Taking Off The Training Wheels

Discussion in 'Homebrew and Kit Projects' started by VK4FFAB, Apr 18, 2017.

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  1. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ^Actually, for RF switching you'd want the slowest possible diodes having the longest possible minority carrier lifetime.

    A "fast switching" diode is terrible as an RF switch -- it will rectify the signal and produce DC to the load. Not something you'd want to do.:p
    AF6LJ and N2EY like this.
  2. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    I know there are ways around that, but you are correct.
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Here's the PIN kW-level RF T/R switch I built for my 4-1000A amp back in 1992.

    It still works and has never required any kind of service. It will outlive me.

    Attached Files:

    VK4FFAB and N2EY like this.
  4. VK4FFAB

    VK4FFAB Ham Member QRZ Page

    Ok, will this work in shunting the induced voltage on the RX Pole of the relay to ground? I was going to put a shunt switch somewhere before the first mixer, but for various reasons, it would be simpler if i hung it off the side of the relay.

    When D1 is biased On and the relay in the TX position, it should essentially short the RX pole to ground. Will it work, Good/Bad/Stupid?
    AF6LJ likes this.
  5. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Good start.
    One thing you need to think about is a means to reverse bias the diode during receive, otherwise the sum total of all the RF on your antenna could (very likely will) be enough to overcome the barrier voltage of the diode you chose to use. When that happens that diode will act like a mixer and you will see a rise in noise and some mixing products in your receiver.

    You can provide an RF path to ground on the cathode end of the diode.
    Use a series resistor something like 1K and feed your reverse bias voltage in that direction.

    You could put another 1K from the cathode to ground.
    This gives the back bias something other than a diode and a bypass cap to work against.
    VK4FFAB likes this.
  6. VK4FFAB

    VK4FFAB Ham Member QRZ Page

    I think i follow what you are saying Sue, something more like this then.
  7. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Got it...
    Now breadbard it and play with the parts values. to get them right.

    You will have the bias terminal that feeds the anode very near or grounded during RX...
    and the reverse bias will have VCC on it...
    TX the boas terminal will have bias and the reverse bias end will be at or very near ground.
    VK4FFAB likes this.
  8. VK4FFAB

    VK4FFAB Ham Member QRZ Page

    Thanks a ton Sue, will build it now on some scrap board and see how I get on.
    AF6LJ likes this.
  9. VK4FFAB

    VK4FFAB Ham Member QRZ Page

    While i am poking your brains Sue, what is the function of L3 in this circuit. To provide a path to ground for DC?

  10. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Yes that the purpose of L-3
    The limiting factor in that circuit is the maximum RF voltage the circuit can switch between the antenna port is a few tenths of a volt.

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