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Help understanding Varactor fm modulation circuit

Discussion in 'General Electronics Discussion' started by Robert Hill, Mar 5, 2015.

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  1. Robert Hill

    Robert Hill

    Mar 5, 2015
    Hello there,

    I'm new to the forum and pretty new to the world of electronics. I've been reading a book called electronics for dummies (I said I was new!). I'm currently on the section which talks about FM signals and there is a circuit to create FM modulations which uses a Varactor diode.You should be able to see a schematic of the circuit in question here:


    The book just gives the circuit as an example FM modulator but doesn't explain how it works beyond the fact that the modulating signal affects the capacitance of the diode which in turn affects the frequency of the FM signal

    I get the concept of the modulating input affecting the capacitance but i'd like to understand both how the circuit works 'step by step' as it were and also understand the design of the circuit for example why the resistors and capacitors are used and placed in series or parallel as they are.

    My current understanding of how this circuit works is as follows:

    R1 and L1 form a Parallel RL circuit (but I can't find any information of what they actually do??)
    I believe that the capacitor in series above the oscillator input may be present to block any DC noise that comes from the Input.
    I believe that the other two sets of resistors and capacitors may be acting as filters.

    I'm happy to go away and read things if anyone can direct me in the right direction to some resources. Ideally ones which avoid all the mathematical formulas and focus on the function of components and the processes going on in the circuit.

    Thanks in advance for any help!

    [Mod edited to show circuit]
    Last edited by a moderator: Mar 5, 2015
  2. Merlin3189


    Aug 4, 2011
    I can't pretend to even begin to understand that circuit, particularly with no values to show the relative magnitude of the impedances and no signal ground. However, there is a phase modulation technique (used to create FM signals) using an RC filter.
    At the cutoff frequency of an RC the output is 45deg out of phase with the input and the phase shift varies with changes in frequency. If the C of the filter is a varicap, as the bias is changed (by the modulating signal) the cutoff frequency varies and the constant frequency input signal from the crystal oscillator finds itself on different points of the phase curve. So the output is varying in phase (and incidentally in amplitude, though that does not matter for FM) according to the modulation.
    Here, the varicap and R2 could be a high pass RC filter to the xtal oscillator input.

    I can't remember the details off-hand of the relation FM and Phase Mod. I think you have to differentiate or integrate the audio (high pass or low pass RC filter) before modulating phase, then you get FM or something like that, but they seem effectively interchangeable..
  3. Robert Hill

    Robert Hill

    Mar 5, 2015
    Firstly, thanks very much for taking the time to respond to my question, Your answer is very helpful.

    It's encouraging to realise that as it stands that circuit diagramme isn't very intelligible so it's not just my lack of knowledge getting in the way.
  4. Merlin3189


    Aug 4, 2011
    First - If anyone else has this circuit with some component values or further description, I hope they may chip in.

    This comment I would say is technically wrong. If it is a phase modulator, as I believe, the point is that the crystal is undisturbed and generates its fixed frequency. This circuit only varies the phase a few degrees, say ±10deg.
    The other sort, the direct FM modulator, places the varicap diode in circuit with the crystal and varies the oscillation frequency of the crystal oscillator. This is generally not preferred because the midpoint bias (DC bias) on the varicap has to be adjusted to put the oscillator on the desired frequency. This means the centre frequency could be set wrongly and may vary with changes in DC bias (mainly due to temperature effects, but also possible supply variations.) The phase modulator does not affect the crystal oscillator, so the transmit frequency is as stable as the crystal can make it.

    I've now found another source for this circuit, but it is exactly the same with no further info nor description (so I guess one copied the other.)

    I'm puzzled by the 3 capacitors and the lack of DC isolation between the modulation input and the modulated output.
    I'm assuming the LR circuit is a low pass filter set just above the carrier to limit higher intermodulation products.
    IF the capacitor connected to the modulation input were grounded at the other end, I would guess that it and the resistor made a low pass pre-emphasis filter. But no ground is shown anywhere.
    I suspect the junction of the resistor and 2 capacitors is ground.
    davenn likes this.
  5. Robert Hill

    Robert Hill

    Mar 5, 2015
    I haven't got any further with the circuit either, though I did go and learn about phase shifting so that was useful. I still need to learn about crystal oscillators.

    The website that has this picture on is: Maybe that's what you found.
    Here's how it is described:
    "The modulating signal varies the capacitance of the diode, which then changes the phase shift incurred by the carrier input and thus changes the phase of the output signal. Because the phase of the carrier is shifted, the resulting signal has a frequency which is more stable than in the direct FM case."

    So you are right that it is a phase modulator.
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