August 1945 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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
strange-looking round schematic symbols are vacuum tubes, which used metallic elements
separated by space as functional elements rather than fused sand containing traces of
impurities. Don't be intimidated, though; just think of them as field effect
transistors (FETs) where the plate is the drain, the grid is the gate, and the
cathode is the source. The next article
in the series discusses hi-fidelity triode detectors; the plate rectifier, infinite-impedance
detectors, grid rectification, and regenerative circuits.
Circuits, Part II in the September 1945 edition of Radio-Craft.
Part I - The Diode Detector.
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
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
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
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
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 12, 2018 (original 12/24/2014)