A PHASE SENSITIVE DETECTION CIRCUIT
Introduction
It is
often found out that the signal which we want is smaller or hidden within the
noise. A good example in this case will be data acquisition systems that
measure physical parameters, such as a vector component measurement using
accelrometer or a wireless signal of specific frequency to be unearthed from
bundle of bandwidths. Phase sensitive
detection is a method whereby the signal of interest is "tagged"
by modulating at a frequency much higher than any normal changes Which the
signal might experience. This tagged signal is then later retrieved amidst the unwanted
noise.
A
simple way of understanding phase sensitive operation is to study the circuit
shown in Figure 1. Here, the input
signal is tagged with a modulating sine wave of a certain frequency. This same
modulating frequency is applied to a switch which alternates from sampling the unaltered
wave every odd half cycle to sampling the inverted wave every even half cycle.
The input and corresponding output are shown on the same time axis.
Figure 1
Phase detection circuit
·
For positive going input, the switch is
in position 1 and the unaltered wave is sampled.
·
For negative going input, the switch is
in position 2 and the inverted wave is sampled.
Therefore,
in both cases, the output is positive.Since noise will also be sampled along
the required signal, but over many cycles it will average out to be zero. Essentially
the signal averaging will be done by RC Filter where the time constant is made
larger to average over as many ouput cycles as possible. But the catch is, this
increase has a limit because variations of signal, occuring due to the changes
in experimental parameters, might also be filtered out. Thus, the time constant
must be made as small as possible so that the experimental variations can be
tracked. Suppose the this circuit is used for detection of voltage across a
temperature sensor, immersed in say, a water bath and if one wishes to measure
every 10 oC as the water is heated upto its boiling point and now
suppose the water gets heated 1 degree per minute, then the time constant has to be small enough to
track the changes every 10 oC.
Block Diagram
Circuit description
Figure
2 is the Phase Sensitive Detector
circuit. Since this is a circuit which will enable us to test realtime
environment, we will create some signals as noise for us. The above circuit has
3 inputs for:
·
A sine
wave which will serve as the noise.
·
A
triangle wave which will serve as the signal of interest.
·
A square
wave which will serve as the modulation input.
The
modulation is controlled by an SW06 FET analog switch. The two boxes in the
diagram each represent 1/2 of the entire 16 pin DIP chip. The triangle wave is
input into the SW06 switch. Note that the two inputs (pins 3 and 14) go to the
same output (pins 2 and 15 which are tied together). Note, however, that the
switch is configured such that only pin 3 or pin 14 is connected to the output at
any one time. The modulating signal at pin 16 switches the output from sampling
the triangle wave to sampling ground. Following the first half of the SW06
switch is a 741 op amp configured as a
summing amplifier. It sums the modulated signal of interest and the
unmodulated noise. Following this first 741, is a second 741 op amp configured as a straight inverting amplifier.
Along with this is a wire which samples the non-inverted wave. The inverted
wave must be exactly equal in magnitude but of opposite polarity to the
non-inverted wave and hence the resistors here should have 1% precision.
The
output of this stage is input to the other half of the SW06 switch. This half
of the switch is modulated with the same frequency (and phase) as the other
half switch (note the connection from pin 9 to pin 16). Therefore, the output
is sampled with the same phase and frequency which has been tagged on the
signal of interest. The last stage is the RC low pass filter. Here, as stated
above, the RC time constant should be made small enough so that real variations
in the signal of interest can be tracked. Therefore the RC time constant should
be much smaller than the period of the triangle wave. The modulation frequency
should be high enough to assure filtering of many cycles within one RC
time constant. Therefore, the period of modulation should be much
shorter than the RC time constant.
Procedure
Part 1, 1/2 SW06 switch and summing
amplifier.
Construct
the first two components of the phase sensitive detector circuit, the first 1/2SW06
switch and the summing amplifier. For the three inputs, we will use a 60 Hz, ~
6 Vp-p sine wave for the noise input, a 50 Hz, 2 Vp-p triangle wave for the
signal of interest, and a 700 Hz, 0 to 5 volt square wave for the modulation input.
Your signal to noise ratio is therefore 1:3. Use the same + 12 V supply
voltages for the SW06 switch and the 741 op amps. The datasheet for the SWO6
switch indicates that it will operate at supply voltages down to + 12 V.
To trigger
the oscilloscope, use an external trigger from the triangle wave input. Draw
this portion of the circuit. Measure the voltages at points A and B of the
circuit.
Part 2 - Remaining circuit
Construct
the remaining portions of the phase sensitive detector circuit, save for the RC
output filter. As an initial check, make sure that the voltages you measured at
points A and B in this circuit are still the same. Now, for the RC output
filter, you would like to try to make the RC time constant large enough to
average over many modulation cycles, but small enough to retain your original
input signal. Therefore, the period of the modulation cycle << RC
<< the period of the input signal. This is hard to do since the input
signal period and modulation period are not that much different. Let's increase
the modulation frequency to 10 kHz. Now make RC such that 10Tmodulation
< RC < 0.1Tinput signal. Record your values for R and C.
Attach your RC filter to the output of the circuit. Draw the entire circuit.
Now, using 10 kHz for your modulation frequency, measure points C, D and
finally the output of this circuit on the oscilloscope.
The final output the triangle wave without
the unwanted noise will be observed.
This is how you retrieve a signal buried in
noise. Now, adjust your signal frequency and verify that you may still
produce an adequate output even when the noise frequency is very close to the
signal frequency.
Part 3 is optional test, need not
necessarily be included in the article. It is just testing circuit. If this is not
included, then in the parts list, Lock-in amplifier should not be mentioned.
Part 3, - Actual signal lock in.
To
illustrate the effectiveness of phase sensitive detection, a circuit using a
commercial lock-in amplifier is set up in the other room and diagrammed below
in Figure 3. A light emitting diode
(LED) is powered by a function generator so that the light emitted is modulated
at a frequency of about 1 kHz. At the same time, a photoresistor op amp circuit
is used to detect the light from this LED. The output of this circuit is
monitored on both an oscilloscope and the lock-in amplifier. Also, the function
generator is connected directly to the lock-in amplifier so the lock-in
"knows" what signal frequency it should be looking for.
Figure 3
Lock-in
amplifier set up to measure the output of a light emitting diode. With the
lights out in the room, draw the oscilloscope output. Label both the voltage
and time axes. Determine the amplitude of the wave that corresponds to the
oscillating signal. In
addition,
record the output of the lock-in. Now, switch the lights on. Again draw the
oscilloscope output labeling the voltage and time axes. The lock-in's output
changes appreciably due to the increased noise. To test whether the lock-in
detects LED or not, place your hand in front of the LED and see what this does
to the signal.
Parts required
·
Oscilloscope
·
Appropriate capacitors & resistors for the RC
filter/time constant
·
1 DIP SW06 JFET quad analog switch
·
2 DIP 741 op amps
·
3 function generators
·
Commercial lock-in amplifier such as AD630
The result on a 1996 CRO at my home
No comments:
Post a Comment