Topband: Norton preamp

[Ford Peterson]

Tom wrote:

...snip...

I find the very high TOI  of 40-50dBm useful in my application but the rest
just makes for meaningless conversation.  If you are in a noise limited
urban environment even a 20dBm TOI might be good enough, with proper input
filtering. Second order intercept, once a reasonable value, is pretty much
meaningless beyond that point.

73 Tom 

*******************

Tom,

Thanks for clarifying my scrambled brain.  When I saw the 10MegaWatt rating
on a preamp design, I knew that reality must somehow lie within reach of the
common man.

Just to be clear, when I evaluate a given preamp using my spectrum analyzer
instead of a slide rule, I can quickly identify the gain function of the
amp.  For discussion, let's say that gain is 12dB.  As I pump more RF into
the amp, I can see that the output is always 12dB higher, until I reach the
1dB compression point.  This is the point at which I don't get 12dB, I only
get 11dB.  Simultaneously, I can visually see distortion products (harmonics
and intermod) developing.  As I push even more RF into the front end, I can
see the IMD develop to the point where any additional RF in produces no
additional desired output, and serves to only develop yet more IMD.  This is
the point I have always concluded to be the TOI of the amp.

In practice, the preamp must be able to develop it's gain in an absence of
these IMD products.  So if my strongest signal is a nearby MW transmitter
that develops -20dBm at my antenna, and the gain of the amp is 12dB, then an
amp with a 1dB compression point higher than -8dBm should be OK.  Correct?

Likewise, if my own station is the source of the strongest signal (e.g. SO2R
ops), and that adjacent band signal can develop +15dBm at the receive port,
then my preamp must be able to produce +27 dBm (i.e. 15dBm + 12dB = 0.5
watts) without distortion.  Correct?  And my receiver must be capable of
handling 0.5 watts at it's input port.  Correct?  So getting too wild with
beefing up the TOI on a preamp may serve to shorten the life of my precious
radio.  Correct?  So if your preamps have a TOI in the neighborhood of
+43dBm, how do you protect your radio from peril?  I would think most
receiver front-ends will fuse well before +20dBm (e.g. 100 milliwatts).

Ford-N0FP
ford at cmgate.com