Page 252 - Complete Wireless Design

Oscillator Design

Oscillator Design 251

The TCXO is a highly temperature stable crystal oscillator, with better than
1 ppm stability. TCXO’s make use of a temperature sensor, normally a ther-
mistor, which generates a correction voltage to the TCXO’s compensation net-
work, a varactor diode, which then overpowers the frequency drifting effects of
the changing temperature within the oscillator’s circuits. TCXOs are lower in
cost, with significantly less current requirements than an OCXO, and warm
up nearly instantaneously.

4.3.5 Testing and optimizing crystal oscillators
To test and optimize the completed physical oscillator for start-up and proper
functioning (frequency, amplitude, spurs, etc) under most real-world condi-
tions as expected in normal operation, duplicate the following tests and opti-
mization procedures.

1. Connect the closed-loop oscillator to a spectrum oscillator.
2. Confirm that the oscillator reliably starts at room temperature by turning
it on and off a number of times. Cool the circuit with a canned cooler and
repeat. Heat the circuit with a heat gun set on low and repeat.
3. Test for the expected output:
a. Check that the oscillator frequency is stable at room temperature, and
that it is stable during and after the above heat/cool test.
b. Check for close-in spurious responses.
c. Check the approximate phase noise level (see “Phase noise tests” on
p. 94).
d. Check that the amplitude is as expected, and that it is stable over time.
e. Check for any wide-ranging spurious responses and excessive harmonic
levels.
It will normally be necessary to improve the amplitude, stability, starting,
and spectral purity of the oscillator by tweaking for optimum performance:
Change the L/C or R/C ratio; vary the transistor’s bias current; tweak any tun-
ing capacitor in series with the crystal. Perform until an optimum point is
reached that satisfies your required specifications. Always send the oscillator
through the full complement of starting, frequency, amplitude, and spurious
response tests after any tuning procedure is completed.

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