Any of the small dual-diode tubes may be used in this circuit. Or, if desired, a duo-diode-triode may be used, with the triode acting as the first audio stage. The left-hand diode serves as the detector, while the right-hand side acts as the a-v-c rectifier. The use of separate diodes for defector and a-v-c reduces distortion when receiving an AM signal with a high modulation percentage.
ages on the b-f-o tube is changed, as the latter circuits usually change the frequency of the b.f.o. at the same time they change the strength, making it necessary to reset the trimmer each time the output is adjusted.
The b.f.o. usually is provided with a small trimmer which is adjustable from the front panel to permit adjustment over a range of 5 or 10 kc. For single-signal reception the b.f.o. always is adjusted to the high-frequency side, in order to permit placing the heterodyne image in the rejection notch.
In order to reduce the b-f-o signal output voltage to a reasonable level which will prevent blocking the second detector, the signal voltage is delivered through a low-capacitance (high-reactance) capacitor having a value of 1 to 2 miid.
Care must be taken with the b.f.o. to prevent harmonics of the oscillator from being picked up at multiples of the b-f-o frequency. The complete b.f.o. together with the coupling circuits to the second detector, should be thoroughly shielded to prevent pickup of the harmonics by the input end of the receiver.
If b-f-o harmonics still have a tendency to give trouble after complete shielding and isolation of the b-f-o circuit has been accomplished, the passage of these harmonics from the b-f-o circuit to the rest of the receiver can be stopped through the use of a low-pass filter in the lead between the output of the b-f-o circuit and the point on the receiver where the b-f-o signal is to be injected.
12-8 Detector, Audio, and Control Circuits
Detectors Second detectors for use in superheterodynes are usually of the diode, plate, or infinite-impedance types. Occasionally, grid-leak detectors are used in receivers using one i-f stage or none at all, in which case the second detector usually is made regenerative.
Diodes are the most popular second detectors because they allow a simple method of obtaining automatic volume control to be used. Diodes load the tuned circuit to which they are connected, however, and thus reduce the selectivity slightly. Special i-f transformers are used for the purpose of providing a low-impedance input circuit to the diode detector.
Typical circuits for grid-leak, diode, plate and infinite-impedance detectors are shown in figure 25.
Automatic Vol- The elements of an automatic ume Control volume control (a.v.c.) system are shown in figure 26. A dual-diode tube is used as a combination diode detector and a-v-c rectifier. The left-hand diode operates as a simple rectifier in the manner described earlier in this chapter. Audio voltage, superimposed on a d-c voltage, appears across the 500,000-ohm potentiometer (the volume control) and the ,0001-/ifd. capacitor, and is passed on to the audio amplifier. The right-hand diode receives signal voltage directly from the primary of the last i-f amplifier, and acts as the a-v-c rectifier. The pulsating d-c voltage across the 1-megohm a.v.c.-diode load resistor is filtered by a 500,000-ohm resistor and a .05-/ifd. capacitor, and applied as bias to the grids of the r-f and i-f amplifier tubes; an increase or decrease in signal strength will cause a corresponding increase or decrease in a-v-c bias voltage, and thus the gain of the receiver is automatically adjusted to compensate for changes in signal strength.
A-C Loading of By disassociating the a.v.c. Second Detector and detecting functions through using separate diodes, as shown, most of the ill effects of a-c shunt loading on the detector diode are avoided. This type of loading causes serious distortion, and the additional components required to eliminate it are well worth their cost. Even with the circuit shown, a-c loading can occur unless a very high (5 megohms, or more) value of grid resistor is used in the following audio amplifier stage.
B-F-O-Equipped frequency oscillator for the Receivers reception of radiotelegraph signals, the use of a.v.c. can result in a great loss in sensitivity when the b.f.o. is switched on. This is because the beat oscillator output acts exactly like a strong received signal, and causes the a-v-c circuit to put high bias on the r-fandi-f stages, thus greatly reducing the receiver's sensitivity. Due to the above effect, it is necessary to provide a method of making the a-v-c circuit inoperative when the b.f.o. is being used. The simplest method of eliminating the a-v-c action is to short the a-v-c line to ground when the b.f.o. is turned on. A two-circuit switch may be used for the dual purpose of turning on the beat oscillator and shorting out the a.v.c. if desired.
Signal Strength Visual means for determining Indicators whether or not the receiver is properly tuned, as well as an indication of the relative signal strength, are both provided by means of tuning indicators (S meters) of the meter or vacuum-tube type. A d-c milliammeter can be connected in the plate supply circuit of one or more r-f or i-f amplifiers, as shown in figure 27A, so that the change in plate current, due to the action of the a-v-c voltage, will be indicated on the instrument. The d-c instrument MA should have a full-scale reading approximately equal to the total plate current taken by the stage or stages whose plate current passes through the instrument. The value of this current can be estimated by assuming a plate current on each stage (with no signal input to the receiver) of about 6 ma. However, it will be found to be more satisfactory to measure the actual plate current on the stages with a milliammeter of perhaps 0-100 ma. full scale before purchasing an instrument for use as an S meter. The 50-ohm potentiometer shown in the drawing is used to adjust the meter reading to full scale with no signal input to the receiver.
When an ordinary meter is used in the plate circuit of a stage, for the purpose of indicating signal strength, the meter reads backwards
RF IF IF
RF IF IF
Was this article helpful?