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PRODUCT SPECIFICATION
RC2798
9
Overtone Mode—Series Resonant XTL
If it is desired to operate a XTL at non-fundamental or
overtone frequency, an AC coupled parallel resonant network
should be connected to feedback input pin, B1. The typical
impedance looking into B1 with B2 AC grounded is approxi-
mately Rin =1K
W
@ 38MHz. It is recommended to design
the value of Qo at approximately 15 to 25. The Lo and Co
values can be calculated from the following equations:
Qo =
w
o
Co Rin
w
o
= 2
p
fo = (1/LoCo)
1/2
The XTL is a series resonant type and it is operated at third
overtone frequency.
Figure 8. Fundamental or Overtone Mode—Parallel Resonant XTL
1
AGC_IN1
220pF
FB
10
μ
F
10
μ
F
.1
μ
F
.1
μ
F
+
+
+5V
Oscillator O/P
XTL
38.886 MHz
Parallel Resonant
Cc
15pF for Fundamental
2pF for Overtone
2
AGC_IN2
3
VAGC
4
VCC MIX
5
OSC_OUT
6
GND
7
B2
8
C1
9
C2
10
20
19
18
17
16
15
14
13
12
11
B1
GND
MIX_OUT2
MIX_OUT1
G1A
G1B
VampIn1
VampIn2
VccAmp
RC2798
OUT1
OUT2
Fundamental or Overtone Mode– Parallel
Resonant XTL
Figure 8 shows the implemenation of parallel resonant XTL at
fundamental or overtone frequency. The XTL is a parallel
resonant type and can be operated at either fundamental or
third overtone frequency depending upon the feedback
capacitor, Cc. When used with Cal Crystal Lab’s XTL,
P/N#CCL-6-38.8860G153, for Cc = 15pF it operates at
fundamental mode and for Cc = 2pF, it operates at third
overtone mode (38.886MHz).
For symmetrical reasons, the following design is recom-
mended for better duty cycle (50 to 50%) output from VCO
(see Figure 9).