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Receiver damage / overload.

Hi,

I'm looking for advice please.

I use my Kiwi SDR with a Wellbrook loop, I'm also a Radio Amateur.

Currently when transmitting (Max 200w), I'm unplugging the power cable from the active loop to protect pre amp circuit but I want to know what tolerence the receiver front end will have to RF Overload and what is recommended if anything to protect it.

Thanks

Glenn M1AQO

Comments

  • I have found the KiwiSDR to be quite durable when it comes to being able to tolerate a nearby transmitter with no damage.

    On my main receive system I have added some external protection - essentially an outboard device that includes both a mediumwave reject filter plus two pairs of series-connected silicon diodes (1N4148 or similar) connected in opposite directions across the RF output (from the filter), giving about 1.2 volts conduction threshold.

    I experience no intermod in doing this - and 1.2 volts of RF at any frequency covered by the Kiwi would surely overload the A/D converter (via the input amplifier) anyway!

    In your case, RF would have to get through your the amplifier of your loop (when it is off!) so any significant amount of RF would have to make its way through a powered-down amplifier in the first place.

    Were I in a similar situation, the situation that would be most likely to cause damage would be a poor connection on the RF input cable - namely an open/loose shield connection - which would allow entry of likely-high circulating currents from a nearby transmitter to enter the receiver.

    73,
    Clint
    KA7OEI
  • By the way, most transceivers have a "Key" output which goes low/high when transmitting.
    It can be used to switch off the loop amplifier or LNAs via a relay during the transmission, to prevent damage from high RF levels.
  • jksjks
    edited April 2020
    Something else worth mentioning: We seen a number of cases where the TVS protection diodes on the Kiwi antenna inputs have gone low resistance (shorted) due to excessive input charge hits. You can tell this is the case by measuring with a DMM across the screw jacks of the green terminal block or between the center pin of the SMA connector and the outer shell. The DMM should read zero resistance.

    This measurement works because the input is capacitively coupled downstream from the diodes. The SMA and green terminal block inputs each have their own TVS close to the connector, and the traces join immediately thereafter (schematic: http://kiwisdr.com/docs/KiwiSDR/kiwi.schematic.pdf)

    If the TVS diodes have gone short just knock them off the board by heating each end repeatedly with a small-tipped soldering iron until they move. The TVS parts are practically microscopic, so instead of trying to replace them you can do this instead: Get a pair of good old axial-lead 1N4148 diodes (or similar) and connect them back-to-back into the jacks of the green terminal block connector. That will conveniently place them across the antenna signal input and ground with no soldering required.
  • isn't there some danger of IMD etc. if you have big signals and use 1N4148 (1N914, 1N4454) etc
  • Yeah could be. Although you see them used like that commonly. They have a Vf of roughly 0.6V which is lower than the TVS (the Kiwi TVS is a CDSOD323-T03C, Vwm 3.3V) So I suppose you could just source an equivalent axial TVS and use that.
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