■ FIGURE 15. Unadjusted
■ FIGURE 16. Crystal oscillator
symbol and dialog box
run; it bears the value
"Cx.” By right-clicking on
it, you can set the values
for Cp, Cs, Ls, and Rs as
shown in Figure 16. And
yes, it does exactly what
you'd expect when used
in the earlier circuit of Figure 10.
I have placed a simulation circuit
in the download for this session
(NV_SPICE_ 37) setting up LTspice
to sweep the crystal with a virtual
VFO (variable frequency oscillator).
You can also do this on the bench
if you have a suitable VFO and an
RF millivoltmeter or oscilloscope,
and a frequency counter. This is a
handy way to identify any unmarked
(using a neon bulb)
Placing a capacitor across the neon
makes a relaxation oscillator, as shown
in Figure 19 and Figure 20 (run
NV_SPICE_ 35 from the download).
You may have noticed that I've
been sloppy with entering component values into the LTspice diagrams.
I did this deliberately as LTspice can
accept and understand just about any
value units. For example, a resistor of
10K can be entered as 10,000 or
10000 or 10K or even 0.01 Meg.
I like to use the EU format, so
a four point, seven kilo-ohm resistor
would be written as 4K7, and a four
point, seven ohms one as 4R7. If
you prefer, you can enter explicit
values, or scientific notation too
('0.001' or '1e- 3' or '1m' or '1m0' or
even '0k000001' are all the same
to LTspice). I did trip myself when
entering lowercase 'm' when I wanted
uppercase 'M.' So, for megohms
Before the dawn of solid-state
electronics, vacuum tubes (or valves)
ruled the day. These are sometimes
called 'hollow state' or 'glass FETs,' and
can be simulated in LTspice. If you're an
audiophile or just curious about forgotten
technology, then simulation tools come
to the rescue. In keeping with our
theme of simulating oscillators in this
session, I've cooked up a very simple
hollow state neon bulb oscillator, with
just one capacitor and one resistor.
Recall that a neon bulb has two
stable states: off and on (glowing).
Applying a low voltage causes little
or no current to flow until a threshold
is reached around 100V DC. Once
the device breaks down and conducts, it has a much lower holding
voltage — around 50V — and almost
unlimited current. If the current was
not limited by an external resistor, the
neon bulb would self-destruct! LTspice
has a nice neon bulb Macromodel,
and we can plot the two stable states
by driving it with a linear ramp through
a current limiting resistor, as shown in
Figure 17 and Figure 18 (run NV_
SPICE_ 34 from the download).
When I get stuck in LTspice, I
turn to a goldmine of knowledge at
the LTspice Yahoo Group (
It's a bit intimidating with almost
16,000 members, but so far I've
found solid answers and downloaded
great Macromodels from the
group's files section. If you need a
Macromodel for a component, go to
that vendor's website and search. All
the major semiconductor companies
have SPICE model support.
■ FIGURE 17. Neon bulb simulation.
■ FIGURE 20. Neon bulb
■ FIGURE 18.
■ FIGURE 19.
February 2009 63