Autodyne mixer/oscillator

Discussion in 'General Electronics Chat' started by KCHARROIS, Nov 16, 2012.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Below I have an autodyne mixer/oscillator circuit that I cant get to work but I don't know why. Pretty sure the transistor is biased correctly so I dont know whats up?
     
  2. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Autodynes were used in the early days of both tube & transistor Superheterodynes to cut the component count,& hence the cost,of economy receivers.

    As you have probably worked out,the same active device acted as both oscillator & mixer.
    "Homebrew"Autodynes were notoriously "cranky"things,& needed a fair bit of fiddling to get them going.

    I suggest that you make sure the oscillator part works,first,then play with the mixer function.
     
  3. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Well see the thing I like about the autodyne circuit is that the incoming signal is proportional to the oscillator signal making it a good mixer. And from what I've read the oscillator works because of the rf signal.
     
  4. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    People of my generation,probably sloppily,meant "Autodyne Mixer for a Superheterodyne" when we used the term "Autodyne".

    There was an earlier meaning for the term,referring to the circuit designed by H.J.Round in the early 1900s:
    http://www.radio-electronics.com/info/radio_history/gtnames/round/hjround.php

    To look at,the circuit on that site looks the same as a Reinartz type of Regenerative Detector,& it is quite similar in operation to that of a Regenerative set with the Regen control advanced until the Radio oscillates.

    In operation,the oscillator is "pulled" by the incoming off air signal,until it is "locked".
    In this condition,the oscillator & carrier are "zero beat" so the only resultant output of the detector is the modulation.

    This is similar to the operation of the "Homodyne" & the "Direct Conversion Receiver",except that they use separate oscillators,& the DC receiver is usually not locked to the carrier.

    A circuit which is superficially similar is used as an "Autodyne Mixer".

    In this case,the oscillator frequency must be different from the received signal by an amount equal to the IF (in many cases,455kHz).

    Unfortunately,here,what was a desirable feature of the Autodyne Detector becomes a problem:-

    If a strong signal close to the current oscillator frequency makes it through the (usually fairly poor) front end RF selectivity,the oscillator may lock to that frequency,& mix the modulation from that signal with the desired signal.

    This,combined with the difficulty in controlling the level of oscillator injection,has lead to the almost complete rejection of this kind of mixer in modern designs.
     
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