An electrical frequency analyzer

An apparatus has been developed by means of which it is possible to obtain a permanent record of the frequency and magnitude of each component of a complex alternating current. The device has two frequency ranges: 20 to 1250 cycles and 80 to 5000 cycles. The power required does not in general exceed 500 microwatts, and the time necessary for making a record is about 5 minutes. An attachment is provided by which, in the same length of time, two simultaneous analyses can be made if desired. In principle, the process consists in introducing the complex voltage to be analyzed into a selective network, the essential feature of which is a sharply tuned circuit whose frequency of tuning is controlled by varying the capacitance in small steps with a pneumatic apparatus similar to that in a player piano. A maximum of response of the circuit occurs at each frequency of tuning which coincides with a component of the complex wave. An automatic device records this response at each frequency of tuning, making a photographic record from which the frequency and magnitude of each component of the complex current may be obtained. For convenience of operation, an automatic control apparatus is provided, so that it is only necessary to connect the complex source or sources to be analyzed and press the starting button. The application of the analyzer in communication problems is shown by a few illustrative uses. A record of the output of a carbon transmitter button driven at an excessive amplitude shows harmonics of the driving frequency and of half the driving frequency. With a condenser transmitter the analyzer can be used to analyze sustained sounds in the air and the application of this method to tones of a few low-pitched organ pipes shows large differences in relative harmonic content. Analyses of the electrical input and acoustic output of a common type of loud speaker give an idea of the amount of distortion. The use of the analyzer in power problems is shown by records taken o- a transformer at load and at no-load, and similar records on a d-c. generator. The no-load transformer record shows the amount of third and fifth harmonic in the exciting current with the secondary open. The d-c. generator records show various parasitic frequencies from 30 cycles to over 4000 cycles, and a consideration of the records leads to the probable causes of these parasitic frequencies