SIGNAL TRACING IN RECEIVER DESIGN [Servicing by Signal Tracing (1939)]

BECAUSE of the basic nature of signal tracing, it is to be expected that it should be of value in receiver design as well as in service work. As we have seen, signal tracing provides information on what is taking place in a receiver while the receiver is functioning and this sort of information is valuable to the engineer as well as to the serviceman. In design work, as in servicing, a knowledge of what is taking place in the receiver circuit is essential. In addition, since trouble-shooting is not limited to the service field but must often be carried out in experimental and development work, signal tracing can be used to simplify this phase of design work.

An elaborate explanation of signal tracing insofar as it is related to design work is not required because of the comprehensive treatment in other sections of this book. The technique of signal tracing is essentially the same whether the problem at hand is one of design work or one of trouble-shooting. In both cases, the desired result is to obtain the best possible performance from the circuit, and as such the procedure and measurements are very similar.

As in servicing, the fundamental instrument in signal tracing is the tuned vacuum-tube voltmeter which we have seen must satisfy certain requirements. It must have a low input capacitance, and should preferably employ a probe cable for convenience in getting at various points in the receiver; it should cover a wide range of frequencies, preferably from about 100 khz up through the broadcast band; it should have sufficient sensitivity so that it is possible to pick off the signal in the low-level stages of the receiver; and it should be provided with a calibrated attenuator.

Essentially the tuned vacuum-tube voltmeter which we have been describing is a further development of the low-capacity untuned type of vacuum-tube voltmeter which employs an acorn tube at the end of a shielded cable. Because of the lower input capacitance and the higher sensitivity of the tuned type, how ever, the usefulness of the latter is considerably extended over that of the untuned type. Quantitatively, the acorn type of vacuum tube voltmeter has an input capacitance of approximately 6 u-u-f, whereas the tuned type of vacuum-tube voltmeter can be made to have an input capacitance of about 1 or 2 u-u-f.

With reference to sensitivity, the untuned vacuum-tube voltmeter has a sensitivity of about 0.1 volt, while the tuned type can be made to have a sensitivity of better than 1 millivolt.

The increased sensitivity made possible through the use of amplification of the signal being measured is often an advantage in design work. In many instances it is desirable to pick off the signal in the low-level stages where its amplitude is not very great. Although it would be impossible to do this with the usual type of vacuum-tube voltmeter, the increased sensitivity avail able with the tuned vacuum-tube voltmeter makes this possible.

The many uses to which the tuned vacuum-tube voltmeter can be put have been covered in the preceding sections of this book.

Measurements made in the course of design work will follow the same general procedure, but in general it will be necessary to use more accurate equipment and to make the measurements to a greater degree of accuracy. Where greater precision is required, it is always possible to calibrate the tuned vacuum-tube volt meter against a standard signal generator. In this way the greater inherent accuracy of the signal generator can be transferred to the voltmeter.

Signal tracing in design work is especially valuable where it is desired to check the signal voltage at points where it is normally not possible or convenient to check the circuit indirectly, by feeding the signal directly across the point in question. For example, it is sometimes desirable to be able to determine the signal present across tertiary windings, trap circuits, etc., and in cases of this sort the tuned vacuum-tube voltmeter makes it possible to pick off the signal at the desired point without interfering with the normal operation of the circuit.

The high sensitivity of the tuned vacuum-tube voltmeter used in signal tracing enables measurements to be made of the relative strength of stray r-f fields, and of the effectiveness of shielding. This can be done conveniently by exploring the region with the voltmeter probe and noting the relative readings of pickup at the various points. Generally there will be sufficient pickup from the short exposed section of the probe; however, where the fields being measured are comparatively weak, the pickup can be increased by attaching a short stiff lead to the probe end.

With proper attention to grounding of the instrument, approximate measurements can be made of the r-f potential at various points along shielding partitions, chassis, etc.

The use of signal tracing in audio design work is well established because of the greater simplicity of the problem. Thus a vacuum-tube voltmeter of high sensitivity can easily be built to cover the range of audio frequencies. Several commercial instruments which have high sensitivity, good accuracy, and a logarithmic calibration are available. Thus, use of instruments of this type in audio signal tracing is similar to that in servicing and requires no further explanation.