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Islamic University of Gaza

Faculty of Engineering

Electrical Department

Digital Communications Lab.

Prepared by:

Eng. Mohammed K. Abu Foul

Experiment # (3)

PCM Modulator

Experiment Objectives:

1. To understand the operation theory of pulse coded modulation (PCM).

2. To understand the theory of PCM modulation circuit.

3. To design and implement the PCM modulator.

Experiment theory:

1. The operation theory of PCM modulation:

PCM modulation is a kind of source coding. The meaning of source coding is the

conversion from analog signal to digital signal. After converted to digital signal, it is easy

for us to process the signal such as encoding, filtering the unwanted signal and so on.

Besides, the quality of digital signal is better than analog signal. This is because the digital

signal can be easily recovered by using comparator.

PCM modulation is commonly used in audio and telephone transmission. The main

advantage is the PCM modulation only needs 8 kHz sampling frequency to maintain the

original quality of audio. Figure 3.1 is the block diagram of PCM modulation. First of all is

the low pass filter, which is used to remove the noise in the audio signal. After that the

audio signal will be sampled to obtain a series of sampling values as shown in figure 3.2.

Next, the signal will pass through a quantizer to quantize the sampling values. Then the

signal will pass through an encoder to encode the quantization values and then convert to

digital signal. In fact, the process of quantization can be achieved at one time by A/D

converter. However, we should pay attention on the quantization levels. For example if the

bits for PCM modulation is 3, then the quantization levels is 23=8, which is 8 steps. If the

bits for PCM are 4, then the quantization levels is 24=16, which is 16 steps. The increasing

of bits of PCM modulation will prevent the signal from distortion, but the bandwidth will

also increase due to the increasing of the capacity of data. From figure 3.2, the encoder

utilizes n output terminals, therefore, we need to convert the parallel data to serial data,

which is the way that satisfy the data format of PCM modulation.

Figure 3.1 Block diagram of PCM modulation

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Figure 3.2 Diagram of PCM modulation

2. The implementation of PCM modulator:

Figure 3.3 is the circuit diagram of PCM modulator. Capacitors C1, C2, resistors R1, R2,

R3, R4 and µA741 comprise a second order low-pass filter. The structure of this low-pass

filter is a voltage controlled voltage source (VCVS) low-pass filter. The gain can be

expressed as

The cutoff frequency is:

Figure 3.3 Circuit diagram of PCM modulation

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If R2 = R3 = R and C1 = C2 = C , then

In this experiment, we use IC CW6694 from Conwise to implement the PCM

modulator. This IC includes the circuits of PCM modulation and demodulation, however,

we only discuss the modulation in this experiment. The analog signal will pass through R5

and input to pin 10, which is the inverting input terminal. Then the signal will pass through

R6 and feedback to pin 9. Therefore, the structure of these two pins is an OPA structure and

the expression of the gain is:

Besides the input gain control of the modulation circuit, the sampler, quantizer and

encoder are built in the IC, therefore, we just need a few components to implement the

PCM modulator. Master clock (MCLK) is the operation frequency of the system, which is

2048 kHz square wave frequency. Figure 3.4 is the circuit diagram of 2048 kHz square

wave generator. From figure 3.4, we use 2048 kHz crystal oscillator to match with the TTL

inverter, which can produce the required signal. Sample clock (SCLK) is the sample

frequency, which supplies the required operation frequency of the internal sampler. The

sample frequency is 8 kHz, i.e. the sampler will sample the input audio signal in every

0.125 ms. The sampling frequency is obtained by using the counter to divided the 2048 kHz

square wave signal by 256.

From figure 3.3, FS0 and FS1 are the data format selection of PCM encoder as shown in

table 5.1. the data format selection of PCM encoder can encode the sample to 8-bit µ-law

format, 8-bit A-law format or 16-bit digital data format. Besides, the above-mentioned

format, the IC CW6694 also provides with encode and decode of the continues variable

slop delta modulation (CVSD) format. The CVSD format can be selected by pin FS0 and

FS1. However, CVSD is not included in this experiment, therefore, the FS1 will be

grounded and FS0 will be in "high" level. At this moment, the output encode data format of

PCM is 16-bit. When FS0 is in "low" level, the output encode of PCM is 8-bit. Pin

is the reset pin of this IC.

From figure 3.3, the data output terminal of pin 26 of PCM modulator will connect to a

buffer U1:B, which is used for impedance matching. The reason is the output of PCM

modulator is bipolar junction transistor type not field effect transistor type, therefore, in

order to prevent the load effect, we need to connect a buffer at the output terminal for

impedance matching.

Table 5.1 Output data format of PCM modulation

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Figure 3.4 Circuit diagram of 2048 kHz square wave generator

Experiment items:

Experiment 1: PCM modulator

1. Refer to the circuit diagram in figure 3.3 or figure DCS5-1 on ETEK DCS-6000-03

module.

2. Let J1 short circuit and from the signal input terminal (I/P), input 250 mV amplitude

and 500 Hz sin wave frequency. Then by using oscilloscope, observe on the output

terminal of low-pass filter (T1), input terminal of audio signal (T2), feedback point of

output signal (T3) and output signal terminal of PCM (OP). After that connect the

output terminal (T4) with 2048 kHz square wave to the CH1 of the oscilloscope and

output terminal (T6) of modulated signal to CH2 of the oscilloscope, then record the

measured results in table 3.2.

3. Follow the input signals in table 3.2, then repeat step 2 and record the measured results

in table 3.2.

4. Let J2 short circuit from the signal input terminal (I/P), input 250 mV amplitude and

500 Hz sin wave frequency. Then by using oscilloscope, observe on the output terminal

of low-pass filter (T1), input terminal of audio signal (T2), feedback point of output

signal (T3) and output signal terminal of PCM (OP). After that connect the output

terminal (T4) with 2048 kHz square wave to the CH1 of the oscilloscope and output

terminal (T6) of modulated signal to CH2 of the oscilloscope, then record the measured

results in table 3.3.

5. Follow the input signals in table 3.3, then repeat step 4 and record the measured results

in table 3.3

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Measured results: Table 3-2 measured results of PCM modulator when J1 short circuit.

Input Signals

Output signal waveforms

T1 T2

500 Hz

250 mV

T3 OP

T4 and T6

T5 and T6

Note: repeat table 3.2 when the input signal is 1 kHz with 250 mV

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Table 3.3 Measured results of PCM modulator when J2 short circuit.

Input Signals

Output signal waveforms

T1 T2

500 Hz

250 mV

T3 OP

T4 and T6

T5 and T6

Note: repeat table 3.3 when the input signal is 1 kHz with 250 mV