THz Absolute Power& Energy Meter System
Installation and Operation Instructions

Thomas Keating Instruments

July 2014
Software version 3.2.2 (1v4)
Thomas Keating Ltd.
Station Mills, Daux Road
Billingshurst, West Sussex
RH14 9SH, England
Telephone: +44 (0)1403 782045
Fax: +44 (0)1403 785464
http://www.terahertz.co.uk/

Contents
Precautions......................................................................................................................... ii
1
1.1
1.2
1.2.1
1.2.2

Introduction ............................................................................................................ 1
The System ........................................................................................................................ 1
Principles of Operation ..................................................................................................... 1
Positioning......................................................................................................................... 2
Modulation........................................................................................................................ 3

2
2.1
2.2
2.3
2.4
2.5

The Support Unit ..................................................................................................... 5
Setting Up.......................................................................................................................... 5
Software Configuration ..................................................................................................... 7
Calibration ......................................................................................................................... 9
Measurement .................................................................................................................. 10
NEP Estimation ................................................................................................................ 11

3
3.1
3.1.1
3.1.2
3.2
3.2.1
3.2.2
3.3

Stand-Alone Use .....................................................................................................13
Power Measurement ...................................................................................................... 13
Setting Up........................................................................................................................ 13
Calibration ....................................................................................................................... 13
Energy Measurement ...................................................................................................... 14
Setting Up........................................................................................................................ 14
Calibration ....................................................................................................................... 14External Responsivity ...................................................................................................... 15

4

Troubleshooting and Maintenance..........................................................................17

A.
A.1
A.2
A.3

Appendix: Software Installation ..............................................................................18
PC Requirements ............................................................................................................. 18
NI-DAQ ............................................................................................................................ 19
Support Software ............................................................................................................ 22

B.
B.1
B.2
B.3

Appendix: Window Transmission ............................................................................24
Factory Measurements ................................................................................................... 24
Measurement Method .................................................................................................... 24
Broadband Transmission................................................................................................. 26

Acknowledgements............................................................................................................27
Index .................................................................................................................................28iPrecautions
NI-DAQ Software Installation
The supplied NI-DAQ software must be installed before connecting the support unit to a PC.
••

Ensure that you are logged into the PC as an administrator.
Install the NI-DAQ software following the instructions in Appendix A, section A.2.

Film Overload
To avoid damage to the metal film within the power meter head, the signal strength must notexceed the following limits, spread over the 40 mm window aperture:••Continuous wave power: 500 mW
Pulsed energy: 100 mJ at a 5 Hz repetition rate

If the beam power is to be concentrated in an area significantly smaller than the aperture, thesebeam power limits should be correspondingly reduced to avoid local damage of the film. Films
have been damaged by relatively small levels of power applied over very small areas.

ii

1 Introduction
1.1 The System
The TK Terahertz Absolute Power Meter System is an auto-calibrating power meter for use with
free-space beams in the frequency range from 30 GHz to 3 THz.
Components• This manual.
• The power meter head (may be purchased separately) which is a photo-acoustic detector
using a closed air-cell and a pressure transducer. Calibration is provided by ohmic heatingof a thin metal film within the cell.
• The support unit is based upon a National Instruments ‘USB-6211’ data acquisition
module encased in metal box. Additional electronics have been added to provide film
heating for calibration and amplify the signal from the head’s microphone pressure
sensor.
- NI USB-6211 based module- 1 USB cable- 2 BNC cables- 1 DVD containing the NI-DAQ software- 1 CD containing the Power Meter software
Specification• The sensitive area of the system is greater than 30 mm in diameter• The system has a typical NEP of about 5 µW/Hz½• For energy measurement the typical NEJ is about 1 µJ
Software features• Automatic calibration• Frequency dependent window loss correction• A set of routines which trap equipment and operator errors

1.2 Principles of Operation
The power meter system measures power in a beam directed into the head window. The headcomprises a closed air-filled cell formed by two closely spaced parallel windows with a thin metal
film in the gap between them. A known fraction (close to 50%) of the power of the incident beam
is absorbed in the metal film, the remainder being partly transmitted through the cell and partly
reflected from it.
1

The beam must be 100% amplitude-modulated either at its source or by a chopper placed in thepath of the beam. The modulation frequency is to be in the range 10 Hz to 50 Hz, and the lowest
NEP is found in the middle of this range. Ideally, this modulation should be square-wave as thecalibration process is based upon a square-wave modulation. The absorption of the power in thefilm produces modulated variations in the temperature of the film and the layers of air in contact
with it. In turn, this produces a modulation of the pressure in the cell, which is detected by apressure-transducer, measured by the lock-in amplifier and displayed on the PC. The modulatedpressure change is closely proportional to the total absorbed power; it depends negligibly on the
distribution of the absorbed power over the film.
The power meter can be calibrated by dissipating a measured amount of ohmic power in the film.
A modulated current is passed through it via electrodes that contact opposite edges of the film.
As shown in Figure 1.1 the head unit provides two independent connections to the electrodes toallow the applied voltage to be sensed accurately. To calculate the power produced, theresistance of the film, ilm , must be known. This should be measured using a digital multimetervia one of the two BNC coaxial cable connectors on the side of the power meter head.
The absorption of the window is close to zero at 100 GHz but increases with frequency — see
appendix B. The support software includes a lookup table giving the window-loss as a function of
frequency based upon TK’s laboratory measurements.

1.2.1 Positioning
The power meter head should be placed in the beam where the beam width is small enough to beclearly accommodated within the aperture of the window. This may require the use of a lens or
mirror to condense the beam to a beam-waist at the power meter head.
To make power measurements which are free of frequency-dependent calibration errors causedby reflections from the windows, the power meter head should be used at the Brewster angle.
There are no reflections from the window if the power meter head is set so that the beam isincident at the Brewster angle (55.5°) with the plane of polarisation in the plane of incidence, as
shown in Figure 1.2. That is to say the angle between the beam axis and the normal to thewindow should be 55.5 ± 5°.

2

Figure 1.1: Sketch of the power meter head identifying the BNC connectors and window
aperture.

If the E-field
field of the beam is in the horizontal plane the meter can simply stand on its base;otherwise
erwise an appropriate support wedge for the meter will be required. If the beam is not
linearly polarised it is necessary to use a wire-grid
grid polariser in the beam and measure separately
the power in the two orthogonal linearly polarised signals.

1.2.2 Modulation
The amplitude modulation of the beam power should have a square/rectangular envelope. The
sensitivity of the measurement is optimal for a 50% duty cycle and for this reason the supplied
software assumes this is the case when calculating the beam power. The
he power displayed willthen show the power when the beam is on.
If the source of the beam can be switched on and off electrically within about 1 ms, good squaresquarewave modulation at 10–50
50 Hz can be produced at source. If the source cannot be modulated inthis way, a mechanical modulator in the form of a rotatingrotating bladed shutter (e.g. a chopper)
chopper shouldbe placed in the beam path. To approximate square-wave modulation the width of each blade in
the chopper would have to be much larger than the width of the beam at the modulator. In
practice, the need to limit the size of the modulator will result in some deviation from the idealsquare-wave form, and the power meter will tend to underestimate the signal power. To assess
the level of error that might be incurred,incu
an extreme case can be considered, namely modulationof sinusoidal form with a peak value corresponding to the unobstructed beam (i.e. the beam
power). In such a case, the power meter will underestimate the beam power by 26%. In practice,of course,, the modulation will usually be much closer to the pure square-wave and the error
incurred very much less.
If a mechanical chopper is being used, care should be taken to ensure that it does not modulate
infrared and visible light. The head windows transmit
it infrared and visible light and the power
meter will detect any changing or modulated power at these wavelengths incident upon it. The
head will also respond to air pressure fluctuations at the reference frequency, perhaps generated
by the modulator blades. To reduce false signals of these kinds it may be necessary to introduce abaffle at the modulator with an aperture in it to pass the beam. Black polyethylene across thewindow aperture can also be used to reduce significantly IR and visible radiation entering the3

head. The reduction in signal level from the power meter when a second, similar, sheet is added
will provide a measure of the attenuation in the first sheet, so that a correction can made to thedisplayed power to compensate for the attenuation.
attenuat

Figure 1.2:: Sketch showing the head aligned at the Brewster angle to the incoming beamin order to minimise reflections from the window.

42 The Support Unit
2.1 Setting Up
1. Following the NI-DAQ
DAQ software installationins(see appendix A, section A.2),
A.2 connect the USB
port of the support unit to your PC, and after a few seconds a green LED light should start
to flash on the hardware.
2. A message (Figure 2.1)
2 may appear on the screen indicating that the data acquisition
device has been detected.
3. To configure and test the device, open the NI-DAQ
NI
configuration application see (Figure
2.2):• NI MAX – in All Programs in the MS Windows Start menu or the
National Instruments subdirectory) or• Measurement
easurement & Automation Explorer – for older versions
version of LabVIEW.
4. In the tree view at the left hand side of the window, open Devices andInterfaces (then NI-DAQmx Devices if it exists). This should include an entry
for the NI USB-6211
6211 device, showing that the drivers have been successfully installed.
5. The support software will, in its default configuration, expect the device connected to thethpower meter to be named ‘Dev1’.
‘
If necessary, it can be renamed at this point in thecentral panel of the window or by opening the right-click menu.

Figure 2.1: Pop-up
up window on device detection.

5

Figure 2.2: Top: NI-DAQ
DAQ Measurement & Automation Explorer.
Explorer. Bottom: NI MAX
application. Both show menu options to rename a device and to perform a self test.

6. Connect the source of modulation (chopper or electrical) to the BNC connector labelled
REFERENCE.. The reference voltage should be TTL, that is a square wave from 0 V to +5
V. The lock-in amplifier will work with lower voltages but is unable to lock into signals lessthan 1 V.
7. Using one of the BNC cables supplied, connect the pressure transducer BNC output on the
top of the power meter head to the BNC connector labelled SIGNAL,, as shown in Figure
2.3.. The second cable should run from one of the two film heating connections on theside of thehe power meter to the BNC cable labelled HEATER.

6

Figure 2.3:: Schematic of the apparatus: connecting the TK power meter head, thesupport unit and the source of frequency modulation.

2.2 Software Configuration
The measured value of the film resistance needs to be entered and saved in the configuration file
before proceeding with measurements. This can be done through the following steps:
1. The power meter software is started by selecting All Programs → TKI → TKI THz
Power Meter 1v4 from the start menu. This brings up the power meter front panel as shownin Figure 2.4.
2. The software starts automatically in idle mode.
3. Erase the name of the configuration file, PM TK-07 config
fig dev1.ini,
dev1.ini from the input
box indicated by an arrow in Figure 2.4.

7

Figure 2.4:: Support software main window. The arrow indicates the location of theconfiguration file name. The
Th signal frequency input is circled.

4. Click the Initialize button. Doing this with no file name specified causes the configuration
window to pop up as shown in Figure 2.5. The configuration file includes pre-calculated
calculated values ofresistances, voltages and timings for the system.
5. Adjust the value of film resistance, labelled R_total (indicated by a red circle in Figure 2.5), tomatch the measured value, ilm . If measurements will typically be made at one signal frequency,
this can be saved in the configuration file (as Freq [GHz])) to avoid having to re-enter it in themain window whenever the software is started.
6. The window transmission as a function of frequency is read from a data file stored in the‘para’ directory within the support software installation directory (further information andupdates can be found on the TH
Hz Power Meter page on the terahertz.co.uk web
b site). The nameof the file can be specified in the box shown by the arrow in Figure 2.5.. The format of the file istab-separated
separated with DOS line termination (CRLF), where frequencies in GHz are given in the firstcolumn and transmission values in the second. The software interpolates between the given data
points.

8

Figure 2.5: Support software configuration, with the film resistance option circled. The
name of the window transmission data file is indicated by the arrow.

7. Click on the save button and choose the filename of the existing .ini file. Then click Replace
to confirm.
8. Click EXIT to close the configuration window.
9. Restarting the software ensures that the required value of the film resistance is made available
for subsequent measurements.
Note: The software need only be configured
conf
once. For subsequent runs of the software you need
only load the desired configuration file (default is PM TK-13
13 config dev1.ini)
dev1.ini by clicking
on Initialize from the menu button.

2.3 Calibration
When the program is running without any power incident on the film, a view similar to that in
Figure 2.4 is seen when the Power Meter tab is selected. Note that before calibration, power
readings give negative and haphazard profiles of measurement. The device automaticallyautomatcalibrates before measuring for the first time, thereafter the measurements tail off to zero.
Manual calibration can also be activated by clicking the Calibrate button located above themeasurement reading on the left hand side of the panel.
••

Ensure that the power meter head is switched on, with no signal beam incident upon it.
Check that the modulation reference is switched on and set to the desired chopping
frequency.
9••Enter the frequency of the radiation to be measured at the lower left of the window(circled in Figure 2.4).
For the best performance, the system should be warmed up for at least half an hour, toallow the lock-in amplifier to stabilise.

Calibration proceeds by first monitoring the head output when no signal is present. Next a known
square wave of ~ 0.8 V amplitude is injected into the film at the reference modulation frequency.
The calibration signal and the head’s response to it can be viewed in the Signal & FT tab,and should resemble that shown in Figure 2.6. During this period no THz signal should — ofcourse — be presented to the head. The calibration procedure takes up to 3 minutes. If thecalibration is unsuccessful the stability of the modulation reference signal should be checked.
After calibration the screen displays the power detected by the head, adjusted for window loss atthe specified signal frequency. Proper measurements can then commence. Small deviations of
the reference frequency away from that used for calibration are displayed as ∆Fref[%]. A chartat the right tracks power levels as a function of time.

Figure 2.6: The signal and Fourier transform display during the second stage of acalibration.

2.4 Measurement
To begin measurement, click the Measure button at the top left of the main window. A numberof controls are available with which to adjust the performance of the instrument.
•

10

A number of buttons control the behaviour of the system. In addition to those alreadydescribed,
Exit halts the program.

•
•

•

•

Save data produces a text file of the chart history including the film resistance,
frequency of measurement and power versus time data. This can be useful in checking
power stability over time.
The control directly below the frequency error display allows the power display to bechanged between microwatts and milliwatts.
The Time Const [s] control, located beneath the chart, changes the digital
integration time used for the values displayed on the meter and the chart. The software
performs a decaying average on the train of measurement values. Increasing the time
constant extends the time. The default integration time is 1 s. To alter this, simply dragthe slider. If a very long time constant is required, the upper limit of the slider’s range canbe clicked and edited.
The Offset [uW] slider, also located beneath the chart, allows an offset to be applied
to the measured data. The value can be adjusted by dragging the slider or by editing thenumber in the input box.
For the chart,– Clicking the secondary mouse button on the chart brings up a menu enabling theuser to control the appearance of the chart.
– The Clear graph button below the graph allows it to be reset.
– If AutoScale Y has been disabled in the chart menu, the power range of thegraph can be changed by clicking the scale limits to edit them.

The software also runs a full set of error trapping routines and will indicate faults such as the lossof a lock-in reference signal or a significant change in the modulation frequency supplied as areference to the lock-in amplifier. This frequency must remain constant during a measurement
session, as the responsivity of the head is dependent upon it. To correct this error the power
meter must be calibrated at the new frequency or the reference frequency must be changed backto that used in the initial calibration. The warning is triggered by a 10% drift in frequency,
corresponding to a change in responsivity at the 10% level — for very accurate work, thefrequency deviation display should be monitored for smaller changes.

2.5 NEP Estimation
The software constantly displays an estimate of the Noise Equivalent Power (NEP) of the power
meter system in the status area to the lower left of the main window. Values of about 10 µW (1 ×
10−5 W) are acceptable. Note that the noise on measurements at high signal frequencies will behigher than the instrument’s NEP by a factor of the window transmission.
The NEP of the power meter system can also be determined manually by looking at the lower
trace on the Signal & FT tab. This displays the noise in the system as a function of thefrequency multiplied by the root of Equivalent Noise Bandwidth (ENBW). The meter at the left of
the window shows the modulation frequency. A guide to the NEP can be found — in the absenceof signal — by looking close by (but not at) the modulation frequency. A precise value requires
division by the square root of the ENBW (lower right of the display, see Figure 2.7).

11

Figure 2.7: The signal and Fourier Transform window, with the Equivalent Noise
Bandwidth (ENBW) indicator circled.

12

3 Stand-Alone Use
3.1 Power Measurement
3.1.1 Setting Up
The output voltage from the power meter is measured using a lock-in amplifier or spectrum
analyser. In the case of a lock-in amplifier, its reference input should be connected to the sourceof signal modulation. The voltage measured will be proportional to the incident power.

3.1.2 Calibration
To calibrate the head, modulated ohmic power is generated in the metallic film. A square-wave
current supply is required with a DC offset which can be adjusted so that the current alternates
between zero and a constant value which produces about 1 mW of ohmic power in the film. The
frequency of the square-wave should be the same as the intended signal modulation frequency.
The current is passed through the film via one of the BNC cable connectors and the potential
difference across the film is measured via the other as shown in Figure 3.1. The current supply
and the DC offset are adjusted so that the voltage across the film alternates between zero and avalue near 10 ilm which gives a current-on ohmic power close to 1 mW. The potential
difference across the film in the current-on periods, , is then measured and the ohmic power = 

ilm

(3.1)

can be calculated. Note that this is the (peak) power when current is flowing, not the mean
power.
The wave-form of the output will be a distorted square wave because the power meter head has
a limited response time. The type of instrument used to measure the output voltage will
determine whether the value recorded, cal, is the RMS value, the amplitude of the fundamental
component, the RMS value of the fundamental component, or some other measure of the output
magnitude. The calibration is therefore to be understood as being specific to the use of the power
meter head with that particular instrument type, at a particular modulation frequency.
The value of the internal responsivity, in units of V/W, of the power meter, pi , is then determinedfrompi = cal =cal ilm
 

(3.2)

Example13Film resistance ilm = 200 W
Voltage applied= 0.8 V
Calibration power  = 0.82 / 200 = 3.2 mW

Figure 3.1:: Circuit for calibration of the power meter head. An oscilloscope or otheropeak voltage measuring device attached to the second film connection allows an
accurate measurement of the applied voltage, .

3.2 Energy Measurement
The head’s time constant is a few milliseconds. Therefore the form of a pulse of radiationradiwhich isshorter than a millisecond has no effect on the response of the head to the pulse. In other words,
the head sees the pulse as a Dirac delta function.
The head can therefore be used to measure energy in pulsed beams whose pulse width is amillisecond
llisecond or less and whose pulse repetition rate is lower than 40 Hz. The head’s sensitivity(NEJ) is of the order of 1 µJ.

3.2.1 Setting Up
The head should be placed in the pulsed beam in the same manner as for continuous wave power
measurements. The signal from the BNC microphone output should be fed to a digital storage
oscilloscope or similar storage device.
Triggering for such a device can often be taken directly from the head’s output when large signals
are present. If the signal energy is close to the NEJ, however, triggering directly from the source
may be necessary.

3.2.2 Calibration
Calibration is performed in a similar way to that for power measurement — via ohmic heating of
the metallic
allic film. An electrical pulse generator is connected to the film as shown in Figure 3.2.
The pulse voltage should ideally be monitored via the second film connection to avoid possible
voltage drops across the source output impedance
impe
or BNC cables. The electrical energy dissipatedin the film is given by
 =

14

 

ilm

(3.3)

where  is the length of the pulse. This should be less than 1 ms and about 250 µJµ of energy
should be generated for calibration.

Figure 3.2:: Circuit for calibration of the power meter head as an energy meter.

Figure 3.3:: Sketch of the response of the power meter head to a pulse of radiation.

The storage oscilloscope should then show a response from the head similar tot that given in
Figure 3.3. The peak output voltage cal (or alternatively the area under the distorted curve) isthen measured using the storage oscilloscope. The value of the internal responsivity, in units of
V/J, of the power meter, ji , is then determined fromcal ilm ."#$ %& $#.#$ %&

 8.66 Vji  cal 
ExamplePulse length  = 0.5 ms
Film resistance ilm = 150 W
Energy required  = 250 µJ
Voltage required =   ilm  (3.4)

Note: The voltage to be used in Equations 3.3 and 3.4 in calculating the calibration power,  ,should be that measured across the film. The output impedance of the signal generator may
cause this to be less than that measured across the generator alone when it is does not have todrive a current.

3.3 External Responsivity
The internal responsivity, pi or ji , can be converted to an external responsivity, e , by making
allowance for the following factors:

15

•

•

Absorption of the film
The absorption of the metal film is 49 ± 2% throughout the submillimetre range — seesection 4.
Transmission of the window
The transmission, +, of the TPX window at the Brewster angle falls from 100% below 100
GHz to 53% at 3 THz — see appendix B.

A measured signal can thus be converted to an absolute power or energy by evaluating

or

16

=

lock-in.012×3pi
peak-or-area.012×3ji

(3.5)
(3.6)

4 Troubleshooting and MaintenanceFilm resistance
Periodic measurement, say, once every six months, of the power meter head’s film resistance isadvisable, to check that the film has not deteriorated or been damaged by excess incident beam
power. The calibration accuracy of the system is based upon a good knowledge of ilm . If theresistance changes by more than 5%, Thomas Keating Ltd. should be informed. A film resistancein the range 110 to 230 Ω is acceptable. Theoretical analysis indicates an absorption rising from
48% when ilm = 110 Ω to 50% at 160 Ω and returning to 48% for 230 Ω. Measurements on such
films confirm 49 ± 2% absorption throughout the submillimetre range.

Battery
A 9V PP3 type battery powers the pressure transducer and lies in the top section of the power
meter head. The transducer draws very little current and will supply power over many weeks offull time operation. However the battery should be replaced, say, once a year. This can be doneby removing the cover on the side of the top section which is held by four Philips type screws.
Replace the battery and refit the cover.

Lock-in failure
If there is a time-out error when the software goes through the calibration cycle, one check thatshould be made is whether the level of the reference voltage is sufficient.

Full scale
The maximum power that can be measured without reaching full scale deflection is 200 mW. If
you wish to measure power higher than this, contact Thomas Keating Ltd. for advice.

17

A. Appendix: Software Installation
A.1 PC Requirements
About 1.1 GB of free space is needed for the installation of the NI-DAQ software and the compiled
power meter program. Microsoft Windows 2000, XP, Vista, 7 or 8 should be installed as theoperating system.
If the user is inexperienced in installing peripherals and configuring PC’s, it may be useful toobtain advice locally. If this is not available, Thomas Keating Ltd. would be pleased to advise.

18

A.2 NI-DAQ
National Instruments’ NI-DAQ software is on two auto-run CDs supplied with the board andprovides the interface between the A/D hardware and the compiled LabVIEW power meter
application.
•

On inserting the first DVD data disk, the menu shown in Figure A.1 should appear.

Figure A.1 NI-DAQ CD menu.

•••

Click Install NI-DAQmx. The software must be installed using a user account withadministrator access, which the software may request.
The next two installer pages allow the target directory (Figure A.2) and installation
options (Figure A.3) to be chosen. Select the Typical installation option if the TK Power
Meter. Throughout the installation, the Next button in the window must be selected toprogress to the next stage.
Continue through the installer questions, shown in Figures A.5 to A.6. If you are unsure
whether or not to allow the National Instruments software to access the Internet (FigureA.5), e.g. for updates, then you should contact your IT support. The licence agreements
must be accepted for the software to be installed but you should make sure that you arefamiliar with them.

Figure A.2 Destination directory choice.

19

Figure A.3 Installation-option selection.

Figure A.4 Product notifications – messages update.

Figure A.5 NI-DAQmx licence agreements.

20

Figure A.6 Microsoft Silverlight licence agreements.

Figure A.7 Installation summary prior to start of installation.

••

Click Next at the summary page (Figure A.7) to begin installation and a window
containing two progress bars will appear whilst the software is being installed (Figure
A.8).
After the installation is complete (Figure A.8) the PC might need to be rebooted (FigureA.9). This ensures that the drivers will be available when the support unit is connected to
the PC.

21

Figure A.8 Completion window.

Figure A.9 Reboot request.

A.3 Support Software
The power meter support software is included on the CD supplied with the Support Unit.
••

When the CD is inserted, the installer should start automatically. Administrator-level
access may be required to install this software.
The panel appears (Figure A.10) that allows the destination directory to be chosen.

Figure A.13 Installation directory choice.

22•A second panel requests you to confirm that the installation should start.

•

Progress bars will be shown during the installation, which should be rapid because thesoftware package is relatively small. A final panel will be shown when the installation isfinished (below).

The CD also contains a copy of the software in the archive file PM13_inst_20140613.zipand a copy of this manual in PDF format. These can be used as an alternative to the automatedinstaller by right-clicking the ZIP file and using the Extract all command.

23

B. Appendix: Window Transmission
B.1 Factory Measurements
The transmission of the TPX window has been measured from 100 GHz–3.5
GHz 3.5 THz, with preliminary
data to 10 THz. The transmission is plotted in Figure B.1. The support software includes a filegiving the data shown in Table B.1, and calculates the transmission coefficient
coefficient for a given signal
frequency by linear interpolation of this data.

B.2 Measurement Method
It is possible to use the power meter to make measurements of the window transmission at aparticular frequency. Users who wish to do this should request a calibrating TPX window from
Thomas Keating Ltd., which will be supplied free of charge. This window is a copy of the windowof the power meter head, with identical thickness.

Figure B.1 Window transmission as a function of frequency.
fre

24Frequency
Ν (GHz)
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
3200
3300

Transmission
T
1
0.975
0.95
0.91
0.9
0.88
0.86
0.84
0.81
0.78
0.75
0.73
0.7
0.68
0.66
0.64
0.63
0.6
0.59
0.58
0.56
0.55
0.54
0.53
0.528
0.53
0.529
0.534
0.535
0.537
0.534
0.531
0.519
0.516

Frequency
Ν (GHz)
3400
3500
3600
3700
3800
3900
4000
4100
4200
4300
4400
4500
4600
4700
4800
4900
5000
5100
5200
5300
5400
5500
5600
5700
5800
5900
6000
6100
6200
6300
6400
6500
6600
6700

Transmission
T
0.514
0.512
0.507
0.503
0.498
0.491
0.478
0.461
0.444
0.434
0.437
0.444
0.447
0.449
0.45
0.452
0.452
0.45
0.444
0.435
0.424
0.412
0.399
0.386
0.371
0.352
0.328
0.311
0.293
0.275
0.255
0.236
0.215
0.195

Frequency
Ν (GHz)
6800
6900
7000
7100
7200
7300
7400
7500
7600
7700
7800
7900
8000
8100
8200
8300
8400
8500
8600
8700
8800
8900
9000
9100
9200
9300
9400
9500
9600
9700
9800
9900
10000

Transmission
T
0.179
0.167
0.16
0.157
0.155
0.153
0.149
0.146
0.146
0.152
0.155
0.154
0.152
0.151
0.147
0.139
0.13
0.121
0.108
0.096
0.091
0.089
0.095
0.111
0.127
0.144
0.157
0.159
0.168
0.168
0.175
0.17
0.174

Table B.1 Window transmission look-up table.

25

The meter is set up as outlined in the standard instructions, at the Brewster angle to the source
under test. If the support software is being used, the frequency parameter should be set to 0
GHz, where the tabulated transmission value is 1. A power measurement, P0, should be made.
Then with the additional window inserted in the beam, close to the head, and also at the Brewster
angle, a second power measurement, P1, is taken. A reduction of the measured power should beobserved, and power values both with and without the additional TPX window recorded. The ratio
5=

$

= 
2
6

(B.1)

can then be used to multiply a measured power (with the software configured for 0 GHz) toobtain the absolute power in the beam.
absolute = 5measured(B.2)

For more convenient operation, measured window transmission values can be stored in a data file
to be read by the support software. This file is formatted and loaded into the software as
described in section 2.2.
ExampleWithout extra window  = 7.3 mWWith extra window $ = 3.2 mW‘Window loss’ calibration factor 5 = 2.28
Window transmission + = 0.438

Bandpass filters
Should a bandpass/lowpass filter be use to exclude frequencies from being detected by the power
meter, an additional calibration factor will need to be introduced, being the reciprocal of thefilter’s transmission at the desired frequency of operation.
ExampleTransmission of the filter +F = 0.83
‘Filter loss’ calibration factor +F $ = 1.205
Combined calibration factor 5 = = 5 × +F $ = 2.75

B.3 Broadband Transmission
When the signal bandwidth is very large, the window transmission may vary significantly over theband. Denoting the signal power spectrum as >(@) the average transmission can be expressed as
E(F)2(F)GF
D$+B = C = band E(F)GF
Dband

26

(B.3)

Acknowledgements
All manufacturers’ trademarks are acknowledged. Thomas Keating Ltd. would like to thank Prof.
Derek Martin of QMW, University of London, who devised the photo-acoustic techniques used in
the Power Meter, and Dennis Moss of the UK National Physical Laboratory for advice in the designof the System.
The LabVIEW support software was developed jointly by Thomas Keating Ltd. and Scientific
Consulting.
Sci-Consulting Sàrl
78 ch. du Bochet
CH-1025 St-Sulpice
Switzerland
Telephone: +41 (0)21 697 0761
Fax +41 (0)21 697 0762
http://www.sci-consulting.ch/27

IndexBandwidth, 30
Battery, 20
Brewster angle, 6
Broadband, 30
Calibration, 12energy, 17
external responsivity, 19
power, 16
Configuration, 11
ENBW, 14Filmabsorption, 19
measured, 6
resistance, 20
Frequency
deviation, 13, 14
modulation, 6
range, 5
signal, 11, 13

28

Lock-in amplifier, 6, 9, 16
NEJ, 5NEP
measurement, 14
typical, 5Powerfull scale deflection, 20
maximum, 4Pulse, 17maximum energy, 4Save, 14Storage oscilloscope, 17
Triggering, 17
Windowreflection, 6transmission, 5, 11, 28