Hi all,

I'm interested in finding out what the state of the art is in low
distortion preamps for HF receiving systems. Anyone out there have
pointers to reasonable designs for a preamp with input IP3 above, say,
+55dBm, with amplifier power requirements limited to half a watt, and a
noise figure better than 10dB? IP2 should also be high--greater than
+100dBm--though it may be acceptable to have a lower IP2 as frequency
increases. Gain around +10dB would be good.

Cheers,
Tom

Hi all,

I'm interested in finding out what the state of the art is in low
distortion preamps for HF receiving systems. Anyone out there have
pointers to reasonable designs for a preamp with input IP3 above, say,
+55dBm, with amplifier power requirements limited to half a watt, and a
noise figure better than 10dB? IP2 should also be high--greater than
+100dBm--though it may be acceptable to have a lower IP2 as frequency
increases.

I certainly hope you're going to be willing to accept an
estimated/extrapolated rating for that last figure, and don't require
an actual performance test!

:-)

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
Hi all,

I'm interested in finding out what the state of the art is in low
distortion preamps for HF receiving systems. Anyone out there have
pointers to reasonable designs for a preamp with input IP3 above, say,
+55dBm, with amplifier power requirements limited to half a watt, and a
noise figure better than 10dB? IP2 should also be high--greater than
+100dBm--though it may be acceptable to have a lower IP2 as frequency
increases.

I certainly hope you're going to be willing to accept an
estimated/extrapolated rating for that last figure, and don't require
an actual performance test!

Somehow this .sig file seems doubly appropriate he

.... from S-30 to ...
Now _there's_ a signal report you don't often enter in your log...
True... most of us are too busy running away from the electric arcs
and dodging the ball lightning :-) -- Dave Platt, in r.r.a.h

--
Mike Andrews, W5EGO

Tired old sysadmin

:-) Yes, I would expect the IP3 and IP2 to be measured with signals in
the -10dBm to 0dBm range. AFAIK, testing IP3 and IP2 is pretty much
always done at levels way below the intercept points. It would be a
pretty non-linear amplifier that had IP3 and IP2 below its maximum
output before clipping. With the performance I'm looking for, it means
detecting distortion products down more than 100dB from the inputs, but
that's something I'm up for. I do need to work on a little better
combiner for my sources, though, so they don't intermodulate before
going into the D.U.T.. I can get fairly close to the sort of
performance I asked about with some modern op amps, but not across the
full HF band.

Cheers,
Tom

With the performance I'm looking for, it means
detecting distortion products down more than 100dB from the inputs, but
that's something I'm up for. I do need to work on a little better
combiner for my sources, though, so they don't intermodulate before
going into the D.U.T.. I can get fairly close to the sort of
performance I asked about with some modern op amps, but not across the
full HF band.

Cheers,
Tom

Hi Tom. Look for Anzac combiners, we used them to measure +55 dBm Ip3 on the
VMP-4 amplifiers and didn't get measureable IMD from them at +6 dBm in.

John Thorpe (AOR 7030 fame) assures us that the simple MCL PSC 2-1 is
blameless to 0 dBm.

http://www.aoruk.com/comments.htm

I had been having what appeared to be hybrid IMD using some Merrimac
combiners, which totally cleared up by using DDS sources instead of low
noise HP 8640's modified to remove the levelling crosstalk. The DDS's do NOT
talk to each other and less attenuation for isolation is required.

Gimmee a hint on your "modern" op-amps. Split PSU's I presume?

Regards
W4ZCB

Do you realize that 100dBm means o power output capability 10 10
MegaWatts? Even backing up 20 dB for improved linearity, not even the
Voice of Ameirca will be able to compete with your LNA!
I suggest you reformulate the question. then we'll be able to give you
some answers.
Saandy 4Z5KS

Hi Saandy,

"+100dBm second order intercept" means that second order distortion
products, following a simple small-signal second order model, would
become equal to the input if extrapolated up to +100dBm. Second order
products increase in dB amplitude exactly twice as fast as the
fundamental, so that means that, say, a -10dBm input would produce an
input-referred second harmonic of -120dBm, or -110dBc. If you decrease
the input to -20dBm, the second harmonic drops to -140dBm, or -120dBc.
If you increase the input to 0dBm, the second harmonic increases to
-100dBm, which is also -100dBc. Extrapolating those numbers, you get a
+100dBm second order intercept. In no way does it imply that the
preamp (not all that low noise at a 10dB noise figure) can put out
+100dBm, or can handle even +10dBm input without clipping or serious
compression (though it would be nice in this case to get to close to
that input level before clipping). Of course, distortion rises very
quickly as you go into compression/clipping.

Similarly, a +55dBm third order intercept implies third harmonic at
-110dBm for a 0dBm signal. In other words, I'm looking for an
amplifier that has about equally good second and third order distortion
products for inputs near full scale. I don't want super-good TOI
without also having second order products low.

Cheers,
Tom

Since I really need wider bandwidth than the super-low-distortion ones
will give me, I'm not remembering those off the top of my head.
Well-at AUDIO, you can get distortion down in the -140dBc range for
outputs at a couple volts RMS with either the OPA627/OPA637 or the
AD797 at low gains, and the '797 also has about a 4dB noise figure for
a 50 ohm source. But for wider-band RF stuff, the OPA847 is one
candidate, if used carefully. It's not quite as good as I'd like to
see, but close.

Cheers,
Tom