[Table
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged. |
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 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.
A Practical CircuitBuilding 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. 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" 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