UNILATERALLY INJECTION-LOCKED GUNN OSCILLATOR PAIR ACTING AS A MICROWAVE ACTIVE NOTCH FILTER

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International Journal of Electronics and Communication Engineering & Technology

(IJECET)

Volume 7, Issue 2, March-April 2016, pp. 25-32, Article ID: IJECET_07_02_004

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ISSN Print: 0976-6464 and ISSN Online: 0976-6472

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UNILATERALLY INJECTION-LOCKED

GUNN OSCILLATOR PAIR ACTING AS A

MICROWAVE ACTIVE NOTCH FILTER

Santosh Kumar Dawn

Dept. of Physics, Viva-Bharati, Santiniketan, West-Bengal, India

Taraprasad Chattopadhyay

Dept. of Physics, Visva-Bharati, Santiniketan, West-Bengal, India

ABSTRACT

In this paper, we present the theory and design of a novel microwave

active notch filter operating at X-band (8 GHz-12.4 GHz). The notch filter has

a notch frequency of 10.22 GHz and a 3 dB calculated bandwidth of 87 MHz,

the corresponding 3 dB experimental bandwidth is 120 MHz. The filter is

designed using a magic tee and two tunable X-band Gunn oscillators

injection-locked to the input interference to be eliminated. The special features

of this filter are that it is of low noise, tunable and signal tracking character

and possesses considerable power handling capacity.

Key words: Gunn Oscillator, Injection locking, Microwave, Notch filter,

Notch Frequency.

Cite this Article: Santosh Kumar Dawn and Taraprasad Chattopadhyay.

Unilaterally Injection-Locked Gunn Oscillator Pair Acting As A Microwave

Active Notch Filter, International Journal of Electronics and Communication

Engineering & Technology, 7(2), 2016, pp. 25-32.

http://www.iaeme.com/IJECET/issues.asp?JType=IJECET&VType=7&IType=2

1. INTRODUCTION

Notch filters are essential components of a microwave communication system.

Microwaves, according to IEEE convention, span over the frequency range 3 GHz-30

GHz of the electromagnetic spectrum. Notch filters are used to eliminate monotone or

narrowband interference in a receiver. This interference can appear automatically

from adjacent channels and it can also be man-made when the interference is

introduced deliberately for jamming a communication receiver. A good notch filter

must have low insertion loss, negligible radiation loss and high power handling

capacity which are of major concern at microwave frequencies. A notch filter can be

treated as a special case of band reject filter where the stop band becomes very narrow

Santosh Kumar Dawn and Taraprasad Chattopadhyay


and the attenuation becomes high. Design of microwave notch filters [1-6] is being

investigated over a few decades. To make the notch filters reconfigurable and to

operate at higher microwave frequencies, research work is going on the design of

notch filter all over the globe till now. Jackowski et al., have proposed a frequency

agile, constant bandwidth, reconfigurable notch filter with a tuning range of an octave

[7, 8]. A notch filter has been designed at 13.2 GHz with a bandwidth of 50 MHz and

25 dB rejection at the notch frequency by Narayana et al., . Notch filters have been

designed [10] using Barium strontium Titanate thin film varactor technology by

Ramadugu.

The low frequency notch filter discussed above are all passive, in general. Active

notch filters [4-6] incorporate one or more amplifying devices such as negative

resistance oscillators. These negative resistance oscillators when operated in the

injection-locked mode possess amplitude noise reduction property as well as higher

power handling capacity.

In this paper, we have used a pair of unilaterally injection locked Gunn oscillator

operating at X-band. The Gunn oscillators are locked to the input interference

received at the receiver and follows the interfering signal. The Gunn oscillator pair is

thus made coherent and their outputs are subtracted at the E-arm of the magic tee

resulting in a cancellation of the interference accompanying the input signal assuming

the Gunn oscillator pair to be identical. The desired signal falls outside the rejection

band of the notch filter and passes through it.

2. MECHANISM OF OPERATION

The schematic circuit diagram of the notch filter is shown in in Fig. 1. It incorporates

a magic tee and two coherent Gunn Oscillators connected with the collinear arms of

the magic tee. The input of the notch filter is the H-arm and the output port is the E-

arm of the magic tee. The schottky diode detector is not any part of the notch filter. It

only detects the microwave power and delivers an output voltage proportional to the

input microwave power. It gives a method of indirect measurement of output

microwave power. The detector output voltage is measured in an oscilloscope (CRO).

Figure.1. Schematic circuit diagram of an active microwave notch filter.

Unilaterally Injection-Locked Gunn Oscillator Pair Acting as a Microwave Active Notch

Filter


3. ANALYSIS

For injection locking at microwave frequencies, there must exist a free running

microwave oscillator, called the slave oscillator, which is injected by the output power

of another microwave oscillator, called as the master oscillator. Depending upon the

frequency detuning of the injection signal frequency from the free running slave

oscillator frequency and the relative power of the injection signal, the slave oscillator

gets injection locked to the master oscillator. Under locked condition, the slave

oscillator frequency becomes identical with that of the master oscillator and the input-

output phase error assumes a constant value. If the injection signal power is much less

than the free running slave oscillator power, the latter is said to be under driven. On

the other hand, the locked oscillator is said to be overdriven if the injection signal

power is comparable or greater than the free running slave oscillator output power.

The injection locked Gunn oscillators (GO#2 and GO#3) have amplitude limiting

property. Any amplitude modulation of the injection signal is shaked-off by the

locked oscillators. So, we will not use amplitude modulation as modulation format of

the input signal. Rather, we will consider FM signal input and study the response of

the sub-system to such frequency modulated signal. The interference to be removed

from the receiver is assumed to be monotone in nature.

We will make a static analysis of the sub-system considering a slow variation of

input master signal frequency. The input signal is delivered by a tunable Gunn

oscillator (not shown in Fig. 1). Let the microwave voltage of the input signal be

written as

tVtv 111 sin)( (1)

where 1V is the voltage amplitude and 1 is the radian frequency. The injection

signal voltage to Gunn oscillators (GO#2 and GO#3) is obtained through half-power

division of the input signal at the magic tee junction and is given by2

)(1 tv.

The Gunn oscillator GO#2 and GO#3 are taken of the same type which oscillate at

the same microwave frequency. The voltages of the free-running GO#2 and GO#3 are

described by the equations

tVtv f 0222 sin)( (2)

and tVtv f 0333 sin)( (3)

respectively. Here, 0302 where 02 and 03 are the radian frequencies of the

Gunn oscillators GO#2 and GO#3 respectively. The free running output powers are

2fP and 3fP where 2

22 ff VP and 2

33 ff VP respectively. The output voltages of the

injection-locked Gunn oscillators are assumed to be of the form

)(sin)( 1 iii tVtv (4)

for i = 2, 3. iV is the voltage amplitude and i is the input-output phase error of

the i- th slave Gunn Oscillator. Neglecting any asymmetry in locking, the phase

equation of the injected Gunn oscillator (GO#2 and GO#3) are given [11] by



)(sin22

0

1

0

0

10 tP

P

Qdt

di

oi

inj

Li

ii

i

ii

(5)

for i = 2, 3. )(ti is the input-output phase error for the i- th Gunn oscillator, LiQ

is the loaded Q-factor and oiP is the output power of the i- th injection locked Gunn

oscillator. Under steady state condition of locking, 0)(

dt

td i. Then,

i

i

inj

oiLii

P

PQ

0

012sin

(6)

assuming ii 001 2 and 2

ioi VP is the output power of the i-th locked

oscillator. The amplitude governing equations [11] of the injected Gunn oscillators are

written as

i

i

inj

Li

i

Li

iii

i

P

P

QQ

Raa

dt

da

cos

221

0

0012 (7)

where i=2,3 and fi

ii

V

Va . The parameter

L

Ld

R

RRRR

1 . Here dR is the

magnitude of the negative resistance of the Gunn diode, R is the cavity resistance and

LR is the load resistance. In the steady state of locking, 0dt

dai . The frequency

response of the injected Gunn oscillator is obtained by eliminating i from (5) and

(6) with 0dt

dai as

201

2

0

2

1

222

0

21 i

i

Liii

i

inj QRaa

P

P

(8)

for i=2,3. For a given value of i, equation (8) is numerically solved to get the

frequency response of the injection-locked Gunn oscillator. The variation of 2a and

3a with frequency detuning i01 is shown in Fig. 2 and Fig. 3 respectively.


Filter


Figure. 2. Plot of 2a with Frequency detuning

021

Figure. 3. Plot of 3a with frequency detuning

031

Using the scattering matrix of the magic tee [11], the total input voltage of the

Schottky diode detector connected with arm-4 (E-arm) of the magic tee is expressed

as

i

i

iDin t

vtv

1

3,2

sin2

)( (9)

The detector characteristics as obtained from earlier work [11] is expressed as

2

2

)(tv

Pv

Din

inDout

(10)

where Doutv is the detector output voltage, )(tvDin is the effective microwave input

voltage of the diode detector, inP is the input microwave power of the diode detector

and is the responsively of the detector. From equation (9), we can write

3232

2

3

2

2

2cos

2

1

4

1)( vvvvtvDin (11)



Under locked condition, both GO#2 and GO#3 have the same oscillation

frequency, 1 . The injection power is same for both the Gunn oscillators GO#2

andGO#3. When 32 ,

232

2

max 4

1)( VVtvDiun (12)

This gives the maximum output voltage of the detector. The detector is not a part

of notch filter. It is only used to measure the output power of the notch filter in terms

of voltage. The normalized output power of the notch filter using equation (10) is

given by

232

3232

2

32

2

.max

2

cos2

)(

)(

VV

VVVV

tv

tv

Dout

Dout

=

3232

2

33

2

22

323232

2

33

2

22

2

cos2

ffff

ffff

PPaaaPaP

PPaaaPaP

(13)

The calculated frequency response of the proposed notch filter is obtained from

equation (13) and is plotted in Figure. 4. The normalizing factor is 315 mv. The

Theory and experiment show a good fit.

Figure. 4. Theoretical and Experimental frequency response of the proposed notch filter.


Filter


4. EXPERIMENT

The Gunn oscillators having model no. XG-11 have been procured from M/S SICO

Ltd; India. Magic tee and directional couples used for microwave power measurement

and variable attenuator for input signal power attenuation have been purchased from

M/S Vidyut Yantra Udyog Limited, India. Schottky barrier diode and the microwave

power meter have been procured from M/S Salicon Nanotechnology Limited, India.

The measured lock band for Gunn oscillator GO#2 with free running frequency of

10.22 GHz and free running power of 8.75 mW is 118 MHz for an injection power of

4.25 mW. This corresponds to a loaded Q factor. QL=60. GO#3 has a free running

power of 7.5 mW at the free running frequency of 10.22 GHz. It has a Q-value of 59.

The notch Frequency can be tuned by tuning the free running frequencies of slave

oscillators GO#2 and GO#3.

5. CONCLUSION

The theory and design of an active microwave notch filter has been presented in this

paper. The notch frequency is 10.22 GHz with a 3-dB experimental bandwidth of 120

MHz. This filter has negligible insertion loss and considerable power handling

capacity. Besides, since injection locking has been used in the design, the notch filter

will strongly reduce amplitude noise [11] of the received signal. The notch filter has

also input signal tracking property.

6. REFERENCES

[1] I.C. Hunter and J.D. Rhodes, “Electronically tunable microwave band stop

filters”, IEEE Transactions on Microwave theory and Techniques, vol. MTT-30,

pp. 1361-1367, Sept. 1982.

[2] R. Levy, R.V. Snyder and G. Mathali, “Design of microwave filters”, IEEE

Transactions on Microwave theory and Techniques, vol. MTT-50, no. 3 pp. 783-

793, Mar 2002.

[3] T-Lin. Wu, “Microwave filter design”, Chap.6, Department of Electrical

Engineering, National Taiwan University, John Wiley & Sons, Inc., 2001.

[4] R.V Snyder, “Evaluation of of passive and active microwave filters”, Microwave

Symposium Digest, pp. 1-3, 2012.

[5] B.Y. Kapilevich, “Variety of approaches to designing microwave active filters”,

27 th European Microwave Conference, Jerusalem, Israel, vol. 1,pp.397-408, 8-

12 Sept. 1997.

[6] C.Y. Chang and T. Itoh, “Microwaves active filters based on coupled negative

resistance method”, IEEE Trans. On Microwave Theory and Techniques, vol.

MTT-38, no. 12, pp.1879-1884, Dec. 1990.

[7] D.R. Jackwoski and A.C. Guyette, “Sub-octave-tunable microstrip notch filter”,

IEEE EMC Society Symposium on Electromagnetic Compatibility, pp. 99-102,

Astin, Texas, VSA, Aug. 17-21, 2009.

[8] D.R. Jackwoski, “Passive enhancement of resonator Q in microwave notch

filters”, 2004 IEEE MTT-s International Microwave Symposium Digest, pp.

1315-1318, June 2004.

[9] M.S. Narayana and N. Gogia, “Accurate Design of a notch filter using

electromagnetic simulators”, Applied Microwave and Wireless, pp. 44-49, vol.

12,Part 11, 2000.



[10] J.C. Ramadugu, Design of microwave bandstop and bandpass filters on Barium

Strontium Titanate thin film varactor technology”, Ph. D. Thesis, University of

Dayton, Dayton, Ohio, USA, Dec. 2013.

[11] Arvind Kumar, A Bhattacharya and D K Singh. Microwave Image

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[12] Prof. B.N. Biswas, S. Chatterjee and S Pal. Laser Induced Microwave Oscillator,

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[13] P. Bhattacharyya, S. K. Dawn and T. Chattopadhyay, “Low noise bandpass filter

using an X-band injection–locked Gunn oscillators”, International Journal of

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UNILATERALLY INJECTION-LOCKED GUNN OSCILLATOR PAIR ACTING AS A MICROWAVE ACTIVE NOTCH FILTER