Nuts and Volts - February 2009

■ FIGURE 15. Unadjusted crystal oscillator breadboard frequency.

■ FIGURE 16. Crystal oscillator symbol and dialog box (NV_SPICE_ 34).

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 crystal units.

The Hollow
State
Oscillator
(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).

LTspice Units

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 use Meg!

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).

On-Line Support

When I get stuck in LTspice, I turn to a goldmine of knowledge at the LTspice Yahoo Group (

http://tech.

groups. yahoo.com/group/LTspice/). 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.