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|February 1952 Radio & Television News|
[Table of Contents]
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
This is part 5 in a series that began in the October 1951 issue of Radio & Television News magazine (see 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.
By David T. Armstrong
Part 5. A discussion of some of the applications of crystal diodes to television receiver circuits.
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 detector element.
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.
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.
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 to meet.
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 mentioning),
4. High degree of linearity,
5. Ability to shield the detector by mounting inside shield can.
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.
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.
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.
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.
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 simplification.
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 video system.
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.
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.
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 video frequencies.
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.
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 unduly affected.
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 r.m.s.
(To be continued)
Posted December 21, 2015