Silicon Chip – July 2019

76 Silicon chip Australia’s electronics magazine siliconchip.com.au

ing them to the board.
This should allow you to mount the
balun close to the board, so it won’t rat-
tle around after the wires are soldered.

Proceeding with

construction

Now fit metal can transistor Q3
close to the PCB, leaving about 1mm
between the bottom of the device and
the upper PCB surface. Don’t install it
firmly down on the PCB because the
metal case of the transistor is inter-
nally connected to the collector ter-
minal of Q3.

Also, before you solder it in place,

check the metal case is not touching

any adjacent component leads.

Next, fit your modified AD9850 DDS

module by soldering two 10-pin head-

ers to the PCB, then soldering the mod-

ule to the pins on top of these headers.

The wire you connected to that

module earlier connects to the lead

of transistor Q1 which is closest to

MOD1. RevB PCBs have a dedicated

pad for this wire.

Otherwise, solder it directly to Q1’s

lead, on the top side of the PCB. Ei-

ther way, trim the wire to length be-

fore stripping and soldering it. This

wire should ideally be routed under

the module for neatness. If you keep

it short, it won’t move around later.

Next, fit output socket CON2. As

it’s an edge connector, push it onto

the edge of the PCB, with the central

pin sliding over the central pad on the

bottom side.

Solder that central pin, plus the

posts on either side, on both the top

and the bottom sides of the PCB.

As this is a fairly substantial chunk

of metal being soldered to copper

planes, you will need a hot iron and

Fig.6: these three SMD resistors must be removed from the AD9850 DDS module. One
of the pads which connected to the now-gone 3.9kresistor makes a handy connection
point for the extra wire needed to connect pin 12 of the IC (RSET) to the collector of
transistor Q1 on the main board, for output level control. See also the close-up photo
at right. CONNECT THE RSET
(PIN 12) WIRE HERE

REMOVE THESE

SMD RESISTORS

Two inter-coil screens, show in red on the overlay) must be
fitted between the coils as shown here. These can be cut from
a scrap of tinplate (eg, a food tin). This photo also shows the
mounting of the 7805 regulator on the case heatsink.

The modified AD9850 module in situ on the main PCB.

The three SMD resistors are all removed and the yellow wire

is soldered to the appropriate pad – the one marked R6.

(make sure it is the one closest to the AD9850 IC).