"Tom Bruhns" wrote in message
...
"Joe" wrote in message
link.net...
"Reg Edwards" wrote in message
...
An 18 nano-henry inductor seems, to me, to be an impractical inductor
value.

;-) You just aren't working with small enough, high enough frequency
circuits, Reg.

3 turns of 28AWG on a 4-40 (USA) screw (remove the screw, of course)
will give you about 18 nH. Unloaded Q at 440MHz will be around 200.

The link to the schematic is:
http://www.radiotronix.com/downloads/433_pair.pdf

Easy enough to use SMT components soldered to a piece of copper-clad
for a ground plane. Just put down the two caps and the shunt resistor
"tombstone" style, one end soldered to the ground plane and the other
up in the air, separated by enough distance to put the series coil and
resistor between them. You can keep the parasitic L and C very low
indeed this way. You can also use tiny squares of copper-clad glued
down to the ground plane as tie points.

It's also possible to mechanically make a simple circuit like this on
copper-clad using surface mount parts, by scribing lines with
something like an X-Acto knife and removing strips of copper. In this
case, I could see scribing four lines across the board, separated by
about 1/16 inch, and removing two copper strips, one from between the
first and second scribe, and one from between the thrid and fourth
scribe. That leaves a trace about 1/16 inch wide. Cut gaps in the
trace, about 1/16 inch long, where you want to solder down series
components. Solder shunt components across from the trace to the
copper ground plane. Bridge across the trace with a copper strap to
connect the two gound plane sides, with the bridge up in the air over
the trace. Nice to have a ground plane on the opposite side, with
some holes and wires to connect to the top-side ground planes, but not
necessary. I'm thinking 0805 parts with the sizes I mentioned...make
things slightly larger for 1206. You can make the trace be a 50 ohm
transmission line if you want, by sizing things right.

Cheers,
Tom

Hi Tom,

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.

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. 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. The noise on the receiver is supposed to
be there according to him. It is between 1Khz and 1.5Khz. he said some
people use the data slicer pin with a holtek ht12D decoder, which i am doing
or you can also use a micro to look for the bit pattern and decode the
signal. I am just using the TV (transmission valid) to set off the rest of
my circuit which, right now, consists of a latch and an LED.

Joe