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Tuner Front End

Discussion in 'Electronic Design' started by Myauk, Apr 13, 2007.

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  1. Myauk

    Myauk Guest

    I have been studying the Tuner Front End Designs.
    According to suggestions from previous discussions, I studied the
    application notes and circuit diagrams of front end modules.
    The following things are those that I am getting confused with:
    1. According to my understanding, there are several allocated channels
    for television according to CCIR, France, USA, or China Tables. Let's
    say a tuner front end is connected to the antenna, the first thing it
    needed to be done is to select a typical channel among the channels
    available. I understand that 'tank circuit' is used with variable
    capacitance to tune to the desired channel.

    So I imagine that a tuning unit with varicap diode will do the
    selection of channel. The capacitance is set by the voltage applied
    (up to 33V range). And this voltage might be supplied by a typical DAC
    and register where the values are set by I2C control.

    But practically, when I studied the tuner application note, I found no
    clue for setting the capacitance for tuner tank circuit.
    What I found is something 'Charge Pump', Prescaler, Divider, 15 Bit
    Counter, Phase Detector, Op-amp unit, detecting Local Oscillators,
    (sorry if I presume wrongly) and the output VTU controls the tuning
    part. (according to a tuner front end design using the IC: SN761678B
    There I am confused!

    Can anybody show me some links or explain to me about it so that I
    could start learning?


    Frank Raffaeli
  3. Phil Allison

    Phil Allison Guest


    ** TV and FM tuner modules typically have just one voltage control input
    for adjusting the operating frequency.

    That input connects to several internal varicaps, controlling tuned circuits
    in the RF & local oscillator stages.

    The design of the module ensures the RF and local oscillator tuned circuits
    track as required.

    ........ Phil
  4. Myauk

    Myauk Guest

    Thank you so much.
  5. Sometimes there is no 33V and a charce pump ciruit could be used
    to make 33 from say 5V.
    The osc frequency is deviced by the pre-scaler and compared to some reference..
    The divider is often programmable so you can set the division rate for example via
    i2c I do not know this chip you mention).
    The opamp would amplify the 0-5V of a DA converter to 0-33V.
  6. Myauk

    Myauk Guest

    I have read the article.

    According to the article, I learned about single conversion and double
    conversion scheme.
    But there is something I was not sure about.
    While Selecting the desired channel among the frequencies in the
    available range, let's say, 45 to 860 MHz, wouldn't it need tuning
    circuit? (something like tank circuit?)
    Or it is like the whole frequency range 45 to 860 MHz is downconverted
    by mixing with Local Oscillator frequency, converting it into the
    range in which the desired channel is sticking around the preferred IF
    frequency (PLL?) and the rest are filtered out by using Band Pass
    Filter? (referring to the block diagram of MC44S803)

    And what is the High side/low side injection?
    Now I am tracing the circuit design of the tuner to understand more
    about it so that I could make my question more clearer for specific

    Sorry for my ignorance in RF field. But I am quite eager to dig into
    more details about it gradually until I understand the whole scheme of
    tuning algorithm.(especially the popular hardware/software interface
    scheme for tuning control)

  7. High side injection, which is more common in single-conversion down-
    converters for TV, means that the local oscillator frequency = RF +
    IF. This is now also the favored design practice in digital and hybrid
    TV designs, using IF's of 44 or 36 MHz.

    The amount of tuning in the front-end (before the mixer) varies by
    design. In low-cost systems that use all-digital front-ends (e.g.
    xceive), there is a band pre-selector. In tracking tuners like LG/
    Innotek there is a varactor-tuned circuit before the mixer.

  8. vasile

    vasile Guest

    The noise at the charge pump output is essential.
    A few years back designing a DBVT board I've learned that.
    MAX5025 is a good choice:

    No you don't need that. Modern circuits have constant 33V on varicaps
    through a resistor.
    Driving low the current through an open collector will allow 0 to 33v

    - Hide quoted text -
  9. That will work, but in case 'opamp' with associated components you can have
    more exact gain I think.
    Anyways DA converter is a wide description here, a XOR phase comparator with
    lowpass is also in a way an DA converter, but I think that was obvious.
    Then perhaps there is a loop filter to be implemented, where the opamp high open
    loop gain may help.

    I must say that I have looked at some of these chips, and a lot has been integrated
    making the tuner construction much simpler, say cheaper.
    I had problems to identify the things that were left...once I designed my own
    How far will integration go? hard to integrate 60 Mhz (ch 3,4 etc VHF), but
    UHF all in one chip? possible? (I mean no more inductors)?
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