Tuner front end question

Discussion in 'Tuners' started by tranguru, May 3, 2017.

  1. tranguru

    tranguru Active Member

    Messages:
    112
    Location:
    Brussels
    Hi all,

    I am in the process of add info to FM tuner section in marantz manuals.
    I need some circuit clarification. hope one of you can help.
    upload_2017-5-3_21-51-30.png
    This if the output of the Front end.
    I think this is a band pass filter to let 10.7 MHz signal pass and provide image rejection.
    how does this kind filter work?

    Thanks for your help
    Robin
     
  2. amptramp

    amptramp Active Member

    Messages:
    360
    Location:
    Mississauga Ontario Canada
    L106 is an L-C tuned circuit at the mixer output with a resistor in parallel to broaden the bandwidth. It has nothing to do with image rejection - that is done by the RF stage tuned circuits. With an IF of 10.7 MHz (which is the most common IF frequency) and an oscillator tuned above the RF frequency by 10.7 MHz, with no RF tuning, an RF input at the frequency 10.7 MHz above the oscillator frequency could also come through - that is the image frequency. For example, we have a strong local station at 99.9 MHz, so the oscillator will be at 10.7 above that or 110.6. But an input at 121.3 MHz will also result in an output at 10.7 MHz and that just happens to be the approach control frequency at Pearson International Airport in Toronto, so low quality radios will pick up air traffic control frequencies.

    It is rare to set things up so the oscillator is below the RF frequency because that would require the difference between the maximum and minimum to be a greater proportion of the maximum frequency so the tuning would not track as well between the RF and oscillator. When you multiply two frequencies together you get the sim and difference frequencies.
     
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  3. tranguru

    tranguru Active Member

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    112
    Location:
    Brussels
  4. amptramp

    amptramp Active Member

    Messages:
    360
    Location:
    Mississauga Ontario Canada
    The mixer functions as an analog multiplier. Each input can be treated as a cosine wave so if you have two frequencies u and v then:

    cos(u)*cos(v) = 0.5*cos(u-v) + 0.5*cos(u+v)

    which are the sum and difference frequencies,

    This is a trigonometric identity where angles are replaced by 2*pi*f*t where f is the frequency and t is the time.
     
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  5. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

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    I used to know this 30 years ago. Thanks for refreshing my memory.
     
  6. Punker X

    Punker X AK Subscriber Subscriber

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    same here.. best to stick with sum, difference and the two originals..
     
  7. tranguru

    tranguru Active Member

    Messages:
    112
    Location:
    Brussels
    Thanks for the input.
    I have another question, about the local oscillator this time:
    upload_2017-5-5_13-4-41.png
    Marantz 4400
    why is the local oscillator tuning circuit before the transistor (H103)?
    what if the function of the transistor? it is connected thru r113.
    is it to stabilize or regulate the output level of the oscillator (is resonator a more appropriate term) or boost the Q factor (make sense?) ?
    what would typical values for C106, the internal capacitor and the self? (I want to calculate)
    I looked at another schematic (Lenco)
    upload_2017-5-5_13-6-57.png
    what does the transistor do here?
    Here the mixer FET has 2 inputs: seems better no? or is it just cheaper?

    Thanks
    Robin
     

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  8. I LIKE MUSIC

    I LIKE MUSIC Super Member

    In basic terms the LO is made up of 2 parts. The tuned network of the inductor and the capacitors (sometimes called the tank circuit) determine the frequency of oscillation and the transistor. The transistor supplies the power (gain to over come the losses in the tank circuit) to keep the tuned circuit oscillating. With out the transistor the tank circuit will not maintain oscillation.

    A small amount of the output of the transistor is fed back to its input to allow for oscillation.
     
  9. tranguru

    tranguru Active Member

    Messages:
    112
    Location:
    Brussels
    Thanks for the info.
    the oscillator signal feeds the base of the transistor thru c121 and the feedback flows back thru the same C121 ? coming from R113 ?

    Do you know what type of oscillator this is?

    Thanks
    Robin
     
  10. I LIKE MUSIC

    I LIKE MUSIC Super Member

    It is a colpitts oscillator.

    The configuration of the transistor amplifier used in the LO is of a common collector amplifier, the feedback signal (at the emitter) supplies the base with positive feed back.

    It is this positive feedback that sustains the oscillation.

    The output of the LO is taken from the secondary winding of L105.

    Just a point of terminology. The inductor and its parallel capacitors are not called the oscillator. They are the tuned circuit that determines the frequency of oscillation as the main tuning capacitor is adjusted.

    The combination of this tuned network and the transistor is called the oscillator (local oscillator).
     
    Last edited: May 6, 2017
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  11. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Here is some additional information. Reposting this picture for reference.

    upload_2017-5-6_4-11-34.png


    The picture below is the frequency response/impedance of the tuned network (the inductor and its associated capacitors) as seen at C121.

    XYZA.JPG

    The impedance of the tuned (LC) network will be the highest at the tuned frequency of the LC network. The impedance seen at C121 and the base of the transistor will be the highest at this frequency and lower on each side of the tuned frequency. This is how the tuned network is able to control the frequency of oscillation of the local oscillator.

    Basically the transistor wants to oscillate because of the positive feedback from the emitter to the base. The characteristics of the tuned LC network (because of the basic characteristics of a parallel tuned LC network) control and determine the frequency of oscillation.
     
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  12. tranguru

    tranguru Active Member

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    112
    Location:
    Brussels
  13. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Your first picture shows a Hartley oscillator. It takes its feedback signal from a split/tapped inductor.

    Your second picture shows a Colpitts oscillator. It takes its feedback signal from split (series connected) capacitors.

    The end result is the same, controlled positive feedback applied to the base of the transistor.

    These two circuits differ from the LO oscillator circuit that you asked about. They take the usable output from the collector. The LO oscillator circuit that you asked about takes the feed back signal from the emitter and the usable signal that goes to the mixer from the secondary winding of the inductor.

    It is the difference between a common emitter circuit and a common collector circuit.

    There is a wealth of information about transistor oscillators and amplifier types on the internet. The same with the functionality of the mixer.
     
    Last edited: May 6, 2017
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