AMATEUR RADIO
By GARRY CRATT, VK2YBX
What's all this talk about SINAD?
SINAD measurements (signal plus noise and distortion)
are now the accepted method of evaluating FM
receiver sensitivity, having replaced the old "20d.B
quieting" method. Here's a quick rundown on what
SINAD measurements are all about.
Many amateurs will be familiar
with valve transceivers. They were
used in large numbers across
Australia by amateurs keen to
operate on the VHF and UHF bands.
To keep costs low, some amateurs
converted retired commercial
equipment, some of which is still in
use today.
When those transceivers were
originally manufactured, receiver
sensitivity was measured using the
"20dB quieting" method. Basically,
this involved connecting an AC
voltmeter across the loudspeaker
output terminals, opening the
receiver squelch, and measuring
the noise voltage with the volume
control set to some convenient level.
An unmodulated carrier was then
applied to the receiver and the
signal level adjusted until the out-
put noise was reduced by 20dB (ie,
by a factor of 10).
When the - 20dB level was
reached, the signal generator level
was noted and this became the
"20dB quieting point". Receiver
sensitivity was then specified as so
many microvolts (µ V) for 20dB
quieting. A typical figure was 5µ V.
This method was popular because it was simple but it could
sometimes give misleading results.
Let's consider an example - a
receiver that has a very narrow
bandwidth due to misalignment. In
this case, the 20dB quieting method
will still indicate normal sensitivity.
In practice, the receiver would be
unable to receive intelligible signals
due to severe distortion.
To receive FM correctly, the
receiver must have adequate band-
width to include all the important
FM signal components. For example, if the receiver is required to
correctly demodulate a ± 5kHz
signal, then it must have a bandwidth of at least 10kHz. If this
bandwidth is reduced by misalignment, the receiver will not recover
intelligible audio.
This was the major disadvantage
of the "20dB quieting" method.
SINAD
measurements
SINAD measurements do not suffer from the above problem.
Basically, SINAD is a measure of the
quality of a communications signal.
When a signal is introduced into a
receiver, the output of the receiver
will consist of the original
modulating signal plus system noise
and receiver distortion.
Thus SINAD is defined as:
Signal + Noise + Distortion
Noise + Distortion
Expressed in dB, the equation
becomes:
20.Log Signal + Noise + Distortion
Noise + Distortion
Because distortion can be considered as a type of noise for this
application, the equation simplifies
to:
20.Log[Signal/Noise)
The "Sinadder Linear 5" is part of a range of SINAD meters made
by Helper Instruments Company, USA. The local distributor is
RF Devices Pty Ltd, 9 Lyn Pde, Hoxton Park 2171.
92
SILTCON CHIP
The big advantage of the SINAD
method is that it measures both
signal and noise, unlike the 20dB
quieting method which only
measures noise. In fact, SINAD
measurements in RF equipment are
similar to THD (total harmonic
distortion) measurements in audio
gear.
To measure SINAD, we apply an
FM SIGNAL
GENERATIJR
ANTENNA
RECEIVER
SPi::ER
With an RF input signal of 20µ,V,
the noise and distortion should be
low. So when the meter is switched
to the SINAD mode, the reading will
be very low, say - 40dB or more.
Reducing the RF signal will progressively increase the noise and
distortion so the SINAD reading will
rise.
For today's amateur gear, the RF
input will have to be reduced to
around 1µ,V or less to obtain a
reading of - 12dB SINAD.
Fig.2 shows the test setup if no
SINAD meter is available.
\
SINAD
METER
lkHz MODULATION
3kHz PEAK DEVIATION
Fig.1: basic test setup for measuring SINAD. Initially, the set level
control on the meter is adjusted for a reading of OdB, then the FM
signal generator output is reduced until a reading of - 12db is
obtained.
o-------FM SIGNAL
GENERATOR
ANTENNA
RECEIVER
1kHz MODULATION
3kHz PEAK DEVIATION
lkHz NOTCH
FILTER
v
Receiver alignment
Fig.2: this test setup can be used if no SINAD meter is available.
SINAD measurements are similar to total harmonic distortion (THD) .
measurements in audio gear.
RF signal modulated at lkHz to the
receiver and connect a SINAD meter
to the receiver output. Fig.1 shows
the basic setup. The peak deviation
(of the lkHz test signal) should be
set to 3kHz while the RF signal level
should be adjusted so that the
receiver hard limits.
Initially, the meter should be
switched to the "AC volts" range
and the set level control adjusted
for a reading of OdB. After that, it's
simply a matter of switching to
"SINAD" and reducing the generator
output until a reading of - 1ZdB is
obtained.
The sensitivity of the receiver
can now be determined simply by
noting the generator output level.
What actually happens inside the
SINAD meter is this: when the instrument is switched to "AC volts", we
get a reading of the combined signal
plus noise output. When the unit is
switched to "SINAD", a lkHz notch
filter is switched into circuit. This
filter removes the lkHz fundamental but allows all other frequencies
to pass.
So, in the SINAD mode, we simply
get a reading of the noise and
distortion components. If no noise is
present, the reading is simply a
measure of the distortion.
Dynamic balancing for fans rite magnet from an old louspeaker.
With one side sitting firmly on
the centre of the piezo transducer,
and the other side resting on the
cabinet, it gave the transducer
quite a workout when the fan was
turned on. The voltage reading with
a 10MO load was a steady 5V AC
(straight into the input of an AC
voltmeter).
Next I mixed up a small amount
of 5-minute epoxy and coated the
underweight joint on the fan. The
"vibration" reading decreased to
about 1VAC. Had I gone too far, or
not enough?
Attaching a small piece of sticky
tape dropped the reading to 0.9V,
so I mixed up some more 5-minute
expoxy and applied what I guessed
to be ten times the mass of the piece
ctd from p.87
of sticky tape. The next reading
was quite a surprise - only 30mV
AC. Bingo!
I had hit near perfect balance
after only a couple of tries - admittedly more by luck than anything
else - but it was reassuring to find
0
Manufacturers often specify
various test points on their circuits
to assist alignment. To align the
transceiver in the conventional
way, a suitable signal is injected
and the circuit is tuned for maximum output.
However, maximum output does
not necessarily indicate the optimum tuning point, particularly
when considering RF amplifier
stages and mixers: Tuning for maximum gain is not the same as tuning
for best signal-to-noise ratio.
In some cases, it's possible to improve receiver sensitivity by as
much as 3dB by realigning the circuit to obtain the best SINAD
reading. This is equivalent to doubling the transmitter power at the
other end!
·~
that putting a small piece of tape
anywhere on the fan rotor only increased the vibration, regardless of
where I put it.
All's well that ends well - in
fact, the cabinet vibration of this
particular air cleaner is now an
order of magnitude better than one
straight out of the factory!
~
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RCS Radio Pty Ltd is the only company which
manufa.c tures and sells every PCB & front panel
published in SILICON CHIP, ETI and EA.
,.'Jc
651 Forest Road, Bexley, NSW 2207
Phone (02) 587 3491 for instant prices
NOVEMBER 1988
93
