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RF preamplifier bypass using spdt switch

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neazoi

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Hi I have this preamplifier **broken link removed** and I would like to remove one of the contacts of the DPDT bypass switch (relay) and use a single SPDT switch.
Obviously, either the input or the output of the preamp must be shorted. But which one should I better short, and will it have a significant effect at the output signal power and impedance (50R now).
 

BradtheRad

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One factor is whether switching causes a loud click. It is especially likely when you connect/ disconnect/ ground/ short an input. It can send a pulse through the amplifier to the output. You might consider adding a component to soften the pulse (such as a capacitor, or series resistor/capacitor, etc.).

On the other hand, suppose you connect/ disconnect/ ground/ short the output. Then will undesired current flow somewhere? Maybe if it's a small signal amp, then it won't hurt.
 

FvM

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But which one should I better short, and will it have a significant effect at the output signal power and impedance (50R now).
In a brief, both ideas are equally bad.
 

neazoi

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One factor is whether switching causes a loud click. It is especially likely when you connect/ disconnect/ ground/ short an input. It can send a pulse through the amplifier to the output. You might consider adding a component to soften the pulse (such as a capacitor, or series resistor/capacitor, etc.).

On the other hand, suppose you connect/ disconnect/ ground/ short the output. Then will undesired current flow somewhere? Maybe if it's a small signal amp, then it won't hurt.

Hm... that is a point indeed. I think the use of the relay will minimize this click, but only because the contact will be done more instantly.
My intention is to switch off power to the preamplifier and bypass it. Maybe I should FIRST switch it off and then bypass it (maube using a capacitor accross the relay contacts for delay), will help eliminating this "click".

- - - Updated - - -

Why do you want a 2P2T relay to a 1P2T?
It is only because I would like to make it with less components, if this is possible. Instead of a relay I would use a 2P2T switch, one end would switch of the preamp power whereas the other end would bypass the signal.
 

neazoi

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Because of the diodes protecting the I/O of the preamp, putting too much RF across these will cause them to either over heat and burn out, or limit your RF out with a diabolical specteral output.
Frank

That is at an instant, when the switch changes stage only. I do not think the input or the output signal level will be ever be more than 0.6v to make the diodes clip. I may be wrong though.
 

Borber

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Why do you want to destroy this preamplifier with automatic switchover from RX to TX?
 

neazoi

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Why do you want to destroy this preamplifier with automatic switchover from RX to TX?


You mean what I did in the original schematic, or what I have asked to do in this thread?

If you mean the original schematic, I wanted the preamplifier to be able to be bypassed by just detecting the RF input to it (from a TX source) no additional control cables. This is mostly the case when the preamplifier is located near the roof antenna to minimize losses. If the preamp is to be used in a transceiver, then you need to bypass it during TX. How else would you do it without an additional control pin and without passing any DC to the RF cable??
 

neazoi

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To ensure that the diodes do not conduct you must have less then .7V peak RF = .5V RMS, so Pout = .5 X .5 /50 ~5mW.
Frank

Do you think it is possible for a broadband RF preamplifier to output more than 5mW? This is the total number of signals added because the preamp is broadband.
 

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