On Jan 6, 9:47*am, "Joel Koltner" wrote:
"K7ITM" wrote in message

...
On Jan 5, 3:10 pm, "Joel Koltner" wrote:

What do you do about the plethora of short wave broadcast
signals, several of which can each be up to perhaps 0dBm out of your
antenna, or the fellow just down the street (or on the same ship,
etc.) who keys up a transmitter and feeds +20dBm to your receiver --
WHILE you want to keep listening to the signal that's only -110dBm at
your receiver?

At work one of the products we sell to the military consists of a handful of
electronically adjustable notch filters for precisely this purpose -- the
output is fed to (someone else's) SDR.

...although our dynamic range isn't the 130dB+ that you'd need for your later
example... (at least in some reasonable bandwidth...)

Your almost-all-digital HF receiver there sounds quite impressive. *Do you
give the user the option to adjust the switcher's frequency away from 600kHz
if they happen to really want the best sensitivity right there? *(...this
seems to be the common approach with many a ham HF rig...)

---Joel

Hi Joel,

I'm curious about the tunable notch filters. Is there a data sheet I
can find somewhere?

There's really no need to get the ~600kHz any lower. Any decent
antenna at that frequency will pick up way more atmospheric noise than
the level of the switcher residual. After all, -144dBm is only about
14 nanovolts RMS at 50 ohms. I think it's fair to say that any of our
customers looking for little signals will be using good antennas for
the job. Admittedly, a stock unit won't do that good, but it will be
in the neighborhood. Of course, the high noise level at ~1MHz is why
we get by with such crumby antennas for our portable and car AM
radios.

Cheers,
Tom