RCT-XXX-AS
Low-Cost SAW-stabilized surface mount OOK RF transmitter
© 2001,2002,2003 All Rights Reserved
The output of the oscillator is derived directly from the collector of the oscillator transistor. It is,
therefore, very sensitive to VSWR. The module is designed to work with a 50 ohm load, which
exhibits a VSWR of 1. The designer must ensure that the antenna exhibits no more than a
VSWR of 2 to guarantee operation. This is particularly true for PCB trace antennas. If the
module does not appear to have any output at the antenna port, it is likely that the antenna does
not meet this requirement.
TROUBLESHOOTING HINT:
If the module appears to have no output at the antenna port, try the following: disconnect the
antenna and put a 1000pF cap in series with a 51 ohm resistor to ground. This will ensure that
the transmitter is properly loaded. Then look at the output with a scope and you should see the
oscillator working. If it is working, the antenna is not properly tuned and that is the reason that
the module is not working properly. If not, then check the power supply and data input for proper
voltage levels.
Data Rate
The oscillator start-up time is on the order of 40uSec, which limits the maximum data rate to 4.8
kbit/sec.
TROUBLESHOOTING HINT:
If the module appears to have a good output, but your data rate is corrupted, that could be a sign
that the start-up time of the oscillator is too long. The load on the antenna affects oscillator start-
up time. To determine the start-up time of the oscillator, use a 2 channel digital storage
oscilloscope. Attach channel 1 to the data input and channel 2 to the antenna output (don’t
worry, it won’t affect the antenna VSWR). Trigger the scope on a rising edge on channel 1. Now,
toggle the data pin from low to high and capture the resulting waveform. You should see a
square edge on channel 1 and the oscillator ring-up on channel 2. Measure the time between the
rising edge on channel 1 and when the oscillator waveform is at its full voltage swing, and you
have the start-up time. To determine if the antenna is the problem, remove it and place a 1000pF
cap in series with a 51-ohm resistor to ground and repeat the test. If the start-up time is correct
with the test load, the problem is the antenna.
Power Supply
The RCT-433-AS is designed to operate from a 1.5 -12V power supply. It is crucial that this
power supply be very quiet. The power supply should be bypassed using a 0.01uF low-ESR
ceramic capacitor and a 4.7uF ceramic capacitor. These capacitors should be placed as close to
the power pins as possible.
Antenna Output
Pin 1 is a 50 ohm antenna output. It will support most antenna types, including printed antennas
integrated directly onto the PCB. The performance of the different antennas varies. There are
many good application notes available that describe external and PCB trace antennas. We
maintain a list in the technical support section of our website.
DESIGN HINT “Antenna traces”:
Any time a trace is longer than 1/8
th the wavelength of the frequency it is carrying, it should be a
50 ohm micro strip. This ensures that a proper match is maintained between the transmitter
output and the antenna.
- 4 -
8/21/2003
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
