Interelectrode Capacity

It has been pointed out, in the section on Single-Stage Oscillation, that the coupling medium for such oscillation is the inter-electrode capacity of the tube.

The method of measuring such small capacities in the presence of much larger capacities in a network that cannot be opened to measure the desired capacity directly may be of interest.

Fig. 23 represents the capacity network that exists in the tube as far as feedback capacities are concerned.

The method of measurement is to have a similar network, Fig. 24, in operation, supplying a voltage to some measuring device and fed by a source with the proper characteristics. The output capacity

ColibroMd Standard Caxtanwr

G«id-to-ptotf Capacity of Tut*

G«id-to-ptotf Capacity of Tut*

Output Capacity of Tuto«

Figure 23

Output Capacity of Tuto«

Figure 23

ColibroMd Standard Caxtanwr

Figure 24 Measurement of Inter-electrode Capocity

of the tube is connected across the measuring circuit and a reading obtained on the output meter; then, when the grid of the tube is connected to the source of voltage, there will be an increased reading on the meter. The calibrated condenser is then reduced in capacity until the meter reads as before. The inter-electrode capacity of the tube is the difference in the capacity of the calibrated condenser at its two settings.

The calibrated condenser for the above measurement is an elaborate device not available to the experimenter or service man, therefore the above method of checking inter-electrode capacity cannot be attempted, but a very similar method can be used to check the uniformity of inter-electrode capacity in the manner shown in Fig. 25. Here a signal generator or service oscillator is used to

Figure 25 Comparison of Inter-electrode Capocity

Figure 25 Comparison of Inter-electrode Capocity furnish a signal to a low-impedance load so that the input-impedance of a tube may be connected across it with negligible change in voltage. The inter-electrode capacity feeds the signal into the radio receiver across whose input a low-impedance load has been connected so that variations in the output capacity of the tube will be swamped out. An output meter on the receiver will serve to indicate the relation between tubes of differing inter-electrode capacity.

The above system for comparing inter-electrode capacities has been successfully used in a number of laboratories desiring checks on inter-electrode capacity but which were unable to purchase complete equipment for making these measurements. It probably is not a measurement that any service man will have occasion to use, but some advanced experimenters, attempting to obtain the maximum possible performance from circuits, may desire to check their tubes for this parameter.

FIDELITY vs. SELECTIVITY

One of the most frequent requests from experimenters working on high-fidelity receivers is for IF components that will permit high-fidelity reception, yet have good adjacent-channel selectivity-.

The following considerations will show that it is impossible to meet borh specifications simultaneously. In order to transmit a single audio frequency by the double sideband transmission method which is standard on all types of broadcast and shortwave entertainment transmissions, a carrier and two additional frequencies are required. These additional frequencies are located one above and one below the carrier frequency bv an amount exactly equal to the audio frequency. For example, if it is desired to transmit a 10 KC note on a 1000 KC carrier, the upper sideband will be 1010 KC and the lower sideband will be 990 KC. It can be shown mathematically and it can actually be demonstrated that in the above case three separate signals exist, if a sufficiently selective receiver is used for the demonstration. Since it is the American practice to assign broadcasting frequencies at 10 KC intervals, it is obvious that the 10 KC transmission of the 1000 KC station above mentioned will fall exactly on the carriers of the two adjacent channels and will produce heterodynes that will give rise to spurious audio responses in any receiver having a selectivity curve wide enough to pass both sidebands on a 10 KC modulation.

Since it is reasonable to assume that there will be modulation on both adjacent channels it will be obvious that the transmissions- of the two adjacent channels will encroach upon the territory that the 1000 KC station is using if it modulates up to 10 KC. Now if all three stations are producing a signal of equal intensity and are all modulating up to 10 KC the receiver will not be able to separate these three programs. If, however, the pass-band of the receiver is narrowed down until it accepts a band of frequencies only 4 KC above and b<»low its mid-frequency, it will acccpt from the adjacent channels only those frequencies above 6000 cycles which frequencies carry a comparatively small part of the energy of speech or music and consequently will not interfere with the desired program to as great an extent.

If the ratio of desired signal strength to adjacent-channel strength is now changed so that the desired signal is many times stronger than the adjacent channel signal strength, the pass-band of the receiver can be increased considerably without introducing appreciable interference.

From the above it can be seen that "High-Fidelity" reception can be used only where the ratio of desired signal to adjacent-channel signal is very great, say 1000 times or more, and that, unless the receiver is confined to the reception of local stations, it must be able to sharpen its selectivity curve when it is desired to select one station whose signal strength is near or below the signal strength of the adjacent channels. In order to accomplish this economically, IF transformers, whose physical and electrical features are shown in Fig. 26, are available. In these transformers, the pass-band is varied

Figure 26

by changing the coupling between primary and secondary, by means of a tapped coil in series with one winding closely coupled to the other. By this arrangement a very high percentage change in coupling can he accomplished with practically no change in the self-resonant frequency of the tuned circuit which has been switched-This arrangement permits one receiver to be adjusted for either uide or narrow pass-band instead of requiring two independent receivers of the desired characteristics.

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