Campbell Scientific RF500M Radio Modem - Manual (Page 29)

Appendix A. Fundamentals of Radiotelemetry
sequences, sets data to be transferred into data blocks, creates signatures of
data blocks, modulates the radio's carrier wave, and stores information on
communication quality.
The user at the computer is responsible for naming the desired communication
path with a setup string. This setup string contains any repeater (MOL)
modem IDs and the destination (EOL) modem ID in sequence. After sending
this information out through the RF system, all of the RF Modems in the
specified link will set themselves in the proper mode. The RF Modem has
different modes to distinguish responsibilities at various localities within a
link.
Establishment of an RF link consists of getting all of the RF Modems in the
proper mode and receiving a verification block from the EOL modem.
A.4 Transceiver
The purpose of a transceiver (radio) is to transmit and receive the modulated
carrier wave.
A radio is both a transmitter and receiver. The main component in the
transmitter is the oscillator of which the frequency of oscillation is provided
electronically or by a crystal. The electronics or crystal oscillates at a desired
frequency, which is specific for the carrier frequency. The oscillator converts
DC power to an AC signal. This signal is then amplified, modulated with the
signal, and transmitted to the antenna system. The receiver consists of an
amplifier, frequency converter to slow signal, limiter to give constant
amplitude but same frequency, and discriminator or demodulator.
The radio has a known impedance, or resistance. Maximum power is
transferred if the impedance of the radio matches the impedance of the antenna
and cable. This impedance is generally 50 ohms. Mismatching of impedance
will cause a lesser transmit power and result in a higher VSWR (Voltage
Standing Wave Ratio).
When the transmission cable and antenna does not match the impedance of the
output circuit of the radio, not all of the energy fed down the cable will flow
into the antenna. A percentage of the energy will be reflected back forming
standing waves on the cable. The ratio of voltage across the line at the high
voltage points to that at the low voltage points is known as the VSWR. When
the VSWR is 3.0:1 or greater, the percentage of errors per data value is greater
than 50%. The VSWR should be kept below 1.5:1 for error free
radiotelemetry.
A-3