Generally, I only take a circuit’s power requirements
back to the voltage and current requirements needed, and
leave it up to the builder to rummage through their junk
box for the parts necessary to complete the raw voltage
supply. Since this project includes an odd assortment of
voltages, I am including a schematic for it. The circuit is so
simple and straightforward that no explanation is really
needed here. Hence, no parts list was written up for it.
However, the transformers will appear in the overall parts
list, and they are quite small and cheap.
Also, you may be wondering about the -13V supply. I
wanted a ±12V supply for the op-amps. The negative
supply is used as a reference voltage at many points on the
sweep generator board (Figure 3). Since the 79L12 can
vary by almost a volt either way in tolerance, I wanted to
make sure I had a specific voltage at this point. This
simplified the design and will always insure that it can
maintain that voltage — even if replacement of that
regulator was required. The regulator voltage can be
adjusted up by virtue of its common pin resistor, but
you can never make it lower. So, by adding resistance
to that pin, a -13V output will always be obtainable —
no matter what the regulator’s tolerance is. The 79L12
I used required a 510 ohm resistor to output exactly
- 13.0V. The 79L12 you use may have a different
tolerance and will require a different value. Shoot for
1% of the target voltage — not difficult to do. The
attenuator is shown in Figures 2 and 6.
As you can see in the photo, I built mine on a
shielded panel that gets enclosed in a box. This was
probably overkill as 40 dB of attenuation is not that
great. I think the switches could merely be panel mounted
and spaced on 5/8” centers without any shielding and
perform just fine. These use garden variety miniature toggle
switches and 5% 1/4 watt resistors. I have had decent
performance in the past with this type of construction in
upwards of 200 MHz frequency. This one checked out with
a worst case error of 3%. Note that the top two attenuator
switches shown on the front panel look a little odd. This
was a temporary change I made to facilitate a battery of
repetitive tests I am making, and normally these would be
side to side toggles as the lower two are.
Figure 7 shows the internals and can guide you as to
overall layout. I built the chassis by forming a 90 degree
bend in a piece of 1/8” aluminum sheet stock. Four 3/4”
aluminum L brackets were cut and mounted to this piece
for attaching the front panel and cover. The front panel was
cut from 1/16” aluminum sheet and fastened to the two
front brackets with two 6-32 screws. The cover was a piece
of 20 gauge steel sheet with two 90 degree bends that was
attached to the side brackets with four 6-32 screws.
The front panel layout points and artwork were made
using a free program from Front Panel Express
( www.frontpanelexpress.com). See my power supply article
in Nuts & Volts’ March 2007 for more detailed info on
Calibration and Usage
For the following tests and procedures, an oscilloscope
and frequency counter are required. Starting with the RF
deck, ideally you will want three voltages to coincide with
those shown in Figure 1. Since Q1, Q2, and Q3 are DC
coupled, the DC current through R4 sets these levels. You
want to be as close to 2. 3 VDC here as possible by
selection of the value of R1. This could be quickly done on
a solderless breadboard with the intended transistor used.
The next point to check is the signal level at the Q2
emitter. This should be within 5% of the 370 mV P-P value
as shown back in Figure 1. Minor changes in R13-R17
(AGC) will set this if necessary for those particular bands.
Finally, the signal level at the C11, R9 junction should
be 185 mV P-P. Make these tests at 2 MHz with your
34 December 2013
■ FIGURE 6. Attenuator.
■ FIGURE 7. Internals.