DN36

Design Note 36
Issue 1 May 1996
ZDC833A Dual Variable Capacitance Diode
C1
(a)
Vbias
(b)
Vbias
(c)
Figure 1.
Basic Tuned Circuits.
Va r i ab le c a pa c ita nc e di odes hav e
established a broad applications base
f r o m v o l ta g e c o n t r o l l e d c r y s t a l
oscillators (VXCO) to television and
cellular communications. They are
capable of providing many advantages
in terms of space and cost savings, and
also allow implementation of such
features as frequency scanning and
stored tuning.
The standard ‘LC’ parallel circuit is
shown in figure 1a). For electronically
tuned s y stems t h e ‘C ’ e l e me n t i s
replaced by a variable capacitance diode
as figure 1b) - this design requires a DC
blocking capacitor C1, and a voltage bias
supply. This method does have some
disadvantages, namely:
1. Under high signal conditions, the
diode can become forward biased
leading to rectification and generation of
distortion products.
2. The maximum frequency of operation
is limited by the minimum capacitance
exhibited by a single diode.
3. The DC blocking capacitor required
needs to have a large value which may
be undesirable in space limited designs.
Many oscillator circuits are designed for
the higher frequency bands where low
values of capacitance are required. In
these cases, a common option is to use
two variable capacitance diodes in
series as shown in figure 1c).
DN36 - 1
Design Note 36
Issue 1 May 1996
This provides a circuit capacitance of a
half that of a single diode, prevents
forward biasing (as each diode is
bl oc k e d by its reverse c onnec ted
partner), and removes the requirement
for a DC blocking capacitor.
To allow further cost and space
reductions, Zetex have produced a
monolithic dual variable capacitance
diode available in the industry standard
SOT23 package. This part is based on the
single diode ZC833A device, and so has
been called a ZDC833A - the capacitance
versus voltage curve (C-V) has been
reproduced in figure 2. As the dual diode
is produced monolithically (Eg. on the
same piece of silicon), the parametric
m a t c h i n g i s i n h e r e n tl y e x c e l l e n t ,
typically within 0.25%.
C - Capacitance (pF)
100
10
1
0.1
1
10
100
VR - Reverse Voltage (V)
Figure 2.
Capacitance v Reverse Voltage for the ZDC833A.
Appendix
PARAMETER
SYMBOL
MIN.
Reverse Breakdown Voltage
VBR
25
Reverse Leakage Current
IR
Temperature Coefficient
η
Diode Capacitance
Cd
29.7
Capacitance Ratio
Cd / Cd
5.0
Figure of Merit
Q
200
TYP.
33
DN36 - 2
MAX.
UNIT
CONDITIONS.
V
IR = 10µA
20
nA
V R = 20V
400
ppm/°C
V R = 3V, f=1MHz
36.3
pF
VR = 2V, f=1MHz
6.5
VR = 2V/20V, f=1MHz
VR = 3V, f=50MHz
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