Download TWO TONE TESTING

TWO TONE TESTING
In these amazing days where one can go out and buy a state of the art
transceiver for just a little less than the cost of a good second hand car, much is
written about such features as price and the number of bells and whistles.
Unfortunately, almost nothing is written about transceiver testing, which is
the only way for the amateur to gain a "feel" for the really important front panel
features such as audio AGC, audio compression, RF power output, and metering
BEFORE a signal is put to air. For beginners this is particularly important,
because a huge gap exists between the knowledge required to pass the amateur
"ticket", and practical transceiver operation. If you are actually constructing
equipment then this knowledge is mandatory.
As the major mode of operation on HF is SSB, two tone testing needs to
be clearly understood. With a two tone tester, a dummy load and an
oscilloscope, a transmitter can be really put through its paces. Two tone testing
consists of applying two clean non-harmonically related sine waves of
approximately equal amplitude to the audio input of an SSB transmitter. The sine
waves are typically around 600 - 700Hz and 1800 - 2000Hz i.e. about 300Hz
from either end of the audio pass band. The result, in a properly adjusted
transmitter, is an RF output that varies from zero to maximum at a rate
determined by the difference in frequency between the two audio inputs.
Consequently, overdrive (which causes splatter), non-linearity, instability and a
host of other problems are easily visible on the CRO. This thorough testing of
the transmitting system becomes even more important when the transmitter is
followed by a linear amplifier- if only to avoid rock throwing, obscene language
and other similar behavior by understandably irate neighbours.
Other test modes can be used for SSB but they are not very realistic. For
example, a single audio tone can be applied to the microphone input of an SSB
transceiver. The oscilloscope display that results is a carrier envelope which
does not vary in amplitude. Overdrive and other problems are not easily
detected unless a spectrum analyser is available (and only a very fortunate few
have one of these). Furthermore, the average RMS output power is equal to the
peak envelope power, which is definitely not the case for speech, where the
relationship between these two is probably around a 1:5 or 1:6 ratio. At full rated
output, power supplies and heatsinks in the typical transceiver generally cannot
cope as they are not designed for this type of operation, and transceiver damage
is a distinct possibility.
In two tone testing, the average power output is one half of the peak
envelope power, and this represents the most severe conditions ever likely to be
encountered in amateur operation, which is the continuous sending of a stream
of morse code dots (50% duty cycle). And this is a pretty savage test. For phone
operation, heat liberated from power supplies and RF power amplifiers is very
much less than that caused by two tone testing. Further, the PEP measured
during typical speech will usually be significantly larger than that measured under
two tone testing conditions (probably 5 - 15% larger) simply because the power
supply rails rise due to the light demands and poor supply regulation.
So, in summary, intelligent two tone testing of a transmitting system and
application of its results will give the amateur all he needs to operate a
transceiver correctly, and put a clean signal on air to the benefit of the entire
amateur fraternity.
Single tone testing is also provided by the generator described later in this
article, and should generally be reserved for AM, double sideband, and FM. The
dummy load and oscilloscope can be used for the first two modulation modes
with the same excellent results as for SSB. FM however, requires different test
gear to analyse the transmitter output. None of these three modes will be
discussed in this article.
HOW IT WORKS
Operation of the generator is quite simple. The two oscillators are based
around a low pass phase shift network which provides a 180 deg. phase shift at
the frequency of oscillation. The circuit is unique, and is the reverse (dual) of the
high pass network normally used in phase shift oscillators. It has the major
advantage that the gain around the oscillator loop does not have to be critically
adjusted to obtain a sine wave. In fact, the whole circuit operation simply relies
on the amplifier having a very high gain to produce a square wave at its output,
which is then low pass filtered to generate a sine wave at the amplifier input.
Both oscillators are run for two tone operation, while one or the other is stopped
for single tone work.
The sine voltages existing at pins 6 and 9 of the op amp are current
summed at pin 2, which is a virtual earth due to the negative feedback from pin 1
of the output amplifier.
In order to get a good sine wave, the positive and negative half cycles of
the square waves existing at either pin 7 or 9 must be closely symmetrical (better
than 2%), and to cause this to happen a slightly larger negative supply is
generated by the power supply splitter circuit section. This compensates for the
fact that the op amp output swings closer to the positive rail than to the negative
rail, and places the op amp inputs exactly at the centre of the output swing.
Note that the current swing into pin 2 can be varied by the 100K pot. This
is an important feature sometimes omitted from two tone testers, and allows the
amplitude of one audio sine wave to be adjusted relative to the other. This
feature compensates for the fact that the transmitter audio response is usually
not flat, and allows perfect zero crossings to be obtained in the RF output (see
CRO photos).
The last stage is simply an inverting amplifier providing a gain which can
be varied over a 50:1 range. The output impedance of this stage is
approximately 600 ohms allowing transmitter low impedance microphone inputs
to be driven. If the transmitter microphone input is high impedance (47K) then
the generator output voltage will be approximately twice that shown on the circuit.
Two diodes are included to protect the op amp output, so that it cannot be
dragged either above the positive or below the negative rails by an external
voltage. Finally high and low voltage outputs are provided.
CONSTRUCTION
Construction is so simple almost no description is required. Make the PCB
(if you feel so inclined) and follow the component overlay using the components
specified. Make sure all wiring is kept well away from the pin 7/8 end of the IC so
that no glitches appear in the output sine waves due to stray capacity coupling of
the fast square wave transitions. USE A METAL BOX to avoid RF feedback
during testing, and coaxial cable between the generator output and transmitter
microphone input for the same reason. The battery is retained by a U loop of
heavy copper wire which passes through the two holes provided in the PCB (do
not solder these). The copper wire is then soldered to the piece of tin plate
shown on the component overlay on top of the battery, through which the battery
hold down screw passes. Before switching on, check battery polarity and the
orientation of electros and the IC. There isn’t much to go wrong…………..
VK5JST
PARTS LIST
Capacitors
2@ 10uF 16V aluminium electrolytics
2@ 0.1uF 100V MKT caps.
6@ 0.047uF 100V greencaps
Resistors
All resistors are 0.25Watt 5%
2@ 560R
1@ 3K3
2@ 3K9
1@ 5K6
2@ 10K
2@ 12K
1@ 22K
1@ 33K
1@ 39K
1@ 180K
1@ 220K
Potentiometers
1@ 100K linear pot
1@ 500K linear pot
Semiconductors
2@ 1N4148
1@ TL074 or TL084
Miscellaneous
1@ box- DSE Catalogue No. H2305 or equivalent
2@ potentiometer knobs
1@ 9V battery clip
2@ BNC panel mount connectors
1@ SPDT switch (on-none-on) DSE. P7654 or Jaycar ST0335
1@ SP3T switch (on-centre off-on) Jaycar ST0336
1@ 14 pin DIL IC socket
2@ 19 mm long standoff pillars tapped 3 mm both ends
miscellaneous screws, hookup wire, cable clips, scrap tinplate