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How did they hear small wavelengths (high frequencies) *before* superheterodyne?

  1. Jan 16, 2017 #1
    Hi, I am researching the early history of electricity and I would love some help.

    So, in the very early 1900s (like 1906) there were several items created that were diodes (crystals, Fleming valve, electrolytic detector). As far as I understand, people would use these diodes to convert the AC from the signal into pulsed DC with a diode and thus were able to hear the radio buzzing with their headsets. Now humans can hear up to around 20 kHz, which implies that the maximum frequency that this method would work for is 40 kHz.

    In 1918, Armstrong invented the superheterodyne to work for super high frequencies (above 500 kHz). So, here is my question. How did they "hear" any frequency between 40 kHz and 500 kHz before then?

    What am I missing? Very confused.

    Thanks
     
  2. jcsd
  3. Jan 16, 2017 #2

    NascentOxygen

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    The superhet allowed gain and selectivity to be preset at the intermediate frequency. Before that, gain and selectivity was achieved at each station's radio frequency (in a TRF receiver), where it was found that a regenerative RF amplifier/detector stage (an amplifier incorporating positive feedback) could really boost the selectivity's sharpness.
     
  4. Jan 16, 2017 #3

    Baluncore

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    You have missed the point that an envelope detector was used to follow the audio envelope of the modulated RF or IF signal.
    https://en.wikipedia.org/wiki/Envelope_detector

    It was Sir Oliver Lodge, wandering the streets of London back in 1897, who invented tuning. That meant many stations could share the same band and so required receivers with improved selectivity.

    Before that, an envelope or power detector was used across a whole band.
     
  5. Jan 17, 2017 #4

    sophiecentaur

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    A simple band pass filter (tuned circuit) would be sufficient to isolate the wanted range of frequencies in that range. Follow that with a diode detector and possibly an amplifier and you could hear sound radio signals perfectly well.
    It is quite possible to design a superhet that will receive signals in the range 40kHz -500kHz. (There is no lower limit, in principle) All that's necessary is to have the appropriate local oscillator frequency and choose a suitable Intermediate Frequency. The superhet design was chosen because it was difficult to achieve the necessary filtering characteristics with a simple variably tenable filter. The thing about a superhet design is that the same channel filter (suitable band pass characteristic) can be used for all received frequencies. An early standard of Intermediate Frequency was 455kHz so the limits you quote could be related to that; a superhet cannot receive signals near its Intermediate frequency and it could have been difficult (but not impossible) to make a receiver tune below and so close to its own IF.
    I don't know about the history of receiver design but the Armstrong system could well have been built around the very first frequency allocation regime. Long Wave (Low Frequency) was possibly not used initially because of the difficulties involved with making a suitably efficient transmitting antenna. The wavelength at 200kHz is 1500m and that could have been hard to produce at significant power. Later radio sets (superhet) all had LF band reception down to around 2000m reception (150kHZ)
    See this Wiki link for a lot of historical notes. It gives a good explanation of the rationale behind the introduction of th superhet design.
     
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