Joe wrote:
Being so new at this, I don't even know what you mean by 'tombstone
style', and it sounds like an awful lot of work to be scoring pcboard
with an Xacto knife to create transmission line effects.
You can easily build what you need using single-sided board: a
continuous groundplane, components soldered on top, and connections made
using sticky-backed copper tape (available at craft stores for
stained-glass work).
If you cut strips 0.1in wide, these will be a good approximation to
50-ohm microstrip, for the relatively low frequencies and short lengths
that you will need. On the reverse side, use broader the copper tape to
link the groundplanes of your extension and the Radiotronix module.
Simply lay the SMD components against each other, and solder them
together... but before you do, mark where the connections through to
ground will need to be, for the two capacitors in the pi-network and
possibly other places. Then drill the board and insert links of thick
wire (14AWG) so that the ground connections will have zero length beyond
the thickness of the board - but I really do mean ZERO, or else your
network will not perform correctly. The ground links must be touching
the ends of the capacitor chips before you solder them.
Likewise, use ZERO lead lengths inside the pi-network. Butt the ends of
the two Cs and the L chips directly together, and solder. Any
"connecting leads" must be restricted to the places that can use 50-ohm
microstrip.
This construction method is very quick and simple. The SMD parts are
well-enough anchored to the board for all normal conditions of use.
I talked with a tech rep from radiotronix today. They called me
because I ordered a half dozen modules from mouser. He explained that
the pi circuit on the TX is so the TX sees 50 ohms, which is pretty
critical.
- so remember what I just said about zero lead lengths.
I have not done anything with the transmitter modules yet (they are
from Velleman because they are thru hole), but I am building a receiver
board on pc board using pcbexpress and single sided copper pcboard. It
will be the first prototype. Now that I have it working on a breadboard
I am going to build one and see how it works. I am planning on using a
straight piece of (20ga) wire for the 18nH inductor and using the T
network of 2 8pf caps beside it. The tech said this is just a low pass
filter, but it seems to make a difference in receiver sensitivity.
On the face of it, this pi-network has a 1:1 impedance transformation
(two equal capacitors) so it shouldn't make any difference. However,
there may be stray capacitances and/or inductances in the module that we
don't know about. Bottom line is, this network is what the Radiotromix
techs have found to work.
Since you don't have the design experience and the test equipment to
measure how the whole thing performs, you would be very wise to copy
EXACTLY what Radiometrix did, which includes building the whole network
using the best possible construction practice.
Also, a straight piece of wire is actually a short length of
transmission line: it does not have quite the same electrical properties
as an inductor. So use an 18nH chip inductor like Radiometrix said.
This is really part of your "contract" with the module manufacturer. It
isn't fair or reasonable to expect their tech support to second-guess
what the effects of every individual user's network might be, if they're
all slightly different. You might get away with it this time, but some
of the worst tech-support problems are caused by people who decla
"I did exactly what you said..."
[Now follows 20 minutes of intensive cross-examination over the phone]
"... oh, apart from all these things that are different."
[Deleted: strange sounds on the phone line. The tech has the curly cord
gripped in both hands. He's puling it out straight, and he has a red
glint in his eyes. Be very thankful that you're several hundred miles
away...]
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek