Diode characteristics and their applications have not changed fundamentally
since this article was published in 1952. Sure, the die are smaller,
power handling and frequency range has increased, package styles
are greatly expanded, and the cost per unit is way down, but if
you are looking for some basic diode information, you will find
it here in this 4th installment of a multi-part series in Radio &
Television News magazine. Don't let the vacuum tubes in schematics
scare you off and think that it makes the story irrelevant for today's
circuits. For purposes of illustration substitute a transistor's
collector (or drain) for the tube's
plate, a transistor's base (or gate)
for the tube's screen grid, and a transistor's emitter
(or source) for the tube's cathode.
Crystal Diodes in Modern Electronics
Part 4. Various applications of germanium crystal diodes
as employed in present-day FM circuitry.
By David T. Armstrong
Several banks of assembly machines at the Clyde,
New York plant of General Electric Company. This factory is devoted
exclusively to the manufacture of germanium assemblies and other
It is assumed here that the reader has a basic understanding
of FM and that he is familiar with the function of limiters, frequency
discriminators, and ratio detectors. These are the important parts
of an FM circuit in which crystals are beginning to play a significant
role. Only those aspects of circuit considerations will be treated
here which deal specifically with the application of germanium diodes
to functions heretofore performed entirely by diode tubes of the
6H6 and 6AL5 types.
Crystals function exceptionally well in any type of FM circuit,
on i.f.'s ranging from the 4.5 mc. of the intercarrier sound system,
through 10.7 and 25.75 mc., to the new 44 mc. frequency now coming
into use in modern television receivers. The FM section may be a
distinct entity of an FM receiver, or it may be the FM sound system
in a modern television circuit. The material presented here applies
equally well to any type of modern FM circuitry.
One of the basic require merits of an FM system is a limiting
device to eliminate amplitude variations before they reach the detector.
The function of the limiter is to remove amplitude modulation and
to pass on to the detector a frequency modulated signal of constant
amplitude. To operate successfully, the limiter must be supplied
with a sufficiently large signal voltage so that the amplitude of
its output will not change with rather wide variations in signal
amplitude. Noise, which causes little frequency modulation but much
amplitude modulation of the received signal, is virtually wiped
out in a limiter stage. Automatic volume control may be used with
an FM receiver, but when a limiter is operating properly, a.v.c.
is neither necessary nor desirable.
The limiter is part of the final i.f. amplifier stage; its main
function is to remove amplitude variations which might reach the
detector and appear as distortion in the audio output. The limiter,
then, is a gate which removes amplitude variations from a signal
above a predetermined level and passes on a signal that is constant
The positive and negative peaks of the FM signal will be truncated
and flattened. See Fig. 1. This does not introduce distortion into
the FM signal as it might in an AM signal because the modulation
component or intelligence is contained in the frequency deviations
of the signal and not in the amplitude variations of the signal.
Frequency deviations due to modulation are not affected by limiter
The actual FM response curve is neither ideal nor flat topped.
Hence the various frequencies making up the total frequency deviation
will not have the same relative amplitude at the input to the limiter.
The center frequency and the frequencies close to it will have greater
amplitude than those considerably removed from the center frequency
due to the action of the i.f. tuned circuits. This is demonstrated
in Fig. 2. The unequal amplitude of the various frequencies appearing
at the input to the limiter would cause severe distortion if something
were not done in the receiver to compensate for it.
Fig. 1 - Limiter action for a strong i.f. signal.
Note that the amplitude of the input wave at the highest frequency
deviation components of the FM wave is above the limiter level,
and that the input level of the i.f. signal over the entire range
of the frequency deviations is above the limiter level. Also note
that positive and negative peaks of the output FM wave are truncated.
The output of the limiter is constant over the entire range of the
A limiter is sometimes regarded as a device for removing all
noise. This is not so. A limiter will function efficiently (but
not perfectly) when the voltage level (amplitude) at the input to
the limiter of the greatest frequency deviation component (this
is the frequency ± 75 kc. from the mean frequency) is greater than
the limiting level. Limiter output will be constant when a total
band of 150 kc. is passed at a constant level, for then all the
frequencies making up the total deviation will be reproduced in
their proper relation, and without distortion due to AM or random
The limiter characteristic represented by the graph at A in Fig.
2 will permit AM distortion because the i.f. signal is below the
limiter level. Note that the limiter level is gauged by the characteristic
curve of the voltage-frequency graph. The signal at B will permit
no distortion because the lowest signal input level is above the
limiter level. Hence all AM components and/or random noise are "hedge
clipped" by the action of the limiter.
Fig. 2 - Effect of limiter action on varying
limiter input signals. (A) Noise voltage reproduced in the output
of limiter. (B) Noise is removed by limiter action, wave is truncated.
Note that when the input level of the i.f. signal over the entire
range of frequency deviation is above the limiter threshold, as
in (B), no noise voltage is reproduced in the output of the limiter
but when the input level of the i.f. signal is below the threshold
of limiter action, the limiter cannot function and the noise will
be reproduced as shown in (A).
Diode Dynamic Limiter
Any residual amplitude modulation and noise riding the FM wave should
be suppressed. The limiter component desired must be an effective
supplement to the action of the FM detector to reduce random noise
and AM interference. This is necessary because a balanced discriminator
completely suppresses AM at but one frequency, and a ratio detector
is critical to align and balance. Maximum AM rejection may not occur
at that alignment adjustment which provides the most desirable linearity.
It is of course recognized that a cascade type grid bias limiter
is capable of nearly complete AM suppression; but two additional
tubes are necessary and this type of circuit is relatively expensive.
A comparative set of curves for one diode, two diodes, and a cascade
limiter is shown in Fig. 3. The single and double diode curves are
variable threshold devices that show AM reduction factors ranging
from 6 to 10 db better than the cascade limiter for signal levels
below the threshold of the cascade type limiter.
Fig. 4 shows a dynamic limiter circuit employing a type 1N48
or 1N56 as the germanium crystal diode. This is a simple and highly
effective type of amplitude modulation limiter for both an FM receiver
and a TV sound channel. This limiter provides a variable threshold
action that extends to small signal levels and effects a significant
degree of quieting on weak signals as well as for interchannel background
Any signal of such peak amplitude as to be above the threshold
level will have its residual amplitude variations suppressed by
this limiter. The ideal limit of suppression may be more nearly
approached by a germanium crystal than by a vacuum tube, because
the crystal exhibits so much greater conductance than a tube. In
addition to high conductance, the crystal diode exhibits extremely
This variable threshold limiter device uses a resistance-capacitance
network with a time constant long compared to the lowest expected
amplitude modulation frequency, and the limiter adjusts itself automatically
to the varying average signal amplitude. A time constant of 0.1
second is sufficient to insure rejection of AM components down to
For a given frequency there is a loss caused by the insertion
of a diode in a transmission system. It is the ratio, expressed
in decibels, of, the power delivered before the insertion to the
power delivered after the insertion; this is commonly referred to
as "insertion loss." Since for any given signal level the insertion
loss of the limiter becomes greater as the resistance is reduced,
the resistance value is governed by the allowable limiter insertion
loss and the desired degree of small signal AM rejection. 10,000
ohms is a reasonable compromise among all the factors which obtain.
To achieve a time constant of 0.1 second the value of the electrolytic
type condenser then becomes 10 μfd.; for a 20,000 ohm resistor
it would be 5 μfd. The small 500 μμfd. mica condenser bypasses
the high frequency i.f. components.
Fig. 3 - Comparison of single and double diodes
with cascade type grid bias limiter. (A) Single diode dynamic limiter
(IN56), (B) Double diode dynamic limiter (2-IN56's) end (C) Cascade
limiter using 2-6SJ7 tubes.
With this type variable threshold limiter AM reduction varies
smoothly with signal level, AM noise decreasing as the signal level
increases, and approaching zero as the signal voltage is increased
by virtue of improvement in crystal efficiency at high signal voltages.
The biased high conductance diode 1N48 or 1N56 is shunted across
the tuned circuit which is the primary of the detector input transformer,
either limiter-discriminator or ratio detector type. Whenever the
"Q" of the tuned circuit exceeds 25, the damping provided by the
diode is effectively integrated over the i.f. cycle. Voltage regulation
is predominantly in the tuned circuit, and the diode helps maintain
essentially constant voltage across the circuit.
This type dynamic limiter is not critical with respect to characteristics
of the particular crystal employed; virtually any germanium diode
will perform well in this circuit. This is a worthwhile consideration
in connection with replacement of one unit by another. Further,
the back resistance of the crystal also serves to augment the action
of the limiter. Finally, a receiver using a dynamic limiter would
require only 1/2 to 1/3 the input signal voltage at the antenna
to produce a given amount of quieting.
Of course this limiter is not capable of as great AM suppression
as the cascade grid bias type limiter. However, the variable threshold
action tends to extend the range of operation to low signal levels.
Thus the use of such a dynamic limiter in simplified FM receivers
is attractive because of the significant quieting on weak signals,
even with but slight over-all gain. In the absence of a signal some
squelch action occurs as a result of partial limiting on receiver
Fig. 4 - Single diode dynamic limiter. The values
of the 10,000 ohm resistor and the 10 μfd. condenser may be chosen
to suit the signal frequency and degree of clipping desired. Values
of the resistor may range from 5000 to 50,000 ohms. Condenser values
will depend on the time constant desired. The time constant of this
circuit is approximately 0.1 second. Where high impedances are desirable
G-E types 1N52 or 1N63 or Sylvania type 1N54 may be employed.
In a TV receiver with intercarrier sound this type dynamic limiter
helps to reduce the audio buzz which sometimes accompanies excessive
modulation depth of the picture carrier.
The double diode dynamic limiter circuit shown in Fig. 5, used
in conjunction with an FM detector, helps suppress residual AM in
frequency modulation type receivers or sound circuits of TV receivers.
A high conductance diode like the 1N56 provides exceptionally effective
limiter action, particularly at signal levels as low as 5 volts
or less. The low dynamic impedance and the low diode capacitance
produce a minimum of reactive loading across the source and minimize
any loss traceable to limiter insertion at low signal levels.
The two biased diodes are so polarized that they conduct in opposite
directions. The net improvement in AM reduction factor (ratio of
the percentage modulation of output signal to input signal) is so
exceptional that it is shown graphically in Fig. 6.
Many television receivers use a limiter stage ahead of the discriminator,
even when a ratio detector is used as the detector. The function
of the limiter is to clip off any amplitude variations of the sound
i.f. signal that may be caused by noise or non-uniform i.f. amplification
over the frequency band. Wherever the normal amplification of the
grid biased limiter is not necessary, a biased diode may be used
The basic limiter circuit in Fig. 4 illustrates this effectively.
The diode with a bias voltage equal to the normal signal level is
placed across a tuned circuit. The diode will conduct only on peaks
that exceed the normal signal level; hence noise peaks will be automatically
shorted out. Harmonic distortion as a result of such clipping action
may be minimized by using two diodes to clip both the positive and
negative peaks, as in Fig. 5. This is, in effect, a full-wave limiter.
The bias is usually obtained from an RC circuit so designed and
with such a time constant configuration that it automatically adjusts
itself to the signal level. This use of crystal diodes is one of
the most inexpensive means of securing desirable limiter action.
These germanium diodes are quite likely to be used widely in discriminator
circuits. They may be wired directly to the transformer and mounted
in the shielded can to facilitate elimination of contact potential
feedback and filament hum problems.
Fig. 5 - Double diode dynamic limiter circuit.
One of the basic requirements of an FM system is that the detector
be a device for converting frequency changes into amplitude variations
which may then be amplified as audio signals. In the widely used
Foster-Seeley discriminator the signal frequency varies back and
forth across the resonant frequency of the discriminator and an
a.c. voltage of the same frequency as the original modulation is
developed and passed on to the audio amplifier.
Fig. 6 - Comparison of AM reduction factor for
single and double diode dynamic limiter.
The discriminator in an FM circuit corresponds to the detector
in an AM circuit in that both demodulate the intelligence from the
carrier wave. The process is different, but the net result is substantially
the same. The voltage versus frequency characteristic for an FM
discriminator is shown in Fig. 7. The total voltage output of a
discriminator varies in a positive and negative direction depending
upon the deviation of the i.f. signal above or below the mean frequency.
The greater the frequency deviation the greater the voltage developed.
Fig. 7 - Characteristic curve for a discriminator.
Note that output voltage of unit is greater for a high input voltage
level as shown in curve B as compared with lower input level for
curve A. Also the quality of response depends on linearity of curve
, from -75 kc. to +75 kc. deviation from center frequency of i.f.
response. This graph demonstrates that output of the discriminator
may vary with changes in signal level (which is AM variation since
curves for B and A show characteristics for different signal levels).
The output voltage is the algebraic sum of the voltages developed
across the load resistors of the two diodes. It should be apparent
from the curve shown that if the straight portion of the discriminator
voltage frequency curve covers a wider range of frequencies than
those generated by the transmitter, the audio output will be reduced
from the maximum value of which the receiver is capable. This must
be so because at its "center" frequency the discriminator produces
zero output voltage. On either side of this center frequency there
is developed a voltage of a polarity and magnitude that depends
upon the direction and amount of frequency shift from the center
Therefore, the voltage output of a discriminator varies in precisely
the same fashion as the audio voltage which modulates the carrier.
The greater the voltage developed across the diode load the louder
the sound coming from the speaker. When there is no modulation on
the FM carrier there is no deviation of frequency and consequently
no audio voltage is developed; hence, no sound comes from the speaker.
But there is an important point to be made in connection with
a discriminator detector. The output voltage of a discriminator
may vary directly with change in input voltage. The curves marked
A and B in Fig. 7 indicate this fact. This is why a limiter circuit
is important. It holds the input level at constant amplitude and
does not permit the discriminator to receive signals that are amplitude
modulated. The reason why amplitude modulated signals might appear
at the discriminator in an FM circuit was discussed before in connection
with the limiter, where it was shown that since the response curve
is not perfectly flat topped, there is some variation in the signal
level which is, in effect, amplitude modulation of an FM signal
The method of conversion of frequency changes into audio voltage
is graphically illustrated as a function of the linear portion of
the discriminator characteristic, shown in Fig. 8.
The circuit of Fig. 9 is a simple discriminator detector circuit.
The better the matching of the diodes the better the performance
of this type circuit; but note the remarks in the caption. This
circuit will operate over the entire range of commonly encountered
i.f. frequencies from the 4.5 mc. used with intercarrier sound to
the 44 mc. i.f. The crystals and associated resistors and condensers
may be mounted under the chassis or they may be enclosed in a small
shield can. In some instances, by careful layout and design, it
is possible to include the diode crystals, resistors, and condensers
in the FM discriminator shield can. Such location is important in
preventing feedback. This makes a most compact assembly although
it does present servicing difficulties.
The circuit shown in Fig. 10 is desirable from the standpoint
that crystal matching is not necessary. The 220,000 ohm resistors
in parallel with standard stock type 1N48 diodes keep the circuit
balanced irrespective of the back resistance of the crystals. The
other circuit values are typical of those found in a discriminator
circuit. The reverse resistance of a crystal diode is subject to
minor variations with changes in ambient temperature, humidity,
and impressed voltage. While in general applications the small changes
in back resistance are of little consequence they are significant
in an FM detector because demodulation depends upon close balance
between the two parts of the circuit. The better the balance the
higher the degree of linearity and the greater the AM suppression
for the discriminator.
Fig. 8 - Relation of frequency deviations to
audio voltages. A large frequency deviation from the mean frequency
generates a large voltage in the discriminator, and a small frequency
deviation from the mean frequency generates a small voltage. In
the discriminator output this frequency deviation from the carrier
frequency appears as a voltage variation
from a zero line which represents the mean or carrier frequency.
Graph at left is the output of limiter and input to discriminator
as measured in frequency deviation from the mean or carrier frequency.
Fig. 9 - FM discriminator circuit using germanium
diode crystals. The IN35 duo-diode, consisting of carefully matched
crystals, is highly satisfactory for this circuit. IN35's are matched
in forward resistance only and since this resistance is small compared
to 100,000 ohm load, balance is unimportant. The necessary balance
is in back resistance which is not very much greater than 100,000
ohms. This is one reason why shunting resistors are suggested in
Fig. 10. Use of shunting resistors will permit the use of less expensive
IN34 type crystals. The 100,000 ohm resistors and 47 μμfd.
condensers should be low tolerance matched components for ideal
balance of two parts of circuit. The de-emphasis circuit network
is shown only to indicate parts values.
The sound circuit of a television receiver is the same as that
found in a typical FM receiver. Detection of the i.f. signal is
accomplished by a discriminator or a ratio detector circuit. Both
types of circuit require two diodes and balanced conditions for
optimum operation. Germanium diodes have been successfully substituted
for vacuum tube diodes in a discriminator circuit; probably the
most widely used discriminator is the Foster-Seeley type. The chief
circuit difference for crystals as compared to the vacuum tube is
the use of shunting resistors with the crystals to maintain fairly
uniform balance between both halves of the circuit with respect
to the back resistance characteristics.
Ratio Detector Circuits
A discriminator detector requires one and preferably two limiter
stages because of discriminator sensitivity to amplitude as well
as to frequency variations. For effective limiting there must be
good amplification of the i.f. signal before it reaches the limiter
in order that all signals have a level sufficiently high to operate
the limiter at saturation. Since a ratio detector does not respond
appreciably to amplitude variations it is, from that point of view,
superior to a discriminator type detector.
Fig. 10 - Frequency discriminator circuit. This
circuit performs as well as that shown in Fig. 9 but eliminates
the need for using matched diodes by using 220,000 ohm resistors
in parallel with IN48 type diode crystals. Circuit is thus balanced
regardless of back resistance of diodes. Other component values
are typical of those found in a discriminator circuit.
The chief advantage of a ratio detector is that for a weak carrier,
on modulation, the voltage ratio is the same as for a strong carrier,
on modulation; therefore, the ratio detector is not responsive to
carrier changes, and hence relatively insensitive to either sudden
or dynamic changes in amplitude of the applied signal. Because a
ratio detector is responsive to slow changes in carrier, a.v.c.
may be desirable. The audio output deriving from frequency modulation
of the applied signal results from the change in the ratio of the
two diode voltages which makes the circuit responsive mainly to
variations in signal frequency and not to dynamic changes in signal
With a ratio detector circuit, balance between the halves of
the system is more critical than for a discriminator type circuit.
The ratio detector provides AM suppression as well as FM detection
and its operation depends, to a great extent, on the balance between
the halves of the system. The back resistance of crystals is not
uniform and changes with temperature and voltage level; the situation
is complicated by the fact that the changes are not likely to be
the same in both diodes, nor to occur at the same time.
It is therefore more difficult to design a ratio detector system
using germanium diodes, but it is not impossible. Variations of
the ratio detector circuit have been designed to minimize any detrimental
and undesirable effects of the back resistance characteristics of
the crystals. Although these circuits do not achieve all the good
inherent in the ratio detector system, they do approach the operating
quality of conventional vacuum tube circuits.
The ratio detector has excellent inherent noise and AM reduction
characteristics, and the conventional circuit using a 6AL5 is economical.
But it is not possible to simplify the conventional ratio detector
circuit just by inserting germanium diode crystals as substitutions
for the separate halves of the 6AL5. The dynamic characteristics
of a crystal are somewhat different from those of a vacuum tube
However, experimental work with the ratio detector circuit has
facilitated the development of a crystal diode ratio detector circuit
that provides performance data approximately equivalent to that
obtainable from a vacuum tube. The crystal diode circuit has excellent
physical advantages over the vacuum tube diode with respect to savings
in weight, power, and space, making possible the development of
battery-operated, portable-type FM receivers.
The ratio detector circuit depends critically upon close balance
between the two individual parts of the circuit in order to obtain
a high degree of linearity and to provide the amount of AM suppression
desirable in an FM receiver. The modified form of ratio detector
circuit here presented will yield results comparable to those achievable
by a vacuum tube circuit, assuming that both units are properly
designed and equally well constructed.
The combined load circuit shown in Fig. 11, has a time constant
long with respect to the period of any AM components present and
causes the sum of the diode output voltages to remain constant as
far as AM components are concerned. Since the sum of the diode voltages
is thus fixed by the long time constant load circuit, the ratio
detector is not responsive to the dynamic changes in the amplitude
of the signal.
The audio output due to frequency modulation of the applied signal
results from a change in the magnitude of the two diode voltages,
the net effect of which is to make the circuit responsive only to
variations in signal frequency and not to dynamic changes in signal
amplitude. Thus AM components due to noise and multipath transmission
effects are largely suppressed in the ratio detector.
To obtain maximum suppression of amplitude variations in the
output of the ratio detector, it is essential that the two halves
of the circuit be balanced and remain so throughout the entire dynamic
range of the input signal. This requires close tolerances in the
resistance and capacitance values and careful design of the input
transformer primary, secondary, and tertiary windings, as well as
close matching of the diode characteristics. The close matching
of the diode characteristics is most critical; for this reason it
is generally necessary to supplement the ratio detector with some
means of AM reduction before the ratio detector stage. In this one
respect crystals have some superiority over vacuum tube diodes.
Additional details on this point will be given later.
Many attempts to substitute crystal diodes for vacuum tubes in
the conventional ratio detector circuit have been unsuccessful,
in that little or no AM reduction was obtained, and the circuit
itself proved to be unstable both with respect to symmetry of detector
characteristic and permanency of alignment. However, with suitable
modification of the basic circuit arrangement the undesirable effects
of the variations in the back resistance of the crystal can be largely
eliminated, and a germanium diode ratio detector exhibiting the
characteristics of the vacuum tube diode circuit can be designed.
Fig. 11 - Crystal diode shunt ratio detector.
Modifications from the conventional type ratio detector circuit
are relatively minor, as can be gathered from a consideration of
the modified shunt ratio detector circuit shown in Fig. 11. The
load resistors for the crystals are shunt rather than series connected.
Electronically, the shunt circuit is equivalent to the series circuit
in that, for given values of load resistance and signal voltage,
the rectification efficiency is essentially the same for both.
Shunt connection of the crystal diodes makes possible the use
of resistances in parallel with the crystals, each of which is of
much lower value than the back resistance of the crystal across
which it is connected; these resistances have the effect of swamping
out the crystal back resistances. This detector circuit is relatively
insensitive to changes in crystal back resistance and tends to reduce
static and dynamic imbalance between the halves of the circuit.
By virtue of their high conductance, crystals tend to provide somewhat
improved circuit efficiency over vacuum tube diodes. Low shunting
resistors, however, reduce efficiency as compared to vacuum tubes.
Diode balance or AM suppression can only be obtained at the expense
There are numerous advantages to be gained by using crystal diodes
to replace the vacuum tubes in this type of circuit:
1. Compactness - The entire assembly can be built into the same
shield can as a plug-in device if one is willing to use the Vector
socket technique. The associated condensers and resistors, in addition
to the two germanium diodes, occupy so little space that there is
plenty of room to make a complete package unit of the entire ratio
2. Filament Elimination - The elimination of heater requirements
materially reduces hum difficulties, particularly with series heater
circuits, where the potential difference between cathode and heater
of a detector tube may be quite large.
3. Parts Elimination - This makes for economy in the number of
component parts, such as the socket and a smaller size filament
transformer, which result in a substantial saving of space and reduction
4. Imbalance Elimination - There is complete elimination of imbalance
resulting from contact potential effects in diode elements. Contact
potential may upset static balance between the halves of the circuit.
In FM detector circuits that are properly balanced maximum AM
suppression occurs at that frequency corresponding to the crossover
of the detector characteristic. A crystal circuit is somewhat more
susceptible to residual amplitude modulation than a 6AL5 duo-diode
The load resistance in a crystal type ratio detector circuit
has some effect upon circuit sensitivity as well as upon AM reduction.
Sensitivity as used here indicates the ratio of the d.c. voltage
across the holding condenser to the r.f. voltage across the secondary
of the input transformer.
On the basis of experimental curves showing circuit performance
with load resistances varying from 5000 to 50,000 ohms, it has been
found that the circuit is most stable when the load resistance is
kept small with respect to the back resistance of the crystals.
Values from 15,000 to 20,000 ohms are a nice compromise among the
variety of factors which obtain.
In some respects a crystal circuit is superior to a vacuum tube
circuit for AM suppression, but for an off-tune signal, background
noise is quite likely to be greater. On the other hand, a crystal
circuit is simpler to align than a vacuum tube circuit, by virtue
of elimination of contact potential imbalance effects.
A ratio detector circuit may be adjusted for virtually any pair
of crystals, but the AM reduction will vary from pair to pair, because
of the degree of variability in the dynamic forward characteristics.
With random selection of crystals an AM reduction factor of about
0.025 is possible, while with careful selection of crystals matched
for similar forward dynamic characteristics the AM reduction factor
is better than 0.010.
Fig. 12 - Effect of single crystal diode dynamic
limiter on AM reduction factor for shunt crystal diode ratio detector.
(A) Crystal diode ratio detector only. (B) Crystal diode ratio detector
and dynamic limiter.
Fig. 13 - The effect of a single diode dynamic
limiter on the AM reduction factor over entire band of operating
Whenever it is desirable to achieve a degree of AM suppression
comparable to that achieved by a vacuum tube grid bias limiter,
a circuit involving a crystal diode dynamic limiter just before
the ratio detector stage is indicated. In addition to providing
a substantial degree of AM suppression in itself a dynamic limiter
tends to minimize the necessity for AM suppression by the ratio
Fig. 14- Dynamic limiter and ratio detector
with (A) single and (B) double diodes.
Fig. 15 - One manufacturer's commercial adaptation
of a ratio detector circuit.
In such a combination germanium diode circuit crystal selection
may be eliminated in the ratio detector stage. This is because a
dynamic limiter extends AM suppression to lower input signal levels
and over a wider frequency deviation range from the mean signal
frequency. Fig. 12 and Fig. 13 show the AM reduction factor as a
function of signal level both with and without the dynamic limiter.
In summary, then, a crystal diode shunt type ratio detector combined
with a crystal diode dynamic limiter will provide an audio output
comparable to that obtainable with the conventional duo-diode tube
ratio detector of the 6AL5 type. A suggested circuit is shown in
Fig. 15 shows a commercial application of the 1N48 to a good
ratio detector circuit.
(To be continued)
Posted September 14, 2015