SL1454
15V
VIDEO
OUTPUT
5
6
4
3
40n
SL1454
1n
140MHz
INPUT
0·1µ
1n
0V
7
8
2
1
82
Q
=
2
33p
Fig. 4 Typical application for 140MHz
APPLICATION NOTES
The SL1454 FM demodulator has a very simple application
with very low external component count. This is demonstrated by
the applications circuit diagram Fig. 4, but as with most integrated
circuits, particularly those working at high frequencies, some
attention to good RF layout techniques and correct component
selection will ensure optimum results.
A good layout can usually be ensured by the simple precau-
tion of keeping all components close to the SL1454, maintaining
short lead lengths and ensuring a good low impedance ground
plane. Double sided board layout enables these objectives to be
easily met, but is not essential for satisfactory operation. All
coupling and decoupling capacitors should be chosen for low
impedance characteristics at high frequencies. A fairly stable
component should be selected for the quadrature coil tuning
capacitor to prevent excessive drift. The power supply decoupling
capacitor from pin 6 to ground should be 0.1µF minimum, but the
input coupling and decoupling values can be smaller, about
330pF being adequate.
The only remaining components to be selected are those
forming the quadrature circuit on pins 2 and 3 and some care in
the determination of values for these is required if maximum
performance is to be obtained.
Choose suitable values for L and C to resonate at the
intermediate frequency you are applying to the device, using:
f=
1
2p=LC
reducing the video output level is to incorporate a dual tuned
circuit in the quadrature network. This can easily be done by
capacitatively coupling another parallel tuned circuit to the
normal quadrature tuned circuit.
Fig. 6 shows an example of this form of dual tuned circuit, both
sections having the same Q factor and coupling capacitors
chosen to give the best linearity (linear phase response). Fig.5(b)
shhows the advantages of the dual tuned circuit. The effect of
varying the Q factor of the dual tuned circuit on bandwidth is also
described by Table 1.
Example
Design a quadrature circuit to demodulate a 140MHz carrier
with centre with 21.4MHZ peak to peak deviation, modulated
with a 25Hz triangular dispersion wave form of 2MHZ peak to
peak deviation. The video bandwidth required is 9MHZ.
Choose L = 40nH
then
C = 32.309pF (nearest preferred value 33pF)
The next value to choose is the Q factor. As dispersion is
employed, linearity over the full 21.4MHz range needs to be
optimised. The graphs in Fig.5 show that either a single tuned
circuit with a Q of 2, or a dual tuned circuit with a Q of 3 is
adequate. The dual tuned circuit has the advantage that the peak
to peak video output is larger than that of the single tuned circuit,
but extra components are required. Both circuits have a larger
video bandwidth than the required 9MHz. The value of the
damping resistor for the required Q is calculated below:
For Q = 2
Total R = Q2πfL
= 2323π3140310
6
30·04310
26
= 70·3717Ω
Allowing for the internal 800Ω resistance between pins 2 and 3
(see Fig. 3), the external resistance should be 77.1Ω Choose
.
82Ω
..
For Q = 3
Total R = Q2πfL
The value of C should by greater than 15pF to prevent stray
capacitance effects introducing errors and distortion of the
demodulation S-curve, but the use of very large capacitances
with small inductance values will lower the impedance of the
tuned circuit at the required Q value, reducing the drive level to
the demodulator and thereby restricting the video output available.
Once suitable L and C values have been determined, the
working Q for the quadrature circuit should be set, the Q value
determining the video output level and bandwidth. Video output
is proportional to Q whereas video bandwidth is inversely
proportional. The effect of Q variations on video bandwidth and
amplitude can be determined from Table 1 and the graphs in
Fig.5.
A value for total damping resistor value to obtain the required
Q can be calculated from:
R = Q2πfL
The internal 800Ω resistance between pins 2 and 3 must be
allowed for when calculating R.
As can be seen from the graphs in Fig.5, for the demodulator
to demodulate a 20MHz peak to peak deviation signal with
optimum linearity a very low Q value needs to be chosen (,2).
However, this has the disadvantage of producing a demodulator
with a very low peak to peak video output level.
One way of increasing the linear region of the S-curve without
= 3323π3140310
6
30·04310
26
=105·56Ω
Allowing for the internal 800Ω resistance, the external resistance
should be 121·5Ω, so choose 120Ω.
When using a dual tuned circuit the value of coupling capaci-
tor is dependent of the Q factor. Table 2 gives a guide to the
values needed for best linearity.
3
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