Issue 316: EQ Answers

Question 1: What is the second grid in a tetrode vacuum tube for? How about the third grid in a pentode?

Answer 1: In a triode, there is a certain amount of capacitance between the control grid and the plate, which contributes to negative feedback and stability problems if there’s significant phase shift in the surrounding circuitry. This often requires “neutralization”, which consists of an external capacitance between the plate and the cathode (often just a metal tab along the outside of the tube) that helps cancel out this effect.

The second grid in a tetrode, called the screen grid, is used to electrostatically isolate the control grid from the plate and eliminate this effect. It is usually tied to a voltage that is close to the plate voltage, but it is heavily bypassed (AC-coupled the cathode or to ground). A secondary effect of this grid is to help intensify the E-field near the control grid and accelerate the electrons in this region.

A problem that crops up in tetrodes, however, is that electrons get knocked loose from the plate by the impact of the cathode current in a process called secondary emission. Some of these electrons get drawn to the second grid, creating a current that is proportional to the plate current and partly negating the intended effect of this grid. A pentode introduces a third grid, called a suppressor grid, that is tied to a more negative voltage (in fact, it is usually tied directly to the cathode) and repels these secondary electrons back toward the plate.

Question 2: Wirewound resistors tend to have an undesirable reactance because of their construction. This series inductance causes the overall impedance to rise with frequency. Sometimes it is suggested to wind the resistor as two separate windings and then connect them so that their magnetic fields cancel. However, this creates a different problem. What is it?

Answer 2: In order to get better magnetic cancellation, the two windings are often done by twisting the two wires together and then winding them together on a form. When you connect the windings so as to cancel, it turns out that the terminals of the resistor are the two wires at the same end of the combined winding. Because of their physical proximity, this creates a great deal of parasitic capacitance that appears in parallel with the desired resistance. This causes the overall impedance to fall off at higher frequencies.

Question 3: What is the relationship, if any, between the GPS master clock and the GPS microwave carrier frequencies L1 and L2? Why are two different frequencies used?

Answer 3: The L1 carrier is 1575.42 MHz, which is exactly 154 times the GPS master clock rate of 10.23 MHz.

The L2 carrier is 1227.60 MHz, or 120 times the master clock.

Two frequencies are used so that receivers can make estimates of the bending effects of the ionosphere, which allows them to make corrections to their time-of-flight measurements. Both carriers are modulated with the C/A (coarse acquisition) signal.

Also, high-resolution GPS receivers can lock directly onto the carrier frequencies in order to establish their position more accurately. The carrier wavelength is just 19 or 24 cm, while the C/A “chip” wavelength (at 1.023 MHz) is 293 m. Such receivers can establish absolute position to within a few cm and make relative position measurements to a fraction of 1 cm.

Question 4: Who, exactly, is “ELI the ICE man?”

Answer 4: Not who, but what. It’s a mnemonic phrase that reminds you that voltage (E) in an inductor (L) leads the current (I), or “ELI”, and that current (I) in a capacitor (C) leads the voltage (E), or “ICE”.

What we’re talking about here is the phase relationships between voltage and current when you apply a sinewave voltage to a coil or capacitor. Mnemonic devices can be handy, but it’s better to have a good basic understanding of what’s going on.

An inductor stores energy in the form of a magnetic field produced by the current flowing through it. Although you can apply an arbitrary voltage across a coil, the current will change only by adding or subtracting energy from the field. This causes the current to lag behind the applied voltage.

Similarly, a capacitor stores energy in the form of an electric field produced by the charge on its plates. Although you can apply an arbitrary current to a capacitor, the voltage will change only by adding or subtracting charge from the plates. This causes the voltage to lag behind the applied current, or equivalently, the current to lead the voltage.