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If we objectively examine most tuning pulser (pecker), some ESSB
discussions, and CW or amplifier forums, we find misconceptions about SSB and CW transmitters behavior.
Understanding how the transmitter works, especially the RF generation process,
goes hand-in-hand with adjusting amplifiers and designing RF equipment.
Discussions of SSB behavior also fit, in significant part, with AM and CW transmitters
and amplifiers. As with SSB transmissions, AM and CW transmitters are amplitude
modulated. This is why
undistorted operation and maintaining original bandwidth of all three
modes, CW, SSB, and AM, requires linear amplification on stages
following the modulation process. Even off-on keyed CW requires linear
amplification. AM and CW transmissions contain sidebands above and below a
carrier frequency. SSB transmissions are missing the carrier and one group of
sidebands, but still convey information through amplitude modulation.
Frequency Conversion View, the Simple Approach
SSB single tone
If we examine frequency domain of an ideal SSB transmitter, we
find the SSB transmitter coverts or moves audio frequencies to much higher radio frequencies.
In effect, the SSB transmitter moves the zero frequency point of audio up to radio frequencies.
LSB (lower sideband) transmissions become inverted, with the lowest audio
frequency highest in spectrum. LSB transmissions are the difference between
audio frequencies and the suppressed radio frequency carrier. USB transmissions
sum the audio to the suppressed carrier frequency.
If we modulate any
HF SSB transmitter
with a single tone,
such as a 50 Hz tone,
transmitter
output
contains no 50 Hz
energy. If there was 50 Hz energy, it would not make it through the coupling
components, including matching networks, filters, and antennas.
Let’s look at an 80-meter band example. With
“carrier” set at
3.800 MHz, a
steady 50 Hz tone is applied to the balanced modulator (or digital equivalent of the
balanced modulator). We now have three new frequencies:
- The carrier at 3.800 MHz
- The upper sideband, at 3.800 MHz plus .00005 MHz, resulting in 3.80005
MHz
- The lower sideband, at 3.800 MHz minus .00005 MHz, or 3.79995 MHz
Assuming we filter the DSB AM signal in a narrow LSB filter, or phase cancel
the carrier and USB out, the result is a pure
radio frequency sine wave at
3,799,950 Hz, or 3.79995 MHz. This is why the 50 Hz tone passes through antenna
tuners, coax, and radiates from a 3.8 MHz dipole!
A 50Hz tone, from an ESSB transmitter, looks like this:
This is a sweep rate of 5uS per division, or 200,000 RF cycles per division. We
see each horizontal division is full, meaning the applied frequency is much
higher than 200 kHz.
Note there is no energy at all at 50 Hz. We have the equivalent of a
3.799,950 MHz steady CW carrier.
The single tone RF
output, regardless
of audio tone input
frequency, is a single
narrow RF spike that
actually is a steady
unmodulated RF carrier!
This leads us to what a single tone signal is.
Audio Pulsed Tone equals CW OFF-ON Keyed Carrier
Years ago, Collins and other
companies
generated CW this
way. Collins injected a “pure” 1000
Hz tone into the
audio chain of the 32S1 SSB
exciter and KWM-2
transceivers. This is because an audio tone is simply converted to a single
radio frequency by the SSB system. Collins had to abandon that idea, because
hum, noise, and distortion, as well as opposite sidebands and carrier, leaked
out in levels sufficient to generate frequent complaints and FCC citations, but
that was due to the physical transmitters being less-than-perfect.
Two-Tone
A two-tone
source is used to simulate SSB performance. A two-tone test is a very marginal
IM distortion test, because it varies load on power and bias supplies at the
tone frequency separation rate. A two-tone test will not normally show power
supply dynamic regulation issues or ALC problems, but a two-tone test is better
than nothing. (A true distortion test would use either real speech, or three
tones. With three tones, one would represent bass, one represent treble, and the
third tone represent the syllabic rate of speech. Peak IMD levels would be
captured over many sweeps and displayed.)
Let’s consider tones of
50 Hz and 2500 Hz.
A 3.800 MHz LSB
transmitter would
now output two
frequencies,
3.799,950 MHz for the 50
Hz tone, and
3.797,500 MHz for the
2500 Hz LSB tone.
The resulting
waveform
is a two-tone
signal:
Notice zeros are reached about every 0.41 milliseconds. This corresponds to 2500
Hz – 50 Hz = 2450 Hz. The actual RF signal never gets below 80 meters, and
never presents audio frequencies to amplifiers, antennas, or anything else.
Audio frequencies will only appear after the signal is detected!
If we trace the outline of crests above zero in a two-tone test, we would draw a
humped waveform. The waveform would be the same as an unfiltered full-wave
rectifier dc output. In normal class AB or B amplifiers, the traced waveform
would load power supplies at that rate, and tend to force current back into bias
supplies at that same rate. This waveform is always at the
difference between RF frequencies. In this case a 50 and 2500 Hz tone would load
power supplies and back feed bias supplies at a 2450 Hz rate. Neither the 50 Hz
or 2500 Hz, or any two original tones, appears at the supplies!
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