Page 239 - Complete Wireless Design

Oscillator Design

238 Chapter Four

B, should load the resonator as lightly as possible to prevent frequency changes
and an excessively decreased gain margin.
The attenuation of harmonics will also be helped by coupling the energy out
of the oscillator with an inductor, since the inductor’s reactance increases with
frequency, thus decreasing harmonic output. If an inductor is chosen to couple
power out of an oscillator, then a blocking capacitor (X 1 ohm) will be
C
required in most applications to prevent DC from entering or exiting the stage.
A single coupling capacitor is cheaper and smaller than the required induc-
tor/capacitor combination, and so is more common in low-cost applications.
The higher the gain margin of an oscillator, the higher will be the allowed
output power that can safely be coupled out to a load, since a high gain margin
will have more room to fall before it becomes dangerously low within the feed-
back loop. This means that a decreased coupling reactance can be utilized (X ≈
50 ohms) for a power output that is not too low for your particular application.
Adding a 6- to 10-dB pad between a VCO coupled output and its amplifier,
filter, or mixer stage is normally required to prevent reflections from occurring
back into the oscillator. The pad will decrease phase noise and load pulling,
which are critical parameters in most LO applications.

Oscillator output power. RF power out of a Class A oscillator (or amplifier) can
be calculated by taking the transistor’s maximum V peak voltage swing and
CE
subtracting the minimum peak V swing, then multiplying this by the maxi-
CE
mum I peak minus the minimum I peak, and dividing by 8:
C C
(V V ) (I I )
CE(MIN)
C(MIN)
CE(MAX)
C(MAX)
P (in mW)
RF
8
This formula demonstrates that the wider the voltage swing between the
collector and emitter, and/or the wider the collector current swing, the higher
the output power in any Class A–biased active device.
Basically, an oscillator will output close to the power of the transistor’s P1dB
when the DC bias is set for this maximum output power. Any output pad at
the oscillator’s output must, of course, be subtracted from this value; while any
buffer amplifier’s gain must be added. However, high oscillator loop gain will
not always mean higher output power dB for dB, since the oscillator may be
reaching hard saturation and cutoff at every cycle. But the power that can be
tapped from the oscillator will be approximately the P1dB of the transistor,
with the actual available output power being lowered by the reactance of the
coupling capacitor or inductor. Yet, if loop gain is excessively low, the available
output power of the oscillator—before any power is tapped for the load—will
be something less than the P1dB of the transistor.
Biasing the oscillator at higher than normal quiescent current levels, and
using a higher-frequency transistor, will increase the oscillator’s output pow-
er. Nevertheless, the higher the output power of an oscillator, the more diffi-
cult it can become to prevent frequency drift due to heat buildup during
warm-up and operation.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.