RC2798G Fairchild Semiconductor, RC2798G Datasheet - Page 9

• Manufacturer Part Number: RC2798G
• Description: Up-Down Converters TSSOP-20
• Manufacturer: Fairchild Semiconductor
• Type: Frequency Down Converterr

Specifications of RC2798G

• Mounting Style: SMD/SMT
• Operating Supply Voltage: 5 V
• Maximum Operating Temperature: + 75 C
• Minimum Operating Temperature: - 40 C
• Package / Case: TSSOP-20
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PRODUCT SPECIFICATION
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 =1KW @ 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:
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).
+5V
Oscillator O/P
10µF
15pF for Fundamental
Parallel Resonant
2pF for Overtone
+
Figure 8. Fundamental or Overtone Mode—Parallel Resonant XTL
38.886 MHz
.1µF
FB
10µF
220pF
XTL
Cc
+
.1µF
OSC_OUT
AGC_IN1
AGC_IN2
VCC MIX
VAGC
GND
10
1
2
3
4
5
6
7
8
9
C1
C2
B2
B1
The XTL is a series resonant type and it is operated at third
overtone frequency.
For symmetrical reasons, the following design is recom-
mended for better duty cycle (50 to 50%) output from VCO
(see Figure 9).
RC2798
Qo = w
w
o
= 2 p fo = (1/LoCo)
o
Co Rin
GND
MIX_OUT2
MIX_OUT1
G1A
G1B
VampIn1
VampIn2
VccAmp
OUT1
OUT2
20
19
18
17
16
15
14
13
12
11
1/2
RC2798
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