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YEDITEPE UNIVERSITY ENGINEERING FACULTY

COMMUNICATION SYSTEMS LABORATORY

EE 354 – COMMUNICATION SYSTEMS

EXPERIMENT 6: FSK & 100 kHz RX/TX ANTENNA

Objective:

Generation and demodulation of FSK modulated signals. Using 100 kHz Rx/Tx

antenna for digital data and voice transfer.

Equipment:

Sequence Generator Module

Adder Module

Variable DC Module

VCO Module

Bit Clock Regen Module

Tunable LPF Module

Multiplier Module

Phase Shifter Module

Audio Oscillator Module

Dual Analog Switch Module PCM Encoder

Master Signal Module 100kHz RX Antenna Utilities

Headphone Amplifier

Speech Module

Oscilloscope

General Information:

FSK:

A Frequency Shift Keyed transmitter has its frequency shifted by the message.

Although there could be more than two frequencies involved in an FSK signal, in this

experiment the message will be a binary bit stream, and so only two frequencies will be

involved. An FSK waveform derived from a binary message is shown in Figure 6.1.

Figure 6. 1 An FSK Waveform, Derived From a Binary Message

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Conceptually, and in fact, the transmitter could consist of two oscillators (on

frequencies f1 and f2), with only one being connected to the output at any one time. This is

shown in Figure 6.2.

Figure 6. 2 An FSK Transmitter

Unless there are special relationships between the two oscillator frequencies and the

bit clock there will be abrupt phase discontinuities of the output waveform during transitions

of the message. In practice, f1 and f2 are integer multiples of the bit rate. This leads to the

possibility of continuous phase. FSK signals can be generated at baseband, and transmitted

over telephone lines (for example). In this case, both f1 and f2 would be audio frequencies.

Alternatively, this signal could be translated to a higher frequency. Yet again, it may be

generated directly at ‘carrier’ frequencies.

Demodulation of FSK:

There are different methods of demodulating FSK. A natural classification is into

synchronous or asynchronous. Close looks at the waveform of Figure 6.1 reveals that it is the

sum of two ASK signals. The receiver shown in Figure 6.3 takes advantage of this.

Figure 6. 3 Demodulation by Conversion-to-ASK

The FSK signal has been separated into two parts by band pass filters (BPF) tuned to

the MARK and SPACE frequencies. We are not free to choose parameters – particularly

frequencies. If they are to be tuned to different frequencies, then one of these frequencies

must be 2.083 kHz (defined as the MARK frequency). This is a restriction imposed by the

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BIT CLOCK REGEN module, of which the BPF are sub-systems. As a result of this, most

other frequencies involved are predetermined. The output from each BPF looks like an ASK

signal. These can be demodulated asynchronously, using the envelope. The decision circuit, to

which the outputs of the envelope detectors are presented, selects the output which is the most

likely one to the two inputs. It also re-shapes the waveform from a bandlimited to a

rectangular form.

In the block diagram of Figure 6.4 two local carriers, on each of the two frequencies

of the binary FSK signal, are used in two synchronous demodulators. A decision circuit

examines the two outputs, and decides which is the most likely.

Figure 6. 4 Synchronous Demodulation,

This is a two channel receiver. The bandwidth of each is dependent on the message bit

rate. There will be a minimum frequency separation required of the two tones. This

demodulator is more complex than most asynchronous demodulators.

The output of a demodulator will typically be a bandlimited version of the original

binary sequence. Some sort of decision device is then required to generate the original binary

sequence as is shown in the figures above

A phase locked loop (PLL) shown in Figure 6.5 is capable of demodulating an FSK

signal as well.

Figure 6. 5 Phase Locked Loop (PLL) Demodulator

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The control signal, which forces the lock, is a bandlimited copy of the message

sequence. Depending upon the bandwidth of the loop integrator, a separate LPF will probably

be required to recover the message. In TIMS, the integrator can be modeled with the LOOP

FILTER in the BIT CLOCK REGEN module.

DATA TRANSFER OVER 100 kHz:

In this part, binary data generated PCM Encoder and voice which is generated Speech

Module will transferred from 100 kHz TX ANTENNA to 100 kHz RX ANTENNA.

Basic Modules:

a) 100 kHz Tx Antenna:

A loop antenna to broadcast signals at or near the TIMS "carrier frequency" of 100

kHz. A single BUFFER AMPLIFIER is normally used to drive the ANTENNA.

b) 100 kHz Rx Antenna Utilities:

A loop antenna designed for operation in the long wave and medium wave frequency

ranges. The UTILITIES module includes a high gain, broad band amplifier and a separate 100

kHz band pass filter.

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c) Speech Module:

The SPEECH module allows speech and audio signals to be recorded and replayed.

Three independent channels are provided: CHANNEL 1, CHANNEL 2 and LIVE. The

module includes an in-built microphone. An EXTernal input is also provided for recording

externally generated signals. The recorded channels’ signals are band limited to 300 Hz and

3.4 kHz. The LIVE channel has user selectable LPF and HPF.

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Figure 6. 6 PCM Encoder and AM Modulation Diagram

Figure 6. 7 100 kHz Rx Antenna and Demodulation Circuit

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Figure 6. 8 Speech Modulation Diagram

Figure 6. 9 100 kHz Rx Antenna and Speech Demodulation Circuit

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Procedure:

1. Obtain the CPFSK signal by the use of VCO and the digital data produced with 2 kHz

clock. Observe and sketch this signal on a scope sheet with its corresponding digital

data.

2. Build the circuit shown as a block diagram in Figure 6.2. Use the VCO as frequency

f1 source and 2 kHz sinus as the signal with frequency f2. Control the switching by

both the digital data produced with divided by n clock and its DC level raised version.

Perform the divide by n with a TTL input from the audio oscillator to the related

module. Observe and sketch the output on a scope sheet with its corresponding digital

data. Compare the resulting waveforms of this step and that of step1.

3. Build the PLL Demodulator shown as a block diagram in Figure 6.5. The LOOP

FILTER performs the feedback and it also provides the control input of the VCO.

Observe and sketch the output of the LOOP FILTER on a scope sheet with its

corresponding digital data.

For Tx Antenna Side

4. Build the PCM Encoder and AM Modulation shown as a block diagram in Figure

6.6. Transfer the bit stream over antennas from Tx module to Rx module by

connecting modulated signal to the 100 kHz Tx Antenna shown as a block diagram in

Figure 6.6.

For Rx Antenna Side

5. Build the 100 kHz Rx Antenna and Demodulation Circuit shown as a block

diagram in Figure 6.7.

For Tx Antenna Side

6. Build the Speech Modulation to generate the voice data. Transfer the voice data over

antennas from Tx module to Rx module by connecting modulated signal to the 100

kHz Tx Antenna shown as a block diagram in Figure 6.8.

For Rx Antenna Side

7. Build the 100 kHz Rx Antenna and Demodulation Circuit shown as a block

diagram in Figure 6.9.

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YEDITEPE UNIVERSITY

DEPARTMENT of ELECTRICAL & ELECTRONICS ENGINEERING E X P E R I M E N T R E S U L T S H E E T

Course: EE 354 Communication Systems Experiment: 6 Semester: 2017 Spring

Group Information

Group No

Date Lab. Instructor’s Notes

Student Signature

Student No Student Name

1

2

3

4

FSK (FREQUENCY SHIFT KEYING) & 100 kHz RX/TX ANTENNA

PROCEDURE 1. Obtain the CPFSK (Continuous Phase Frequency Shift Keying)

signal by the use of VCO module whose TTL input data is coming from the digital data

sequence produced by SEQUENCE GENERATOR module, with 2.083 kHz analog clock.

CPFSK

CHANNEL 1

(CPFSK Signal)

............. V/Div

............. s/Div

CHANNEL 2

(Digital Data)

............. V/Div

............. s/Div

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PROCEDURE 2. Build the circuit shown as a block diagram in Figure 6.2, in the

experiment sheet. Use the VCO module as frequency f1 source and 2 kHz sine as the signal

with frequency f2. Control the switching of the DUAL ANALOG SWITCH module by both

the digital data produced with divided by “n” clock and its -reversed polarized & DC level

raised- version. Perform the divide by “n=8” with a TTL input from the audio oscillator to

BIT CLOCK REGENERATOR module.

FSK

CHANNEL 1

(FSK Signal)

............. V/Div

............. s/Div

CHANNEL 2

(Digital Data)

............. V/Div

............. s/Div

First Carrier Signal :

Second Carrier Signal:

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PROCEDURE 3. Don’t unplug the modulator circuit. Build the

demodulator circuit (Synchronous Demodulator) shown as a block in below. Multiply

separately the FSK signal by the carrier signal with frequency f1 and f2. Take one of the

outputs, apply PHASE SHIFTER module for 180° phase shift.

Demodulation of FSK

CHANNEL 1

(Demodulated Signal)

............. V/Div

............. s/Div

CHANNEL 2

(Digital Data)

............. V/Div

............. s/Div

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PROCEDURE 4 – 5. For Tx Antenna Side

Build the PCM Encoder and AM Modulation shown as a block diagram in Figure 6.6.

Transfer the bit stream over antennas from Tx module to Rx module by connecting modulated

signal to the 100 kHz Tx Antenna shown as a block diagram in Figure 6.6.

For Rx Antenna Side

Build the 100 kHz Rx Antenna and Demodulation Circuit shown as a block diagram in

Figure 6.7.

100 kHz .......... Antenna Side

CHANNEL 1

(………………)

............. V/Div

............. s/Div

CHANNEL 2

(…………….)

............. V/Div

............. s/Div

Comment: Please write the bit stream of the PCM coded DC voltage level as 1’s and

0’s. (in 7 bits)

PROCEDURE 6 – 7. For Tx Antenna Side

Build the Speech Modulation to generate the voice data. Transfer the voice data over

antennas from Tx module to Rx module by connecting modulated signal to the 100 kHz Tx

Antenna shown as a block diagram in Figure 6.8.

For Rx Antenna Side

Build the 100 kHz Rx Antenna and Demodulation Circuit shown as a block diagram in

Figure 6.9.

Comment: Explain briefly, what is done at speech modulation side and demodulation

side? Can you improve the quality of the voice? Which switches and knobs affect the quality?