IndianJournalofEngineering
& MaterialsSciences
Vol. 1,June1994,pp. 149-152
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Structure
If
and application
of a modified
filter using solid-state microwave
lock-in
notch
,.j.,
oscillator
SuvraSarkarI
PhysicsDepanment,
DurdwanUniversity,
Burdwan713104,India
Received
31 December
1993;accepted
15March1994
\
,
A modified lock-in notch filter (MLINF), based on an injection synchronizedsolid-state microwave (SSMW) oscillator and a voltage-controlledmicrowave phase shifter (VCMPS) has been proposed.The improved strong-signalrejection capability of the MUNF and the better weak signal amplifying property of the system,based on MllNF have been observed through analytical studies.
The algorithmof the MUNF has beenexperimentallytestedin the RF band.
' ,..
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-put
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In the present age of communication explosion, a
frequency channel is being shared by a number of
users and a communication receiver has to identify the desired signal in presence of co-channel
unwanted signals. In some cases, the desired signal may be weak compared to the interference
signal and to reject the unwanted strong signal the
lock-in notch filter (UNF) is often used. A UNF
in the RF band has been describedl, where a 10cal oscillator is synchronized to the stronger component of the input signal and the output of the
synchronized oscillator (SO) is subtracted from
the
input
to eliminate
the strong
signal.
Butsignal
its rejection
capability
is notunwanted
uniform
Theoretical Analysis
Fig. la shows the block diagram of an ISGO
based MUNF, where the input signal is sum of a
weak wanted signal (WWS) and a strong co-channel tone interference signal (SCIS). Assuming the
WWS is free from angle modulation, one can approximate the effective input signal as
..
S(t)=Esm((J).t+xsm~(J)t)
.., (1)
where E and (J), are respectively the amplitude
and angular frequency of the SCIS, while xE and
( (J),+ ~ (J))are those of the WWS. The GO of the
system
be locked
SCIS and producing
the WWS
can
be will
considered
astoa the
perturbation
throughout the lockband of the SO. When the SO
output signal phase lags or leads, the synchronizing signal (SS) phase, depending upon the initial
frequency detuning (FD) of the two signar, the
performance of the conventional UNF (CUNF)
degrades appreciably. In the proposed MUNF
the phase difference between the synchronizing
and synchronized signals is detected using an additional phase detector (PD), then the phase of
the SO output signal is brought in phase with the
SS by means of a variable phase shifter controlled
by the PD output. Subtracting the phase shifted
SO output from the SS, rejection of the strong incomponent can be ensured throughout the
whole lockband of the SO. The additional sub-'
phase modulation of the, ISGO. Following3, the
steady state phase modulation index (M) of the
i...
hedoutput,Fig la-Block diagramof the solid-statemicrowaveoscilla-!
tor basedMUNF
circuits (viz. PD and VCMPS) are available in the
state-of-art of microwave technology. The algori-
(:.-~(- )+SCIS MLlNF ~w
'
thin
of
the
MUNF
has
been
analytlca
II
' fi
y
ven
d
e
in the case of a system comprised of injection
synchronized (IS) Gunn oscillator (GO) and experimentally tested in the RF band.
~
II
WWS{I'\j
(Ws'~-
-s-
d
Notch
at
~
Dulred
Signal~d)
1560
Wrdw
(~I
Mod~~ed
by
Fig, 1b-Schematicdiagramof a MUNF basedsystemused
to extractWWSassociated
withSCIS.~
150
INDIANJ. ENG.MATER.SCI.,JUNE1994
ISGO can be expressed as a complicated function
of x, A,W and the static phase error (;0)' Again;o
is
of FD constant
and M, within
respectively.
Although
M function
is considered3
the lockband
of
When the output of the UNF is applied to a 10cal oscillator (LO) of free running frequency equal
tothewantedsignalfrequency(wd)(Fig.1b),theLO
output can be looked upon as synchronized ampli-
the SO assuming ;0 as [(.7l'/2):t M] but this condition is valid only at the end points. Table 1
gives calculated values of M for different FD and
x.
From these data, it is evident for a given x, M
increases slowly with FD. Now a voltage proportional to the phase difference of the effective input si~
and the output of the ISGO can be obtained by a microwave phase detector (MPD) (designed using hybrid tee, square law detector and
mixer4) and this error voltage is used in a VCMPS
to shift the phase of the SO output accordingly to
implement the MIlNF. The notched output can
then be obtained using the algorithm described
above. Considering output power of the SO large
compared to the input signal, different spectral
components of the output of the MLINF and the
CUNF can be calculated5. Writing (ws+A,w) as
Wd (wanted signal frequency) the following equations may be obtained:
fied version of the WWS applied at the UNF input
and the SCIS will produce angle modulation of
the LO output signal. The performance of the
whole system in extracting the WWS embedded in
the background of the SCIS can be estimated by
evaluating the angle modulation index of the angle
modulated output of the LO of frequency Wd'
Considering Gm(t) and Gc(t) as the inputs to the
LO, the respective values of the angle modulation
indices have been calculated4 and are represented
by Mom and Mac, respectively. Fig. 3 shows variation of Mom and Mac with the FD for different x.
Small magnitudes of Mom compared to Moc confirm better performance of the proposed system.
Gm(t) = 'Imsin(w, t+ alm)+ '2msin(wdt+ a2m)
+ '3msin[(ws- A,w) t+ a3m]
...(2)
and
Gc
( t ) =,
sin
( w.t+
,
a
) +,
sin ( w
.3
lc
2c
+'3csm[(ws-A,w)t+a3c]
Ic
t +
d
a
)
Experimental Procedure
The effectiveness of the proposed MLINF has
been verified by an experiment performed in the
RF band. The output of a synchronized Wienbridge oscillator (WBO) has a relative phase :t..7l'
with the input SS. It is properly attenuated and
phase shifted using opamp based RC phase shifters (PS). such that perfectly notched output can
be obtamed t,hroughout the lockband of the
WBO. Figs 4a and 4b show theoretical variation
of
2c
.., ()
where the suffixes m and c are for the modified
and the conventional system, respectively. , nmand
aRm (n = 1, 2, 3) are complicated function of M
and x while' nc and aRcdepend on M, x and ;0'
Then it is possible to obtain the amplitude ratio of
the wanted signal and the interference signal at
the notch filter output. Defining this parameter as
Ym(= '2m/'lm) and Yc('2c/'IC)' the magnitude of Ym
and Yc for identical input conditions in order to
compare the responses of the MUNF and the
CUNF may be computed. The computed results
have been shown in Fig. 2 which ensures better
performance of the modified system.
LO
output
. ..
phase
and
required
phases
of
lockband (K)
M
the
combination of two PS-one .7l'/ PS 1 and other
(.7l'/2 =1=
;) PS2. Using a particular capacitor, variation of the resistor values (R) required for these
phase shifts can be computed for different values
of the input signal frequency. Computed values of
R have been found to be in close agreement with
the experimentally used resistors (Fig. 4c). Thus it
is observed that using a variable phase. shifter after the SO and controlling the amount of phase
shift as a function of steady-state phase error (;)
produced in the SO depending on the FD, considerable improvement in the performance of the
UNF can be obtained.
0.0048
0.1833
0.2909
0.577
0.6482
0.72
0.8626
0.9703
x -0.1
0.09427.
0.09445
0.09497
0.09607
0.09656
0.09713
0.09846
0.09966
x = 0.3
0.28309
0.2836
0.2852
0.2886
0.2901
0.2919
0.296
0.2977
"
--'t
Phase shifters WIth mput Signal frequency. Th e actual phase shifter has been implemented as a
Table I-Calculated valuesof M for different FD and x
FD
Normalizedby the
,.--
~
.
SARKAR: MODIFIED
LOCK-IN NOTCH FILTER
151
I
~
e
0.19091t
~4'
NUNFI
CllI*"
~
I
1.0
"
..
~
,
~ 0.8
-a.
~
r
t
-~
';&. 0.6
0
4 (f Ifo )
,,
\
0.19091t
.
'.
\.
Fig. 4a- Theoretical variation of the steady-state phase error
of the SO with the input signal frequency.
\
\.
,
~
c
..-
,
.;;:
..c
\"
0.'
0 .005
~
O.
.2
1/1
..
..
0.05
0.10
,
0
~
0.2
Norm
r ~
(G)
.FD,
Fig. 2-Curves showing relative performance of the MUNF
and the CUNF
in extracting WWS [X axis: Norm.
~
~
FD=(w,
~
-wo)/ K, Wo= centre frequency of the SO used in
UNF, K-Lockband
of the SO; Y axis: Ratio of the WWS
arnplitude-to-tfte
SCIS amplitude
at the MUNF
and the
CIlNF output normalized to the corresponding maximum values, x ~ 0.3, (Wd -w,)/ K = 2.8234, sync. signal to oscillator
power = 0.017].
r PS I
~
(b)
095
...(
0
e -Experimental
--Predicted
51
~
~
-I
~
.9. 2.0
G.
NUNF
.
I
i
CUtF
..
" /
1
g 1.0
4
""
(f/fo)
Fig. 4c-Analytically
predicted and experimentally used values of the resistors employed in the PSI and PS2 to imple-
"
ment the MUNF
for different input signal frequency.
[fo=Centre frequency of the oscillator, f=Frequency of the
input signal].
"
~.O05
~
(C)
/.;'
I
:"'i1}J¥
';::~"
.."'{t
~,,"'"
I
2
.
7
f I fo
Fig. 4b-Required phase shifts in the PSI and PS2 as a function of the input signal frequency to implement the MUNF.
).0
~
104
o.
throughout
the lockband
of the SO used in it. The
best performance
is obtained
at the zero FD but
".
N«m FD
Fig. 3-Curves showing performances of the MUNF based
and the CUNF based WWS extracting system. [X axis: Norm.
FD = ( w, -wo)/ K, Wo= Centre frequ7n~y of the SO used .in
UNF, K-Lockband
.of ~e SO WIthin the UNF; ~ axiS:
the
Output
corresponding
phase modulatIon
minimum
Index values.
of the ISGO
x = 0.3,
normalIzed
(Wd-w,)ito
it degrades with increase of the FD due to steady
state phase error between the SS and the output
of the SO.
2 The proposed
UNF
is very simple to implement and has better performance
over CUNF.
Its~
Interference
rejectIon
capabIlity
IS almost same m
K= 2.8234, sync. signal-to-oscillator power=0.017].
Conclusion
On the basis of the above studies,
it may be
concluded
that
..ence
1 The performance
of the CUNF
m extractIng
the whole lockband
of the SO.
3 The proposed
MLlNF-based
system would)
be more suitable
to extract and to amplify
the
weak ~ignal in the background
of strong interfer-:
sIgnal compared to the CUNF based system.
4 Preliminary
experiments
confirm
the possibi-~"I
WWS
lity of realisation
in the
environment
of SCIS
is not uniform
of the practical
MUNF.
152
INDIAN J. ENG. MATER. SCI.,JUNE 1994
Acknowledgement
The author is grateful to Prof. B N Biswas for
his interest in the work. Financial assistance from
the Council of Scientific & Industrial Research,
lh.. at
full
k
I d d
New De I, IS so grate
yac nowe ge .4
R fi
e erences
1 Pramanik K & Ray S K, Indian J Pure Appl Phys, 15
(1977).
j
2 Pramanik K, Studieson unilaterallyand bilaterallycoupled
oscillators,Ph D Dissertation.The University of Burdwan,
1981,199.
3 BiswasB. N, Phase-Lock Theories&Applications (QxfordmM,Indla), 1987,108.
Biswas B N, Chatterjee S, Sarkar S, BhattacharyaA K &
Ray S K, IEEE TransMicrowave Theory Tech,35 (1987)
812.
5 Biswas B N, Sarkar S & ChatterjeeS, IEEE TransMicrowaveTheory Tech,37 (1989)627.
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