ESSB HiFi SSB transmitter bandwidth splatter



Transmitter Bandwidth and Splatter


Home IC751 measurements


 

Related pages: Checking
signals 
 
Receivers    
Emission rules Part
97.307 

Mixing
Modes

Technical
Comments about
Transmitter
Intermodulation,
Distortion, and ESSB
(or Hi Fi Wide Fi
SSB) audio or over
equalization

Bandwidth

The most common
cause of excessive
width in a SSB or
ESSB signal is not
just the bandwidth
or audio fidelity
applied to the
microphone input or
filter bandwidth,
but includes several
factors. Let’s look
at some of the
common causes of
excess bandwidth. When
checking for
splatter or
reporting splatter,
we should be sure:

We
know our
receiver is

good 

We
are not using a
noise blanker

We
actually understand
what normal
bandwidth is

We
do not have
excessive receiver
gain 

IM Distortion

Distortion
products are a major
cause of excessive
bandwidth. Many
forms of distortion
products causing
splatter cannot be
heard when listening
closely on
frequency, and
cannot be detected
with a oscilloscope
displaying the
envelope!
The
splatter (excessive
bandwidth) we
observe most
commonly comes from odd-order
products



Note:
Some ESSB or
High Fidelity
good audio
proponents
think the
bandwidth of
filters sets
the bandwidth
of their
signal. They
also think
they can hear
the distortion
that causes
splatter, and
if they
“sound
clean” on
frequency to
friends they
have no
splatter
bothering
off-frequency
neighbors. We
will see why
this isn’t
true as we go
through this
article.  


Mixing between
various tones in the
RF signal caused by
even the slightest
amount of
non-linearity (just
as in audio systems)
will generate new
frequencies. With
RF, something is
different. Odd-order
mixing in the SSB RF
system creates 
new
“off-channel”
distortion products.
By saying
“off-channel”,
I mean outside the
passband of the
filters of a
receiver tuned to
your operating
frequency.

The above
distinction is very
important. I don’t
want to be offensive
to anyone, but the
fact is many
“audio
experts” claim
they or their
friends would know
they have splatter.
They base this claim
on the fact they
listen closely to
audio quality
through good
receivers, and would
notice
splatter-causing
distortion on the
desired
signal. 

The cold hard
fact is we actually
can’t hear
troublesome
odd-order IM
products listening ON
FREQUENCY

even when they are
at offensive levels
no matter how
“golden”
our ears, and we
probably cannot
detect non-linearity
that causes
low-level IM
distortion with a
scope. A fraction of
a percent
non-linearity can
create noticeable
harmful adjacent
channel QRM. No
one could even hear
distortion 30 dB
down from desired
audio unless they
were listening above
or below the other
station’s main
signal. IM 30dB down
can be devastating
to QSO’s on the
adjacent channel up
or down.

Again, there’s
nothing wrong with
wanting to listen to
high fidelity audio
as long as we don’t
take up three or
more normal
communications
channels on a
crowded band to do
it. 



Remember
these rules of
thumb for IM3
bandwidth:

The maximum
frequency
spacing of new
intermodulation
products is
the difference
between the
lowest and
highest
pitched tones
modulating the
transmitter.

The total
bandwidth
occupied by a
SSB signal,
when we
include IM3
products, is
approximately three
times
the
audio
bandwidth of
the
system.  


What is
Odd-Order
Intermodulation?
 

In SSB systems,
the RF
harmonic
of
an RF carrier
created by one audio
pitch or tone can
mix with the third
harmonic
of
another RF carrier
caused by another
modulating pitch or
tone. The action or
mechanism of this
mixing is much like
the mixing in an
intentional
frequency conversion
scheme. As a matter
of fact, all of our
regular receivers
have a stage
intentionally driven
into non-linearity
(saturation) by a
local oscillator. A
controlled local
oscillator frequency
is combined in a
mixer to produce a
sum and difference
frequency with any
signal frequencies.
We don’t hear the
distortion caused by
this non-linear
mixer because it
primarily develops
out-of-band or
off-frequency mixing
products that are
filtered out or
rejected by tuned
circuits. A
well-designed high
dynamic range mixer
will actually
terminate these
unwanted frequencies
in a dummy load, so
they do not reflect
back into the
mixer!  

Our SSB rigs
include a multitude
of amplifier stages
conducting much less
than 360 degrees (as
well as some
mediocre mixers or
frequency
converters). Most
stages are class AB.
The full sine wave
isn’t amplified,
only a portion of
it. 

Linearity

The most
important factor is
linearity, which can
be expressed as a
transfer function of
input vs. output
level. If transfer
function was
perfect, an X
percent input change
would produce an
identical output
power percentage
change. In that case
IM would not be much
of an issue. 

Unfortunately
manufacturers and
designers compromise
linearity in
transmitters to
reduce expense,
size, heat, and
power requirements.
Transmitter
IMD 
performance has
taken a backseat,
and we have come to
accept poor IMD
performance as a way
of life. If you have
any doubt about
this, look at
equipment test
reviews published in
QST or
manufacturer’s
specs. Most newer
radios have
odd-order
transmitter IMD
products in the 30
dB below PEP range.
(Numbers like that
would be considered
terrible in
receivers.)

My old Collins
KWM-2 measured -47dB
below PEP for IM3, a
new IC-756 I tested
was -30dB. The
TS-870, often used
by ESSB proponents,
has IM3 as low as
-20dB PEP on some
bands.  That’s
about 400 times more
power in the
off-channel
distortion power
levels in the 870S
compared to an old
1950’s Collins
rig! Most
interesting is a
TS870S sounds great
on frequency.
Off-frequency is
where we notice the
distortion. I can
detect spits and
splats as far as
20kHz away from
normal modest
strength SSB
stations when they
use some of the
poorer modern
transceivers and
NORMAL audio
bandpass. 
  

How odd-IM
Frequencies are
Created

Amplitude
linearity describes
how closely the
input to output
transfer response
(gain) of an
amplifier (or mixer)
resembles a straight
line. When an
amplifier’s input
level increases by a
certain percentage,
its output level
must increase by the
same percentage,
otherwise distortion
is produced. The
deviation from a
straight-line can be
represented by a
power series. When a
single carrier input
signal is
substituted into the
above expression,
the output waveform
will contain the
original carrier and
harmonic distortion
products. For most
communications
applications, (with
bandwidths of less
than an octave),
harmonics can be
eliminated by
filtering.  

SSB is different.
We can consider each
speech
“tone” a
carrier that changes
amplitude and
frequency with our
voice. In other
words the SSB
generator in our
rigs simply
upconverts 
baseband audio
applied to the
microphone input to
radio frequencies.
When more than one
audio input tone is
present, more than
one RF output
“carrier”
varying in frequency
and level is
present. Beat
products are
produced in the
vicinity of these RF
output
“carriers”.
The new signals are
known as
intermodulation
distortion (IMD)
products. They
are located at
frequency intervals
equal to the
separations of the
desired carriers.
 
Filtering cannot
eliminate IMD
products, because
the IMD is located
on the same
frequency, or nearby
the desired output
signals.

As two or more
pure RF
“tones” or
carriers from our
voices pass through
a
less-than-perfectly
linear stage or
component, harmonics
are generated. It
doesn’t matter if
devices are
push-pull or
single-ended, it
doesn’t matter if we
can’t hear the
distortion, it is
always there to some
extent. Various
harmonics created,
even though greatly
attenuated, mix with
the fundamental
desired tones and
other harmonics of
those tones. Most
undesired products
fall far outside the
band we are using
and are easily
cleaned up.
Unfortunately some
products fall in
band, just 
outside the desired
occupied bandwidth.
These are the
odd-order products.
The odd-order
products are the
problems that are
difficult or
impossible to
filter.  

We identify
troublesome products
by the harmonic
relationship of the
tones being mixed.
We have a lowest
order harmful
product creating
splatter. The
third-order
product
is
the lowest order
product that is a
problem. This is
where the 2nd
harmonic of one tone
mixes with the
fundamental of the
other desired tone.
This is called the third-order
product

because mixing of 2
times F1
and one
times F2 makes the
new undesired
signal. The harmful
products are 2F1-F2
where F1 can be
either tone, as can
F2.  

There are also
other products.
Consider the fifth
order
intermodulation
product, or
“IM5”.
This mix is caused
by 2*F1 minus 
3*F2. It’s called
the fifth-order
because it is a
combination of a
second harmonic and
third harmonic, and
2nd + 3rd is
“5”.

Looking at a
Practical
Transmitter

An 1850 kHz LSB
transmitter
modulated with audio
tones of 500 Hz and
3000 Hz would have
main signals at
1849.5 and 1847kHz.
This is the carrier
or dial frequency of
1850, minus 500 Hz
for one tone and 3
kHz for the other
tone. The
third-order IM
products of this
particular
configuration will
fall at:

(1849.5*2)-1847
= 1852
kHz  

(1847*2)-1849.5
= 1844.5 kHz

You can see we
have two new
frequencies at 1852
and 1844.7 kHz, both
OUTSIDE the
bandwidth occupied
by desired pure
channel tones of
1847 kHz to 1849.5
kHz. This is why
Golden Ears, no
matter how
good,  can’t
hear
splatter-causing
distortion listening
to the desired
signal! It only
bothers the other
people up or down
the band.

The fifth-order
products would fall
at:

(1849.5*3)-(1847*2)=1854.5kHz

(1847*3)-(1849.5*2)=1842kHz 

You can see every
increase in order
spreads the signal
another
(F1-F2)  up and
down the band. In
the above case F1-F2
is 2.5kHz.  The
7th order product
would be 2.5kHz
above and below the
5th order products.
Any odd product adds
bandwidth to the
signal that is
outside the passband
of the original
audio! 

What if most of
our speech energy is
at 1849 and 1848, in
the 1000-2000 Hz
frequency range? In
this case the
stronger IM3
products would be
(1849*2)-1848 = 1850
and (1848*2)-1849 =
1847 kHz. The IM3
extends the
bandwidth only 1kHz
lower than the
highest pitched LSB
tone! The more we
restrict bass, the
less overall
distortion bandwidth
we
have!    

You
won’t find this on
many (if any) ESSB
web sites, but the
worse case for
bandwidth and
splatter is when
bass and treble are
simultaneously
increased! This
gives the widest
spread between strong
frequencies in the
RF signal, moving IM
products the
greatest possible
distance from our
“carrier”
frequency. This is
true even when we
use good filters,
and low distortion
audio chains driving
the
transmitter. 

If
somebody above or
below you is running
enhanced bass you
will almost
certainly notice
greatly increased
adjacent channel
interference
problems, and they
aren’t likely to be
your receiver’s
fault. Enhanced bass
increases distortion
product bandwidth,
and do let anyone
kid you…modern SSB
transceivers all add
very noticeable
distortion products
in the RF sections.
Most tetrode
grid-driven
amplifiers are also
much worse than most
cathode driven
amplifiers, just
look at
reviews. 

Even-order
Distortion

If the product is
direct mixing or
even-order mixing,
mixing would be
(1*F1)-(1*F2),
(3*F1)-(1*F2),
(2*F1)+(2*F2), etc.
The “harmonic
order” in the
mixing would total
an even number.
Let’s try that:

(1849.5*1)-(1847*1)=2.5
kHz. 2.5 kHz is well
outside the passband
of the transmitter
and antenna! It
isn’t even RF
anymore.

(1849.5*1)+(1847*1)=
3696.5 kHz, again
well outside TX
passband. 

(1849.5*3)-(1847*1)=3701.5
kHz again outside
the passband of the
antenna and
transmitter’s RF
section. This was
the fourth order.

Even-order mixing
clearly isn’t a
problem in RF
transmitters. This
is the reason why
push-pull RF
amplifiers don’t
help audible
distortion and don’t
help bandwidth.
Push-pull designs do
reduce harmonic
content
(distortion). This
in turn relaxes
output filter
requirements, but
output filtering is
generally is a
non-issue anyway.
Even a simple pi can
provide
adequate harmonic
suppression.   

Keep these rules
in mind:


bullet Any
increase in
frequency
difference
between the
highest and
lowest
modulation
frequency
increases BW
greatly.
bullet An
increase in
level increases
the strength of
the IM product
in even greater
proportion than
we might
expect.  

Odd-order
products create most
of our SSB
headaches. This
occurs because
odd-order products
can fall outside the
normal passband of a
typical SSB
transmitter,
spreading unwanted
and undesirable
distortion energy
throughout adjacent
voice channels. This
is why Enhanced SSB
or increased bass
and treble is a very
poor idea on crowded
bands.  

Three Greatest
Sins

The three
greatest sins
creating unnecessary
bandwidth are:

  1. Turning
    up a radio’s
    internal power
    or “drive
    limit” pot.
  2. Enhancing
    Bass and Treble
  3. Under-loading
    an amplifier

Radios 

Some modern
radios are
especially poor,
even when operated
at rated power
levels.  For
example, the TS-2000
and IC-756 series
radios are
particularly bad on
160 meters. I can
hear some of these
radios, when signals
are strong,
producing weak
spurious emissions
(splatter) 10-20 kHz
away from the
operating
frequency. If
you look at ARRL
test reports
of
transmitters, you
will see many radios
are just a bit above
class-C performance
levels. (Remember
the ARRL uses dB
below PEP, which
improves results by
6 dB compared to
commercial test
methods.)

IM3 levels of -30
dB are really very
poor. An old KWM2 I
tested was -47 dB
using ARRL
standards. Compared
to something like an
IC-756, the Collins
had about 50
times LESS power
in
total adjacent
channel distortion
products!

Compression and
Processing  

The multiple
tones in our voices
contain low and high
pitched tones that
mix. The strength of
the distortion
products outside the
desired
communications
channel depends
heavily on the
average power level
of low and high
pitches and the
frequency spread
between the lowest
and highest pitches
modulating the
transmitter. Since
the average level of
lowest and highest
modulating
frequencies increase
with speech
processing, any form
of speech processing
or compression (even
ALC) increases
off-channel average
IM power
levels. 

Processing is a
bit of a
double-edged sword.
It isn’t all bad.
Processing that
controls peak levels
reduces chances of
overdriving stages
following the
processing.
Decreasing the ratio
of peak to average
power produces a
more steady load on
power supplies. It
also can prevent
later stages from
limiting or
clipping. Although
processing brings
the average power
level of lows and
highs up, it can
also decreases
overdrive problems.

Light or
modest 
processing is
actually beneficial
in reducing
splatter!

ALC

ALC is normally
plagued with
inherent problems.
Filters in radios
add group delay (the
signal takes
noticeable time to
move through
filters), and the
ALC loop adds a
time-delay of its
own. The result can
be a leading edge
signal power
overshoot, which
often shows up as an
adjacent channel
“spit” or
“pop” on
leading or rising
edges of voice or
CW.

Some rigs like
the early 775DSP and
IC706 are plagued
with very high
levels of overshoot.
A new 775DSP I had
actually overshot to
around 300 watts on
leading edges for a
few milliseconds.
Kenwood and other
radios also have
this problem. This
problem is not only
harmful for
bandwidth, it also
can damage
amplifiers
. The
problem often gets WORSE
as power level is
turned down! 

Gain should be
set so ALC just
starts to take
effect, if a drive
control is
available. Rigs like
the FT1000D include
a “Drive”
adjustment.

WideFi or
Enhanced Audio

Enhanced SSB
audio is a generally
bad idea, since it
adds and boosts
unnecessary lows and
highs. Audio
response flattening
brings levels of
unnecessary low and
high frequencies up,
and this rapidly
increases power
level in unwanted
off-frequency
products compared to
normal
communications
audio. The frequency
difference between
lows and highs is
wider, so the
“junk”
extends further than
normal. The level of
lows and highs are
significantly
stronger than levels
in normal
communications
audio, and this
makes IM products
much stronger. As a
matter of fact,
energy in IM
products does not
follow a linear
increase as base and
treble are
increased! Unwanted
power on adjacent
channels increases
at several times the
rate of the power
increase in base and
treble! 

Make no mistake
about it, enhanced
SSB or Hi-fi SSB
audio, even with
perfect “brick
wall”
filtering, is always
going to have significantly
more
unwanted energy on
adjacent channels
when compared to
regular
communications audio
through the same
system.

Many of the
radios popular with
the ESSB crowd are
among poorer radios
for IM performance!
My own opinion is
Hi-Fi audio is OK on
emptier bands, but
we should do all we
can to discourage
enhanced audio on
crowded bands or
near weak signal
areas.

Transmitter
Tests

The standard
transmitter test is
IM3 or higher order
products. The
general test uses
two tones of equal
level. If it is a
radio, the two tones
are fed into the
audio port. If it is
an amplifier, the
tones must generally
be from two separate
transmitters
generating steady
carriers. The
carriers are mixed
through a combiner
and used to drive
the amplifier. The
reason we use two
separate
transmitters in the
test is most
amplifiers,
especially cathode
driven triodes, are
far cleaner than
most modern radios.
The test radio’s
two-tone IM would
establish the IM
distortion limit (
not the amplifier,
in most
cases.     

Confusing
Conflicting
Standards

In most
commercial tests, we
find the maximum
power of one tone
and compare that
level to the level
of third-order
spurious signals
created in the
transmitter. The
ARRL for some reason
adopted a different
reference. The ARRL
compares PEAK power
of both tones to the
spurious, rather
than the level of
one test tone to the
spurious. This
inflates transmitter
IM results, making
everything appear
6dB better than the
standard
dB-below-one-tone
test used
commercially. 

If you look at
Eimac data sheets,
the IM specs are
dB-below-one-tone of
the two tones. If
you look at data
sheets from other
sources, they might
use dB below PEP.
One manufacturer’s
US importer of
Russian tetrodes
used dB below PEP to
compare the quality
of their product to
Eimac. 

Unfortunately
Eimac used dB below
one tone, while the
other tetrode test
used dB below PEP.
This results in a
6dB change in
results. The tetrode
manufacturer wrongly
proclaimed their
tetrodes
“cleaner”
when in fact they
were not. If you
look at QST tests of ETO, QRO, and
Ameritron amps you
will see the
3CX800’s in the
AL800H greatly
surpass the 4CX800’s
in the other amps
for IM3 and IM5.
This confusion is a
clear example of how
mixed standards gets
us in
trouble.   

It’s a Poor
Test Anyway

Two-tone IM3 (or
higher orders, like
IM5) transmitter
tests generally show
us the very BEST
a transmitter will
do. In general,
two-tone tests are
pretty poor tests
for system designed
to process speech.
The two-tone test
uses two steady
signals (normally
with wide spacing),
but actual
modulation has many
frequencies that
vary at syllabic
rates. 

In two-tone
tests, the test
spacing is generally
a few kHz. The
varying load on
power and bias
supplies is at the
separation of the
two frequencies.
Small capacitors
filter the
time-varying load,
while the long term
(or low frequency)
dynamic load remains
constant. Two-tone
tests do NOT
show power supply
deficiencies.

Slow variations
in speech level load
and unload power and
bias supplies. This
causes supplies to
“wobble
around”. The
conclusion of some
is that screen or
bias regulation is
“unimportant”,
but that conclusion
is mostly rooted in
the fact the tester
actually used a
flawed test method
that does not show
low frequency
dynamic regulation
problems. We can’t
test for distortion
created by poor
low-frequency
dynamic regulation
when the test method
provides a constant
load on the
supplies! 

A Better Test

There are two
tests that are
better. One test is
an adjacent channel
power tests, with
normal voice
modulation of the
transmitter, another
test I developed
uses a three-tone
signal. 

The three tone
test injects a third
low-frequency tone
into the system. The
third tone is
anything from a
warble to  a
low-pitched hum
causing a slow
variation in power
levels of the two
major tones. The
analyzer reads the
peak amplitudes of
mixing in higher
frequency tones
while the level is
varied at a syllabic
to low pitched audio
rate. The low pitch
amplitude modulation
is varied in
frequency until the
worse case IM is
produced.

The variation
causes the power to
change at a speech
rate, testing supply
regulation effects
on wide spaced
distortion at all
important
frequencies for
speech.

The adjacent
channel power test
simply uses normal
voice operation and
compares the long
term peak power in
an adjacent channel
to the peak power in
the desired channel.

Either test gives
a much more reliable
indication of
transmitter
bandwidth than a
two-tone test.

Remember, a
two-tone test is
generally a
“best
case” scenario.

Summary

A normal two-tone
test is not really
very effective in
measuring a SSB
signal, because
there is no slow
dynamic change
typical of a voice.
Voltage regulation
problems are masked
and do not show up
when a two-tone test
is used, because
load currents all
average the same
amount. Filter
capacitors and other
energy storage
components mask any
voltage regulation
problems. We can, as
a general rule, be
confident the actual
SSB voice
performance is LESS
than a two-tone test
indicates.

A large
improvement occurs
with a three-tone
test, when levels
are varied at
syllabic rates as
well as higher
frequencies in the
voice range. This
tests voltage
regulation problems
otherwise hid in a
two-tone test,
because all speech
frequencies ranges
are included. The
most accurate test,
of course, is with
actual speech. The
FCC  now
requires some
commercial radios
used on congested
bands to be tested
with actual speech.

We can help ease
spectrum pollution
by:


bullet Strongly
discouraging use
of enhanced bass
and treble on
crowded bands.
It has no place
on crowded band
or near weak
signals. Wide-Fi
is selfish and
inconsiderate
even when it
uses a 3kHz
filter because
of the increase
in level and
frequency spread
of IM
products.  

 


bullet Discouraging
and chastising
people would
turn the power
limit or drive
limit control
inside radios
up. This is CB
behavior! 
There isn’t a
radio made that
will tolerate a
user-increase in
power limit
without a
serious
degradation in
IM performance.
If you have a
friend who peaks
up the power
control inside a
radio, tell him
why it is bad.
Radios are bad
enough without
removing even
more headroom.
Even the best
transistors
cannot be driven
more than about
half of
saturated power
before IM
becomes
unacceptable.    

 


bullet Making sure
we use
processing and
ALC, but only at
modest levels.
We should just
see the needles
start to show
compression.

 


bullet Making sure
we tune
amplifiers
correctly
,
and avoiding 12
volt transistor
amplifiers or
grid-driven
tetrode
amplifiers
whenever
possible. 


Hit Countersince
March 19, 2005