Some Aspects of Radio Reception at Ultra-High Frequency: Part II. Admittances and Fluctuation Noise of Tubes and Circuits

The signal-to-noise ratio of radio receivers depends in part upon tube and circuit admittances and upon noise sources present in tube and circuit elements. Tubes operating in a linear fashion can be represented by a 4-terminal network consisting of admittances, of which the most important are the input, output, and feedback admittance, and by two constant-current generators of which the more important is the one which determines the transadmittance. The input admittance of conventional type tubes is determined-largely by: 1. ohmic losses; 2. interelectrode capacitances; 3. electrode self and mutual inductances; 4. lead self and mutual inductances; 5. the space-charge conditions within the tube; and 6. the magnitude of the cathode-control-grid transit angle. The cathode lead inductance, which may be the most important, results in an input admittance term which varies as the first power of the transconductance, and as the square of the frequency. The cathode-control-grid transit angle introduces a similarly varying admittance term which is also proportional to the transit angle, (provided the latter is not too large). These added admittance terms are positive for the case of space-charge-limited emission and negative for temperature-limited emission. Similarly, the input capacitance is increased over the "cold-cathode" value in the space-charge-limited emission case and decreased in the temperature-limited emission case. The transadmittance does not vary greatly in magnitude as a function of frequency, although its phase angle may vary considerably. The feedback admittance (primarily a capacitance) may reverse in sign at a particular frequency.