• VCR Triplexer to TV Input

    From G8MNY@21:5/101 to TECH on Fri Aug 2 07:10:07 2019
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    From: G8MNY@GB7CIP.#32.GBR.EURO
    To : TECH@WW

    By G8MNY (Corrected Mar 06)
    (8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

    LA6EAH asked for a circuit to feed 3 RF signals from VCR to a TV..

    With the systems options below if there is an external aerial, then 2 clear RF channels are needed per modulator, as they are normal AM not VSB. Also check on digital TV signals as these often appear as just high noise on your VCR channel!

    If you can check on a spectrum analyser, you will clearly see how good or poor the setup is...
    Strong
    10 mV┤Car Sound Sound .│. Sound
    3 mv┤ │. FM │ .│. │
    1 mV┤ │. │ Nicam Weak Signal Unwanted │ .│. Col│ Unwanted 0.3mV┤ │. Col│║ _______ Sound │/\.. │ ../\│ Sound 0.1mV┤.│ ../\│║ │ COFDM │ 2x │ │ │ 2x
    30uV┤ │ │║ ------│ │------ | Unwanted│ │ |
    10uV│ │ │║ │2nd ord -20dB growth │ | L S B │ │ |
    └─┬──────┬─ ─┬──────┬───────┬──────┬─ ─┬──────┬──────┬──────┬──────┬
    8MHz ch 1ch +2 dirty ch! 2 channels + products
    NORMAL V.S.B. TV C.O.F.D.M. DIGITAL TV D.S.B. (AM) VCR OUTPUT


    It is easy to put the wide VCR output over a wanted channel, or have a digital channel under the chosen VCR channel seen as noise on the VCR output.

    Also amplifying a digital signal before the Rx often leads to corruption of the digital signal, as amplifiers have to be VERY LINEAR to handle the weak digital signals in the presence of stronger analogue signals.


    OPTION 1 The normal way
    ┌──────┐ ┌──────┐ ┌──────┐ ┌────┐
    AERIAL>─┤ VCR1 ├───────────┤ VCR2 ├───────────┤ VCR3 ├───┤ TV │
    └──────┘ Attenuate └──────┘ Attenuate └──────┘ └────┘
    3-6dB 3-6dB

    Because most VCRs have 3-6dB RF gain in their aerial isolating amplifiers, attenuators are often needed, so that the following VCRs are not overloaded with the sometimes stronger modulator outputs. Some older VCRs have local- distant switches to help improve their sensitivity & through linearity.

    The attenuators can just be made up of the thin lossy leads you normally get with the VCRs!

    There are no losses in this system just RF amplifier problems.


    OPTION 2 The abnormal way
    ┌──────┐ ┌────────┐ ┌────┐
    AERIAL>──┤ VCR1 ├──┤ ├───┤ TV │
    └──────┘ │ │ └────┘
    ┌──────┐ │COMBINER│
    │ VCR2 ├──┤ │
    └──────┘ │ │
    ┌──────┐ │ │
    │ VCR2 ├──┤ │
    └──────┘ └────────┘

    The combiner can be any of several types:-

    using 2 x low loss (3.5dB ea) standard splitters, although matching is dubious!

    3.5dB loss (o)________________________
    _______ │ )| FB
    7dB loss (o) │ )| FB 150 )────(o)
    150 )─────(o)(o)_│___)|
    7dB Loss (o)___│___)|


    or a dedicated 3 way resistive splitter eg 4x 39Ω in a star network properly matched to 75Ω.

    (o)-39─┐
    (o)-39─┼─39-(o)
    (o)-39─┘ │
    │_________│ Port to any port 10dB loss


    or slightly less loss with a dedicated matching ferrite bead transformer.

    (o)-47─┐ ┌──────(o
    (o)-47─┤__)| 2t │
    (o)-47─┘ )| 1t │ Port to common port loss 7.5dB (0.5dB in Xformer)
    │_______│______│


    OPTION 3 The switch way
    ┌──────┐
    AERIAL>──┤ VCR1 ├──┐
    └──────┘ │
    ┌──────┐ o ┌────┐
    │ VCR2 ├──o<───┤ TV │
    └──────┘ o └────┘
    ┌──────┐ │
    │ VCR2 ├──┘
    └──────┘
    Good switches at UHF are rare! Most have 1-2dB insertion loss & only 20dB isolation.


    Why Don't U send an interesting bul?

    73 de John G8MNY @ GB7CIP

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