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Help needed understanding the ANC-4 design

Discussion in 'General Electronics' started by Jason Hsu, Oct 15, 2003.

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  1. Jason Hsu

    Jason Hsu Guest

    The schematic is on the last page. Has anyone here worked with a
    similar circuit? I'll explain what little I do understand so you can
    build on this.

    My questions:
    1. What is the Q1 transistor for? Is Q1 an amplifier or just a
    buffer? At DC, I notice that it's fed by a supply voltage just under
    12V (only 20 ohms and some inductance away from 12V). The CR1 and CR2
    diodes, of course, are there to prevent overloading from unexpected
    large signals. Capacitors C3 and C2 couple the RF to ground. I
    assume that R3, R4, and R8 play a role in determining the gain. (I
    can't remember enough of FETs to show the exact relationship.) The RF
    output must be the voltage across T1 and is not connected to ground.
    The inductance provides RF impedance between Q1 and the power supply.
    2. What does transformer T1 do? I notice that the not-quite-12V DC
    (I'll call it 11.8V DC) is connected to two of the T1 windings. So if
    the input of T1 is .01V RF, it extends from 11.79V DC to 11.80V DC.
    Then the output would extend from 11.80V DC to 11.81V DC. Until I get
    an answer from Timewave, any idea what the turns ratio and inductance
    values might be? Does it play a role in the noise phase adjuster? Do
    I need to treat this as a current transformer rather than a voltage
    3. How does the noise phase shifter work? How does noise phase
    shifter work? Adjusting the value of potentiometer R6 is supposed to
    change the frequency response. The phase shift should vary between
    about -pi/2 and +pi/2 depending on how the pot is adjusted, but I'm
    not getting anywhere in the simulations. The FREQ RANGE switch should
    control where the phase shift equals 0, but I can't make any sense of
    this, either. Unfortunately, information on the transformers is not
    provided. But even assuming 1:1, 10:1, and 1:10 turns ratios on
    FT50-43 toroids isn't helping. I must be thinking about the circuit
    in the wrong manner. The adjustment for the noise gain seems
    straightforward enough to me - the pot controls a simple voltage
    4. What is the Q2 transistor for? It's also fed by a supply voltage
    just under 12V (only 10 ohms and 2 inductors away from the 12V power
    supply). Capacitor C8 seems to be like capacitor C2 - coupling
    capacitor. I guess R9 and R10 play a role in setting the gain, like
    R8, R3, and R4 in Q1.
    5. What does transformer T2 do? The strange thing is that the DC
    power supply is connected to the OUTPUT winding. I'm lost.
    6. How does the noise gain work? Is the R12 pot a simple voltage
    divider? I notice that C11 lies between R12 and Q3, so it has no DC
    connection to it. I also notice that part of this pot is in parallel
    to R37.
    7. What does Q3 do? I don't remember my BJT transistor theory very
    well either. The collector is 2 inductors away from the 12V power
    supply and is thus connected at DC but isolated at RF. C12 is a
    coupling capacitor, so R16 plays a DC role. R13, R14, R15, and R16
    determine the gain.
    8. How exactly does T3 work? Again, there is a DC supply voltage on
    the output side.
    9. What does Zener diode CR12 do? Do it, CR4, and CR3 play a role in
    dealing with unexpected large RF signals, or am I off base here?

    I don't yet understand the T/R switch at the bottom left of the page,
    but I'll save that for another post.

    Jason Hsu, AG4DG
  2. Guest

    Is this part of your research for your post graduate work,
    or were you doing something on DSP? In any event, thanks
    for posting the schematic! I'll look at it in detail when
    I can, but I'll comment on the xsistors: for the FETs, the
    voltage (signal) at the gate controls the current thru the
    source-drain. For the bipolar, the current at the base
    controls the current thru the emitter-collector. All of
    them are amps that drive the transformers. At a glance,
    the xformers appear to be auto-transformers to boost the
    signal voltage.
  3. Guest

    I wonder if they use fets as variable capacitors to tune the
    resonant circuits? Works well in VFOs.

    I will study it. I have one but this is the 1st time I've
    a schematic. Thanks

    Murray vk4aok
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