Let’s review for a moment. First, oscillators need gain (A), a
frequency-selective filter, and a
positive feedback loop (β) that all
combine to satisfy the Barkhausen
criterion, Aβ = 1.1 Even if you didn’t
intend to create an oscillator, if those
three conditions are present, an
oscillator you will have!
At RF, the two fundamental
oscillator types were devised to
create feedback through reactances
that formed a voltage divider with
inductance (the Hartley oscillator) or
capacitance (the Colpitts oscillator).
That’s where we left things.
Practical RF
Oscillators
The two schematic snippets in
the previous column showed the
basic idea but weren’t practical
circuits, omitting such important
items as power supplies, biasing, and
output connections. So, how do we
make an oscillator, really? Figures 1
and 2 show a pair of actual
functioning oscillator circuits.
You can build them and listen to
their output signal on a world band
or ham receiver between 7. 5 and 8
MHz. (You’ll need to listen in SSB or
CW mode since the steady output
has no modulation for an AM radio
to detect.)
Each of the oscillators has a
parallel-LC “tank” circuit that is a filter
at its resonant frequency. The filter, in
turn, determines the oscillator’s
frequency. Those components have
the designators Ct and Lt.
Why is an LC circuit called a tank
circuit? Because it stores energy like
an electrical flywheel. The energy
sloshes back and forth from the
inductor to the capacitor at the
circuit’s resonant frequency. The
positive feedback supplies a little
energy and the JFET’s gate takes a
little out. The resonant frequency of
the tank circuit is:y
For the Hartley circuit (Figure 1),
Lt = Lt-a + Lt-b = 1.48 µH because the
two inductors are in series, thus fo =
7.96 MHz. In the Colpitts circuit
(Figure 2), Ct = (Ct-a x Ct-b) / (Ct-a + C
t-b) = 410 pF because the two
capacitors are in series. Thus, fo = 7. 5
MHz. Both circuits are from an
excellent electronics design
reference, Experimental Methods in
RF Design by Hayward (W7ZOI),
Campbell (KK7B), and Larkin
(W7PUA).
What is the purpose of the other
circuit components? Cbyp is a bypass
capacitor to keep the drain of the
hwardsil@gmail.com
■ BY WARD SILVER N0AX THE HAM’S WIRELESS WORKBENCH
RF Oscillators
In the previous column, we learned what makes
an oscillator do what it does, and tried a simple
low frequency example. Now, it's time to move
up — in frequency — to the oscillators which
make the signals that drive the ham's wireless
world.
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
May 2015 19
FIGURE 1. The Hartley oscillator will operate around 7. 9 MHz. J310 JFETs in
through-hole TO-92 packages are available from Mouser Electronics
( www.mouser.com) and in SMT packages from many suppliers. Inductor Lt-a
consists of 17 turns of #24 or #26 solid wire on a 0.5” diameter, type 6 powdered-iron toroid. Inductor Lt-b is 10 turns on a 0.3” diameter, type 6 powdered-iron
toroid. For best stability, Ct should be a polystyrene film or silvered-mica capacitor.
Powdered-iron types refer to Amidon Corporation products
( www.amidoncorp.com). +V can be from 6V to 15V with excellent results.
fo = 1
2π√LtCt
