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Although the Vacuum Triode was a magnificent invention, it was not without limitations. Although it could amplify weak signals, the ratio of amplification was low. Early Vacuum Tubes had amplification ratios µ as low as 3 to 5. This required voltage step up transformers between stages to boost the overall ratio, perhaps by another factor of 5. Even so, to match typical signal gains found in modern broadcast receivers, 20 or more vacuum tubes and transformers would be needed.
Another difficulty occurred in radio frequency amplification, needed before the radio frequency (RF) and audio detector stage in order to improve overall sensitivity (i.e. the ability to receive weak signals rather than just make those received appear loud). The inter-electrode capacitance between anode and grid created a feedback path inside the tube, leading to instability. In old war movies etc, radio's often are heard with whistles in the background, a symptom of instability.
However just as the diode evolved to the triode, it seemed reasonable to add yet another grid and make a tetrode valve.
This additional grid was given a positive potential and helped attract electrons emitted from the cathode to flow through on to the anode. This has two important advantages,
* The voltage amplification immediately rocketed to several thousand or more and
* The new screen grid acted as an electrostatic shield, almost eliminating capacitive feedback from anode to grid 1.
The feedback capacitance of triodes is around 5 pF, in contrast the tetrode feedback capacitance is less than 0.05 pF.
As time went on, even more grids were added, creating pentodes, hexodes and heptodes. Unfortunately it was soon found that each additional grid added significant noise to the amplified signals.
The earliest AM broadcast radios used a Tuned Radio Frequency, or TRF approach. This required one or more tunable Radio Frequency (RF) amplifiers, followed by a RF to Audio demodulator, then an audio amplifier. Several tuned circuits could be incorporated to improve selectivity, that is the ability to receive a signal on one frequency and reject those on a nearby frequency. These had to be tuned in unison, so a common shaft with multiple plate stacks per section was needed. Sometimes these would not track exactly, leading to reduced sensitivity and impaired selectivity.
This approach was very much in vogue by the early 1930’s, but required much skill in tuning, especially when “ganged” tuning capacitors were not available and independent tuning adjustments were needed for each tuned circuit.
Later, various people such as Hartley and Colpitts had experimented with the concept of amplifier "instability" and considered if this problem could be turned into an asset. The redefined this instability as an "oscillation" in which a pure single tone is produced, much like a note played on a whistle. Although "oscillators" had occurred by accident previously, they both devised circuits that could better control it and apply it usefully. Oscillators today have many different topologies, which is a name for the order in which components are connected together.
A further concept emerged called frequency conversion. When two frequencies are mixed together a "beat note" is produced. For example, a guitar string can be tuned to a given note by listening to the slow beat note that goes up and down in loudness when they are not quite in tune. When the tuning is exact, the beat note disappears, i.e. it becomes static.
Armstrong reasoned that oscillators could provide a note that could be combined with the wanted station frequency and produce a lower frequency beat note that was easier to amplify. In addition, this could be fixed, listening to a single frequency, (i.e. tone). Stations could then be selected just by changing the oscillator frequency.
This beat note is called a heterodyne. Since the radio frequencies involved where several hundred kHz and higher, much higher than the 20 kHz hearing of people with exceptional hearing, the term super-heterodyne was coined.
Armstrong called these new frequencies “Intermediate Frequencies” or IF. Several important advantages were evident.
This, to Armstrong, was the best thing since sliced bread, and he knew he had a winner. This superhet technology made VHF and UHF reception possible, and inexpensive. Only two high frequency devices were needed to process RF, everything else could work down at much lower, and easy to process frequencies. Still, some drawbacks existed. The method of introducing the LO signal required a common electrode, and this not only caused oscillator loading (and frequency pulling) effects, but also allowed a sizable LO signal to leak out to the antenna, causing potential interference to other spectrum users. Armstrong needed a better mixer device.
Various configurations were tried, ranging from injecting the LO signal into the Cathode, or Screen grid 1 and even Screen grid 2. All three methods gave good results and the combination of Screen grid 1 as a “virtual Anode” and the Cathode even allowed a simple oscillator to be constructed inside the mixing valve. However such configurations were never optimum in terms of convenience.
Soon people trigged that the LO signal could be injected into any of the valve’s grid terminals, and that sum and difference frequencies would be generated, they began to see the process of frequency conversion as a multiplication function. Voltage on one grid could control the gain of the device to a signal on a different grid. This quickly led to the generation of a special mixer valve called a Hexode (6 terminals) and Heptode (7 terminals).
These new mixer valves used control grid
1 for RF signal input, grid 2 was given a positive charge to help attract
electrons, grid 3 was used for LO injection. A forth grid was also added
after grid 3 to further attract electrons by adding additional positive charge,
and to help shield the anode from grid 3, in case unwanted instability might
occur. This resulted in 6 electrodes total, i.e. a hexode. A final grid, added
after grid 4 and connected to ground was then added to reduce an effect called
"secondary emission" which is not covered here, except that it caused
undesirable instability by a negative resistance mechanism.
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