Standard-Signal Generator lias proved to be a most useful instrument for these tests. Figure 1 on the next page shows them in the final production line at the Roister Radio Corporation's factory in Newark, New Jersey.
In order to take care of the peak production from this plant, ten test positions are necessary. Each one has its Type 601-A Standard-Signal Generator mounted upon a cabinet containing two loud-speakers (one is a stand-by in case of trouble) and an output meter. The completely assembled chassis are received in the test room from the slowly moving conveyer which has carried them down from the assembly departments. The power and loud-speaker leads are plugged in and the radio-frequency input from the standard-signal generator, working through a standard dummy antenna, is connected to the proper terminals.
Each standard-signal generator has two oscillator coils, one for the intermediate frequency and one for the broadcast band, either of w hich may be selected by a switch oil the panel. While working at the low- frequency,
HERE is no other single test that will give the figure of merit of a radio receiver more accurately than the measurement of its over-all sensitivity. Because it is one of the fundamental characteristics of any receiver, laboratory information oil the sensitivity is usually well known. A quick comparison of the sensitivity of the sets as they progress along the final inspection line with the predetermined laboratory standards has proved to be a most satisfactory indication that their performance is acceptable.
It is almost always necessary to align the ganged variable condensers in a receiver during the final testing operation and a good radio-frequency oscillator is required for this. For superheterodynes, the alignment process must be made at frequencies both in the broadcast band and at the intermediate frequency. Since sensitivity tests must also be made, the standard-"signal generator is often used as the aligning oscillator.
The General Radio Type 601-A
the intermediate-frequency system of the receiver is lined up. After this adjustment the generator is switched to the high frequency and the general line-up of the receiver is made.
The next step is to determine the over-all sensitivity at several points in the broadcast band. These test frequencies are indicated by lines on the main tuning dial. The generator is set step-by-step to each of these frequencies, the receiver tuned to them one at a time, and the sensitivity of the receiver measured in the usual way by observing the audio-frequency output for a given signal input.
Standard output is marked on the output meter in the lower cabinet and just above it is the meter and multiplier of the standard-signal generator showing microvolts input. In this way, the operator has the whole story before him. At times, he records his observations on the charts provided and these are most useful to the production supervisor.
As a final check, a loud-speaker is turned on for a listening test.
Although the tests were originally designed for broadcast-receiver measurements, sets intended for operation at the police or aircraft frequencies could also be quickly checked by changing the coils of the standard-signal generators.
Figure 2 is a view of the rear of the Type 601-A Standard-Signal Generator with the cabinet and batteries removed. The two toroidal coils mounted on the subpanel are for the radio-frequency oscillator. They are mounted on pin jacks for interchangeability with
Figure 2. A view behind the panel of a Type 601-A Standard-Signal Generator. Either or both of the plug-in inductors may be replaced with others to cover special bands
Figure 3. Schematic wiring diagram for a Type 601-A Standard-Signal Generator
Figure 3. Schematic wiring diagram for a Type 601-A Standard-Signal Generator other coils for different frequency ranges. Selection between them is accomplished by a four-point double-throw switch controlled from the front panel.
After some experimentation, toroidal coils were selected instead of solenoids because, although a little more difficult to build, they have so little external field that shielding is much simplified.
The radio-frequency amplitude is measured by means of a vacuum-tube voltmeter. This is a tube operating in the usual way by observing the incremental change in plate current due to changing amplitudes of radio-frequency voltage on its grid. The direct current is read by means of the micro-ammeter on the panel. All of the radio-frequency circuits except attenuator are located on the shelf and are covered by a shield.
Fastened to the under side of the shelf is the audio-frequency modulator circuit. It is a standard tuned-plate oscillator operating at 400 cycles per second with an amplitude sufficient to provide modulation at either 30 or 50 per cent. Normally, the Type 601-A Standard-Signal Generator is supplied with 30% modulation, but if desired General Radio can make the adjustments necessary for 50% modulation before the instrument is shipped.
The toroidal oscillator coil is tapped and a small part of the total voltage across it is led off through a shielded conductor to the attenuator. In Figure 2 the casting in back of the modulator circuit at the lower left corner of the panel houses the complete attenuator assembly. It is divided into three separate compartments between which the attenuation units are divided so that the total voltage reduction in each does not exceed 40 decibels. Due to stray admittances, it is virtually impossible to exceed this attenuation within one shield without encountering serious errors.
The whole attenuator assembly is in contact with the front panel at only one point—where the low-voltage output jack is located. This helps to reduce circulating panel currents to a point where they do not affect the measurements at high frequencies to any extent. The output voltage lead to the receiver under test is shielded and enters the attenuator through a plug and jack construction that maintains the continuity of the shield directly to the attenuator circuit.
Two output jacks are provided, one connected to the variable voltage output and the other to a fixed point 011 the attenuator system at a higher voltage. The former provides outputs variable in discrete steps from 1 to 20,000 microvolts. The fixed tap is at 100,000 microvolts. All of these ranges can be multiplied by a factor of 1.5 by increasing the radio-frequency oscillator amplitude to the correct point as indicated by the vacuum-tube voltmeter.
Reference to the schematic wiring diagram shown in Figure 3 will indicate the arrangement of the circuit elements.
As will be noted, the modulation voltage is introduced in series with the plate-supply battery of the radio-frequency oscillator. With this method of modulation, it is necessary to provide a highly stable high-frequency oscillator, otherwise difficulty is encountered due to frequency modulation. That is, the plate voltage applied to the radio-frequency oscillator tube, varying at an audio rate, may shift the carrier frequency by a considerable amount unless the most stable high-frequency oscillator circuits are used.
The vacuum-tube voltmeter is connected across one half of the oscillator coil in series with a very small variable condenser, which is used to adjust the reading of the voltmeter. The attenuator voltage is taken across a part of the coil.
Only one of the two radio-frequency oscillator coils is shown in the diagram for the sake of simplification. The switching between these coils is arranged so that the one that is not operating is completely detuned by shunting a large condenser across it. Thus, no reaction can occur between it and the coil in use.
In order to provide a means for checking the voltages of the various batteries without a multiplicity of meters, the micro-ammeter is connected to a multi-point switch with suitable series resistors for making direct-current-voltage measurements on the A- and B-batteries.
The General Radio Company wishes to acknowledge its indebtedness to Mr. C. E. Brigham and Mr. G. Elcock of the Kolster Radio Corporation for their kindness in supplying the photographs of their plant used in this article.— Arthur E. Thiessen
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