74 Silicon chip Australia’s electronics magazine siliconchip.com.au
An AM/FM/CW
Scanning
HF/VHF RF
Signal Generator
Part 2
by Andrew Woodfield, ZL2PDWe introduced this RF signal generator last month. It is an ideal entry-
level test instrument for anyone into radio: capable, yet low in cost and
quite easy to build. None of the parts are too hard to come by, either...
Now let’s get into building it – and getting it up and running. We also
have some performance plots and instructions on how to use it.
T
he signal generator is built on
one double-sided PCB coded
04106191, measuring 152.5 x
102mm. Refer to the PCB overlay dia-
gram, Fig.5.
Most of the top (component-side)
surface has been retained as a ground
plane for added shielding. No SMD
parts are used in the construction of
the signal generator, making it rela-
tively easy to build.
Start by fitting all the resistors where
shown. It’s best to check each part with
a DMM set to measure ohms before
fitting them, as the colour bands can
be hard to distinguish (eg, brown can
look like red, as can orange). Don’t for-
get the 47Ω resistor hiding under S4!
Then mount diodes D1 and D2, en-
suring they are orientated as shown.
Next, mount the socket for IC1, with
its notched end facing the top of the
board.
Now fit the ceramic and MKT ca-
pacitors, which are not polarised.
Don’t get the different values mixed
up though. There’s also one of these
under S4. Follow with trimpot VR1
and plastic package transistors Q1, Q2,
Q4 & Q5. Q4 is a different type than
the other three.
Next, solder 6-pin header CON3
and two-way headers CON4 and JP1
to the board, followed by the power
socket (CON1) and then the electro-
lytic capacitors. These are polarised;
in each case, the longer lead must go
to the pad marked with a “+” on the
PCB. The stripe on the can indicates
the negative side.
Fit the three pushbutton switches,
with the flat side orientated as shown
in Fig.5, ensuring they are pushed
down fully onto the board before sol-
dering their pins. S3 is red while S1
and S2 are black. You now have almost
enough components mounted to test
the power supply.
It is recommended that you attach
REG1 to the case for heatsinking, but
we haven’t built the case yet. Any-
way, the easiest way to do this is to
cut the three regulator leads short,
then solder 25mm lengths of medium-
duty hookup wire to the stubs, using
some small diameter heatshrink tub-
ing to insulate the solder joints and
the lead stubs.
You can then solder these three
leads to the regulator pads on the PCB,
ensuring that it is soldered the right
way around - ie, so that if you hold it
up above the board with the wires not
crossing over, the tab is facing away
from the board as shown in Fig.5.Early testing
Now you can apply 12V power toDC input connector CON1 and make
some checks. Unfortunately, there is
no power-on indicator LED at this
stage (there will be when MOD1 is fit-
ted), so the simplest check is to meas-
ure the voltage at the right-hand pin of
JP1 relative to a ground point such as
the mounting screw hole in the mid-
dle of the board. At this stage, there
should be little to no voltage yet.
Now briefly press power switch S3,
and you should measure close to 5V
on the right-hand pin of JP1. Press S3
again and that voltage should drop
away to almost zero. That confirms
that the power supply section is work-
ing correctly.Modifying the AD9850
module
Minor modifications are required to
the AD9850 module before mounting it
on the PCB. Three SMD resistors need
to be removed and a thin wire soldered
to one of the free pads. These changes
are shown in Fig.6 and the accompa-
nying photo of the modified module.
The module I used is, I believe, the
most common version but there appear
to be other versions available that use
the same circuit but a different layout.
So if your module does not look ex-
actly the same as mine, don’t panic!
You can use a DMM set on continu-