WCE2009_pp376-381.pdf

Proceedings of the World Congress on Engineering 2009 Vol I

WCE 2009, July 1 - 3, 2009, London, U.K.

A Simple Digital VHDL QPSK Modulator

Designed Using CPLD/FPGAs for Biomedical

Devices Applications

Gihad Elamary, Graeme Chester, Jeffrey Neasham

Abstract — we proposed a new simple design for a Quadrature

Phase shift Keying (QPSK) modulator applied for implantable

telemetry applications as demonstrated. VHDL programming

code is used to generate QPSK digital signal. The input test

signals data and carrier are interfaced to the CPLD and FPGAs

board from Agilent function generator (E8408A). We used the

local clock oscillator for test, which is operating at 25.175 MHz

and used 12.5MHz for the carrier and 2Mbps reduced for data

source. The modeled Modulator has been designed and simulated

and performance was evaluated by measurements. The design

has low power consumption and size for biomedical applications.

Furthermore, the advantages of this modulator are it can be

reconfigured and upgraded to enhance the data rate.

Index Terms—VHDL Modulator (QPSK); Biodevices,Passive

filter, CPLD/FPGA.

are unsuitable for medical purposes, this type of modulator

provides digital synthesis and the flexibility to reconfigure and

upgrade with two most often languages used

VHDL-and-Verilog (IEEE standard) based as hardware

structures language described [6, 7, 14, 15].

II.

M ETHOD MODULATOR D ESIGN

All analogue or hybrid analogue/digital QPSK modulators

work with phase shift carrier angle ( j ), as a key of

modulation [4, 16]. The phase signal is most important part in

the modulator to acquire two discrete signals (Sine and

Cosine) [21]. Practically, it use Direct Digital Synthesizer

(DDS) or Numerical Control Oscillator (NCO) for perform the

carrier transitions [11, 12, 17]. However, the NRZ format is

essential for mapping I and Q . The analogue QPSK signal can

be represented mathematically as in Equation (1) and I/Q are

defined in Equations (2, 3):

I. I NTRODUCTION

Biomedical implant telemetry devices are increasingly applied

today in various areas in medical applications, such as

telemedicine, biotelemetry, and health medical care treatments

for chronic diseases epilepsy and blindness patients; which are

using wireless infrastructure environment [1]. The biodevices

are one of these technologies applied with transcutaneous

wireless implant telemetry (TWIT).Wireless inductive

coupling link is common way for transfer the RF power and

data to communicate between readers and a battery-less

implant [2, 3]. Demand for higher data rate for the acquisition

data returned from the body are increasing, and require an

efficient modulator to achieve high transfer rate and low

power consumption. In such applications QPSK modulation

has advantages over other schemes, and double symbol rate

with respect to the BPSK over the same spectrum band.

Contrast to analogue modulators for generating QPSK signals,

where the circuit complexity and power dissipation

QPSK

(

)

(

)

cos(

p −

)

(

)

sin(

)

(1)

cos[(

−

]

(2)

sin[(

−

]

(3)

These types of technique are not suitable for medical

applications, which essential work with the input data in NRZ

signal form at conventional modulators. The proposed QPSK

VHDL modulator is programmed generate a carrier phase

which acquires four discrete states (0, 90,180,270). Two

separate streams in-phase ’I’, and quadrature phase Q for

mapping the data for controlling the four phase different

carriers interfaced to multiplexer. The output is selected by

multiplexer to provide a digital QPSK signal, which passes via

a passive filter before a transmission (TX) to eliminate the

high frequencies [9, 13]. Fig.1 demonstrates the proposed

VHDL modulator comparing to analogue modulator. The

digital QPSK signal of the multiplexer output can be

represented in Equation (4):

Manuscript received March 23, 2009; (revised April 28, 2009). This work

was supported in part by School of Electrical, Electronic and Computer

Engineering (EECE). Newcastle University /UK.

The authors are with Department of EECE, Newcastle University (e-mail:

gihad.elamary1@ncl.ac.uk ; Graeme.chster@ncl.ac.uk ;

j.a.neasham@ncl.ac.uk ). School of EECE- Merz Court, Newcastle upon

Tyne – Newcastle University-NE1 7RU

−

Mux

(4)

out

180

270

ISBN: 978-988-17012-5-1

WCE 2009

Proceedings of the World Congress on Engineering 2009 Vol I

WCE 2009, July 1 - 3, 2009, London, U.K.

clearly it demonstrates the response of filter comparing to the

Butterworth and Chebyshev I. Practically, a simple BW-LPF

is designed to omit the harmonies and DC component. The

size is implant where the filter is implanted with the modulator

in biomedical devices.

Figure 3. The LPF simulation with MATLAB/Simulink

(

)

(6)

Figure 1. The block diagram for the proposed QPSK Modulator

SRC

III. F ILTER DESIGN AND SIMULATION

B. Chebyshev II HPF design and simulation

In wireless transmission we cannot transmit the digital signal

directly without harmonics separation. The output of the

multiplier is producing a QPSK digital signal “square signal”

form. It is essential to use a filter to complete the process for

the modulator “off-chip”. We designed an analogue passive

filter for this purpose as it has zero power consumption. Two

types of filters were investigated Low pass Filter (LPF) and

Band Pass Filter (BPF) [5, 10,18], as appropriate for medical

purpose the Butterworth LPF was given enhanced

performance than other types of LP-filters, to eliminates the

harmonics from the QPSK digital signal. While the second

choice was the Chebyshev II Filter BPF this was observed to

give better performance then other types of BP-filters. As

demonstrated in Fig.2 and Equation (5).

The second prototype choice filter is Chebyshev II analogue

passive LC filter 5 th order. The multiplexer output signal is fed

into the designed filter. The simulation result with

MATLAB/Simulink FFT is presented in Fig. 4. Which

compares the Chebyshev I, Chebyshev II and Elliptic for

performances type and characteristics. Obviously it gives a

high separation, over 50dB.

S QPSK

( LPF

)

Mux _

Digital

QPSK

S QPSK

( BPF

)

Figure 4.

The BPF simulation with MATLAB/Simulink

Figure 2. The filters types used for harmonics eliminates

The simulation performances for other types of filters are

presented in Table I as: (A) best performance (B) is less

performance and (C) weak performance, and (NP) is Not

Perfect performance.

(

QPSK

(

)

(5)

Mux

(

)

out

TABLE I. Influence of the investigation filters simulation

A. Butterworth LPF design and simulation

Filter Types

Our prototype analogue filter selected is a Butterworth 4 th

order to filter the input QPSK digital signal. The transfer

function of LC filter can be represented in Equation (6). The

simulation is presented in Fig. 3; with MATLAB/Simulink

Butterworth Cheby I Cheby II

Elliptic

Bessel

LPF

BPF

ISBN: 978-988-17012-5-1

WCE 2009

)

Proceedings of the World Congress on Engineering 2009 Vol I

WCE 2009, July 1 - 3, 2009, London, U.K.

IV. LE SIMULATION

MATLAB/Simulink simulation

sin((

−

)

(7)

Carrier

(

)

(

−

)

The QPSK modulator was designed and simulated with

MATLAB/Simulink to verify and validate the modulator

specifications [19]. The modulator is consists of carrier source

to produce a periodic pulse signal ( carrier

The Tx and the Rx signals are presented in Fig. 5 shows the

spectrum of the transmitted RF signal (CH1) and the received

RF signal (CH2) in the presence of noise (AWGN).

f ), fed to a carrier

phase shifter; which shift the input carrier into four different

phase signals (0°, 90°, 180°, 270°) interfaced to multiplexer.

While the data source was generated with PN_suqance, fed to

data mapping to generate I and Q signals to influence the four

phase different carries. The output is selected by multiplexer

which provides digital QPSK signal, this signal filtered with

analogue filter before transmitted to pass fundamental

frequency (

f o ± ) and eliminates the higher frequencies

associated with the square signal. The architecture block

diagram of Tx_Mod is shown in Fig .6. The simulated random

data signal (Data_in) which is generated by a PN sequence can

be represented by Fourier series analysis as in Equation (6).

data

= Ã

(

)

(

−

)

−¥

(6)

Figure 5. The specrum of QPSK transmit and rceive signals

at Data rate 2Mbps,carrier 12.5MHz

Where the input carrier signal is a periodic pulse train signal

and mathematically expressed by the Fourier series as in

Equation (7).

Figure 6. The propoused Modulator with Mathlab /simulink daigram

ISBN: 978-988-17012-5-1

WCE 2009

Proceedings of the World Congress on Engineering 2009 Vol I

WCE 2009, July 1 - 3, 2009, London, U.K.

Figure 7.

The propoused Modulator with Mathlab /simulink daigra

B. VHDL programming code simulation

V. Experimental results and discussion

The proposed modulator was built by the Altera UP2

development kit board [8], Programmed with the VHDL

language for modeling, design and analysis of the proposed

QPSK modulator. The simulated result of this modulator is

presented in Fig.7. This demonstrates the output signals

waveforms indicating the transitions (180°, 270°) of the

carrier signal influence by input data signal. The carrier

frequency 12.5 MHz was generated from the local clock

signal on the board, which operates at 25.175 MHz. The data

signal was reduced to 2 MHz by a frequency divider then fed

into a random PN_sequence generator (behavioral

described).The modulator was implemented and comparing

two different designs structural and behavior descriptions; for

efficient performance. The generated VHDL “Behavioral”

block diagram of the QPSK modulator is illustrated in Fig. 8

In this part of paper, we provide the measurements which

were conducted using the Altera UP2 Development kit board,

for testing the VHDL code modulator and comparing the

performance with the simulated QPSK modulation. The

Agilent digital demodulator (E8408A VXI) is used to receive

the filtered RF QPSK signal, and analyzed the parameters of

the transmitted QPSK signal (Tx) as demonstrated in Fig. 9

[22]. The desired carrier signal was generated from the master

clock on the circuit board that operates at 25.175 MHz, using

12.5 MHz as carrier. The carrier phase acquires four discrete

states (0,

Figure 8. The porporsed VHDL modulator

Figure 9. Illustrated the setup Lab measurement withUK2 Alter

ISBN: 978-988-17012-5-1

WCE 2009

p ). This corresponds to mapping I

and Q data source generated with VHDL code inside the

CPLD/FPGAs at 2Mbps. The signal passes as digital QPSK

through the passive LPF for harmonics separation.

Proceedings of the World Congress on Engineering 2009 Vol I

WCE 2009, July 1 - 3, 2009, London, U.K.

We investigated two prototypes of filters in this paper, LPF

and BPF. The BPF has a better performance characteristic

then LPF. However, the measurement result was illustrated in

Fig. 10 as; (a) PN_code signal generated by VHDL code, (b)

the QPSK digital signal, (c) the filtered signal output. While

the spectrum of Tx signal was captured with signal analyzer in

Agilent (8408A) at center frequency 12.58MHz as

demonstrated in Fig 11. The demodulator is also constructed

using the Matlab/Simulink tools to examine the performance

of the proposed modulator. The performance has measured

using Agilent Education version to demodulate the received

signal “QPSK Demodulator” to demodulate the information

data, which was transmitted with VHDL modulator. The

measurement results are given respectively in Fig .12, the

constellation diagram for QPSK Rx signal, and Fig.13

illustrating the spectrum and the demodulated data at 2Mbps.

Ultimately, the whole bench test system is illustrated in Fig

.14

Figure 12. Constellation diagram of QPSK demoulator received

from proposed QPSK modulator

Figure 10. Measured QPSK digital siganl (a); PN-code

;(b)Digital signal (c ) filter signal throgh LPF

Figure 13. Spectrum of the QPSK deomulator recivced signal

from proposed QPSK mod at carrier (12.58Mhz), data (2Mbps)

Figure 11. The Tx Spectrum of the QPSK transmitted signal at

carrier 12.5MH

Figure 14. Illustrates the test bench Lab measurement for transmit

data over wireless inductive link using Agilent demodulator

ISBN: 978-988-17012-5-1

WCE 2009

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