LAB 6 – BINARY FREQUENCY SHIFT KEYING (BINARY
FSK) MODULATOR
1. Introduction
Frequency shift keying (FSK) is a relatively simple, low-performance form of digital
modulation. Binary FSK is a form of FSK where the input signal can have only two
different values (hence the name binary). Binary FSK is a constant-envelope form of
angle modulation similar to conventional frequency modulation except that the
modulating signal varies between two discrete Voltage levels (i.e., l’s and 0’s) rather than
with a continuously changing value, such as a sine wave. Binary FSK is the most
common form of FSK. With Binary FSK, the center or carrier frequency is shifted
(deviated) by the binary input signal. Consequently, the output from an FSK modulator is
a step function in the frequency domain. As the binary input signal changes from a logic
0 to a logic 1 and vice versa, the FSK output signal shifts between two frequencies; a
mark or logic 1 frequency and a space or logic 0 frequency. In this experiment, the XR2206 monolithic function generator is used for the FSK modulator.
2. Materials Required
Equipment
1
1
1
1
1
protoboard
dual dc power supply(+l2 Vdc and -1 to +3 Vdc)
medium frequency function generator (100 kHz)
standard oscilloscope (10 MHz)
assortment of test leads and hookup wire
-
-
-
-
Parts List
1
3
1
1
1
XR-2206 function generator
4.7-k resistors
8.2-k resistor
12-k resistor
33-k resistor
1
1
2
2
1
47-k resistor
1-k variable resistor
0.001 F capacitor
1 F capacitors
10 F capacitor
3. Pre-lab
Bring the XR-2206 data sheet to the lab session.
In the Frequency Sweep and Modulation section of the data sheet, find and record the
equation for the theoretical frequency of operation, f, as a function of R, C, RC, and VC.
(Note that the components have different designations in Circuit 1, below.)
In the FSK Generation section of the data sheet, find and record the equation for the
theoretical frequencies, f1 and f2, as a function of R1, R2, C, and the FSK input Voltage.
LAB 6 – BINARY FREQUENCY SHIFT KEYING (BINARY FSK) MODULATOR
4. Circuit 1 – Frequency-Shift-Keyed, Frequency Sweep Mode
In this section, the XR-2206 function generator operating in the sweep mode is used to
produce binary FSK. The Frequency Sweep and Modulation section of the XR-2206 data
sheet discusses this mode. The schematic diagram for the FSK modular circuit used in
this section is shown in Figure 1.
R3 1 K
+12 V dc
4
1 F
16 15
14 13
0.001 F
VCC
2
+1
1
Multiplier/
Sine Shaper
Vout
4.7 K
Mult.
5
3
C1
0.001 F
VCO
11
+12 V dc
6
R1
+
VC
-
9
7
12 K
47 K
XR-2206
Current
Switches
Bypass
8
10
R2
33 K
1 F
4.7 K
GND
12
10 F
4.7 K
Figure 1. FSK Generation, Sweep Mode.
Procedure
IMPORTANT – when you construct this circuit, confine it to half the protoboard. Only a
few components change when you convert it to Circuit 2, and you will re-use Circuit 2 in
Lab 7. Lab 7 requires both an FSK modulator and a phase-locked loop detector.
1. Construct the binary FSK modulator circuit shown in Figure 1.
2. Connect the two Vc terminals to each other and adjust R3 to obtain a sine wave with
minimum distortion at Vout.
3. Measure the frequency of the signal at Vout. This is the unmodulated carrier
frequency, fc. Calculate the theoretical fc, using the design equations in the XR-2206
data sheet. Compare this to your measured data
4. Remove the short across the Vc terminals and apply +1 Vdc (logic 1), and record the
frequency of Vout. This is the output mark frequency (fm).
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LAB 6 – BINARY FREQUENCY SHIFT KEYING (BINARY FSK) MODULATOR
5. Adjust the Vc to -1 Vdc (logic 0), and record the frequency of Vout. This is the output
space frequency (fs).
6. Calculate the theoretical values for fm and fs, using the design equations in the XR2206 data sheet. Compare these to your measured data from steps 4 and 5.
7. Replace the dc bias supply with a function generator, and set the output of the
function generator to a 10 kHz, 2 Vp-p square wave (i.e., ±1 V).
8. Observe the binary FSK waveform at Vout. (It will be necessary to use the single
sweep mode of the oscilloscope as the Vout frequency is not synchronized to that of
the function generator..)
9. Sketch the digital waveform observed at the function generator output and the binary
FSK waveform observed at the output of the modulator (Vout). Be sure that a
transition of the signal generator waveform is about in the center of the waveforms.
10. Describe the relationship between the two waveforms sketched in step 9.
11. Measure the mark and space frequencies, and calculate the modulation index using
the formula below. (To do this, adjust the oscilloscope to trigger on a positive
transition of the signal generator signal, expand the trace so that only a few cycles of
Vout are visible, and measure the mark frequency. Then adjust the oscilloscope to
trigger on a negative transition of the signal generator signal and measure the space
frequency. Be sure to keep the oscilloscope in single-sweep mode. These will
probably be slightly different than the values you measured in steps 4 and 5.)
f  fs
m m
fb
where
m = modulation index (unitless)
fm = mark frequency (Hertz)
fs = space frequency (Hertz)
fb = binary input bit rate (20 kbps)
12. Using a Bessel table, determine the approximate number of significant side
frequencies, and sketch the output frequency spectrum for the FSK waveform
observed in step 7. (The result will be only approximate because the Bessel table
shows the side frequencies when the modulating signal is a sinusoid. Here, the
modulating signal is a square wave.)
13. Determine the minimum bandwidth required to propagate the FSK signal.
14. Determine the baud rate.
15. Vary the function generator output frequency. Describe what effect this has on the
FSK waveform.
16. Measure the mark and space frequencies with the signal generator output set to a 10
kHz, 1-Vp-p square wave (i.e., ±0.5 V) and set to a 10 kHz, 4-Vp-p square wave (i.e.,
±2 V). Describe what effect varying the signal-generator amplitude has on the FSK
waveform.
5. Circuit 2 – Frequency Shift Keying, Timing Resistor Mode
In this section, the XR-2206 function generator operating in the timing resistor mode is
used to generate binary FSK. The FSK Generation section of the XR-2206 data sheet
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LAB 6 – BINARY FREQUENCY SHIFT KEYING (BINARY FSK) MODULATOR
discusses this mode. The schematic diagram for the FSK modulator circuit used in this
section is shown in Figure 2. Note that except for the components connected to pins 7, 8,
and 9, this is the same circuit as Figure 1.
R3 1 K
+12 V dc
4
1 F
16 15
14 13
0.001 F
VCC
2
+1
1
Multiplier/
Sine Shaper
Vout
4.7 K
Mult.
5
3
C1
0.001 F
VCO
11
+12 V dc
6
+
VC
9
-
R1
12 K
7
47 K
XR-2206
Current
Switches
Bypass
8
10
R2
8.2 K
1 F
4.7 K
GND
12
10 F
4.7 K
Figure 2. FSK Generation, Timing Resistor Mode.
Procedure
IMPORTANT – do not dismantle this circuit when you finish the lab. You will use it
again for Lab 7.
1. Construct the binary FSK modulator circuit shown in Figure 2.
2. Set Vc to 0 Vdc and adjust R3 until a sine wave with minimum distortion is observed at
Vout.
3. Calculate the mark and space frequencies using the design equations in the XR-2206
data sheet.
4. Determine the actual mark frequency by setting Vc to +3 Vdc and measuring the
frequency of the waveform at Vout.
5. Determine the actual space frequency by setting Vc to 0 Vdc and measuring the
frequency of the waveform at Vout.
6. Determine the switching Voltage by slowly increasing the magnitude of Vc until the
output switches from the space to the mark frequency. Record the Voltage at which
the frequency change occurs.
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LAB 6 – BINARY FREQUENCY SHIFT KEYING (BINARY FSK) MODULATOR
7. Replace the power supply that provides Vc with a function generator. Set the
amplitude of the function generator output to 3-Vp-p, 10-kHz square wave and adjust
its offset to 1.5 Vdc. This will produce a waveform that switches between 0 and +3 V.
8. Observe the FSK output waveform at Vout. (Use the same method as in step 8 of the
circuit-1 procedure.)
9. Sketch the binary waveform observed at the function generator output and the FSK
waveform observed at the modulator output (Vout).
10. Measure the mark and space frequencies, and calculate the modulation index using
the same method as in step 11 of the circuit-1 procedure.
11. Describe the relationship between the two waveforms sketched in step 9.
12. Adjust the signal-generator frequency to 5 kHz and then to 15 kHz, while keeping the
levels at 0 and 3 V. Describe what effect varying the signal-generator frequency has
on the FSK signal.
13. Adjust Vc to a 10-kHz square wave switching between 0 and 2 V and then to a 10kHz, 4-Vp-p square wave switching between 0 and 4 V. Does changing the amplitude
of the square wave affect the FSK signal?
6. Discussion Questions
1. In Figure 1, what determines the modulation index?
2. In Figure 2, what determines the modulation index?
3. Considering your answers to questions 1 and 2, if the input were a digital signal,
which circuit would be better for an FSK modulator?
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