Project
Figure 3. The rates and widths are marked on the front panel.
of the components were installed on the board with
the exception of C5 through C15, which I will address
shortly.
Q2 was chosen for its high-speed operation and
power handling ability. Most high-speed switching transistors will work fine, and even the lowly 2N3904 performed satisfactorily in this circuit. C16 should be
mounted close to this collector. No circuit layout problems were encountered in this construction, even with
the front panel wires grouped and laced. J1, J2, and J3
are all accessible through a cutout in the back wall of
the housing.
As for the timing capacitors, C5 through C15, I
elected to mount these between the decks of rotary
switches S6 and S7. When the associated variable controls (P2 and P3) are in their calibrated position (fully
counterclockwise), the rates and widths will be as
marked on the front panel switch positions. These are
as follows:
200 mA.
I built the actual circuit on a perf board with traces
that would accept dual inline package (DIP) sockets. I
always use sockets in my projects for future modifications or troubleshooting. This board was 3. 25 x 5. 5 inches and was laid out as shown in Figure 2. Almost all
RATE:
2 µsec, 10 µsec, 100 µsec, 1 msec, 10 msec, 100 msec
WIDTH:
1 µsec, 10 µsec, 100 µsec, 1 msec, 10 msec
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48
Circle #128 on the Reader Service Card.
The variable controls will take you from one range up
to the next, with a 10 percent overlap to ensure complete
coverage. In the calibrate position, accuracy will depend
on how close the capacitor values are to the required
values.
On some ranges, I nailed it on the first capacitor
picked. On others, I had to sneak up on it with two capacitors (one large, one small). I aimed for one-percent accuracy on all ranges, and it was not difficult to obtain this.
There is no point in trying for greater accuracy than this,
as capacitor stability will not guarantee the timing will
remain that stable. If you want greater stability and accuracy, you will have to use expensive polystyrene or similar
capacitors.
These extreme accuracies are really not necessary, as
you will usually be using this unit in conjunction with other
test equipment (scopes, etc.) for cross-checking. The
parts list shows the target values of capacitors C5 through
C15. These values may vary in your particular circuit, but
offer a close starting point.
Also, the potentiometers (P2 and P3) can be shunted
with high-value resistors to slightly alter their values and
bring the low-end timing (P2 and P3, fully clockwise)
more in line with the expected rates and widths at these
points. The important function in these two circuits is not
so much that you have front panel accuracy, but rather
that you have complete coverage across the switched
bands.
The actual output drive current that the generator will
produce depends on transistor Q2 and the duty cycle (per-
APRIL 2005