WEBVTT NOTE Components at HF 1 00:00:00.000 --> 00:00:13.418 Components behave differently at RF. Every component is only approximately the thing its symbol says. At HF the approximations start to matter. 2 00:00:13.418 --> 00:00:20.085 Question. Why should wire-wound resistors not be used in RF circuits? 3 00:00:20.985 --> 00:00:27.026 Answer. The resistor's inductance could make circuit performance unpredictable. 4 00:00:27.626 --> 00:00:41.755 A wire-wound resistor is a coil. At DC it is a resistor; at 14 MHz it is a resistor in series with an inductor whose reactance may exceed the resistance. 5 00:00:41.755 --> 00:00:50.066 This is also why a dummy load must be non-inductive, the same problem, at a hundred watts. 6 00:00:50.066 --> 00:00:57.586 Question. What happens when an inductor is operated above its self-resonant frequency? 7 00:00:58.486 --> 00:01:01.545 Answer. It becomes capacitive. 8 00:01:02.145 --> 00:01:11.093 Every inductor has stray capacitance between its turns, so it is a parallel resonant circuit with itself. 9 00:01:11.093 --> 00:01:28.138 Below self-resonance it behaves inductively; above it, the stray capacitance dominates and the component behaves as a capacitor. An RF choke used above its self-resonance does the opposite of its job. 10 00:01:28.138 --> 00:01:41.773 Choosing capacitors. Type, Electrolytic; Characteristic, high capacitance for a given volume. Type, Low-voltage ceramic; Characteristic, comparatively low cost. 11 00:01:41.773 --> 00:01:55.153 Electrolytics are polarised, lossy at RF, and limited in life, but nothing else gives microfarads in a small can, so they are the power-supply filter choice. 12 00:01:55.153 --> 00:02:11.516 Ceramics are cheap, small, and good at RF, which is why bypass capacitors are ceramic. Diodes. Two forward threshold voltages worth memorising: Silicon junction diode: approximately 0.7 volts. 13 00:02:11.516 --> 00:02:25.918 Germanium diode: approximately 0.3 volts. The lower germanium drop is why germanium diodes persist in low-level detectors, where 0.4 V of difference is the whole signal. 14 00:02:25.918 --> 00:02:36.145 The Technician semiconductors lesson noted that drop varies by type; these are the two numbers behind that. Transistors. 15 00:02:36.145 --> 00:02:43.905 Question. What are the operating points for a bipolar transistor used as a switch? 16 00:02:44.805 --> 00:02:48.417 Answer. Saturation and cutoff. 17 00:02:49.017 --> 00:03:00.490 Cutoff is fully off, no collector current. Saturation is fully on, as much current as the circuit allows, with minimal voltage across the device. 18 00:03:00.490 --> 00:03:11.251 A switch lives at the two extremes and passes through the middle as fast as it can, because the middle is where the power is dissipated. 19 00:03:11.251 --> 00:03:16.811 Question. Which of the following describes MOSFET construction? 20 00:03:17.711 --> 00:03:24.443 Answer. The gate is separated from the channel by a thin insulating layer. 21 00:03:25.043 --> 00:03:32.666 That insulating layer is why a MOSFET gate draws essentially no current, and why it is fragile. 22 00:03:32.666 --> 00:03:45.586 A few tens of volts of static across a layer that thin punches through it permanently, which is why MOSFETs ship with their leads shorted together. Vacuum tubes. 23 00:03:45.586 --> 00:03:50.963 Still present in HF amplifiers, and the pool still asks about them. 24 00:03:50.963 --> 00:03:59.643 Question. Which element of a vacuum tube regulates the flow of electrons between cathode and plate? 25 00:04:00.543 --> 00:04:03.372 Answer. Control grid. 26 00:04:03.972 --> 00:04:18.407 Electrons flow from the heated cathode to the positive plate; a small voltage on the control grid in between modulates that flow. Same function as a transistor's base or gate. 27 00:04:18.407 --> 00:04:24.887 Question. What is the primary purpose of a screen grid in a vacuum tube? 28 00:04:25.787 --> 00:04:31.067 Answer. To reduce grid-to-plate capacitance. 29 00:04:31.667 --> 00:04:38.084 A second grid between control grid and plate, held at a fixed positive voltage. 30 00:04:38.084 --> 00:04:51.407 It screens the control grid from the plate, and the capacitance it removes is exactly what would otherwise feed output back to the input and cause self-oscillation. 31 00:04:51.407 --> 00:05:04.973 That is the connection to the next lesson's neutralisation: the screen grid solves in construction what neutralisation solves in circuit design. Batteries at HF power. 32 00:05:04.973 --> 00:05:14.274 Question. What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery? 33 00:05:15.174 --> 00:05:18.934 Answer. 10.5 volts. 34 00:05:19.534 --> 00:05:35.414 Below 10.5 V a lead-acid cell begins sulphating irreversibly. It is the number built into every low-voltage cut-off, and the reason a "dead" battery left flat often never recovers. 35 00:05:35.414 --> 00:05:41.854 Question. What is an advantage of batteries with low internal resistance? 36 00:05:42.754 --> 00:05:45.788 Answer. High discharge current. 37 00:05:46.388 --> 00:05:59.854 Internal resistance limits current and drops voltage under load, which is the same voltage-sag problem as undersized DC wiring, from inside the battery. 38 00:05:59.854 --> 00:06:16.599 A radio drawing 20 A on transmit needs a battery whose internal resistance is low enough that the terminal voltage holds up. Check yourself. Why must a dummy load resistor be non-inductive? 39 00:06:16.599 --> 00:06:31.660 Your RF choke works on 80 metres and seems to do nothing on 10. Why? A silicon and a germanium diode in the same detector circuit. Which passes a smaller signal, and why? 40 00:06:37.660 --> 00:06:47.401 A wire-wound resistor is a coil; its inductance makes the load reactive rather than 50 ohms resistive at RF. 41 00:06:47.401 --> 00:07:05.620 You are probably above its self-resonant frequency, where stray winding capacitance dominates and the choke behaves capacitively. Germanium, with about a 0.3 V forward threshold against silicon's 0.7 V.