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August 1945 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
This is the first of a three-part series on radio detector circuits by Mr. Robert Scott. He begins in this article with describing diode action and progresses to uses in various types of signal detectors in radio receivers. A discussion of modulation and distortion sources is included as well. The next article in the series discusses hi-fidelity triode detectors; the plate rectifier, infinite-impedance detectors, grid rectification, and regenerative circuits.
See Detector Circuits, Part II in the September 1945 edition of Radio-Craft.
Fig. 1 - Circuit of a typical diode detector.
Fig. 2 - Characteristic chart for diode tube.
Fig. 3 - Diode action at three frequencies.
Fig. 4 - Full-wave detector. 4a - Detector and A.V.C. circuit for minimum distortion.
By Robert F. Scott
A detector has been described as a means of separating speech or other intelligence components from a radio frequency carrier signal. Detection or "demodulation" is necessary for practically every type of communication which utilizes a basic carrier signal of a frequency well above the audio scale.
There are several methods of separating the intelligence from the carrier. Each of these has its own particular advantages and disadvantages which will be discussed in turn. The most important of these traits are: Sensitivity, Fidelity, Signal handling capacity, and Circuit loading.
Sensitivity of a detector is its ability to respond to comparatively weak signals and this ability is measured as the ratio of R.F. signal input to audio signal output.
Fidelity is the ability to handle audio signals without discrimination against frequency or amplitude. Thus a high fidelity detector will give faithful reproduction of the intelligence envelope of the modulated signal.
Signal handling ability of the detector is its ability to handle signals varying from maximum to minimum signal strength without deleterious effects from insufficient input voltage and overloading.
The circuit loading is the load which the detector circuit imposes upon the preceding stage. It is this factor which must often be carefully calculated; because a low impedance often means that the detector will draw current, and not all preceding stages are designed to furnish the driving power.
The Diode Detector Circuit
Perhaps the simplest and most often used detector is the diode. This employs a tube having only a cathode and anode or plate. Fig. 1-a illustrates a typical diode detector circuit as commonly employed in the receivers of today. A grid and triode plate are included in many such tubes, but play no part in the detector action. Figs. 1-b-c-d show the shape of the modulated input signal, condenser charging voltage and diode current flow respectively.
The modulated signal voltage is applied to the combination of L-C and hence between (diode) plate and cathode of the detector tube. It is well-known that the plate attracts electrons (or draws current) only when it is positive with respect to the cathode. As the input signal increases from zero in a positive direction, the plate is charged positively and electrons flow from the cathode, resulting in a current flow. This current flow passes through the load resistor, R, and there is a voltage drop across this resistor. The voltage across this resistor will be a replica of the positive halt of the modulated input signal. Condenser C1, will take on a charge equal to the voltage across R which is slightly less than the peak voltage of the input cycle.
On the negative portion of the input cycle, the plate is negative with respect to cathode and there will be no current flow. This current flow is also prevented by the presence of the negative charge on the plate of the condenser which is connected to the plate through the L-C network. For the current to commence to flow, it is necessary for the peak charging voltage to exceed the, voltage on the condenser for the voltage on the plate, for subsequent cycles will be the algebraic sum of the voltage on the condenser and the peak charging voltage.
In this manner, the effects of the R.F. will be removed from the output and the voltage across R will constantly follow the shape of the modulating envelope.
For the highest detector efficiency or sensitivity, it is necessary that the value of R be made as high as practical when compared with the value of plate resistance. The ratio of Rp to R may be made from 20 to 100 for efficiencies from 80 to 95 per cent.
Use of Characteristic Curves
The average vacuum-tube manual will supply the characteristic curves of the diode detector when sine-wave voltages are applied to the input circuit with various values of load resistances. The conditions demonstrated in these charts demonstrate only the static characteristics of the tube, but are helpful in determining the dynamic conditions under which it will operate most efficiently. Due to many factors, the detector will react very differently from its static characteristics when it is fed the complex wave forms of speech or music. Even casual study indicates that the highest values of output voltage will be available with the highest values of load resistance. Such a chart is shown in Fig. 2.
The circuit in Fig. 1-a shows the second detector of a popular A.C. receiver using a 12SQ7 tube as half wave rectifier or detector, A.V.C. and first audio stage. It will be noted that in this circuit, the diode load consists of two resistances having a total resistance of .3 meg. 250,000 ohms of this resistance is employed as the volume control for the receiver. The .00025 condenser is used to filter out the pulsations which would result from the R.F. in the circuit. The direct current flowing through the load resistance is also tapped off to supply negative automatic volume control voltage for the I.F. stages of the set.
Figures 3-a-b-c show equivalent circuits at 100, 400 and 5000 cycles. At various audio frequencies, the reactance of the various condensers will change inversely as the frequency (as the frequency increases, reactance decreases). The principal offender of the high frequency shunting is the by-pass condenser, C1. Its reactance at 100 cycles is almost six and one half megohms. This value of reactance shunting 300,000 ohms will have negligible effect on the audio output at this low frequency. At 400 cycles the audio output will be still lower and at 5,000 cycles, the reactance of the R.F. by-pass condenser is 127,000 ohms. When we consider this value paralleling the load resistance of 300,000 ohms we have an equivalent resistance of only 89,227 ohms. Then, considering the A.V.C. resistor and the grid leak in parallel with the load resistance, even this value will be lowered somewhat.
High Modulation Percentages
It has been stated that the voltage drop across the load resistance is somewhat lower than the peak charging voltage. Now, if the resistance offered to the flow of A.C. is less than that offered to the flow of D.C. then the current caused by the flow of A.C. will be greater than that caused by the D.C. When the shape of a modulated signal is studied, it will be noted that as the modulation percentage approaches one-hundred per cent the instantaneous current flowing through the diode becomes smaller and reduces to zero at maximum modulation. When the diode input contains signals having high modulation percentages and containing high frequencies, the R.F. by-pass condenser cannot dissipate its charge through the load resistor fast enough to follow the shape of the modulation envelope. Thus there will be frequency and amplitude distortion.
To determine the value of the R.F. by-pass condenser, the highest modulation frequency to be received will have to be considered as well as the inter-electrode capacity of the tube and the broadcast frequencies to be covered by the receiver. If its reactance is from 2 to 3 times the load resistance at the highest modulation frequency, then it will be possible to receive signals which have been modulated up to 94 per cent without distortion. Higher modulation peaks may be received without distortion becoming noticeable. However, the reactance of this condenser should be as small as possible, because for maximum output from the detector, it is necessary for the maximum R.F. voltage to be applied to the diode plate. If the reactance of the condenser is fairly large compared to the load resistance, a large percentage of the R.F. voltage will be lost across it. It is for this reason that the diode detector is seldom employed for low frequency receivers.
Some Causes of Distortion
The shunting effect of the various condensers and resistances in the circuit has the effect of reducing the effective load resistance of the tube. The dynamic load line of the tube's characteristic curve will pass through the operating point but will have a slope such that it will have a cut-off characteristic at an input voltage less than zero and the distortion will be severe at modulation percentages where the instantaneous current approaches zero. Theoretically, the diode would not be able to handle successfully a signal having a high degree of modulation, but fortunately there is another factor that serves to nullify this effect.
It has been found that the maximum degree of modulation that can be placed upon an R.F. signal and be detected by the diode without distortion is equal to the equivalent impedance at the highest modulation frequency divided by the diode load resistance. When the efficiency of the detector is high, the load resistance offered to R.F. is equal to the load resistance, R, divided by the efficiency. Since the impedance is lower for A.C., the resistance offered to A.C. is equal to the effective resistance divided by the efficiency. The modulation percentage will therefore apparently be reduced and the distortion produced by the diode in the actual detection of highly modulated signals cut down.
The tubes selected for diode detector service should have a low interelectrode capacitance and a low plate resistance. These conditions can be met by employing practically any of the especially designed diodes such as the 6H6 or the multi-purpose tubes as the 6Q7, 6B7, 6B8 and many others.
Fig. 4 illustrates the use of the diode as a full-wave detector. In this case, both halves of the input cycle are utilized. The output of this type of detector is only one-half as great as the output of the half wave-type for the same value of input voltage. This circuit has one advantage. Very little R.F. is placed across load resistor, due to the fact that the center-tap of the input inductance is at zero R.F. potential just as is the cathode.
In Fig. 4-a we see a circuit which has been developed to overcome the effects of shunting of high modulation frequencies caused by low value. A.V.C. resistances and the usual coupling condenser and grid leak for the audio stage. In this case, the detector is a half-wave affair. The second diode plate is capacitively coupled to the plate of the preceding stage. A D.C. drop appears across its load resistor, RL2, to be used as A.V.C. bias. In this case, a section of the load resistor for the detector is employed as the grid leak and volume for the following audio amplifier stage. This method will supply ample audio voltage to the grid of the following stage, since the diode should not be operated at voltage inputs which are lower than 10 volts R.M.S. and this condition may be met by any receiver employing A.V.C. The second part of this article will discuss triode detectors. It will appear in an early issue.
Posted December 24, 2014