Use of varactors
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Varactors have been used with tube-cavity combinations to obtain an electronically tuned oscillator for modulation and/or afc. The varactor is usually inserted as a susceptance stub into the high RF signal voltage point inside the cavity. The varactor Qs are low compared with cavity Qs, and a significant portion of the available output power is absorbed by the varactor. The useful tuning range appears to be about 1 percent and depends on both the degree of the varactor coupling and the amount of acceptable nonlinear tuning. As varactors improve, the useful electronic tuning range will increase.
Tube selection and use
Many tube types with similar shapes but different characteristics are available. The circuit designer is challenged to select the best tube for his application and should seek advice from all available sources. Many designs have internal features not immediately recognizable. A good example of this is the type Y-1124 bonded-heater planar triode shown schematically in Fig. 8. The heater is encapsulated inside a suitable coating and binder is physically bonded to the back of the active cathode surface. This greatly reduces the heater-wire temperature (the hottest spot in the tube), and results in longer life at higher current density and faster heater warm-up (3-5 s). The design gives extra mechanical ruggedness which helps with shock and vibration.
The important design considerations in selecting and using a gridded tube are:
- For best efficiency, choose the smallest tube with suitable ratings and power ouput.
- Pick the best tube geometry for the desired tube-cavity combination and the type of connectors to be used.
- Check for the desired thermal and mechanical capabilities.
- Determine that suitable power with sufficient regulation is available. Are isolated heaters necessary?
- For pulse applications, determine that sufficient peak emission is available for all possible heater voltages.
- For good gain-bandwidth, choose a tube with low capacitance and high transconductance.
- In high current-density applications, check for suitable tube stabilization for the desired early-life stability.
- Check tube and cavity stability over the desired temperature ranges when operating under shock and vibration.
- For maximum performance, several hours of prequalification operation is desirable.
- Derate as much as possible for longest life and best reliability.
- For cw applications using pulse rated types, reduce the heater voltage for longer life.
Higher-power broad-band tubes
Development work under government contract has proven the feasibility of applying high current densities to relatively large planar tubes. The performance specifications of two developmental tubes are shown in Table 1. Active cathode area for the Y-1430 is about 60 mm2; for the Y-1498, 2.0 cm2. The designs were predicated on their use for the last two stages of a four-stage phased-array module now being developed for 1.3 Gc. The objective performance of the module is:
- Over-all efficiency—15% min.
- Over-all gain—50 dB min.
- Pulsed power output—5 kW.
- Duty factor—0.07.
- Pulse duration—500 μs.
- Over-all size max—4- 1/2 x 4- 1/2 x 24 in.
- Over-all passband—1215 to 1400 Mc at 1 dB.
Noise figure and high current densities
Vacuum-tube noise performance is very dependent on the level of induced grid noise caused by transit-time effects. At any given frequency, the higher the current level the lower the noise figure. A typical relationship between noise figure and cathode-current density at 1.2 Gc is shown in Fig. 9. These data were obtained on a type 7077 planar triode. The higher the current densities, the greater the stage gain and useful dynamic range.
Factors in tube life
The practical limit for high-current-density operation depends primarily upon the extended tube life. Two basic problems are the ability to dissipate the heat generated and the maintenance of high-level emission at relatively low cathode temperatures. The first problem has been approached by design of special anode structures to provide efficient heat transfer from the tube.
Providing high cathode emission at relatively low cathode temperatures requires special effort. Some preliminary life test results measured on several types of tubes operating at high current density are shown in Fig. 10. These tubes required careful cathode processing and stabilization at bell-jar vacuum levels and very high temperatures.
The diodes show good life at high current densities. Triode life tests on the Y-1430 have not been completed. The 7486 data were taken at 450 Mc in an oscillator test circuit. Nine tubes were specifically stabilized for operation at the indicated current density prior to life test. The first tube failed at about 3000 hours; but the last of the nine was still operating at its original output power at 8500 hours. There was little change in power output for any of the tubes. All failures were from arcing, which was caused by heater-cycling without removal of other tube operating voltages. Power output was about 4.5 W.
The Y-1094 curve was obtained from X-band tests on one cavity. The power fell off at approximately the same rate as the tube plate current. The Y-1251 results were obtained at 5.7 Gc. Further heater-voltage optimization could have given better results. The Y-1266 life test was conducted in a unit oscillator at about 900 Mc. No noticeable change in power output was measured up to 5000 hours.