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KF5JRV > TECH 23.01.17 14:23l 16 Lines 2800 Bytes #999 (0) @ WW
BID : 9232_KF5JRV
Read: GUEST
Subj: Direct Conversion Rx
Path: IW8PGT<IZ3LSV<IK6ZDE<F1OYP<KQ0I<N3IP<NS2B<KF5JRV
Sent: 170123/1216Z 9232@KF5JRV.#NWAR.AR.USA.NA BPQK6.0.13
With the outbreak of the First World War, there was a great impetus to develop wireless technology further. Both sides of the conflict recognised the benefits it could bring, both in terms of improved communication and intelligence. One of the first areas on which development activity was focused was the valve itself. In these early days of thermionic technology their performance was poor. They lacked gain, especially at high frequencies and they were prone to oscillate when they were used at frequencies anything above a few kilohertz.
Originally it was thought that small amount of gas in the envelope was key to their operation. However an American named Langmuir disproved this and as a result a new generation totally evacuated "hard" valves were introduced. Not only was the operation of valves improved by the complete evacuation, but it also allowed the heaters to have coatings applied to them to improve their emission. In the old "soft" valves the gasses in the envelope contaminated the coatings making them unusable.
The other problem with valves was their susceptibility to oscillation. One of the main reasons for this was the level of capacitance between the grid and anode. A number of attempts were made to reduce this. H.J. Round undertook some work on this and in 1916 he produced a number valves with the grid connection taken out of the top of the envelope away from the anode connection. This proved to give a major improvement, but the final solution did not come until the 1920s.
Although the TRF receiver represented a major improvement in performance over what had been available before, it still fell short of the needs for some of the new applications. To enable receiver technology to meet the needs placed upon it a number of new ideas started to surface.
One of these was a new form of direct conversion receiver. Here an internal or local oscillator was used to beat with the incoming signal to produce and audible signal that could be amplified by an audio amplifier. Although the basic principle of the direct conversion had been known about for many years many considered the system was wasteful of valves because the oscillator and mixer did not contribute to the gain of the set. Even in military circles this was a consideration because of the size and cost of the valves and their associated batteries.
The problem was solved by one of Britain's leading wireless engineers, a man named H J Round. He developed a receiver he called an autodyne in which the same valve was used as a mixer and an oscillator, Whilst the set used fewer valves it was difficult to optimise the circuit for both the mixer and oscillator functions. To make the next leap forward in receiver technology a new type of set was needed.
73 Scott KF5JRV
KF5JRV.#NWAR.AR.USA.NA
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