Campbell Scientific RF310, RF312, RF313 Narrowband Radios - Manual (Page 61)

Appendix D. Fundamentals of Radiotelemetry
D-3
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. These
modes are described in Section 3.1.6.
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.
D.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 by
a crystal. The 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.