The Electro-Magnetic Spectrum Part Three In This Issue: All about Tubes How a diode rectifier works How a triode amplifier works How the mixer in your receiver works Dear Friends: For quite some time I have often wondered how what used to be referred to as the BFO (beat frequency oscillator) in my receiver worked. It was later referred to as the mixer. It was obvious to me that you just couldn t take two radio frequencies and beat them together. When you re dealing with electronics it s not the same as dealing with musical notes where airwaves are involved, but the results are the same, which is, you get the sum and the difference along with the original two frequencies. The difference frequencies are referred to as the second harmonics. The second harmonics beat together and produce the third harmonics, etc. Now in nature you don t get something for nothing, so the power of the harmonics is half that of the two frequencies that produced them. Now that we have an understanding about harmonics let s see if we can figure out how the mixer in our receiver works. The goal is to convert the incoming signal that we are tuned to, no matter what it is, to an intermediate frequency of 455 KHz which can be amplified more efficiently than if we tried to amplify a wide range of frequencies. Also by having intermediate stages of amplification we have a large degree of isolation in keeping unwanted signals from being amplified. To do this we need a local oscillator whose frequency changes in step as we tune in different frequencies. This is done by mechanically connecting the tuning capacitor of the local oscillator with the tuning capacitor in the RF stage connected to the receiving antenna. This is simply done by connecting both capacitors to the same shaft that the tuning knob is on. Now that we have established the facts and the mechanics let s do something with them. I would like to start out with an explanation using vacuum tubes and later explain using transistors. Why vacuum tubes you might ask. There are still a lot of older rigs in use that have vacuum tubes and even modern rigs use them in the final stages. Not only that, understanding how vacuum tubes work enhances our knowledge of electronics, in my opinion. Also, due to the fact that I was a volunteer examiner participating in more than seventy test sessions over the years I am well
aware of the diverse background of the hams in today s world. So let s get into an explanation of how vacuum tubes work. Tubes are generally referred to by the number of elements they contain such as Diode (two elements), Triode (three elements), Pentode (five elements) etc. The first number in the designation is the operating voltage of the filament or heater, such as 6L6 (6.3 volts). The simplest tube is the diode which has one anode (plate) and one filament. Later as technology improved the filament was given a cover referred to as the cathode and the filament became a heater. The purpose of this was to prevent interference from the 60-cycle filament current by providing a steadier source of heat in the form of the cathode. The cathode is usually coated with a mixture of oxides such as calcium, aluminum and barium. The heating element is usually made of tungsten. As the cathode heats up the oxides slowly flake off providing a source of electrons. The air has to be removed from inside the tube and this is done by connecting it to a vacuum pump. The vacuum pump in itself cannot remove all the air so when the tube is constructed a small cup is attached to the inner framework into which a chemical called getter is inserted. The getter is ignited by subjecting it to a source of high radiation which ignites it burning off the remaining air. This causes a silvery mirror like coating on the inside of the tube. The elements inside the tube are connected to pins which protrude from the bottom of the base. These pins are arranged in such a way that the tube can only be plugged into the proper socket in one way. In the case of a four-prong tube two of the pins are larger than the other two. The filament supply is connected to these two pins. The same thing applies to all the other pin arrangements except the octal base which has a key that only allows the tube to be plugged in one way. The pin numbering is viewed from the bottom of the tube and runs in a clockwise direction. See Figure 1. Figure 1 Now that we know what the tube is all about let s see how a diode changes alternating current into direct current. Referring to Figure 2 we find a schematic drawing of a full wave rectifier. One of the big differences between using tubes as opposed to transistors is the high voltages used on the tube plates. These DC voltages are in the 350-volt range so it is therefore necessary to have a step-up transformer whose secondary winding provides the necessary output so when it is rectified and filtered it will meet the requirements of the rectifier tube being used. Such a transformer is shown in Figure 2. It also includes a winding for the heater voltage. Both windings are center
tapped. The parallel lines shown between windings indicate it is wound on an iron core. The input voltage is 120 V.AC.The tube has two plates, one connected to pin 2 and the other to pin 3 with the filament connected to pins 1 and 4. During the first cycle pin 2 will be positive and pin 3 negative. This allows current to flow to pin 2. When the cycle reverses the current flows to pin 3. The current flows thru a load resistor (not shown) and into a filter which is a large capacitor. The return path is thru the center tap of the heater winding. The capacitor which is in the range of 8 microfarad charges and in between cycle changes fills in any fluctuation in the DC voltage. This completes the description of a full wave rectifier. Figure 2 - Full Wave Rectifier Before going on to the mixer I would like to describe the use of a triode as an amplifier. Referring to Figure 3 we see a schematic of a triode using batteries as a source of voltage. The A battery is the source of heater voltage, the B battery the plate voltage thru a load resistor, the positive end to the plate and the negative end to the heater. The C battery is bias voltage connected to the control grid via a resistor in the input signal source (not shown). The control grid is on pin 3 and the plate on pin4. When the input signal arrives on the control grid in the form of a low voltage alternating current, current flows to the plate returning to the heater via the load resistor. The voltage drop across the load resistor is much larger than the input signal hence amplification takes place. The purpose of the bias voltage placed on the grid is to control the current flow thru the tube so that it follows the input signal faithfully giving an output signal exactly like the input. Very simple!
Figure 3 - Triode Amplifier Next, the frequency converter: Please refer to the schematic on the following page. There are many different circuit arrangements designed to convert the incoming signal to an IF frequency of 455Khz. Using tubes. In all cases a local oscillator and a mixer are required. Some circuit designs use a separate oscillator and mixer and others use a single tube incorporating both features. One such tube is the 6BE6 a 7 pin miniature pentagrid converter. If we persevere and learn how this one works we will be able to understand all others. So let s look at the oscillator section first. The cathode and the first two grids act like a triode and are the local oscillator. This circuit is called a Hartley oscillator because it uses a tapped inductor. The cathode is connected to the tap, and the first grid, called the oscillator grid, receives the signal which is coupled to the top of the oscillator coil via a 100pf capacitor. The cathode current flows thru the tap to chasse ground inducing a small voltage into the other half of the coil. The coil is tuned to a frequency which is 455 KHz lower than the incoming frequency. When the positive half of the cycle appears on the oscillator grid it drives the tube to cut off, stopping current flow. When the current flow stops the oscillator grid goes negative again and current resumes. This action turns the oscillator on and off at the frequency that it is tuned to. This action is what causes the mixing of the incoming signal. Now for the mixer itself. Grid two is connected to B+ and acts like a plate, but a leaky plate which allows electrons to pass thru. The incoming signal appears on grid three. What happens next is the incoming signal is not always in phase with the turning on and off of the oscillator and these phase differences is what causes the sum and difference frequencies to be generated. The difference frequency of 455 KHz passed thru the tuned if stage and the unwanted frequencies are filtered out. This concludes the tube section. Next solid-state design and all about how solid state works coming up in the near future in part four of EMS Please direct any questions, comments pro or con to Walter Kunz AA3FC at kunzham@verizon.net.
SOURCES RCA Receiving Tube manual (series rc21) Max Robinson http://www.funwithtubes.net/