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Sound Technology FM generator question.

Discussion in 'Electronic Repair' started by David Farber, Feb 19, 2009.

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  1. David Farber

    David Farber Guest

    I have a trusty old Sound Technology ST1000A FM generator and I have a
    question regarding the variable FM output level. The owner's manual states
    that the output impedance is 50 ohms, VSWR<1.3, 200Vdc isolation. My
    question is if I am using this device to check the sensitivity of an FM
    tuner, will the output level be affected if I connect the output terminal to
    an RG-58 cable and then use a 75 ohm to 300 ohm matching transformer to make
    it compatible with the old FM tuners? It seems to me the 2:1 step up ratio
    of the matching transformer should affect the level not even considering
    the fact that the 75 ohm input of the matching transformer does not match
    the 50 ohm output of the cable.

    Thanks for your reply.
  2. Ian Jackson

    Ian Jackson Guest

    The fact that the generator 50 ohm output and coax has a 75 ohm load
    will give you a voltage of 0.6 of the generator open circuit voltage. If
    the load had been 50 ohms, you would get 0.5 of the open circuit
    voltage. The increase is 0.6/0.5 = x1.2 (+1.58dB).

    75-to-300 ohm transformer should give you a 2-to-1 voltage step-up

    The transformer will have some loss. This should not be more than about

    So, the voltage at the FM tuner 300 ohm input will be the generator
    output (into 50 ohms) + 1.58dB + 6dB -0.5dB = Vout + 7.08dB.

    This assumes, of course, that the tuner input impedance really IS 300
    ohms (which it probably isn't!).
  3. David Farber

    David Farber Guest

    I believe the output level dial is calibrated for a 50 ohm load. So using
    your numbers if the output dial is set to 10µV then I calculated a 7.08dB
    gain to be 22.6 µV. In effect, the sensitivity of the tuner at this point is
    7 dB worse than what the dial indicates. Is that right?

    Thanks for your reply.
  4. Ian Jackson

    Ian Jackson Guest

    The calculation looks OK. However, I suppose it depends on what your
    'standard' impedance is.

    If you have a 'normal' halfwave dipole at (say) 100MHz, the impedance at
    the centre will be about 75 ohms. It would be normal to connect it (via
    a 75 ohm feeder) to a tuner with a 75 ohm input impedance. [Note: It
    might be more correct to say that the tuner is designed to work best
    when fed from a 75 ohms source. In practice, it might not have a very
    good 75 ohm input.] Anyway, let us assume that the level of a received
    100MHz FM radio signal level (into 75 ohm) is 1μV.

    Now, if you replace the 'normal' dipole with a folded dipole, you would
    use 300 ohm feeder and connect it to a tuner with a 300 ohm input
    impedance. Ignoring distractions like differences in feeder loss, the
    tuner input voltage will be 2μV.

    However, despite being fed with twice the voltage, the 300 ohm tuner
    won't work any better with the folded dipole than the 75 ohm tuner works
    with the 75 ohm 'normal' dipole. In both cases, the input power is the
    same. Internally, the electronics will be basically the same. The only
    difference will be in the matching circuit between the input and the RF

    So, it is reasonable to conclude that, when you specify the sensitivity
    of a receiver, you have to specify the impedance. In your test, if your
    standard is 300 ohms, then you would say that your tuner was receiving
    22.6μV (7.04dB more than indicated on the generator dial). If it was 75
    ohms, it would be 11.3μV (6dB less).

    Well, I think I'm correct! What do you reckon?
  5. David Farber

    David Farber Guest

    I don't think you need to mention the impedance of the receiver when you
    mention FM sensitivity. If it is has the correct antenna and the correct
    impedance matching circuit, the sensitivity of the circuitry should be
    enough. No? That way I can compare one receiver/tuner to another and not
    care what the input impedance is. It's kind of like stating that the power
    of a speaker is independent of what type of amp it's hooked up to.

    I did a little more digging in the owner's manual. There is an intermediate,
    almost lossless, antenna matching network (which I don't have) that is
    supposed to be hooked up between the tuner and the generator to make things
    go together nicely. There is also a simple schematic diagram of how to
    construct one yourself if you don't have the Sound Technology matching
    device but the drawback is the ~6dB attenuation. So to go from the 50 ohm
    unbalanced output of the generator to the 300 ohm balanced input of the
    tuner would require three resistors connected as follows:

    R1 130 ohms from the center terminal output of the generator to one of the
    300 ohm inputs of the tuner.
    R2 150 ohms from the shield side of the generator to the other 300 ohm input
    of the tuner.
    R3 62 ohms which goes directly across the generator output.

    In this configuration, the readings on the dial are approximately twice the
    actual output. In other words, if the dial is reading 10?V, then there is
    only 5?V going to the tuner.

    Thanks for your reply.
  6. Tim Schwartz

    Tim Schwartz Guest


    Sound Technology offered a 50 to 300 ohm transformer, model 100, that
    allowed you to direct read the dial on the 1000A. there was also one
    for 50 to 75 ohm conversion, but I've never seen one.

    Tim Schwartz
    Bristol Electronics
  7. Ian Jackson

    Ian Jackson Guest

    No. If you have a 75 ohm receiver which requires 'V' volts for a given
    SNR, a 300 ohm receiver would require '2V' volts to give the same SNR.
    If you were comparing the two, you would need to mention what impedance

    For all you know, the 300 ohm receiver is simply the same model as a 75
    ohm version, except that it has an internal 300-to-75 ohm step-down
    transformer. For the same performance, the 300 ohm model would
    definitely need twice the input voltage.

    If you have 75 ohm signal source and a 300 ohm receiver, you can get
    'free' gain by simply using a step-up transformer to give you more
    source voltage. The limit is when the transformer matches the source to
    the load. If you try to us a step-up transformer between a 75 ohm source
    and a 75 ohm receiver, you will actually get LESS into the receiver than
    you would with direct connection.
    Ah, but....
    With audio systems, the output impedance of the source is very low
    compared with the load impedance. The power fed into the speaker only
    really depends on the source voltage and the speaker impedance.
    This will probably OK if the receiver input is completely floating, with
    no reference to ground (including a centre tap on receiver input coil or
    transformer - if it has one).
    I reckon that you are better investing in a simple ferrite-cored
    75-to-300 ohm balun / matching transformer (or make one - dead easy) and
    allow for the small corrections discussed previously. I see that Tim
    Schwartz has suggested 50-to-300 ohms but, to me, 75-to-300 ohms is much
    easier as it is a simple turns ratio of 2:1. If you really want
    50-to-300 ohms, it would be simpler to add a couple of resistors to act
    as a resistive 50-to-75 matching pad. But, in over 40 years in cable TV,
    I never worried too much about mixing 50 and 75 ohm impedances (but only
    where it didn't matter, of course!).
  8. David Farber

    David Farber Guest

    If I'm understanding you correctly, this is analogous to the ratings on a
    volt meter where you would need to know the internal impedance of the meter
    to be able to factor in circuit loading. Yes?

    Thanks for your reply.
  9. Ian Jackson

    Ian Jackson Guest

    Hopefully the opposite (I think). With luck, the impedance of your
    voltmeter will be sufficiently high that has negligible loading effect
    on any circuit you are likely to come across.

    Just to recap, I think that you don't need to worry too much about the
    difference of impedances between the source and the load. Even if it is
    balanced, provided that the input of the receiver is floating, you
    should be able to feed it directly from an unbalanced generator (with or
    without any additional resistors).

    But what you do need to take into account is that, for the same
    performance, a high impedance input receiver expects a high input
    voltage, and a low impedance input receiver expects a low input voltage.
    It doesn't really matter how the voltage gets to the receiver input - as
    long as you know what it is (whether by measurement or by calculation).

    If you are concerned about whether the receiver input is floating, you
    can overcome this with a simple balun. This can be a 1:1 ratio
    transformer so, effectively, the receiver will still be fed from a 50
    ohm source. You simply use a few corrections (as discussed) if the
    receiver input is not 50 ohms. However, as you know that you have a 50
    ohm generator and a 300 ohm receiver, you might as well use a home-made
    or an off-the-shelf matching transformer and balun. You can get these
    with an F-connector for the 75 ohm end and a couple of spade connectors
    for the 300 ohm end (I have a couple myself). I see that Radio Shack
    sell them:


    Although the technical spec is more-or-less non-existent, the insertion
    loss should be about half a dB.
  10. Tuner sensitivity is measured in dBf -- dB with respect to a femtowatt. 2uV
    into 300 ohms is the same power as 1uV into 75 ohms.
  11. Ian Jackson

    Ian Jackson Guest

    Ah! That is what I would have expected.

    The point is that, in order to measure the sensitivity, you don't have
    to feed one from a 75 ohm signal generator, and the other from a 300 ohm
    signal generator. But you do need know what the input voltage really is.

    Of course, for this sort of equipment, you probably can't rely on the
    input impedances being anywhere near the nominal values. What you
    calculate and what you get could be quite different. For greatest
    accuracy, it would be best to use a resistive and/or transformer
    matching circuit.

    For matching between 50 and 75 ohms, a cheap cable TV in-line attenuator
    (the type with F-connectors) suffices. 6dB is enough, and (if my brain
    is working correctly) the attenuation would be 6db - 1.58dB = 4.42dB.
    [The 1.58dB is the voltage rise when going from a 50 ohm source to a 75
    load.] If you use a transformer to get to 300 ohms, you'll need to
    subtract the loss of around 0.5dB, which makes an attenuation of around
    4dB. So you're 6dB up because of the 75-to-300 ohm conversion, and 4dB
    down because of the attenuator pad and transformer loss - making a
    convenient +2dB higher than what the output meter on the signal
    generator indicates.
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