shows me at work making the basic DB6NT 3.4GHz transverter module.
The kitset comes in two flat, plastic, compartmentalised boxes in
which all the components are sorted and labelled. In fact, DB6NT
scores 100% for presentation! A full construction manual is also
provided but the constructor is expected to have some previous experience
at soldering small surface mount devices (SMDs)onto pcbs, as well
as sufficient knowledge about safe handling procedures in relation
to static sensitive items such as the GaAsFETs and HEMTs that come
with the kit.
The photo also
serves to illustrate some basic needs in the way of tools and other
aids. Note the headband binocular magnifier that I am wearing.
I find this absolutely essential! The components are often very
small and it needs a steady hand to solder them in place. The extra
"dimension" provided by the magnifier makes all the difference.
The soldering iron I am using here is a variable temperature
controlled one marketed by Vann
Draper for around £60. The temperature is accurately
set by the adjustable control on the front of the white base unit
to my right. It's usually set to 300 degrees Celcius but can readily
altered to suit particular components. The other iron is a temperature
controlled Weller TCP-D type but the temperature is fixed by the
type of bit used at the time.I much prefer the Vann Draper iron,
which has a 0.5mm diameter bit, ideal for delicate soldering
include precision tweezers. Once you have had a minute surface mount
component fly across your workshop after your cheap tweezers slipped
you will understand what I mean about "precision"! Such
items can often be found in surplus medical equipment shops and
even at amateur radio rallies and "hamfests".
Ideally you might
use silver solder for microwave work but I have only done this at
frequencies above 10GHz. Below that I use normal cored solder of
about 0.5mm diameter and with a low melting point rating.
I find the average
desk type workbench rather low for sustained electronic construction
work so I lift the work a little higher by using a small makeshift
"platform" (literally two bits of wood with a 25cm x 25cm
square of hardboard nailed to them!)on the workbench. This brings
the project to a comfortable height where I don't get an aching
neck after hours of working on it!
The DB6NT kitset
comes complete with a prefabricated, double-sided printed circuit
board of very high quality. All connections to the groundplane
side of the board from the circuit side are done via plated through
holes, ensuring high stability.
manual (in English with export kits) should be carefully followed.
Do not attempt to do your own thing! If you follow the instructions
and solder carefully, your basic transverter module should work
first time and should be completed in a few days. In fact my
first DB6NT module, for 5.7GHz, was completed in just two
days. This latest one took a little longer, five days, because I
spent less time each day on it.
The method I use
to solder the various SMD components, such as chip resistors and
capacitors and SMD transistors, involves the use of small lengths
of O.5mm diameter solder that I cut off in dozens from a reel of
solder before construction begins. These 1 to 2mm lengths are kept
in a surplus 35mm film cassette container when not in use. I find
this is a much better way of obtaining a good, neat soldered joint.
To solder in a component I lightly "tin" one end of it
with the iron. Then, using one of the 1mm lengths of solder, I flow
a very small amount of solder onto one of the connection pads on
the pcb. I then place the component on the pcb and quickly "tack"
it to the presoldered connection pad, making sure the component
lies absolutely flat on the pcb. Next, I lay one of the 1 to 2mm
pieces of solder right up against the other end of the component,
which component is gently held down with the precision tweezers
I used to pick it up. The tip of the soldering iron is quickly moved
along the pcb stripline towards the unsoldered end of the SMD. The
small 1-2mm piece of solder quickly melts and the iron is removed,
leaving a very neat joint. The other end is then reflowed to ensure
a good contact. All this takes longer to describe than to actually
perform! The main thing is to avoid having to pull a length of solder
from a reel when fitting the components. This can result in too
much solder being applied to the job.
The DB6NT transverter
kitset modules are housed in tinplate boxes, into which the
printed circuit board is soldered BEFORE any pcb components
are mounted. Before that though, the tinplate box needs to have
its seams soldered and the four sma connectors and feedthrough capacitors
fixed to the end wall. This requires careful measurement and accurate
drilling since the pcb is soldered up to the centre spiggots of
the sma connectors.
to mount the board at the height stated in the manual or spurious
resonances may be encouraged and/or some of the taller components,
such as the silver-plated "pill box" filters, might even
project higher than the edge of the box, preventing the lid from
fitting! The photograph below shows the "groundplane"
side of the pcb, mounted in the tinplate box. You can see that some
relatively large components are fitted here .. two "pillbox"
cavity filters and several helical filters, as well as a couple
of standard voltage regulators, trimpots, the Local Oscillator crystal
(in its heater) and a large input attenuator resistor (50 ohm).
All these need a hotter iron temperature setting than the surface
mount components on the other side of the board. The sma connectors
are screwed to the end plate of the box.
pillbox filters is perhaps the trickiest job of the lot. It
is one of the first jobs to be done after the the pcb has been soldered
in to the tinplate box. There are small locating points marked on
the pcb and the filters MUST be accurately located to them as each
filter has two short probes coming up through the board from
the circuit side. The filter must be symmetrically located with
respect to them. The probes are fitted and measured acurately BEFORE
the filters are finally soldered into place.
To make sure the silver-plated
filters are soldered correctly I heat them up first on a kitchen
hotplate! When they are too hot to touch I quickly apply 0.8mm diameter
low melting point solder all around their bases to pre-tin them.
Then, one by one, with the adjusting screws fully inserted in the
top of each filter I lift them with a small part of pliers and I
press the the cavities onto the pcb, quickly wrapping a ring of
solder around their bases. My soldering iron, with a broader bit
than that used for the SMDs and set at 420 degrees Celcius(its highest
temperature), is then applied to the base of the filter to encourage
the solder ring to flow. All this time it is essential to keep the
filter absolutely firmly in place, for if you move it out of position
you may have to use a small flame torch to free it later! This method
usually results in quite a neat installation. The one shown above
is NOT my best!
Other components such as
the helical filters also have to be soldered to the groundplane
using a hot iron. In this case it is essential to solder quickly
as you could melt the formers inside the coil housing if you applied
your iron for too long. I usually cool the component down with a
damp cloth, AFTER I am certain the actual solder joint has
Once all the components
have been soldered into place, I gently apply some alcohol ("Methylated
Spirit") with an old toothbrush to both sides of the board
to clean away any flux deposits. The module is then left overnight
near a warm radiator to complete dry out. The screws are taken out
of the cavity filters during this time.