The degenerative act/on of cathode lead inductance tends to reduce the effective grid-to-cathode voltage with respect to the voltage available across the input tuned circuit. Cathode lead inductance also introduces undesirable coupling between the input and the out-put circuits.
low, the local oscillator must furnish considerable power to the diode mixer. A good diode mixer has an overall gain of about 0.5.
The Triode Mixer A triode mixer has better gain and a better noise figure than the diode mixer. At low frequencies, the gain and noise figure of a triode mixer closely approaches those figures obtained when the tube is used as an amplifier. In the u-h-f and v-h-f range, the efficiency of the triode mixer deteriorates rapidly. The optimum local oscillator voltage for a triode mixer is about 0.7 as large as the cutoff bias of the triode. Very little local oscillator power is required by a triode mixer.
Pentode Mixers and The most common multi-Converter Tubes grid converter tube for broadcast or shortwave use is the penta grid converter, typified by the 6SA7, 6SB7-Y and 6BA7 tubes (figure 20). Operation of these converter tubes and pentode mixers will be covered in the Receiver Fundamentals Chapter.
As the frequency of operation of the usual type of electron tube is increased above about 20 Mc., certain assumptions which are valid for operation at lower frequencies must be reexamined. First, we find that lead inductances from the socket connections to the actual elements within the envelope no longer are negligible. Second, we find that electron transit time no longer may be ignored; an appreciable fraction of a cycle of input signal may be required for an electron to leave the cathode space charge, pass through the grid wires, and travel through the space between grid and plate.
Lead Inductance ance is two-fold. First, as shown in figure 21, the combination of grid-lead inductance, grid-cathode capacitance, and cathode lead inductance tends to reduce the effective grid-cathode signal voltage for a constant voltage at the tube terminals as the frequency is increased. Second, cathode lead inductance tends to introduce undesired coupling between the various elements within the tube.
Tubes especially designed for v-h-f and u-h-f use have had their lead inductances minimized. The usual procedures for reducing lead inductance are: (1) using heavy lead conductors or several leads in parallel (examples are the 6SH7 and 6AK5), (2) scaling down the tube in all dimensions to reduce both lead inductances and interelectrode capacitances (examples are the 6AK5, 6F4, and other acorn and miniature tubes), and (3) the use of very low inductance extensions of the elements themselves as external connections (examples are lighthouse tubes such as the 2C40, oilcan tubes such as the 2C29, and many types of v-h-f transmitting tubes).
Transit Time erated at a frequency high enough that electron transit time between cathode and plate is an appreciable fraction of a cycle at the input frequency, several undesirable effects take place. First, the grid takes power from the input signal even though the grid is negative at all times. This comes about since the grid will have changed its potential during the time required for an electron to pass from cathode to plate. Due to interaction, and a resulting phase difference between the field associated with the grid and that associated with a moving electron, the grid presents a resistance to an input signal in addition to its normal "cold" capacitance. Further, as a result of this action, plate current no longer is in phase with grid voltage.
An amplifier stage operating at a frequency high enough that transit time is appreciable:
(a) Is difficult to excite as a result of grid loss from the equivalent input grid resistance,
(b) Is capable of less output since trans-conductance is reduced and plate current is not in phase with grid voltage.
The effects of transit time increase with the square of the operating frequency, and they increase rapidly as frequency is increased above the value where they become just appreciable. These effects may be reduced by scaling down tube dimensions; a procedure which also reduces lead inductance. Further, transit-time effects may be reduced by the obvious procedure of increasing electrode potentials so that electron velocity will be increased. However, due to the law of electron-motion in an electric field, transit time is increased only as the square root of the ratio of operating potential increase; therefore this expedient is of limited value due to other limitations upon operating voltages of small electron tubes.
4-7 Special Microwave
Due primarily to the limitation imposed by transit time, conventional negative-grid electron tubes are capable of affording worthwhile amplification and power output only up to a definite upper frequency. This upper frequency limit varies from perhaps 100 Mc. for conventional tube types to about 4000 Mc. for specialized types such as the lighthouse tube. Above the limiting frequency, the conventional negative-grid tube no longer is practicable and recourse must be taken to totally different types of electron tubes in which electron transit time is not a limitation to operation. Three of the most important of such microwave tube types are the klystron, the magnetron, and the travelling wave tube.
The Power Klystron The klystron is a type of electron tube in which electron transit time is used to advantage, Such tubes comprise, as shown in figure 22, a cathode, a focussing electrode, a resonator connected to a pair of grids which afford velocity modulation of the electron beam (called the "buncher"), a drift space, and another resonator connected to a pair of grids (called the "catcher"). A collector for the expended electrons may be included at the end of the tube, or the catcher may also perform the function of electron collection.
The tube operates in the following manner: The cathode emits a stream of electrons which is focussed into a beam by the focussing electrode. The stream passes through the buncher where it is acted upon by any field existing between the two grids of the buncher cavity. When the potential between the two grids is zero, the stream passes through without change in velocity. But when the potential between the two grids of the buncher is increasingly positive in the direction of electron
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