December 1966 QST
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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This passive limiter is a simple combination of cascaded "T"
type resistive attenuators that are switched in and out of the
circuit based on the power level in the line. The design takes
a bit of thinking due to needing to retain a reasonable impedance
match at the input and output throughout various stages' conduction
states. Arriving at an optimal value for resistors would require
a circuit simulator with a mathematically based optimizer, but,
especially for amateur radio work, close is good enough. That
is not to say Hams are a bunch of slackers - they're not - it's
just that component and software resources are not as readily
available (aka 'prohibitively expensive')
for doing the analysis and testing. In 1966 when this article
was published, software did not even exist for people without
access to university or corporate computers. For most users
these days, it is cheaper to buy a limiter for 2- to 3-score
dollars from suppliers like
Pasternack,
Mini-Circuits, and others. Note the distortion apparent
in the limited waveform.
A Passive RF Limiter
By George Schleicher.* W9NLT
An interesting audio limiter circuit using diode switching
of resistive attenuators. It does not "slice the top off the
signal" sharply the way simple diode clippers do, and thus has
relatively little effect on the bandwidth of a speech signal.
A limiter circuit can be constructed with passive elements;
the design of this one is such that it will not generate high-order
harmonics and it need not be frequency sensitive in the audio
range. The limiter uses a multiplicity of T-section attenuators
in tandem; each section is unusual in that a pair of diodes
is connected in series with the shunt arm. The diodes function
like switches that open in the absence of a potential but close
when the voltage applied to any section of the attenuator rises
to a predetermined level. The closing of the shunt path causes
the loss of the attenuator section to increase to its design
value. The switching action is illustrated in Fig. 1.

Fig. 1 - The basic limiter circuit. Closing
S1 increases the attenuation without changing the
frequency-transmission characteristics. S1 should
close when Ein reaches a predetermined value.
As a result of the switching action each section of the attenuator
will offer a small loss to the low-amplitude portion of an electrical
signal and a higher loss to amplitudes of higher level. The
maximum loss of any attenuator section is governed by its design.
The maximum amount of compression that the limiter can provide
is determined by loss of each attenuator section and the number
of sections that are connected in tandem. Good results have
been obtained by using ten or twelve sections in tandem, each
section having a maximum loss of two or three decibels. The
maximum amount of compression that will be realized from a limiter
of this type will be equal to about half of the total loss of
the attenuator sections.
When a voice signal is modified by limiting action there
is necessarily a change in the harmonic relationships within
the signal. Listening tests indicate that heavy limiting using
a limiter of this type causes a voice signal to become somewhat
"bassy," but this effect is hardly noticeable if the voice signal
has been limited to a bandwidth of only 3 kc. by means of a
filter.

These scope pictures show the effect of limiting
on waveform. (A) Sine wave (765 cycles) before limiting; (B)
Same signal after 8 db. of limiting.
Diode Action
Solid-state diodes exhibit resistance in the forward conduction
mode. This resistance can vary from a fairly high value (over
10,000 ohms) to less than 100 ohms. It will depend on the voltage
across the diode and the materials of which the junction is
made. The materials also determine the manner in which the diode
will begin conduction. For example, copper-oxide junctions begin
conduction more slowly than germanium or silicon.

Fig. 2 - Test setup for measuring diode resistance.
R1 is a linear control.
Design Principles
The characteristics of the diodes and the design of the attenuator
sections should be complementary. The diode resistance when
conducting should be low enough to be negligible in the shunt
arm of the attenuator; in the nonconducting mode it should be
high enough to make the shunt appear as an open circuit. Pairs
of diodes are used so that the positive-going and the negative-going
portions of a wave will be similarly affected. The voltage at
which the diodes begin conduction determines the range over
which the limiter will be effective. The limiter circuit should
be driven from a source having an impedance at least as high
as the design impedance of the attenuator sections, and it should
be terminated in a similar impedance. Since the diodes are connected
in the shunt arm of the attenuator the basic limiter design
can be applied to both balanced and unbalanced (one side grounded)
attenuators. The circuit described here uses unbalanced T sections
for simplicity.

Fig. 3 - Resistance of three types of diodes
measured with the test circuit shown in Fig. 2.
A Practical Circuit
Building a limiter of this kind can start with the acquisition
of about two dozen diodes of a given type. Their forward resistance
should be measured using an arrangement similar to that shown
in Fig. 2. Measurements should be made in increments of 0.05
or 0.1 volt starting at zero and continuing until the current
through the diode reaches its maximum rated value for the type
of diode under test. A graph can then be drawn plotting junction
voltage against resistance (resistance is first computed by
dividing the voltage by the resultant current). Fig. 3 shows
the kind of curves that result when different diodes are measured
this way. Using the curve for the 1N34A as an example, it is
evident that the resistance will drop to about 200 ohms and
that there is a "knee" in the curve at a potential of 0.45 volts.
The potential is significant because it corresponds to the input
voltage at which limiting action is maximized. The diode resistance
at the knee (250 to 300 ohms) is used in designing the attenuator
sections.1 The shunt resistance used in the attenuator
should be about ten times the diode resistance at this point,
or 2700 ohms if the nearest standard resistor value is chosen.
Knowing that the shunt resistor will be 2700 ohms and desiring
a loss of about 2 db. in the attenuator leads to the conclusion
that the characteristic impedance of the attenuator should be
72 ohms. (These conclusions are arrived at through the help
of the formulas given below.) The resulting limiter circuit
is shown in Fig. 4. It should be noted that between attenuator
sections the output series resistor of one section has been
combined with input series resistor of the following section
(72 + 72 = 144 ohms). Again the nearest standard resistor value
(150 ohms) has been chosen for use in the circuit. The waveform
photographs show how compression changes the shape of a sine
wave.

Fig. 4 - (A) Practical circuit for a single
section.

Fig. 4 - (B) Cascaded sections; note that
the 75-ohm series arm on the output side combines with the 75-ohm
series arm on the input side to make the single value of 150
ohms between adjacent shunt arms. Half-watt resistors are satisfactory.
In this circuit T1 is assumed to have a turns ratio
such that the plate resistance of the preceding amplifier tube
is transformed to a value of resistance that is low compared
with the characteristic impedance, 600 ohms, of the attenuator.
Likewise, the input impedance of the device to which the limiter
is connected is assumed to be high compared with 600 ohms. When
this is not true, R1 and R2 should be
selected so that total input and output impedances are 600 ohms.
Appendix
Attenuators are lossy resistive networks. They are usually
designed to have the same impedance at their input and output
terminals. Unbalanced attenuators are usually referred to as
"T" or "π" attenuators since these letters describe the circuit
configuration. Their balanced counterparts (for use in ungrounded
circuits) are referred to as "H" or "O" attenuator
Only four simple formulas are needed in designing T attenuators;
they are as follows:
Loss (expressed in db.) =
{1}
n =
{2}
a (the series resistor value) =
{3}
b (the shunt resistor value) =
{4}
(Z is the characteristic impedance of the attenuator).
As an example of the use of these formulas, assume that you
are designing an attenuator of 150 ohms impedance with a loss
of 6 db.:
6 =
{from 1}
6/20 =
{from 1}
0.3 =
{from 1}
antilogarithm of 0.3 = 2.0 } from slide rule or log table
2.0 =

= 1/2 = n = 0.5 {solving for n}
a = 150
= 150
= 50 ohms {from 3}
b = 150
= 150 (1/0.75) = 200 ohms {from 4}

Fig. 5 - Attenuator used as an example for
calculation as described in the Appendix.
A single attenuator section of 150 ohms impedance and 6 db.
loss is shown in Fig. 5.
Some representative attenuator section values are shown below.
They are included as an aid in designing limiters of the kind
described here.
1 |
57.5 |
8500 |
2 |
115 |
4310 |
3 |
171 |
2840 |
4 |
224 |
2100 |
These values are based on an attenuator impedance of 1000
ohms. For other impedances the values should be increased or
decreased proportionately.
1 Th The resistance measured
in this way is a "d.c." resistance, and while for higher accuracy
in circuit design the dynamic resistance should be determined,
its measurement is considerably more difficult. The extra complication
would not be warranted unless it were necessary to know the
exact attenuation at different voltage levels.
Posted March 19, 2015