Re: LF: Receiver protection

[Andy Talbot <andy.g4jnt@gmail.com>]

The softrock is a stiff FET gate switch. designed for 3.3V digital busses running at hundreds of MHz.    If the input exceeds the 0/3.3V rails, it'll just clamp to them.  The opamps are ... well ... opamps, but they have an input series resistor the keep them safe

You'd have to try pretty hard to destroy a Softrock

Andy

www.g4jnt.com

On 10 July 2018 at 20:47, N1BUG <paul@n1bug.com> wrote:

Hi Andy,

It is the Softrock I am concerned about. I don't know what the
FST3253 can handle but I don't fancy replacing them if something
were to happen.

I tried putting the diodes at the output of the preamp but that
didn't work. After a TX cycle, the preamp had to be powered off and
back on to get things working again. So I reluctantly moved the
diodes to the input where I have had no further trouble.

As you say, +5 dBm input to a 2N5109 is likely to be +25 dBm output
which worries me going into the FST3253.

73,
Paul


On 07/10/2018 03:19 PM, Andy Talbot wrote:
> That preamp looks as if it would laugh at +5dB input and just
> amplify it linearly - giving your Softrocka  run for itsmoney.
>
> A  can see that preamp  easily surviving well over +20dBm input
> leakage.  Its dissipating something like 1 Watt in its own right, so
> you can quite reasoably to expect it to be happy with even that sort
> of input.
>
> So you could even contemplate pairs of series diodes to limit at a
> HIGHER voltage and that way less of a compromise on strong signal
> handling that input diodes always give you.
>
> Mind you , on my 400W 475kHz Class-E amp, I do use a pair of relays,
> terminating Rx in 50 ohms and measure 70dB attenuation.  But I do
> run Rx into an SDR-IQ which is a bit more expensive to replace.  But
> even that can survive over +20dBm.  I know, I've done it!
>
> Andy
> www.g4jnt.com <http://www.g4jnt.com>
>
>
> On 10 July 2018 at 20:03, N1BUG <paul@n1bug.com
> <mailto:paul@n1bug.com>> wrote:
>
>     Hi Stefan,
>
>     > I suggest to discuss along a concrete schematic that you could
>     present
>     > along with the question(s).
>
>     Here is a simplified description of the setup. RX antenna (at the
>     moment, but soon to change or have more than one choice)
>
>     http://www.n1bug.com/lnv.jpg
>
>     Then there is a two way splitter. One output feeds the LF RX system:
>     band pass filter, preamp, band pass filter, receiver. The other
>     output has the same line up but for MF.
>
>     The preamps, with diodes added at the input
>
>     http://www.n1bug.com/w1vd-pre2.png
>     <http://www.n1bug.com/w1vd-pre2.png>
>
>     Finally the receivers (one for LF, one for MF)
>
>     http://www.n1bug.com/srlite2.png <http://www.n1bug.com/srlite2.png>

>
>     These receivers may look too simple but with the filters and preamps
>     have done quite well for me!
>
>     I was hoping for something better than the diodes to put there at
>     the preamp input. Something which would limit to a much lower level.
>
>     For HF, a company called ICE used to make RX protectors (ICE-196)
>     which use some kind of very small transformer on a ferrite core. I
>     think the idea was the core saturates to provide limiting. But there
>     were other parts, including diodes, resistor, capacitor. I have two
>     of these, one is permanently connected to the RX antenna input on my
>     FT-2000 transceiver. Unfortunately these do not work below 1 MHz.
>     The insertion loss becomes very high at MF and LF.
>
>     Probably the best, as many people have said, is to give up this
>     silly idea of limiting at the RX front end and use a fast relay to
>     disconnect RX antenna and put instead 50 ohm resistor on RX input.
>     This we can say is quite safe for the receiver. :-)
>
>     It's just some work. I have a FT-2000 which I use with home built
>     down converters for CW and digital QSOs on MF and LF. I think there
>     is 15 ms between FT-2000 PTT output going low and the beginning of
>     RF output, so with a fast relay it should be OK.
>
>     I also have a Ultimate 3S beacon transmitter. It provides +5V output
>     for external T/R switching, supposedly with a programmable delay. I
>     have it set to 50 ms now. I am sorry to say I do not completely
>     trust this as there are some things which do not work quite as they
>     should with this device. I don't know if the T/R delay is correct
>     and reliable. I will try to measure it.
>
>     The FT-2000 and all LF/MF receiving stuff is in one room, the ham
>     shack. MF and LF down converters, amplifiers, and U3S are in another
>     room, the lab or work shop. There was no more space in the ham
>     shack. ;-) It's really quite silly to call that little room the lab
>     or shop, because there is a ongoing radio construction project in
>     every room of my house! The whole place is a work shop!
>
>     Some time in the future I may be able to use my HP 3325B synthesizer
>     with a linear PA to TX Ebnaut. I don't have any idea how I will
>     provide proper timing to switch the RX input relay with that, but  I
>     will worry about it later.
>
>     For now I think I should just make the decision and do the work to
>     make both the FT-2000 and the U3S (if its T/R delay is reliable) to
>     switch a relay disconnecting the RX antenna during TX. Unless
>     somebody has a better idea about serious limiting at the RX...
>
>     >> I need to stop being lazy. ;-)
>     >>   
>     > Oh oh, i know what you mean. Good point :-) I can observe the
>     same
>     > behaviour here. Sitting in front of the monitor, watching
>     grabbers,
>     > watching the WSPR map, programming some Linux stuff, playing with
>     > SpecLab can be quite comfortable and relaxing. When looking
>     back, 10
>     > years ago i have been more spontaneous in doing portable
>     experiments.
>     > But can this tendency be stopped or is it hopeless? It could
>     be helpful
>     > to analyse the causes instead of 'fighting' against oneselve.
>
>     Sounds like fun! I'm a little bit tired from building. I built all
>     the LF and MF RX stuff including receivers, preamps, many filters,
>     splitters and so on. I built the U3S, TX converters, TX antenna,
>     variometers, transformers. I built a few LF and MF PAs before
>     finding success... and success came after many weeks of tracking
>     down trouble. Now I'm just finishing up the build of a power supply
>     for the new LF PA, also doing a lot of work to make new and better
>     cables to connect all this stuff together. I enjoy building stuff,
>     but I am ready for a little break. All this has been a bit of a rush
>     trying to get it all done as quickly as possible. After I finish the
>     power supply, RX protection, and new RX antenna maybe I can rest and
>     do some operating... ;-)
>
>     73,
>     Paul