SL1452
15V
0·04µ
VIDEO
5
6
7
8
4
3
2
1
OUTPUT
330
27p
SL1452
1n
612MHz
INPUT
0·1µ
1n
0V
Fig. 4 Typical application
SL1452 QUADRATURE DEMODULATOR
The SL1452 FM demodulator has a 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 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
A good layout can usually be ensured by the simple precau-
tion of keeping all components close to the SL1452, maintaining
short lead lengths and ensuring a good low impedance ground
plane. Doublesidedboardlayout enablestheseobjectivestobe
easily met, but is not essential for satisfactory operation. All
coupling and decoupling capacitors should be chosen for low
impedance characteristics at high frequencies, multilayer ce-
ramic types usually providing small size and adequate high
frequencyperformance. Forthequadraturecoiltuningcapacitor
afairlystablecomponentshouldbeselectedtopreventexcessive
drift.Thepowersupplydecouplingcapacitorfrompin6toground
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.
allowed for when calculating R.
Example
Design a quadrature circuit to demodulate a carrier on pin 8
with centre frequency 480 MHz and video bandwidth of 10MHz.
For L = 40nH, fQUAD = 120MHz,
C = 43·98pF (nearest preferred value 47pF)
From Table 1, Q required is approximately 6,
therefore total R required is:
R = Q2πfL
= 6323π 3480310630·0431026
4
=181 ohms
Allowing for the internal 800Ω resistance between pins 2 and 3
(see Fig.3), the external resistance required is 234Ω. ; choose
270Ω.
It should be remembered that the internal 800Ωresistance is
subject to production tolerances and if fairly close control of
video bandwidth is required, the L and C ratio may require some
adjustment to ensure that the external R is sufficiently low to
swamp the effect of internal resistance changes. The value of
270Ω obtained in the example is low enough to allow adequate
control.
First determine the quadrature circuit operating frequency,
which is a quarter of the input frequency on pin 8 due to the two
internal 42 stages (see Fig.2).
Choose suitable values for L and C to resonate at the correct
frequency using:
1
f =
2p=LC
The value of C should by greater than 15pF to prevent stray
capacitance effects introducing errors and distortion of the
demodulation 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.
In general, for operation in the 400MHz to 600MHz range, an
inductance value between 40nH and 60nH is recommended.
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.
In order to overcome the effects of component tolerances, it
will usually be necessary to make either the L or C a variable
component, the value being adjusted to obtain best linearity.
Q
10
6
Bandwidth
7·5MHz
14MHz
4
23MHz
Table 1
3
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