This is part 5 in a series that began in the October 1951 issue
of Radio & Television News magazine
part 4). Previous articles dealt with crystal diodes
in AM and FM radios, and this article shift gears by moving
into television applications. Crystal diodes were and are still
used in frequency generation, envelope detection, frequency
mixing, and AC signal rectification. Vacuum tubes could be used
for the latter three applications but many physical issues such
as size, weight, power consumption, and heat dissipation proved
to be major drawbacks as designers strived to reduce the size
of electronics assemblies, make them more energy efficient,
lower the cost of manufacturing, increase reliability, and decrease
weight. Demands for portability was the motivation for much
of the work. Early crystal diodes could be noisy and fragile
if not mounted carefully, but as will all technology, continual
R&D has refined and improved crystals significantly. These
early articles give great insight into the work that went into
adopting and promoting a new type of device.
Crystal Diodes in Modern Electronics
By David T. Armstrong
Part 5. A discussion of some of the applications of crystal
diodes to television receiver circuits.
A conventional i.f. transformer assembly
using a 1N64 germanium diode. G·E uses this unit in its TV receivers.
In earlier articles of this series we discussed the uses
of crystal diodes in AM and FM applications. In this and the
next article we will consider several of their applications
in modern television receiver circuits.
Fig. 3 shows several applications of crystals in TV receivers.
As far as it is possible to discover, no manufacturer uses crystals
at all these points, but some manufacturer uses crystals at
each of these points. This block diagram indicates the possible
applications of known germanium diode crystals in modern television
receivers, based on present knowledge of circuitry and the performance
characteristics of crystals. In general, the number of germanium
diodes that may be used in a given television circuit is limited
only by the number of diodes required.
To be quite optimistic, with the recent development of crystal
triodes, which may be used in place of certain vacuum tube triodes,
the future is bright with possibilities for small TV receivers
that may become virtually tubeless!
Germanium diode type crystals have a definite place which
they are now assuming on a large scale. It will be well for
the experimenter and technician to know something about them
for they are quite likely to supplant the 6H6's and the 6AL5's
in many TV circuit designs.
When color comes along the fellow who knows the fundamentals
of the application of germanium crystals will be able to apply
this knowledge to the uses of germanium and silicon crystals
in the ultra-high frequency color spectrum. Be ready for it.
That will be the day for germanium and silicon diodes! While
eventually crystals are likely to displace tubes at high frequencies,
even at low frequencies, certain types of crystals are proving
more stable than presently available tube type diodes.
Basic Video Detector Circuits
At the present time the most widely accepted application
of germanium diodes in television receivers is as video detectors.
The chief function of a video detector is to demodulate the
high frequency i.f. signal to obtain the video modulation. The
most common component used for this purpose, until recently,
has been half of a 6AL5. With but minor circuit changes a germanium
diode may be substituted for the vacuum tube as a high quality
This substitution is not a simple problem, however. It is
necessary to find ways and means of eliminating the other half
of the 6AL5 vacuum tube diode in order to dispense entirely
with the tube, socket, and associated wiring. This problem has
been solved in various ways, such as using another germanium
crystal as the diode d.c. restorer, sync clipper, or a.g.c.
peak detector. Of course it is possible to design and use a
full wave detector circuit using germanium diodes, but no manufacturer
seems to have done this. It is possible to design a very fine
full-wave video detector, but most design engineers feel that
the improvement in greater output and higher efficiency would
not be worth the cost nor the circuit complication.
In Part 2 (November 1951 issue) both series and shunt rectifier
circuits were described. Both can be used in video detector
applications. Consider the simple germanium diode TV detector
circuits shown in Fig. 1. A is a series type circuit and B is
a shunt type circuit. Both types of circuits are widely used
and both will perform equally well in properly designed systems.
Fig. 1 - Basic TV detector circuits. (A)
A series and (B) a shunt type of circuit.
The shunt circuit shown at B in Fig. 1 is used primarily
when a closely coupled i.f. transformer is used and capacitive
coupling to the detector is desirable to prevent "B+" voltages
from being impressed upon the diode crystal. With the diode
crystal connected in such a shunt arrangement it provides its
own d.c. return path; this path is normally restricted by the
coupling condenser in the series hook-up.
For a shunt circuit the back resistance characteristic of
the diode is important. It is necessary that the back resistance
be at least ten times the load resistance to maintain the achieved
gain. However, very high back resistance values may sharpen
the "Q" of the tuned circuit; then bandwidth may have to be
restored by a change in value of the coupling condenser or with
a compensating choke.
For a series type circuit, such as that shown at A in Fig.
1, the forward dynamic resistance of the diode is important
since it may be so large in comparison to the load as to form
a voltage divider and reduce the output voltage. Because germanium
diodes have lower dynamic resistance than vacuum tubes, additional
gain may be realized in the crystal type video detector circuit.
The "Q," or the sharpness of the resonance of the tuned circuit,
will be broader as a result of the lower resistance of a germanium
diode compared to that of a vacuum tube. While this may reduce
the gain of the last i.f. stage, it can be restored by increasing
the load resistance.
For both the series and the shunt circuit the load impedances
are determined primarily by the video bandwidth requirements;
therefore, these load impedances must necessarily be low values.
The load condenser should be small enough to present a reasonably
high impedance to the highest video frequency of 4 mc. and at
the same time be sufficiently large to hold the charge from
one peak to the next with a 24 mc. or 44 mc. i.f. signal. The
load resistor must be large enough so as not to lower the impedance
of the condenser and small enough to permit the condenser to
discharge at video frequencies. Typical values are 5 to 10 μμfd.
capacitance and 1500 to 5000 ohms resistance.
It should be realized that there are wider variations in
the dynamic resistance of germanium diodes than there are in
vacuum tubes. For this reason detector type germanium diodes
are selected in the manufacturing process by test in an actual
video detector circuit, see Fig. 2. This helps assure uniformity
in actual performance. The design engineer attempts to select
circuit values that minimize individual diode variations.
Fig. 2 - The special test circuit for the
1N64 as used by General Electric Company.
As a video second detector the germanium diode must convert
an i.f. of 20 to 50 mc. into d.c., with the video signal and
synchronizing pulses being passed on to the amplifier while
the i.f. is rejected. This requires crystals capable of withstanding
voltages higher than 1 or 2 volts; hence types like the 1N60
and 1N64 are used because these crystals are able to withstand
voltages on the order of 5.0 to 10.0 volts.
Because both the dynamic resistance and the crystal capacitance
of the germanium diode are very low, the crystal provides excellent
demodulation in video detection circuits. The crystal provides
exceptional linearity at low signal levels and is free from
any undesirable contact potential effects. The excellent linearity
characteristic of germanium diodes at low voltages and the absence
of contact potential effects help achieve improved video output
with reduction of distortion factors in low modulation regions.
Hence the quality of the signal representing white is improved
and the over-all picture presents more natural rendition of
various shades from white to gray to black.
Since the picture carrier is amplitude modulated, a TV detector
circuit is similar to the detector circuit found in AM receivers.
In both instances the chief function of the detector is to demodulate
the picture carrier. Crystals perform the detection function
remarkably well at the AM broadcast frequency and crystals will
perform the detection function better in TV since crystals operate
better at higher frequency. The reason for this is that the
efficiency of germanium diodes does not fall off as rapidly
as the efficiency of tubes with an increase in frequency at
which the circuit is operating. But a video detector imposes
more severe requirements on the detector diode than an AM broadcast
type detector or an FM receiver type detector is called upon
The trend in video detector design has been from the 6H6
to the 6AL5 to the germanium diodes. The input signal to the
detector is such that current flows through the diode when the
diode plate is positive with respect to the cathode. While the
polarity of the picture signal is essentially a design problem
and not a service problem, it must be remembered that whenever
a germanium diode is replaced in a TV receiver the original
polarity must be maintained, otherwise white and black objects
will be reversed and synchronization will become extremely critical.
In general, picture phasing considerations are the same for
a vacuum tube diode. It is necessary to achieve correct polarity
of the crystal diode according to whether the blanking pulses
are negative or positive and whether the signal injected to
the CRT is to the grid or to the cathode. When the signal is
to the grid it must end up with sync negative; when the signal
is to the cathode, it must end up with sync positive. Whether
the sync will be positive or negative depends upon the number
of video amplifiers and the polarity of output of the detector;
there is a phase shift of 180 degrees for each video amplifier.
Detector Circuit Considerations
Many modern TV receivers use a germanium diode as the video
detector for various reasons as listed below:
1. Simplicity of design,
2. Ability to handle a large dynamic signal range,
3. Minimum amplitude distortion (not too important, but worth
4. High degree of linearity,
5. Ability to shield the detector by mounting inside shield
Fig. 3 - Possible applications of germanium
diodes in modern television receivers.
One important requirement of a video detector is that the
output level be approximately flat for frequencies from 30 cps
to 4.0 mc.; the video amplifier should be designed to pass this
range of signal without attenuation. Therefore, the value of
each component in the detector circuit is usually the result
of careful selection by the design engineer; if replacement
of any component is necessary, a technician should be careful
to use resistors, condensers, and coils of the same value. The
coupling network may be of the peaking coil resistor type, or
it may be a low pass filter type. Many receivers are now using
a low pass filter type. It is worthy of note that no d.c. restoration
is necessary with direct coupling from the video detector to
the video amplifier because the d.c. component is preserved
with direct coupling. But there must be direct coupling all
the way from the detector circuit to the picture tube.
The signal at the output of a video detector is not quite
strong enough to drive a picture tube. In this respect it is
similar to AM receiver detectors which require one or two stages
of audio amplification for satisfactory sound. Video amplifiers
following the detector are usually RC amplifiers similar to
those found in AM receivers.
Fig. 4 - Rectification efficiency vs. signal
level. The curve for the 1N34 may be considered representative
of the 1N60. These measurements were made with a fixed driving
impedance and voltage which is not exactly the case in a video
detector. In that particular instance the loading on the last
i.f. coil is of most importance.
Signal amplification following the detector is generally
small because most of the receiver video gain is obtained in
the i.f. strip. While it is possible to have two stages of video
amplification after the detector, it is common practice to simplify
the circuit by using just one stage of d.c. coupled video amplification.
This means that a detector must cover a wide range of signal
amplitudes from 0.5 to 5.0 volts.
The 44 mc. i.f. frequencies may involve some reduction in
predetection gain (although with tubes like the 6BC5 and the
6CB6, the gain at the higher frequencies is greater than was
thought possible); thus, the detector may be called upon to
work efficiently at low signal levels and high frequencies.
The video detector for such a circuit would have to provide
good linearity at low signal levels so that correct over-all
highlight gamma (a numerical indication of the degree of contrast
in a received television picture) may be maintained.
The video detector may be any of the usual types such as
half-wave, full-wave, plate circuit, grid leak, or infinite
impedance type. By virtue of simplicity the diode detector is
so common that it is used almost exclusively; practically all
are of the half-wave type. The additional circuit complication
for full-wave detection does not warrant the expense involved.
Detector rectification efficiency of a typical half-wave
diode tube type video detector circuit might be on the order
of 35-40%, that is, with an i.f. input voltage of 1.4 r.m.s.
to the diode the video output is on the order of about 1.5 volts
peak-to-peak, or from maximum white to synchronizing pulse tip.
The detection efficiency of a germanium diode type video detector
circuit may approach about 52%. Small time constant load circuits
involving small capacitances and low values of load resistance
are necessary in order to preserve the high frequency video
components in the detector output; these affect rectification
and account for some loss in over-all efficiency.
Because these remarks may seem misleading to some engineers
it should be understood that they are made with the following
considerations in mind. The term "rectification efficiency"
does not indicate whether or not more useful video output will
be obtained by germanium diodes than by tubes. It is necessary
to design the circuit specifically for crystals or tubes in
order to maintain proper bandwidth as well as a.c. output. If
two optimum circuits are compared there is likely to be little
difference in output for crystals over tubes.
There seems to be no disagreement that the germanium diode
has decided advantages over a vacuum tube where the detector
is required to operate with signal levels on the order of 0.5
volt peak or less. For a bandwidth of 4.0 mc. the germanium
crystal 1N34 shows a 5.5 db gain over a 6H6 and approximately
a 0.5 db gain over a 6AL5 at a signal level of 5 volts. See
Fig. 4. A 1N60 will show slightly better gain. Small signal
rectification of the crystal diode for low values of load resistance
is much better than for the 6AL5.
Fig. 5A shows a video detector circuit using a 1N34 type
crystal. The resistor in series with the 250 μhy. coil and ground
may vary from 3900 to 4700 ohms. The performance of the circuit
is better when the resistor is 4700, as shown in the comparative
sets of curves in Figs. 4 and 6. These curves are more dependent
upon load circuit values than upon tubes or crystal, diodes.
Fig. 5 - Series type video detectors. (A)
Using the 1N34. The inductance, L1, is tuned to the
i.f, frequency with the total shunt capacitance. If the tuning
coil is on the plate side of the circuit, L1 is a
10 μhy. r.f, choke. (B) Using a 1N60. Either L1
or L2 may be the tuning inductance. The value should
be about 0.3 μhy. When tuning inductance L1 is on
plate side of tube. L2 should be a 10 μhy. r.f. choke.
Because of the great interest in the use of germanium diodes
in modern television circuits newer and better types of crystals
are being designed and manufactured. Fig. 5B illustrates a Sylvania
type video detector circuit designed especially for television
applications. The type 1N60 was specifically designed and is
tested for this type of service in the circuit shown in Fig.
8. This germanium diode provides high circuit efficiency and
exceptionally good linearity at low signal levels. Low interelectrode
and stray circuit capacitances make for improved video response.
Increased over-all gain is obtained by virtue of reduced capacitive
loading of the detector input circuit. When a circuit is designed
with the component values specified, a full 4 mc. video bandwidth
may be maintained at the output of the detector.
This circuit has high dynamic efficiency, low shunt capacitance,
and excellent linearity at low signal levels of 0.5 volt peak
or even less signal voltage.
For preservation of the high frequency video components in
the demodulated picture carrier envelope the time constant of
the vacuum tube detector load circuit should not exceed approximately
0.08 microsecond. This time constant should be observed even
with elaborate types of high frequency compensation networks.
To achieve this time constant the diode load resistance is generally
4000 ohms or less, and the load capacitances are correspondingly
small. It is for these reasons that the efficiency of a vacuum
tube detector circuit is generally low.
The dynamic impedance of the diode is an appreciable portion
of the total circuit impedance. With the 1N60 germanium diode
there is a substantial improvement in detection efficiency because
the dynamic impedance of the crystal is materially lower than
that of an equivalent vacuum tube diode of the 6H6 or 6AL5 type.
Since with a crystal the shunting capacitance is substantially
less it is possible to increase the effective load resistance
without sacrificing bandwidth.
Fig. 6 - Video bandpass curves. The curve
for the 1N60 would be the same as that shown for the 1N34. The
shape of the bandpass curves is more dependent on the load circuit
values than on tube or diode used.
Increasing the diode load resistance results in material
improvement of the over-all detection efficiency. The circuit
in Fig. 5B has been carefully designed to provide a bandwidth
of not less than 4.0 mc. The component values have been chosen
to work into an effective load capacitance of about 11 μμfd.
The 9 to 10 μμfd. condenser should be a low tolerance component,
or the tolerance should be on the low side rather than the high
side so that the capacitance does not exceed 10 μμfd.; the additional
1 or 2 μμfd. is the shunt capacitance of the germanium crystal.
Fig. 7 - Static diode characteristics graph.
Note that the curve for the 1N34 and the one for the 1N64 qo
through the zero point for voltage and current on the graph
and that the 6AL5 draws some current at the point of .zero volts
In this circuit the detector polarity is such that the demodulated
video signal at the grid of the video amplifier is sync negative.
There are good reasons for recommending this type circuit:
1. There is some noise limiting in the video amplifier by
virtue of driving the tube to cut-off on the noise peaks.
2. The use of a d.c. coupled video amplifier between "the
detector and the picture tube preserves the d.c. component and
eliminates the necessity for d.c. restoration. It is better
to retain the d.c. than to block it with a condenser and then
attempt to restore it.
3. This circuit presents a high quality picture with receiver
4. The use of the 1N60 provides improved response in the
direction of white, better background illumination levels, excellent
highlight detail, and improvement in the over-all gamma of the
Fig. 7 shows static characteristics for the 6AL5, 1N34, and
the 1N64 over a small voltage range near the origin of the curves.
The following aspects of this graph are noteworthy:
1. The linear portion of the crystal curves extends to considerably
lower voltage signal levels than the 6AL5 is able to achieve.
2. This improvement in linearity at low signal levels has
the over-all effect of improving highlight detail.
3. The better the linearity the more reduction of amplitude
compression in the direction of a white signal.
Fig. 8 - Sylvania's test circuit for the
4. For small value signals, rectification efficiency of crystals
is better with low values of load resistance than with any comparable
6AL5; it is possible to use higher load resistances with the
crystal without sacrificing any element of picture quality.
Circuit Applications of the 1N64
General Electric has developed a special second detector
diode, the 1N64. This was designed specifically for use as a
second detector in television receivers. The physical characteristics
of this diode are identical to those of the general purpose
types. It is primarily selected for maximum efficiency as a
detector at high frequencies because only in this way can proper
detection and uniformity be assured. The minimum d.c. output
current in the circuit of Fig. 9B is 100 microamps, the peak
inverse voltage is 20 volts, and the maximum shunt capacity
is 2.0 μμfd. In addition, to assure uniformity of bandwidth,
the diode is tested to have more than 50,000 ohms resistance
at -1 volt and less than 4000 ohms resistance at +0.25 volt.
Fig. 9 - (A) Series type 1N64 video detector.
(B) A shunt type 1N64 video detector. (C) A commercial type
video detector as used by Calbest Engineering and Electronic
The schematic shown in Fig. 9B was designed to use a 1N64
germanium diode with the 44 mc. i.f., and this is the circuit
used in most G-E model television receivers. The small size
of the diode makes it possible to mount it inside the last i.f.
can for maximum shielding. The 1N64 provides optimum efficiency
in this shunt type detector circuit. The circuit components,
9 μμfd. condenser and the 31.5 microhenry coil, may be
varied if it is desired to change the bandpass characteristic.
Similarly, variations of the 5 μμfd. condenser and the 3600
ohm resistor will affect the video output as a function of the
Fig. 9A shows a series type detector circuit in which the
low forward dynamic resistance of the 1N64 enables it to perform
exceptionally well. Since any variation of the forward resistance
of the diode will effect changes in the bandpass characteristic
of the detector stage, the load resistance should be maintained
relatively high with respect to the dynamic forward resistance
of the diode for the purpose of minimizing variations in the
bandpass. For this reason the components should be chosen with
great care. Low tolerance values will stabilize any crystal
diode detector circuit.
Because of the great interest in the use of germanium crystals
as video detectors a number of commercial applications of the
basic circuits discussed have been illustrated. Fig. 9C is an
application of the 1N64 to a TV receiver by the Calbest Engineering
and Electronics Company. For convenience, the associated circuitry,
involving a.g.c., contrast, and a video amplifier is presented.
Fig. 10 - Circuit of the Garod Series 101A,
101B, 101C, 101D, 103, 103A, and 105 using a germanium crystal
as a video detector and a diode load resistance of 8200 ohms.
Teletone has been interested in TV receiver simplification
because they are designing and manufacturing low price budget
sets for a mass market. Fig. 11 shows a Teletone circuit using
a crystal with an absolute minimum of associated components.
The use of a resistor with a value of 5600 ohms would not be
possible with a 6H6 or a 6AL5 because the bandpass would be
Fig. 11 - The Teletone TAP-2-UL chassis.
Garod makes excellent use of a germanium crystal with a relatively
high value diode load resistance of 8200 ohms. This is one of
the highest values found in any of the commercial circuits available.
This is also an excellent example of a video, detector feeding
a two-stage video amplifier with a single tube, see Fig. 10.
Note also the use of the 1N64 between the second video amplifier
stage and the picture tube. The 1N64 was selected for specific
d.c. characteristics for this circuit; it helps eliminate hash
in the sync circuit.
In the Freed-Eisemann circuit of Fig. 12 there is a complete
schematic for the fourth picture i.f., the crystal video detector,
and the first video amplifier. This is a quality circuit and
it follows the standard recommendations for the choice of the
component values, as indicated elsewhere in this article. While
this circuit uses the 24 mc. i.f., it will work equally well
with the 44 mc. i.f. and with a signal input voltage of 0.1
Fig. 12 - The Freed-Eisemann Models 101,
102, 103, and 104 use this circuit as the fourth video i.f.
(To be continued)
Posted December 21, 2015