Page 249 - Complete Wireless Design

Oscillator Design

248 Chapter Four

Increase reactance value of C to decrease coupling power output to
COUP
load; decrease reactance value to increase coupling. Start with 50- to 200-
ohm X for a 50-ohm load, then simulate to confirm proper loop gain.
C
Pierce overtone crystal mode oscillator (Fig. 4.27). It may be necessary to employ
a lower-frequency crystal than what is required in our design because of crys-
tal cost or availability issues, or we may simply want a higher-frequency oscil-
lator than can normally be obtained by running a crystal in its fundamental
mode. A problem encountered in high-frequency crystal oscillator design is
that as the frequency of the oscillator is increased, the crystal’s internal resis-
tance decreases, and a point is soon attained where it becomes troublesome to
obtain a suitable impedance match for the crystal within the oscillator’s cir-
cuit. This demands running a crystal at one of its overtone frequencies, which
increases the crystal’s series resistance, as well as more frequency accuracy
and stability. However, we must always choose a crystal that has been cut
specifically for overtone operation.
To design a Pierce overtone oscillator:

1. C C 1 ohm (X )
BYPASS C C
2. C 2000 pF/(1
f ) (f is the actual overtone frequency we wish output
2 r r
from the oscillator, and not the crystal’s much lower fundamental fre-
quency).
3. C 0 to 6 pF trimmer.
3
4. R R 3/2 f C
C r 1
5. V value in the S-parameter files for the transistor chosen.
CE
6. I value in the S-parameter files for the transistor chosen.
C
(I R V ) V
CC
CE
C
C
7. R
VCC I
C
8. R [(V 0.6)/I ]
f CE C
9. Increase the reactance value of C to decrease coupling; decrease the
COUP
reactance value to increase coupling. Start with 100- to 200-ohm X for 50-
C
ohm load and simulate.
10. Use proper overtone crystal (odd modes only), while modeling the crystal’s
equivalent circuit values according to the manufacturer’s specifications for
the overtone and frequency required in the linear software simulator pro-
gram.
11. L and C will be a tank circuit resonant at:
1 1
1
f
mid 2 L C
1 1

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