If you would like a
circuit that is useful
in obtaining a
distortion RF source
capable of driving a
50 ohm load, then
you’ve come to the
The circuit in
Figure 1 is the
schematic for a
■ FIGURE 1. Schematic of a Series
Tuned Colpitts Crystal Oscillator.
DESIGN YOUR OWN
Photo 1 shows the oscillator
circuit board with an output
filter board. It was constructed on a
piece of double-sided printed circuit
board (PCB) material. The topside of
the PCB was etched leaving “islands”
on which to solder the components.
The copper foil on the reverse side of
the PCB is connected to ground and
forms a ground plane for the circuit.
When constructing this circuit, it is
important to keep all of the component leads as short as possible.
What is “Q”
tant to understand its definition.
The Q of a resonant circuit is
known as the “Quality” factor. Q
determines the sharpness of the
frequency response curve at a
given resonant frequency. Higher
Qs provide greater frequency selectivity. The equation for Q is
Q = Fc/(- 3 dB bandwidth)
The - 3 dB bandwidth is also known
as the half-power bandwidth. This
can also be re-written as
Q = Fc/(Fh - Fl)
We will be using the term “Q”
in the following text, and it is impor-
C5 R6 R7
BNC 50 Ohms
where Fc is the center (resonant)
frequency, Fh is the higher
- 3 dB frequency, and Fl is
the lower - 3 dB frequency.
These - 3 dB points are also
known as the cutoff
frequencies (fc). Note that
this is a lower case “f.” For
example, let Fc (center
frequency) = 10 MHz, Fh =
10. 5 MHz, and Fl = 9. 5
MHz. Solving for Q, Q =
10 / ( 10. 5 - 9. 5) = 10.
This Q should not be
confused with component
Q which is used to
measure the quality of an
inductor or capacitor.
This circuit uses a
series resonant crystal that