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NZART CODE OF PRACTICE ON The New Zealand Association of Radio Transmitters Incorporated

NZART CODE OF PRACTICE ON
THE CONSTRUCTION AND MAINTENANCE OF AMATEUR
RADIO REPEATERS

Developed: 1998, 1999 and 2000 by Doug Ingham, ZL2TAR

Version 2.5, April 2000

PURPOSE OF THIS BOOKLET

This booklet aims to provide advice on the construction of reliable, high performance,
low maintenance, repeaters, and to condense, into a few pages, the RF design
experience of several professional communications engineers.


MEASURING UNITS USED IN THIS BOOKLET

All signal levels in this booklet are expressed as power levels, in dBm, in other words,
decibels relative to 1 milliwatt. This is the most common calibration of modern test
equipment. Occasionally, where this helps with understanding, the dBm signal levels
are also expressed as the equivalent voltages across a specified impedance, usually 50
Ohms.


REPEATER DYNAMIC RANGE

A modern, well designed, 16 kHz wide FM receiver (16KF3 emission designator) will
deliver good but noisy audio, corresponding to 20 dB "SIgnal to Noise And
Distortion" (SINAD), at -123 dBm input (0.16 µV in 50 Ohms). At this level of
wanted signal it is possible to detect interference from unwanted signals of less than -
136 dBm.

The output of a 25 W transmitter corresponds to +44 dBm. This is 180 dB more than
the weakest detectable signal. Put another way, any unwanted repeater transmitter
outputs, such as white noise, on the receive frequency, need to be suppressed by 180
dB. Similarly, repeater receiver spurious responses, on the transmit frequency, also
need to be suppressed by 180 dB.

Modern mobile and home transceivers, and equipment certified to RFS25, are
designed to only receive OR transmit at any one time, so do not need to be, and, in
fact, are not designed for the 180 dB dynamic range needed for repeaters.

The requirement for repeater equipment is more severe than RFS25. A repeater has to
receive and transmit, on nearby frequencies, simultaneously. Special means, of
isolating the transmitter and receiver from each other, have been devised, to ensure a
transmit-receive dynamic range of more than 180 dB, while simultaneously receiving
AND transmitting. These include special low-noise transmitter and receiver designs,
and high-isolation duplexer filters. Cost-effective repeater designs divide the 180 dB
2
dynamic range requirement into about equal portions: typically about 90 dB dynamic
range for the repeater transmitter and receiver, and about 90 dB isolation for the
duplexer.

There are special requirements for receivers and transmitters used in the National
System.


PHASE NOISE - CRYSTAL VERSUS SYNTHESIZED OSCILLATORS

Transmitters and receivers using synthesized oscillators have much worse, by
typically 30 dB, phase noise performance than those using direct crystal oscillators.
Repeaters made from synthesized receivers and/or transmitters have inferior
performance and/or require more expensive input and output RF filtering to overcome
this inherent performance limitation.

Transmitter oscillator phase noise introduces two performance limitations.

First, transmitter noise radiated in adjacent channels "noises-up" nearby receivers
tuned to weaker, more distant, adjacent channel transmitters. Second, and more
important for repeaters, transmitter phase noise extends to frequencies beyond the
associated receiver frequency, "noising-up" the repeater's own receiver. Extra filtering
is required, between the transmitter output and the antenna, with a pass response at the
transmit frequency and a notch at the receive frequency.

Receiver down-conversion oscillator phase noise also introduces another "noising-up"
performance limitation. Strong transmitters on nearby frequencies (including the
associated transmitter, in a repeater) mix with the phase noise of the down-conversion
oscillator, to produce noise at the receiver's first intermediate frequency (IF),
"noising-up" the receiver. Extra filtering is required, between the antenna and the
receiver input, with a pass response at the receive frequency and a notch at the
transmit frequency.

Transmitter and receiver phase noise (and lack of duplexer isolation) is one of the
main causes of "kerchuffing" on weak input signals. The other main cause is de-
sensing of the receiver input stages, by excessive amounts of transmitter energy,
causes a change in the DC operating conditions, or clipping, in the receiver pre-
amplifier or mixer devices. The mechanism is as follows:

Initially the repeater transmitter is off, so there is no noising-up or de-sensing of the
received signals. A weak signal appears on the receive frequency. The repeater
receiver is operating at full sensitivity. The control system turns on the transmitter;
causing noising-up and de-sensing of the receiver, to the extent that the control system
thinks the input signal has disappeared. After the "tail" time-out the transmitter is
turned off, the receiver resumes full sensitivity, and the cycle repeats indefinitely. The
only cure is to fix the transmitter, duplexer or receiver performance deficiencies.


3
RELIABILITY - CRYSTAL VERSUS VARIOUS TYPES OF SYNTHESIZED
OSCILLATOR

Crystal oscillators are simple and reliable.

Synthesized oscillators are more complex, affecting their reliability, and come in two
types, called parallel load and serial load.

The parallel load synthesizer has higher reliability than the serial load type, because of
the way in which the frequency data is loaded into the programmable frequency
divider IC. In effect, the frequency data is continuously applied, as DC voltages, to
the input pins of the divider IC. As a result the divider IC needs to have a large
number of input pins, requiring a large package.

The serial load synthesizer has lower reliability than the parallel load type. The
frequency data is loaded once, at equipment switch on. Usually, only two pins are
required to serial load the frequency data. The IC package is small, which makes it
popular for inclusion in compact equipment. Virtually all Amateur transceivers use
serial load synthesizers. The power fail detection circuitry MUST be 100% reliable,
otherwise a partial re-load, and corrupted frequencies can result, following brief
power interruptions.

Serial load synthesizers have been found to be particularly sensitive to Electro
Magnetic Pulses (EMP) produced by nearby lightning strikes. The resulting off-
frequency operation interrupts service, causes interference to other communications
services, and requires a visit to the site to reset. Our hilltop sites can experience many
nearby strikes (cloud to cloud or cloud to ground) per year.

Many commercial site owners prohibit the use of serial load synthesizers.

All synthesizers should have lock detectors, but many don't, to mute the transmitter
and/or receiver output when the synthesizer is out of lock.


TRANSMITTER FREQUENCY MODULATORS VERSUS PHASE
MODULATORS

Legend has it that crystal oscillators are difficult to frequency modulate; this is not
true. Most transmitter manufacturers haven't bothered to perfect frequency modulated
crystal oscillators. Instead, they have attempted to use phase modulators.
Unfortunately, phase modulators produce high levels of distortion at normal deviation
(5 kHz) and at low audio frequencies (<1 kHz). The performance of phase modulators
is unacceptable for FM voice or data use.

Properly designed, crystal oscillators can be designed which produce low distortion
and 6 kHz deviation at 6 metres, 16 kHz deviation at 2 metres, and 50 kHz deviation
at 70 cm. Voice and data transmissions in all these bands normally use 5 kHz peak
deviation, so there is plenty of margin.


4
TRANSMITTER RF STAGE ALIGNMENT

A spectrum analyser must always be used.

A spectrum analyser allows inspection and minimisation of unwanted multiples of the
crystal oscillator, parasitic oscillations and unwanted carrier harmonics. The wanted
output is maximised when the unwanted outputs are minimised.

Unfortunately, SWR bridges, and other broadband power detecting devices, such as
the Bird wattmeter, tell a different story. Both device