Page 237 - Complete Wireless Design

Oscillator Design

236 Chapter Four

4.2.4 Testing LC oscillators
To confirm proper operation of the physical prototype:

1. Supply the oscillator with its nominal V . Only the fundamental frequen-
CC
cy and its harmonics (at 10 dBc or less) should be present at the oscillator’s
output. Spurious small-amplitude signals from external EMI entering the
oscillator may be present, and are usually acceptable. In a fundamental
(nonmultiplied) oscillator, there should be no subharmonics (0.25f , 0.5f ) at
r r
the output due to parametric oscillations.
2. Smoothly change the voltage of the oscillator’s V from 0 V to the maxi-
CC
mum safe operating voltage. During these changes, both up and back down
again, there should be only uniform variations in the oscillator’s output fre-
quency and power, with no unexpected jumps—except at some low V
CC
where oscillations will cease.
3. In a VCO, any tuning voltage that is applied to the varactor diode should
show up as a smooth oscillator output frequency—with no severe power
dips or peaks—across its full monotonic tuning range.
4. The oscillator should function within frequency, power, and harmonic specifi-
cations when operated over wide temperature, load, and vibration deviations.

4.2.5 LC and VCO oscillator issues
Oscillator output coupling. There are multiple ways of coupling the oscillator’s
output signal into another device, with different power, phase noise, and 50-
ohm match conditions. Since it is impossible for most oscillators to supply a
perfect match (50 j0) directly into the next stage without decreasing the gain
margin to dangerously low levels (and even to the point of quenching oscilla-
tions completely), a compromise must be found. The following examples are all
of the viable options for coupling power out of an oscillator, depending on
requirements:
1. Tapping the output of the oscillator with a directional coupler is simple
when practiced with narrow tuning VCOs and with crystal oscillators. This
method will allow medium output power (depending on the dB coupler cho-
sen), great phase noise, and a good 50-ohm match at the coupler’s output.
2. Tapping the output of the oscillator with a high-reactance capacitor or induc-
tor (50 to 200 ohms for a BJT), followed by a 50-ohm 10-dB pad into a discrete
common-emitter or MMIC amplifier, will supply medium to high power, with
good phase noise and a very good 50-ohm match at the amplifier’s output.
3. Tapping the oscillator’s output with a low-reactance coupling capacitor (X
C
1 ohm) into a high-impedance common-collector buffer amplifier will
supply medium power, poor phase noise (phase noise is created by complex
interactions between the oscillator and its less-than-perfect match to its
buffer), and a good to excellent 50-ohm match at the buffer’s output.

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