by tuning the probe detector, if tunable. Reduce probe penetration to
keep meter on scale.
8. Peak the meter reading by moving the probe carriage along the line. Reduce probe
penetration to keep meter on scale.
9. Adjust gain controls and or output power from the signal source to obtain exactly
full-scale reading.
10. Move the probe carriage along the line to obtain a minimum reading. Do not retune
probe or detector circuit.
11. Read SWR, which is indicated directly on the instrument scale.
b) HIGHER SWR MEASUREMENTS: -
The straightforward measurement of SWR with conventional methods is
generally applicable when measuring nominal SWR of up to 10, but at higher SWR, special
techniques are desirable.
When the SWR is high, probe coupling must be increased if a reading is to be
obtained at the voltage minimum. However, at the voltage maximum this high coupling may
result in a deformation of the pattern, with consequent error in reading. In addition to this
error caused by probe loading, there is also danger of error resulting from the change in
detector characteristics at higher RF levels.
DOUBLE MINIMUM METHOD: -
In the double minimum method, it is necessary to establish the electrical distance
between the points where the output is double the minimum, as shown below: -
1. Repeat steps 1 through 7 in the low SWR measurement procedure.
2. Move the probe carriage along the line to obtain a minimum reading and note the probe
carriage position.
3. For reference, adjust gain controls to obtain a reading of 3.0 on the db scale. If a linear
detector is being used, adjust gain controls for an indication of 1.5db on the db scale.
4. Move the probe carriage along the line to obtain a reading of full scale (‘0’) on the db
scale on each side of the minimum.
5. Record as d1 and d2, the probe carriage positions at the two equal readings obtained in
step 4.
6. Short the line and measure the distance between successive minima. Twice this distance
is l, the guide wavelength.
The SWR can then be obtained by substituting this distance into the expression: -
SWR= L / (d1 – d2).
Where l is the guide wavelength, d1 and d2 are the locations of the twice minimum
points.
This method overcomes the effect of probe loading since the probe is always set around a
voltage minimum where large probe loading can be tolerated. However, it does not
overcome the effect of detector characteristics.
CALIBRATED ATTENUATOR METHOD: -
Another method for measuring high SWR is to use a calibrated variable RF
attenuator between the signal source and the slotted line. Adjust the RF attenuator to keep the
rectified output of the crystal diode equal at the voltage minimum and voltage maximum
points. The SWR in db is the difference in the attenuator settings.
1. Repeat steps 1 through 7 in low SWR measurements procedure.
2. Move the probe carriage along the line for a voltage minimum, adjust the RF attenuator
to give a convenient indication on the meter and note the RF attenuator setting.
3. Move the probe carriage along the line for a voltage maximum, adjust the RF attenuator
to obtain the same indication on the meter as established in step 2, and note the RF
attenuator setting.
4. The SWR may be read directly in, db, as the difference between the first and second
readings.
While this method overcomes the effect of detector variations from a square-law
characteristic, the effect of probes loading still remains. Always use minimum probe
penetration.
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CHECKING OF SQUARE-LAW RESPONSE: -
The square-law response of either a crystal diode or bolometer is easily checked
with slotted line equipment.
A simple method of calibrating a detector is by increasing the power level in the slotted line in
known steps and noting the detector response on the SWR meter.
Any new crystal being used for the first time should be checked, as there is often a significant
variation between crystals. Data should be taken in both Xtal positions, so that the meter setting
may be determined for any individual crystal diode.
LOCATION OF VOLTAGE MAXIMUM OR MINIMUM: -
It is more desirable to locate the voltage minimum than the voltage maximum
since the effect of probe loading is less at the minimum. However, the location of a voltage
minimum by a single measurement, particularly on low SWR, is usually inaccurate because of its
broadness, thus making the true minimum position hard to determine. An accurate method of
locating the voltage minimum is to obtain the position of the probe carriage at two equal output
readings on either side of the minimum and then averaging these two readings.
PRECAUTIONS WITH SIGNAL SOURCES: -
Signal sources can introduce at least three undesirable characteristics that will
affect slotted line measurements. These include presence of RF harmonics, frequency
modulation and spurious signals.
Signals sources used for standing wave measurements should have relatively low harmonic
content in their output. The standing wave ratio at a harmonic frequency may be considerably
higher than at the fundamental. Spurious frequencies in the signal source are also undesirable,
for, unless very slight, they will obscure the minimum points at high SWR values. The below
figure shows a plot of an SWR pattern made with signal source producing unwanted fm.
Instances are common where the presence of RF harmonics has led to very serious errors in
SWR measurements. Such harmonics are usually present to an excessive degree only in signal
sources that have coaxial outputs. Coaxial pickups of a broadband type will often pass harmonic
frequencies with greater efficiency than the fundamental. In wave guides systems, signal sources
such as internal cavity klystron have a more or less fixed coupling and in addition do not have
pickups extending into the tuned cavity to cause perturbations of the cavity fields. Consequently,
the harmonics problem is generally limited to coaxial systems. Harmonics become especially
troublesome when the reflection coefficient of a load at a harmonic frequency is much larger
than at the fundamental frequency-a common condition. When the harmonic content of the signal
source is high, the reflection coefficient of the load at the harmonic frequency can cause the
harmonic standing wave fields to be of the same order of magnitude as the fields at the
fundamental frequency. Thus, a device having a SWR of 2.0 at the fundamental frequency will
often have a SWR of 20 or more at the second harmonic frequency. If such a device is driven
from a signal source having, say, 15% second harmonic content will be about one fourth the
amplitude of the peaks at the fundamental frequency. Below figure shows a typical pattern
obtained when the RF signal contains harmonics.
STATEMENT OF THE PROBLEM: - To calibrate SWR meter for measuring power.
SETUP BLOCK DIAGRAM FOR CALIBRATING SWR METER: -
METHOD OF OPERATION: -
1. Connect all the equipments as shown in setup block diagram.
2. Set the frequency of signal generator says, at 8.0GHz.
3. Raise the power level of signal generator to maximum value.
4. Set fine & coarse gain controls of SWR meter to maximum so as to provide a gain of
10db.
5. Set the range input selector switch to 30db.
6. Peak the meter to obtain full scale deflection on the meter scale by adjusting modulation
frequency of the signal source, or by reducing probe penetration, so that meter scale
remains on the position marked ‘1’ on the meter scale.
7. Note the power meter reading in dBm for meter scale reading of SWR meter positioned
on marked ‘1’.
8. Reduce the power level and note the position of scale on SWR meter with respect to
power meter reading.
9. When the meter scale reading goes to left of marking ‘3’ on top scale, set the range
switch to next (40db) range and read the indication on the second SWR (3 to 10) scale.
10. Reduce power level in consequent steps and note power meter reading with meter scale
position.
11. Again, when meter scale goes to left of marking 10 on scale, then increase the gain to
50db position. Now read the SWR on the top scale. Note that the range switch is changed
in two steps, so use the top scale; however, all indications on this scale must be
multiplied by 10.
12. Convert SWR meter scale position reading in db.
13. Plot graph between SWR meter scale reading in db and power meter reading in dBm.
14. Similarly, set signal generator at different frequencies, i.e. 8.5GHz, 9.0GHz, 9.5GHz, and
so on.
15. Repeat the same procedure as above.
16. Note reading of the power meter with respect to position of indicator on SWR scale.
17. Plot the graph between SWR meter scale reading in db and power meter reading in dBm
at different frequencies.
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TABLE SHOWING RELATIVE VALUES OF VSWR, REFLECTION CO-
EFFICIENT, RETURN LOSS, & MISMATCH LOSS: -
VSWR RETURN LOSS (dB)
% POWER LOSS
REFLECTION COEFFICIENT
MISMATCH LOSS (dB)
1 INFINITE 0 0 0.00
1.15 23.1 0.49 0.07 0.021
1.25 19.1 1.2 0.111 0.054
1.5 14.0 4.0 0.200 0.177
1.75 11.3 7.4 0.273 0.336
1.9 10.0 9.6 0.316 0.458
2.0 9.8 11.1 0.333 0.512
2.5 7.4 18.2 0.429 0.880
3.0 6.0 25.1 0.500 1.25
3.5 5.1 30.9 0.555 1.6
4.0 4.4 36.3 0.600 1.94
4.5 3.9 40.7 0.636 2.25
5.0 3.5 44.7 0.666 2.55
10 1,7 67.6 0.818 4.81
20 0.87 81.9 0.905 7.4
100 0.17 96.2 0.980 14.1
Readings taken at different frequencies and graphs plotted are shown on the next pages: -
Various implications of SWR meter: -
SWR meter or VSWR meter measures the standing wave ratio in a transmission line.
SWR meter is an item of radio equipment by which we can check the quality of the
match between the antenna and the transmission line.
SWR meter scale can be calibrated for measuring power but since transmitted power
varies a little bit at different frequencies so for calibration of meter scale, average can be
taken.
Graphs plotted between SWR meter scale reading (db) and power meter reading (dBm) at
different frequencies are linear.
VSWR meter should be connected in the line as close as possible to the antenna. This is
because all practical transmission lines have a certain amount of loss, causing the
reflected power to be attenuated as it travels back along the cable, and producing an
artificially low VSWR reading on the meter. If the meter is installed close to the antenna,
then the problem is minimized.
SWR meter does not measure the actual impedance of a load (i.e. resistance and
impedance), but only the mismatch ratio. To measure the actual impedance an antenna
analyzer or other similar RF measuring device is required. Note also that for accurate
readings, the SWR meter must be matched to line impedance.
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