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 <[email protected]
> <mailto:[email protected]>> 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
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