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A Passive Limiter
By George Schleicher.* W9NLT
Fig. 1 - The basic limiter circuit. Closing S
_{1} increases the attenuation without
changing the frequency-transmission characteristics. S
_{1} should close when E
_{in}
reaches a predetermined value.
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.
Fig. 2 - Test setup for measuring diode resistance. R
_{1} is a linear control.
Fig. 3 - Resistance of three types of diodes measured with the test circuit shown in Fig. 2.
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.
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.
Diode ActionSolid-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.
Design PrinciplesThe 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.
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
T_{1} 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, R_{1} and R_{2}
should be selected so that total input and output impedances are 600 ohms.
AppendixAttenuators 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" attenuators.
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}
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.
Fig. 5 - Attenuator
used as an example for calculation as described in the Appendix. Loss,
db.
a resistance
b resistance
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} 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 3/5/2013