INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY,

Effectiveness of Linear FM Interference Signal on Tracking

Performance of PLL in Monopulse Radar Receivers

Harikrishna Paik*, Dr.N.N.Sastry, Dr.I.SantiPrabha

Assoc.Professor, Dept. of E&I Engg, VRSEC, Vijayawada, India

Professor & Dean, R&D Wing, VRSEC, Vijayawada, India

Professor, Dept. of ECE, JNT University, Kakinada, India

E-mail: pavan_paik2003@yahoo.co.in, netisastry@yahoo.co.in, santiprabha@yahoo.com

Abstract—Monopulse radar receivers employing

phase locked loop (PLL) invariably track the

target in range and angle (frequency) domains. In

this paper, the tracking performance of PLL for a

highly application specific airborne tracking radar

in frequency domain is presented. Two different

signals such as the linear frequency modulated

(LFM) interference signal from a repeater source

and the radar echo signal are injected into the PLL

simultaneously with an assumption that initially,

the PLL locks onto the echo signal frequency. The

effectiveness of key parameters of LFM signal such

as frequency deviation, modulation rate and LFM

signal power on break-lock are demonstrated. The

simulation result shows that break-lock is achieved

at a frequency deviation of 0.36 MHz for a typical

LFM signal power of -14 dBm and 200 kHz

modulation rate when the radar echo power at the

PLL input is -14 dBm. The break-lock is studied

for a typical loop bandwidth of 200 kHz and

different values of radar echo signal power at the

PLL input. The computer simulation is carried out

using visual system simulator (VSS) AWR software

and potential conclusions are demonstrated.

Index Terms—frequency modulation, jamming,

monopulse, radar receiver, radar echo, tracking

I. INTRODUCTION

Several advanced techniques have been

employed in modern tracking radar against

electronic attack (EA) threat in electronic warfare scenario. These include pulse compression, Pulse

Doppler, Monopulse, Ultralow sidelobe antennas

and Coherent sidelobe cancellation etc. All these

techniques make modern radars difficult to jam and require special EA techniques for effective

jamming. Of these techniques, monopulse

technique is invariably used in all the tracking

radars and modern missile seekers [1]. However,

monopulse angle (frequency) tracking systems are difficult to jam because this technique

provides an inherent resistance to amplitude-

modulated jamming waveforms from point target

sources [2]. Several jamming techniques have been employed against monopulse tracking

systems which include noise jamming, deception

jamming, transponder jamming and support jamming. In these techniques, the radar receiver

is jammed either by introducing imperfections in

the monopulse design or by using multiple repeater sources in order to distort the angle of

arrival of the echo signal. Thus, the monopulse

tracker is caused to move away from the target

and results break-lock in the missile radar [3]. In this paper, the jamming of monopulse radar

receiver using deception jamming is illustrated.

The basic objective of the deception jamming scheme is to cause frequency (angular) break-

lock by injecting suitable modified replica of

radar echo signal into the tracking radar receiver.

The linear frequency modulated (LFM) signal from a repeater source is used as an interference

signal which is injected into the phase locked

loop (PLL) along with radar echo signal and break-lock in the PLL is analyzed.

The effects of LFM interference signal on

tracking performance of PLL in monopulse radar receiver have been presented in several studies

[4-7]. The statistical parameters of the monopulse

receiver such as steady state gain; tracking index

and range Doppler coupling coefficients in the presence of linear frequency modulation signal

have been derived in [4]. The time-frequency

characteristics of non-stationary LFM signal is estimated more precisely using Wigner Ville

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Transform to detect the presence of wrong

frequency component [5]. The maximum

likelihood angle estimation technique is analyzed [6-7] for the detection of two closely unresolved

targets in the sea clutter environment by

implementing modified generalized likelihood ratio. Here, we propose the generation of the

LFM signal by frequency modulating the

sinusoidal carrier by sawtooth waveform using a

frequency modulator. The PLL is assumed to be operating at an intermediate frequency (IF) of 50

MHz with a typical bandwidth of 200 kHz. The

radar echo and LFM jamming signal are applied to the PLL simultaneously. Initially, it is assumed

that the PLL locks onto the radar echo signal

frequency. The frequency deviation of LFM signal is increased such that the frequency

separation between these two signals increases.

Thus, the PLL loses the frequency lock from the

echo signal and locks on to the jamming signal frequency. The frequency deviation required for

break-lock is estimated as a function of LFM

jammer power for different modulation rates such as 200, 300, 400 kHz, and so. The break-lock is

studied for different values of jammer signal

power in the range between -14 and -2 dBm and the echo signal powers of -14 and -10 dBm.

II. SYSTEM CONFIGURATION ANDMODELLING

A. Monopulse Radar Receiver

The basic block diagram of two channel

monopulse receiver is shown in Fig.1.

Fig.1. Block diagram of Monopulse Receiver

As shown in Fig. 1, the antenna feeds at the

receiver front end receive the echo signal

reflected from the target and the repeater interference signal. The outputs of antenna feeds

are then given to two inputs of hybrid junction,

which is a four port microwave device with two

input and two output ports. When the signals from two antenna feeds are applied at the input

ports, the sum and difference of the two are

obtained at the output ports. The resulting signals in each sum and difference channel are

heterodyned to an intermediate frequency (IF)

and then amplified by an IF amplifier as necessary. The amplitude of the difference signal

indicates the magnitude of the angular error, and

its phase indicates the direction of the off-axis

error. The sum channel signal is applied to the PLL to predict the break-lock behavior of the

receiver. The monopulse radar receiver

implemented using visual system simulation (VSS) AWR software is shown in Fig.2.

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Fig.2. Monopulse receiver implemented using VSS

As shown in Fig.2, the sinusoidal CW radar echo

and the LFM interference signal are applied at

the receiver input simultaneously. The quadrature hybrid coupler at the receiver front end divides

the input signal into sum and difference channel

signals which are 900 out of phase to each other.

The signals in each channel are then amplified by

an RF amplifier and heterodyned to an IF

frequency centered at 50 MHz. The sum channel

signal is applied to the PLL and break-lock is predicted.

B. Phase locked loop

The monopulse radar receiver invariably employs

PLL as a frequency tracking subsystem. The PLL mainly includes a phase detector (PD), loop filter

and a voltage controlled oscillator (VCO). In a

classical PLL, when the reference signal and

repeat jamming signal are simultaneously applied at its input, the PLL acquires lock onto the

reference signal if the strength of the reference

signal is larger than the interference signal strength. When strength of interference signal

exceeds the echo signal strength, the PLL loses

the frequency lock from the reference signal and

locks onto certain other frequency [8]. Thus, break-lock is said to occur in the PLL. For our

simulation, the PLL with charge pump PD and a

third order passive loop filter is designed and

implemented VSS software. The charge pump

PD is chosen due to the fact that the charge pump

PD provides infinite pull-in range and zero steady state error [9]. Furthermore, a third order

filer is generally recommended for the most of

RF applications and it is rare that a PLL is constructed with a filter higher than third order.

In addition, the passive loop filter has the

advantage over active filter that there is no active

device to add noise into the PLL. The loop filter is designed using exact method. The exact

method of filter design involves with solving the

time constants and then determining the loop filter components from these time constants. The

different key parameters considered for design of

the loop filter are phase margin (), loop

bandwidth (fc), phase detector gain (K in mA), VCO gain (Kvco in MHz/volt) and pole ratio

(T31). The pole ratio is the ratio of third order pole and reference pole of the loop filter. The

phase margin determines the loop stability which

is typically chosen between 48 and 55 degree.

The loop bandwidth is the crucial parameter in filter design which determines the spur rejection

and lock time of the loop. The selection of pole

ratio has an impact on reference spur in the loop. The time constants of filter are determined from

phase margin () of the loop forward gain [G(s)] given by [10].

)311(tan 1)1(tan 1)2(tan 1TTTT

(1)

where, T1,T2 and T3 are the filter time constants.

The loop forward gain G(s) is given by:

)()( sZS

sGkk vco

(2)

where, Z(s) is loop impedance, K is phase detector gain, Kvco is gain of VCO. In equation

(1), the value of phase margin () and the pole ratio (T31) are known, so an equation containing

T1 and T2 can be obtained. Another equation of

T1 and T2 can be obtained by finding the maximum value of phase margin at a frequency

equal to the loop bandwidth. It is seen that the

loop maximizes the phase margin at a frequency equal to loop bandwidth [11]. So, we can write

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0

d

d

c

(3)

Solving the above equation, we can express

)3

(2

1

3

)1

(2

1

1

)2

(2

1

2

Tc

Tc

Tc

Tc

Tc

Tc

(4)

Now, solving equation (1) and (4) for two unknowns, the time constants T1 and T2 can be

determined. The time constant T3 can be obtained

by using the relation

TTT 3113 (5)

Once the time constants are determined, the loop

filter components are obtained by defining the

constants k1, k2, k3 and k4 given as:

c totk 1

(6)

kTTk 1)

31(

2 (7)

T

kTT

k

2

131

3

(8)

c

c

k

1

3

4 (9)

By solving the above four equations, the filter components R1, R2, C1, C2 and C3 are determined.

The typical parameters chosen for design of the

loop filter are: phase margin () = 55 deg., input frequency (fcomp) = 50MHz, loop bandwidth (fc) =

200 kHz, VCO output frequency (fout) = 4 GHz,

phase detector gain (K) = 2.5 mA and VCO gain

(Kvco) = 40 MHz/volt. The filter is designed using Hittite Microwave Corporation PLL Design and

Analysis Tool. The design window which shows

the filter component values is shown in Fig. 3. The filter components are found to be R2= 1 kΩ,

R3= 7.5 kΩ, C1= 100 pf, C2=2.2 pf and C3= 15 pf.

The third order PLL implemented using VSS

software is shown in Fig. 4.

Fig.3. HMC design window

Fig.4. Third order PLL

C. LFM signal generation

Several methods have been used for the

generation of LFM signal. The LFM signal is generated by fractional synthesizer [12-13] in

which the fractional logic controls the division

ratio (N) of the programmable frequency divider

to generate highly linear LFM signal. The main limitation of this method is quantization noise

caused by division ratio as it takes only integer

value. The proposed method generates the LFM signal

by frequency modulating the sinusoidal carrier

signal by a sawtooth waveform using an FM modulator. For computer simulation, the LFM

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signal is generated with a sinusoidal carrier

centred at an IF frequency of 50 MHz and

modulating sawtooth waveform of 2 V (peak) as shown in Fig. 5.

Fig.5. LFM signal generation

As shown in Fig. 5, the FM modulator block

frequency modulates the sinusoidal carrier signal by a low frequency sawtooth modulating signal

and produces baseband LFM signal. Thus, the

instantaneous frequency of carrier signal is shifted by an amount equal to the modulating

signal times the frequency sensitivity (MHz/volt)

of the FM modulator. Specifically, if the modulating input signal is v(t), and carrier has

amplitude (A) with center frequency (fc), then the

output signal, y (t) is given by [14]:

t

foff

dvtjty kfA0

)])(22(exp[)(

(10) where, foff is offset frequency from the carrier and

kf is the frequency sensitivity(MHz/volt). The

LFM signal is generated with following key parameters: modulating voltage (vm) = 2 V,

modulating frequency (fm) = 200 kHz, carrier

center frequency = 50 MHz, carrier power = -14

dBm. The modulating signal and LFM spectrum are shown in Fig. 6. (a) and (b).

Fig.6. (a). Modulating signal

Fig.6. (b). LFM spectrum

The spectrum of the LFM signal shows that the

signal is centred at 50 MHz and signal bandwidth

is about 5 MHz.

III. COMPUTER SIMULATION

The designed configuration of the PLL is modeled using VSS simulator AWR software.

The elements are modeled using the elements

available in VSS library. The computer simulation is carried out at the IF stage of

receiver with the sum channel signal at PLL

input. The radar echo signal with a typical power

of -14 dBm at 50 MHz is applied at the PLL

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input along with the LFM interference signal

centered at 50 MHz (200 kHz modulation rate).

Initially, the power of the LFM signal is assumed to be relatively less compared to echo signal

power as a condition for good tracking. Initially,

it is seen that the PLL locks onto the radar echo signal frequency. Then the frequency deviation

of LFM signal is adjusted such that the frequency

separation between these two signals increases.

At a certain value of frequency deviation, the PLL loses the frequency lock from the echo

signal and locks onto the jamming signal at

certain other frequency. The break-lock is observed through the frequency spectrum of the

PLL. The deviation required for break-lock is

measured as a function of jammer power and different modulation rates such as 200, 300, 400

kHz and so. The simulation is also carried out

with the echo signal power of -10 dBm applied at

the PLL input.

IV. RESULTS AND DISCUSSION

The simulation results of break-lock measured

through frequency spectrum of the PLL are

shown in Fig. 7(a), 7(b) and Fig. 8(a), 8(b). The results are presented for a typical jammer power

of -14 dBm, modulation rate of 200 kHz when

the echo powers at the PLL input are -14 and -10 dBm.

Fig.7. (a) PLL spectrum at 0.25 MHz frequency

deviation with echo power of -14 dBm

Fig.7. (b) PLL spectrum at 0.36 MHz frequency deviation with echo power of -14 dBm

Fig.8. (a) PLL spectrum at 0.40 MHz frequency

deviation with echo power of -10 dBm

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Fig.8. (b) PLL spectrum at 0.51 MHz frequency

deviation with echo power of -10 dBm

It is clear from Fig. 7(a) that the radar echo signal

power at 50 MHz is -14 dBm and the PLL output

signal power at 4 GHz is 10 dBm when the

frequency deviation is 0.25 MHz. This shows that the PLL is locked onto the echo signal

frequency as the PLL output frequency is equal

to the VCO frequency. From the PLL spectrum as shown in Fig. 7(b), it is seen that the PLL

output signal is at 4.08 GHz (different from 4

GHz) when the frequency deviation is 0.36 MHz. So, it demonstrates that break-lock in the PLL is

achieved at a frequency deviation of 0.36 MHz

when jammer power is -14 dBm and the echo

signal power at the PLL input is -14 dBm. Fig. 8(a) and (b) show the PLL spectrum at frequency

deviations of 0.40 MHz and 0.51 MHz,

respectively when the echo power at PLL input is -10 dBm. It is clear from Fig. 8(b) that the PLL

output signal is at 4.08 GHz demonstrating that

break-lock is achieved at a frequency deviation of

0.51 MHz. From the above results, it can be estimated that larger value of frequency deviation

is required for break-lock when the echo signal

power is large. The simulated results of frequency deviation

required for break-lock as a function of jammer

power for different modulation rates are shown in Fig. 9(a) and Fig. 9(b).

Fig.9. (a). Frequency deviation Vs jammer power

with echo power of -14 dBm

Fig.9. (b). Frequency deviation Vs jammer power

with echo power of -10 dBm

From Fig. 9(a), it is clear that for a typical modulation rate of 200 kHz, the frequency

deviation required for break-lock is 0.36 MHz at

-14 dBm jammer power and it is 0.19 MHz at -2 dBm jammer power demonstrating that the PLL

loses the frequency lock at higher value of

frequency deviation when the jammer power is

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less. Similarly, when the jammer is high, the

break-lock is achieved at lower value of

frequency deviation. It is also estimated that when the modulation rate of LFM signal is low,

break-lock is achieved at lower value of

frequency deviation. Similarly, larger value of deviation is required for break-lock when

modulation rate is high. From Fig. 9(a), it is seen

that the break-lock is achieved at 0.36 MHz when

modulation rate is 200 kHz and it is 0.91 MHz when modulation rate is 500 kHz. So, from the

above results it can be demonstrated that the

jammer power and frequency deviation are the key parameters for breaking the frequency lock in

the PLL. Thus, for effective jamming of the

monopulse receiver, the LFM jamming signal with suitable power and frequency deviation is to

be injected into the receiver along with the radar

echo signal.

V. CONCLUSIONS

The break-lock of phase locked loop in monopulse receiver in the presence of LFM

jamming signal has been presented. It is

demonstrated that the break-lock is achieved at a frequency deviation of 0.36 MHz for a typical

modulation rate of 200 kHz when both the echo

and LFM signal powers are -14 dBm. Furthermore, it is estimated that at jammer power

of -14 dBm, the frequency deviations required for

break-lock are 0.36 and 0.91 MHz when the

modulation rates of LFM signal are 200 and 500 kHz respectively. It is also verified that break-

lock is achieved at lower value of frequency

deviation when the modulation rate is less and at higher modulation rate, larger value of frequency

deviation is required for break-lock. So, from the

above simulation results, conclusions can be

drawn that for effective jamming of monopulse receiver using LFM jamming signal, the jammer

power and frequency deviation are to be selected

suitably. ACKNOWLEDGMENT

The authors would thanks to Siddhartha Academy of General & Technical Education who

provided the research facilities and technical

support. We are also indebted to Director and

Principal of the institute who extended support in

many useful technical discussions during the progress of this work.

REFERENCES

[1] D. C. Schleher, Introduction to Electronic

Warfare, 1st ed., Norwood, MA: Artech House, 1986.

[2] Samuel M Sherman, Monopulse Principles and Techniques, 2nd ed., Artech House, 1984.

[3] D. C. Schleher, Electronic Warfare in the Information Age, 2nd ed., Norwood, MA: Artech House, 1999.

[4] Winnie Wong, W. D. Blair, “Steady-State Tracking with LFM Waveforms,” IEEE

Transactions on Aerospace and Electronic Systems, Vol. 36, no. 2, pp. 701-709, April 2000.

[5] Christophe De Luigi, Eric Moreau, “An Iterative Algorithm for Estimation of Linear Frequency Modulated Signal Parameters,” IEEE Signal Processing Letters, Vol. 9, no. 4, pp. 127-129, April 2002.

[6] A. Sinha, T. Kirubarajan, Y. Bar-Shalom,

“Tracker and Signal Processing for the Benchmark Problem with Unresolved Targets,” IEEE Transactions on Aerospace and Electronic Systems, Vol. 42, no. 1 pp. 279-300, Jan. 2006.

[7] Blair W.D., Brandt-Pearce M., “Monopulse DOA Estimation of Two unresolved Rayleigh Targets,” IEEE Transactions on AES, Vol.37, no.2, pp. 452-469, April 2001.

[8] F M. Gardner, Phase lock Techniques, 2nd ed., John Wiley, 1979.

[9] Gardner F.M., Charge-Pump Phase-Lock Loops, IEEE Trans. Comm., Vol. COM-28, pp. 1849-1858, 1980.

[10] Keese, William O., An Analysis and Performance Evaluation of a Passive Filter Design Technique for Charge Pump Phased Locked Loops,

Application Note, National Semiconductor, 2001. [11] Franklin F., Powell D., and Emami-Naeini A.,

Feedback Control of Dynamic Systems, 3rd ed., Addison-Wesley, 1994.

[12] T. Musch and B.Schiek, “A highly linear frequency ramp generator based on a fractional divider phase locked loop,” IEEE Trans. Instrum. Meas., Vol. 48, pp. 634-637, April 1999.

[13] T. Musch, I. Rolfes and B. Schiek, “ Fractional divider concepts with phase locked control for the generation of precise linear frequency ramp,” in 28th EUMC Proc.., Amsterdam, Netherlands, pp. 451-456, Oct. 1998.

[14] S. Haykin, Communication Systems, 3rd ed., John Wiley & Sons, New York, 1994.

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