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What is an intermediate frequency transformer ?

  1. Feb 10, 2010 #1
    Can intermediate frequency transformer be classified as a variable inductor ?
    What is its actual use in radio ?
  2. jcsd
  3. Feb 10, 2010 #2


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    Can intermediate frequency transformer be classified as a variable inductor ?

    An IF transformer usually has two windings in it so it does contain variable inductors, but these may be tuned to a fixed frequency by placing capacitors across the inductors.

    If you have two parallel tuned circuits close enough to each other, then they will pass energy between them but only at a very limited range of frequencies. Other frequencies are mostly rejected. A common IF frequency is 455 KHz.

    IF transformers like that were mostly used for valve amplifiers because they had high impedance inputs and fairly high output impedances.

    IF transformers for FETs can have a low impedance input winding and a tuned secondary. This has the advantage that there is a voltage step-up and a lot of the voltage gain of the amplifier can come from this step-up

    What is its actual use in radio ?

    A signal is mixed with an oscillator in a MIXER and the output of this will usually be the difference of the two frequencies. This will be a lower frequency than the input signal or the oscillator.
    An IF amplifier is used then which is only tuned when the receiver is built. It amplifies signals at one frequency or small range of frequencies.

    Between stages of this amplifier, there are IF transformers which actually provide the selectivity while the active device ie transistor, FET, Valve or IC provides the power gain.

    You might like to read about superheterodyne receivers.

    http://upload.wikimedia.org/wikipedia/en/f/f1/Superhet2.png [Broken]
    Last edited by a moderator: May 4, 2017
  4. Feb 10, 2010 #3
    There is usually a tiny capacitor inside the IF metal can inductor which completes the LC resonator. Also 10.7 MHz is very common.

    Its job is to filter output from the mixer. The mixer converts higher frequencies to lower frequencies or vice-versa.

    For example, if you listen to FM radio let's say a station at 100 MHz. In the radio circuit, the mixer would convert 100 MHz to 10.7 MHz. And depending on the design, a second mixer might convert 10.7 MHz to 455 KHz.

    What's important to note is that 10.7 MHz and 455 KHz is always held constant. But what is changing is the way the mixer down-converts the incoming signal. And the way the mixer down-converts is by subtracting the incoming signal from the local oscillator (AC generator).

    So to tune to a 100 MHz station, set the local oscillator to 89.3 MHz. That way: 100 MHz - 89.3 MHz = 10.7 MHz

    to tune to 105 MHz station, set the local oscillator to 94.3 MHz. That way: 105 MHz - 94.3 MHz = 10.7 MHz
    Last edited: Feb 11, 2010
  5. Feb 10, 2010 #4
    The circuit in the attachment is an old vacuum tube AM radio (from www.antiqueradios.com). T4 and T5 are 455 kHz IF transformers. T3 is the local oscillator tank circuit. The 6A8 is the mixer tube. The 6K7 is the 455 kHz IF amplifier.

    Bob S

    Attached Files:

  6. Feb 11, 2010 #5


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    This is what an IF transformer looks like inside its metal can.

    http://www.radio-workshop.co.uk/images/bush-if.jpg [Broken]

    Those coils are tuned by moving the powdered iron slugs inside the coils. Doing this changes the inductance of the coils.
    You can see the slugs at the bottom although they have been broken.

    The tuning capacitors are the flat yellow things near the coils. They are fixed capacitance (about 400 pF) and tuning is done with the slugs in the coils.
    Last edited by a moderator: May 4, 2017
  7. Feb 11, 2010 #6


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    Answering the OP.
    An IF transformer is, essentially, a bandpass filter. Receivers use an intermediate frequency because it is difficult to make a tunable filter which has a good frequency characteristic. All incoming signals are frequency shifted (see the earlier comments about Mixers) to the chosen Intermediate Frequency so that this IF filter can shape the signal spectrum. It sounds a long winded method but it is the only way to get good selectivity and frequency response but the 'SuperHet' (Superhetrodyne) receiver technique is very successful.
    The filtering can be done better these days but, until quite recently, the filtering was done using two resonant circuits (two LC resonators), coupled to one another by the mutual inductance between two coils. (See the earlier picture) This constitutes a transformer. Another good reason for using this form of filter is that you can use it to couple two stages of amplification and match the output of one to the input of the next. There can be several stages of filtering which can achieve a nice looking 'rectangular' frequency response (sharp edges and a flat frequency response at the top).
    "Is it a variable Inductor?"
    It contains two variable inductors - adjustable by screwing a magnetic core in and out. But, if you want a variable inductor, you don't want an IF transformer because it has other components inside it as well which will change it's characteristics.
  8. Feb 11, 2010 #7
    can anyone tell me how to design intermidiate frequncy transformar? i want to tune it for 140 KHz
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