Experiment 4: Radiation
1. Introduction
In measuring technology it is a common problem that the desired signals are overlaid
by noise. The noise amplitude can be substantially higher than that of the wanted
signal. Thus a weak desired signal may disappear in the background of the noise
signals. A procedure to reduce or eliminate the noise is the lock–In – technology. The
desired signal can be faded out of noise signals which may even be up to 5 orders of
magnitude higher.
2. Basics
2.1 Lock – In
A Lock-In amplifier is an AC voltage measuring instrument. The low bandwidth is
typical, i.e. only spectral shares with or very near in the measuring frequency are
measured. Therefore, disturbances are suppressed very effectively. The measuring
frequency is given the Lock-In amplifier by a reference signal.
A harmonious alternating voltage to be measured is characterised by amplitude U R
and phase angle  against the reference signal. In addition to the phase angle  the
reference signal can be shifted. All together:
U (t )  U R  cos(t     )
(1.1)
With the addition theorem
cos(a  b)  cos a cos b  sin a sin b
(1.2)
result in
U (t )  U R cos(   ) cos(t )  U R sin(   ) sin(t )  U X cos(t )  U Y sin(t )
(1.3)
with
U X  U R cos(   )
(1.4)
U Y  U R sin(   )
A one channel Lock-In amplifier is measuring UX. A two channel Lock-In amplifier
contains two measuring channels, so that at the same time UX and UY are measured.
Especially comfortable devices can calculate from UR from UX and UY
(UR2 = UX2 +UY2).
As with alternating voltage measuring instruments commonly also the Lock-In
amplifier indicate root-mean-square values (UXeff = UX /2, UYeff = UY /2, UReff = UR
/2).
1
The Lock-In amplifier works after one of the following procedures:
- Rectification of the modulated measuring signal by means of a synchronous
rectifier sensitive to phase (the changeover switch which is synchronised on
frequency and phase of the measuring signal) and next smoothing with a
lowpass filter
- Multiplication of the modulated measuring signal with a sine alternating
voltage
of the same carrier frequency and next smoothing with a low-pass filter
A Lock-In amplifier cannot measure DC voltages. Therefore, DC-signals must be
modulated if necessary. In our experiment the Ir radiation is modulated by a cutting
wheel (chopper). The reference signal comes from a light barrier which is interrupted
by the same chopper. There must be paid attention to the fact that only the
measuring signal itself is modulated, because all additional signals (e.g., scattered
light) are amplified together with the measuring signal.
2.2 Bolometer
Bolometers contain several thermocouples which are connected in row. Their
advantage consists in the fact that they work as almost ideal black receivers, i.e. her
sensitivity in the Infrared–area is nearly steady. Their disadvantage is the long set
time (time constant), in the area between 10ms to 1s.
3. Procedure
In the experiment the infrared radiation of a current-heated silicon carbide – hanger
are measured with a Bolometer. Standard operating voltage for the IR emitter is 8 V.
The current may not cross 2 A. Therefore, put the current limitation to 2 A (short
circuit), before you connect the IR emitter.
3.1 Use of the Lock – In – amplifier
Heat the IR emitter by putting on a voltage of 8 V (current limitation on 2 A). Put the
Bolometer with minimum distance before the IR emitter. Use sensor no.2. Put the
height of the used sensor above the table possibly on the height of the IR emitter, so
that the ray way is in parallel with the table surface.
- Measure the output voltage with the oscilloscope and multimeter
Turn the ray on/off by hand.
Switch on the chopper, and increase the chopper frequency, change afterwards also
the distance of the Bolometer in front of the IR – source.
- Check the measuring signals, and write down the observations
- Draw in each case the temporal change of the sensor signal for chopper
frequency of 0.1 Hz and 10 Hz.
- Measure now with the Lock–In – amplifier:
Lock–In settings: Measuring area 1mV, time constant 1s, signal input:
A, Coupling: DC, Slope: 12db/oct
- Choose a frequency of round 8Hz. Adjust the phase in such a way that the x
component of the output signal becomes zero (simulation of 1-channel
Lock–In – amplifier).
2
- Shift the phase around 90° and read them then the x component. The
y component of the output signal must become zero.
- Measure now the components with any phase situation and calculate the
amplitude.
3.2 Dependence of the output signal versus the chopper frequency
Measure the output voltage with chopper frequency between 0.1Hz and 60Hz. Vary
the frequency in steps of possibly factor 2.
3.3 Dependence of the output signal versus distance of the Bolometer
Set a suitable chopper frequency.
- Measure the output voltage with distances of 10... 100cm between emitter
and
Bolometer. Take up at least 10 value pairs with expressive distances.
3.4 Dependence of the output signal versus the electric power
Set the Bolometer again with minimum distance in front of the IR emitter. Regulate
the voltage at the IR emitter to 0V and wait so long, until the measuring signal
disappears. Increase then the voltage in steps of 0.5V up to 8V. Wait in each case,
until the output signal of the sensor becomes stationary. Measure then the sensor
signal as well as the current which flows by the IR emitter.
3.5 Dependence of the time constants versus the filling gas
Repeat the measurement from 3.2.
- Use sensor 3 (filling gas neon)
- Use sensor 4 (filling gas nitrogen)
Pay attention to the fact that sensor and source are aimed precisely on each other.
3.6 „Unknown sensors“ No.1 and No.5
Repeat the measurement from 3.2.
- Use sensor 1 (filling gas helium)
- Use sensor 5 (filling gas nitrogen)
Pay attention to the fact that sensor and source are aimed precisely on each other.
The orientation marks are right on the rider's foot.
4. Evaluation
4.1 Handling with the Lock-In – amplifier
- Which conclusions do you get from the observations of the measurements with
the oscilloscopes or the multimeter?
- How high are the absolute values of the x- and y-components with adjusted
phase, minimum distance and 8Hz of chopper frequency?
- Estimate the radiation power on the Bolometer by two different considerations.
1. Assume that the electric power is radiated isotropic.
2. Use the sensitivity which is given in the data sheet of the Bolometer
(sensitivity of the Bolometer see /25/, TS-100, filling gas is a krypton)
- To which temperature difference between measuring element and comparative
element does this correspond? (Thermo tension of the used materials see
table 3)
- The IR - signal is modulated by the chopper roughly rectangle-shaped. Which
influence does this have on the measuring result in comparison to for example
sine-shaped modulation?
3
4.2 Dependence of the output signal versus the chopper frequency
Draw the sensor voltage as a function versus the chopper frequency (transference
identity line). (Logarithmic x-y-axis).
- Which transference behaviour shows the Bolometer? (Analogy to filters in the
electrical engineering)
- How high is -3dB – cutoff frequency?
- How big is the time constant? (Comparison with/25/)
- Which statements can be derived from the time constant for the application
of a Bolometer?
4.3 Dependence of the output signal versus the distance of the thermo column
Draw the output voltages as a function of the distance in a twice logarithmic graph.
- Which dependence do you recognise from the graph which physical principle
can you see?
4.4 Dependence of the output signal versus the heating achievement
Draw the output signal as a function of the heating power. Describe the dependence
qualitatively.
4.5 Dependence of the time constants versus the filling gas
Calculate the time constant of the sensors, compare this together, and explain
occurrent differences.
4.6 „Unknown sensors“ No.1 and No.5
Compare the measured thermo voltages to those of the sensors 2 – 4 with same
measuring conditions. Explain available differences? Use for your argumentation the
data sheets in the appendix.
5. References
/20/
/21/
/22/
/23/
/24/
/25/
/26/
Niebuhr, J., Lindner, G.: Physikalische Meßtechnik mit Sensoren.
Oldenbourg Verlag München, Leipzig, 1996
Ehrhard, D.: Verstärkertechnik.
Vieweg Verlag Braunschweig, 1992
Friedrich, W. [Begr.], Rohlfing, H. [Hrsg.]: Tabellenbuch Elektrotechnik,
Elektronik, Dümmler, Bonn, 1993
Manual Chopper (Firma Scientific Instruments)
Manual Lock-In Verstärker (Firma Stanford Research)
Datenblätter Thermosäulen (Firma IPHT Jena)
Tabelle Thermospannungen (Quelle: Friedrich, W.: Tabellenbuch
Elektrotechnik, Fachbuchverlag Leipzig, 1976, S. 149)
6. Appendix
6.1 Manual SR 830
You find prospectus and manual on the experiment place.
6.2 Manual Oszilloskop HM 1507-2
4
You find the manual on the experiment place.
5
6.3 Data sheet and description TS-72
FEATURES
1. High Sensitivity
2. Rugged Construction
3. Low Cost
4. Broad Spectral Response
5. Self-Generating Voltage
6. No Bias Required
7. No 1/fNoise
8. Ambient Temperature Operation
9. High Reliability
10. Hermetically Sealed
11. Excellent long-term Stability
TECHNICAL DESCRIPTION
The model TS-72 sensor is a miniaturised multijunction thermopile made by modern
thin-film technology on Si wafers. It consists of 72 radially arranged junction pairs
formed from evaporated antimony and bismuth thin films. The centrally located active
(hot) junctions comprising the active area of 0.24mm 2 are blackened by metallic
smoke or an interference absorption multilayer System.
The element is hermetically sealed in a small modified TO-5 package under an inert
gas atmosphere. In the case of the metallic smoke absorbing layer the thermopile
sensor’s broad and flat spectral response from UV to far lR is limited only by the
transmission characteristics of the window material selected. Standard window
materials are KRS-5 (0.6 ... 40µm) or band pass filter (8 ... 14µm), other materials on
request. The output e.m.f. of the sensor is proportional to the temperature difference
between the active and the reference junctions. The thermopile requires no cooling
and no bias voltage or current for Operation. It generates no 1/f noise but only the
thermal resistance (Nyquist) noise. The sensor can be used for DC and low
frequency AC measurements. The model TS-72K is a compensated variant of the
model TS-72 with all the same Parameters. It contains additionally a contact
temperature sensor (e.g. a spreading resistance, IC, or thin-film resistor) near the
thermopile reference (cold) junctions for compensation ambient temperature changes
(- 20 ... 70°C) by including this additional sensor in a special external circuit.
6
TYPICAL SPECIFICATIONS TS-72
0.24 (0.5mm )
72
10 ...25
-0.2 ...-0.4
 20nV
12
N2
Krypton
130...150
280
-0.35...-0.55
-0.40...-0.65
15...20
30... 40
8
4x10
7.5 x108
2)
with Standard window KRS-5
Flat from UV to FIR (dependent on Window
Material)
Standard: KRS-5; 8-14µm Filter
Other Materials on Request
83 ° (KRS-5 Window  4mm)
TO-5 Package (modified)
- 20 ... + 70°C
2grms
Active Area
mm2
Number of Junctions
Resistance
kΩ
Resistance TC
%/K
Noise Voltage
nV/Hz1/2
Max. Irradiance
mW/mm2
1)
Filling Gas
DC Responsivity 2)
V/W
Responsivity TC
%/K
Time Constant
ms
2)
D* (500 K, DC)
cm Hz1/2/W
1)
other filling gases : He, Ne, Xe
Spectral Response
Window Materials
Field of View
Case
Operating Temperature
Weight
Window Material useful Spectral
Range (µm)
Transmission
(%)
KRS-5
CaF2
Quartz
Glass
Silicon
BP Filter
LWP Filter
70
90
90
90
 55
 75
 75
0.6 ... 42
0.2 ... 9
0.2 ... 3
0.4 ... 2
1.1 ... 15
8 ... 14
7.2 cut on
APPLICATIONS
12. Non-Contact Temperature Measurements
13. Radiometry
14. Imaging System Requiring Small Spot Size
7
6.4 Data sheet and description TS-100
FEATURES
1. High Sensitivity
2. Rugged Construction
3. Low Cost
4. Broad Spectral Response
5. Self-Generating Voltage
6. No Bias Required
7. No 1/fNoise
8. Ambient Temperature Operation
9. High Reliability
10. Hermetically Sealed
11. Excellent long-term Stability
TECHNICAL DESCRIPTION
The model TS-100 sensor is a miniaturised multijunction thermopile made by modern
thin-film technology. It consists of 100 radially arranged junction pairs formed from
evaporated antimony and bismuth thin films. The centrally located active (hot)
junctions comprising the active area of 1mm2 are blackened by metallic smoke.
The element is hermetically sealed in a small modified TO-5 package under an inert
gas atmosphere. The thermopile sensor's broad and flat spectral response from UV
to far IR is limited only by the transmission characteristics of the window material
selected. Standard window materials are KRS-5 (0.6 ... 40µm) or band pass filter
(8 ... 14µm), other materials on request.
The output e.m.f. of the sensor is proportional to the temperature difference between
the active and the reference junctions. The thermopile requires no cooling and no
bias voltage or current for operation. It generates no 1/f noise but only the thermal
resistance noise. The sensor can be used for DC and low frequency AC
measurements.
The model TS-100K is a compensated variant of the model TS-100 with all the same
parameters. It contains additionally a contact temperature sensor (e.g. a spreading
resistance, IC, or thin-film resistor) near the thermopile reference (cold) junctions for
compensation ambient temperature changes (- 20 ... 70°C) by including this
additional sensor in a special external circuit.
8
TYPICAL SPECIFICATIONS TS-100
1 (1.13mm )
100
18 ...43
-0.2 ...-0.4
 30nV
12
N2
Krypton
60 ...70
100 … 125
-0.35...-0.55
-0.40...-0.65
40...50
70... 100
8
1x10
2 x108
2)
with Standard window KRS-5
Flat from UV to FIR (dependent on Window
Material)
Standard: KRS-5; 8-14 um Filter
Other Materials on Request
83 ° (KRS-5 Window  4mm)
TO-5 Package (modified)
- 20 ... + 70°C
2grms
Active Area
mm2
Number of Junctions
Resistance
kΩ
Resistance TC
%/K
Noise Voltage
nV/Hz1/2
Max. Irradiance
mW/mm2
Filling Gas 1)
DC Responsivity 2)
V/W
Responsivity TC
%/K
Time Constant
ms
D* (500 K, DC)2)
cm Hz1/2/W
1)
other filling gases : He, Ne, Xe
Spectral Response
Window Materials
Field of View
Case
Operating Temperature
Weight
Window Material useful Spectral
Range (µm)
Transmission
(%)
KRS-5
CaF2
Quartz
Glass
Silicon
BP Filter
LWP Filter
70
90
90
90
 55
 75
 75
0.6 ... 42
0.2 ... 9
0.2 ... 3
0.4 ... 2
1.1 ... 15
8 ... 14
7.2 cut on
APPLICATIONS
12. Non-Contact Temperature Measurements
13. Radiometry
14. Imaging System Requiring Small Spot Size
9
6.5 data sheet and description TS-116
FEATURES
1. High Responsivity
2. Rugged Construction
3. Low Cost
4. Broad Spectral Response in the IR
5. Self-Generating Voltage
6. No Bias Required
7. No 1/fNoise
8. Ambient Temperature Operation
9. High Reliability
10. Hermetically Sealed
11. Excellent long-term Stability
TECHNICAL DESCRIPTION
The model TS-116 sensor is a miniaturised multijunction thermopile made by modern
thin-film technology on Si wafers. It consists of 116 rectangularity arranged junction
pairs formed from evaporated antimony and bismuth thin films. The centrally located
active (hot) junctions comprise an area of 0.25mm2. The whole membrane (1.2 x
1.2mm2) supporting the active layers is coated with an interference absorption
multilayer system. The element is hermetically sealed in a small TO-46 package
under an inert gas atmosphere. Standard window material is a band pass filter (8 ...
14µm, other materials on request.
The output e.m.f. of the sensor is proportional to the temperature difference between
the active and the reference junctions. The thermopile requires no cooling and no
bias voltage or current for operation. It generates no 1/f noise but only the thermal
resistance (Nyquist) noise. The sensor can be used for DC and low frequency AC
measurements. The model TS-116K is a compensated variant of the model TS-116
with all the same parameters. It contains additionally a contact temperature sensor
(e.g. a spreading resistance, 1C, or thin-film Thermistors) near the thermopile
reference (cold) junctions for compensation ambient temperature changes (- 20 ...
70°C) by including this additional sensor in a special external circuit.
10
TYPICAL SPECIFICATIONS TS-116
Active Area
mm2
Number of Junctions
Resistance
kΩ
Resistance TC
%/K
Noise Voltage
nV/Hz1/2
Max. Irradiance
mW/mm2
Filling Gas 1)
DC Responsivity 4)
V/W
Responsivity TC
%/K
2
DC Output (38µW/mm )
mV
Time Constant
ms
2)
D* (500 K, DC)
cm Hz1/2/W
1)
other filling gases : He, Ne, Xe
1.44 (1.2mm x 1.2mm)
116
60 ...65
-0.2 ...-0.4
 33nV
12
N2
Krypton
28 ...32
55 … 60
-0.35...-0.55
-0.40...-0.65
2) 3)
0.8
35...40
65... 75
8
1.1x10
2.1 x108
2)
with Standard window 8 … 14µm
3)
500K, 4) KRS-5 Window
Flat from 5.5 ... 10µm
> 90 % from 5 ... 15um
Standard: 8... 14µm Filter
Other Materials on Request
80 ° (KRS-5 Window  2.4mm)
TO-46 Package
- 20 ... + 70°C
<1grm
Spectral Response
Window Materials
Field of View
Case
Operating Temperature
Weight
Spectral Absorbency of the Multilayer System
APPLICATIONS
12. Non-Contact Temperature Measurements
13. Radiometry
14. Imaging System Requiring Small Spot Size
11
6.6 Design
In the experiments application, a single Beam is chopped by the outer row of slots,
and the reference output from the right BNC is used to lock the Lock-In amplifier to a
chop frequency. Note, that inner row of slots could be used, in which case the
reference from the left BNC would be used. In either case, the REFERENCE MODE
switch is in the “up” position.
Detector
TS-Sensors
IR Source
Chopper
Controller
SR 830
Lock-In
f
Reference Input
Chopper Wheel
Abbildung 13: Single Beam Experiment /23/
6.7 Lock–In – amplifier SR510 /24/
The Lock-In Technique
The Lock-In technique is used to detect and measure very small ac signals. A LockIn amplifier can make accurate measurements of small signals even when the signals
are obscured by noise sources which may be a thousand times larger. Essentially, a
Lock-In is a filter with an arbitrarily narrow bandwidth which is tuned to the frequency
of the signal. Such a filter will reject most unwanted noise to allow the signal to be
measured. A typical Lock-In application may require a centre frequency of 10kHz and
a bandwidth of 0.01 Hz This filter has a 0 of 106 - well beyond the capabilities of
passive electronic filters
In addition to filtering, a Lock-In also provides gain. For example, a 10 nanovolt signal
can be amplified to produce a 10V output - a gain of one billion.
All Lock-In measurements share a few basic principles. The technique requires that
the experiment be excited at a fixed frequency in a relatively quiet part of the noise
spectrum. The Lock-In then detects the response from the experiment in a very
narrow bandwidth at the excitation frequency.
Applications Include km level light detection, Hall probe and strain gauge
measurement, micro-ohm meters, C-V testing in semiconductor research, electron
spin and nuclear magnetic resonance studies, as well as a host of other situations
which require the detection of small ac signals.
A Measurement Example
Suppose we wish to measure the resistance of a material, and we have the
restriction that we must not dissipate very much power In the sample. If the
resistance is about 0.1 Q and the current is restricted to 1 µA then we would expect a
100nV signal from the resistor. There are many noise signals, which would obscure
this small signal. 60Hz noise could easily be 1000 times larger, and dc potentials
from dissimilar metal junctions could be larger still.
12
In the block diagram shown below we use a Vrms sine wave generator at a frequency
r as our reference source. This source is current limited by the 1 MΩ resistor to
provide a 1 µA ac excitation to our 0.1 Ω sample.
Two signals are provided to the Lock-In. The Vac reference is used to tell the Lock-In
the exact frequency of the signal of interest. The Lock-In's Phase-Lock Loop (PLL)
circuits will track this input signal frequency without any adjustment by the user. The
PLL output may be phase shifted to provide an output of cos(rt +Φ).
A high gain ac coupled differential amplifier amplifies the signal from the sample
under test. The output of this amplifier is multiplied by the PLL output in the PhaseSensitive Detector (PM). This multiplication shifts each frequency component of the
Input signal, s, by the reference frequency, r, so that the output of the PSI) is given
by:
Vpsd = cos(r t + Φ) cos(s t)
= ½ cos[(r + s ) t + Φ] + ½ cos[(r + s ) t + Φ]
The sum frequency component is attenuated by the low pass filter, and only those
difference frequency components within the low pass filter's narrow bandwidth will
pass through to the dc amplifier. Since the low pass filter can have time constants up
to 100 seconds, the Lock-In can reject noise, which is more than .0025 Hz away from
the reference frequency input.
For signals, which are in phase with the reference, the phase control is usually
adjusted for zero phase difference between the signal and the reference. Maximising
the output signal can do this. A more sensitive technique would be to adjust the
phase to null the signal. This places the reference oscillator at 90 degrees with
respect to the signal. The phase control can now be shifted by 90 degrees to
maximise the signal. Alternatively, since the phase control is well calibrated, the
phase of the signal can be measured by adding 90 degrees to the phase setting,
which nulls the signal.
Understanding the Specifications
The table below lists some specifications for the SR510 Lock-In amplifier. Also listed
are the error contributions due to each of these items. The specifications will allow a
measurement with a 2% accuracy to be made in one minute.
We have chosen a reference frequency of 5kHz so as to be in a relatively quiet part
of the noise spectrum. This frequency is high enough to avoid low frequency “1/f”
noise as well as line noise. The frequency is low enough to avoid phase shifts and
amplitude errors due to the RC time constant of the source impedance and the cable
capacitance.
The full-scale sensitivity of 100nV matches the expected signal from our sample. The
sensitivity is calibrated to 1 %. The instruments output stability also affects the
measurement accuracy. For the required dynamic reserve, the output stability is
0.1%/°C. For a 10°C temperature change we can expect a 1% error.
A front-end noise of 7nV/√Hz will manifest itself as a 1.2nVrms noise after a 10 second
low-pass filter since the equivalent noise bandwidth of a single pole filter is 1/4RC.
The output will converge exponentially to the final value with a 10 second time
constant. If we waft 50 seconds, the output will have come to within 0.70% of its final
value.
The dynamic reserve of 60dB is required by our expectation that the noise will be a
thousand times larger than the signal. Additional dynamic reserve is available by
using the band pass and notch filters.
13
A phase-shift error of the PLL tracking circuits will cause a measurement error equal
to the cosine of the phase shift error. The SR510 phase accuracy will not make a
significant contribution to the measurement error.
Specifications for the Example Measurement
Specification
Full Scale Sensitivity
Dynamic Reserve
Reference Frequency
Gain Accuracy
Output Stability
Front-End Noise
Output Time Constant
Total RMS Error
Value
100nV
60dB
5kHz
1%
0.1%/°C
< 7nV/√Hz
> 10s
Error
1%
1%
1.2%
0.7%
2%
Shielding and Ground Loops
In order to achieve the 2% accuracy given in this measurement example, we will
have to be careful to minimise the various noise sources, which can be found in the
laboratory. (See Appendix A for a brief discussion on noise sources and shielding)
While intrinsic noise (Johnson noise, 1/f noise and alike) is not a problem in this
measurement. Other noise sources could be a problem. Proper shielding can reduce
these noise sources.
There are two methods for connecting the Lock-In to the experiment: the first method
is more convenient, but the second eliminates spurious pick-up more effectively.
In the first method, the Lock-In uses the 'A' input in a 'quasi-differential' mode. Here,
the Lock-In detects the signal as the voltage between the centre and outer
conductors of the A input. The Lock-In does not force A´s shield to ground, rather it is
connected to the Lock-In’s ground via a 10Ω resistor. Because the Lock-In must
sense the shield voltage (in order to avoid the large ground loop noise between the
experiment and the Lock-In) any noise pickup on the shield will appear as noise to
the Lock-In. For a low impedance source (as is the case here) the noise picked up by
the shield will also appear on the centre conductor.
This is good, because the Lock-In’s 100dB CMRR will reject most of this common
mode noise. However, not all of the noise can be rejected, especially the high
frequency noise, and so the Lock-In may overload on the high sensitivity ranges.
Quasi-Differential Connection
The second method of connecting the experiment to the Lock-In is called the “true
differential” mode. Here, the Lock-In uses the difference between the centre
conductors of the A & B inputs as the input signal. Both of the signal sources are
shielded from spurious pick-up.
With either method, R is important to minimise both the common mode noise and the
common mode signal. Notice that the signal source is held near ground potential in
both cases. A signal, which appears on both the A & B inputs, will not be perfectly
cancelled: the common mode rejection ratio (CMRR) specifies the degree of
cancellation. For low frequencies the CMRR of 100dB indicates that the common
mode signal is cancelled to 1 part in 105, but the CMRR decreases by about
6dB/octave (20dB/Decade) starting at 1kHz. Even with a CMRR of 105, a 10mV
common mode signal behaves like 100nV differential signal.
True-Differential Connection
There are some additional considerations in deciding how to operate the Lock-In
amplifier: Dynamic Reserve (DR) is the ratio of the largest noise signal that the Lock14
In can tolerate before overload to the full-scale input. Dynamic reserve is usually
expressed in dB. Thus a DR of 60dB means that a noise source 1000 times larger
than a full scale input can be present at the input without affecting the measurement
of the signal. A higher DR results in degraded output stability since most of the gain
is DC gain after the phase sensitive detector. In general, the lowest DR, which does
not cause an overload, should be used.
The Current Input has a 1kΩ input impedance and a current gain of 106 Volts/Amp.
Currents from 500nA down to 100fA full scale can be measured. The impedance of
the signal source is the most important factor to consider in deciding between voltage
and current measurements.
For high source impedances, (>1 MΩ or small currents, use the current Input. Its
relatively low impedance greatly reduces the amplitude and phase errors caused by
the cable capacitance source impedance time constant. The cable capacitance
should still be kept small to minimise the high frequency noise gain of the current
preamplifier. For moderate source impedances or larger currents, the voltage input is
preferred. A small value resistor may be used to shunt the source. The Lock-In then
measures the voltage across this resistor. Select the resistor value to keep the
source bias voltage small while providing enough signal for the Lock-In to measure.
The Auto-Tracking Band pass Filter has a 0 of 5 and follows the reference
frequency. The pass band is therefore 115 of the reference frequency. The band
pass filter can provide an additional 20dB of dynamic reserve for noise signals at
frequencies outside the pass band. The filter also improves the harmonic rejection of
the Lock-In. The second harmonic Is attenuated an additional 13dB and higher
harmonics are attenuated by 6dB/octave more. You may wish to use the band pass
filter and select a low dynamic reserve setting in order to achieve a better output
stability. Since the processor can only set the band pass filter's centre frequency to
within 1% of the reference frequency, this filter can contribute up to 50 of phase shift
error and up to 5% of amplitude error when R is used. In addition, the band pass filter
adds a few nanovolts of noise to the front end of the instrument when it is in use.
Line Notch Filters should be used in most measurement situations. The filters will
reject about 50dB of line frequency noise (about a factor of 300). If your reference
frequency is one octave away, then these filters will introduce a 100 phase shift error,
and a few percent amplitude error. Their effect on your signal is negligible if your
reference frequency is more than two octaves away. The frequency range of the
SR510 Lock-In amplifier extends from 0.5Hz to 100KHz. No additional cards are
required for the instrument to cover its full frequency range. The SR510 can be used
to detect a signal at the reference frequency or at twice the reference frequency to
allow for convenient measurement of the harmonic of the signal.
Noise measurement is a feature, which allows direct measurement of the noise
density of the signal at the reference frequency. This is a useful feature to assess at
what frequency you should run your experiment.
Output Filters can have one pole (6dB per octave) or two poles (12dB/octave). A
two-pole filter provides a signal to noise improvement over a single-pole filter due to
its steeper roll off and reduced noise bandwidth. Single-pole filters are preferred
when the Lock-In is used in a servo system to avoid oscillation.
In many servo applications, no output filtering is needed. In this case, the SR51 0
may be modified to reduce the output time constant to about 20 µS. Contact the
factory for details.
Ratio Capability allows the Lock-In’s output to be divided by an external voltage
input. This feature is important in servo applications to maintain a constant loop gain,
and in experiments to normalise a signal to the excitation level.
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Computer Interface allows a computer to control and to record data from the
instrument. This is the single most important feature for extending the Lock-In’s
capabilities and it's useful lifetime. Measurements, which are impractical without a
computer become simple when a computer is, used to co-ordinate various parts of
the experiment.
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