[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 4

[Mike Feher]

Walt -

How are you measuring IP3 (the 3rd order intercept point)? To the best of my
knowledge it is not a measurable quantity, but is extrapolated from other
measurements? Right now I am still having trouble with your numbers, which
of course does not make them wrong, it is just that I would like to
understand. Thanks & 73 - Mike



Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-901-9193



----- Original Message -----
From: <waltah@earthlink.net>
To: <hbr@mailman.qth.net>
Sent: Saturday, April 12, 2003 9:16 PM
Subject: Re: [HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 4


> Mike Feher said:
>
> > it sure seems to me that a 13 dBm IP3 is pretty darn good. I have yet
> > to think about the adequacy, one way or another about the bottom end.
> > Regardless, 13 dBm as you know is 20 milliwatts, measured in a
> > wideband sense utilizing your techniques, and is essentially
> > equivalent to the same power integrated over the bandwidth outside of
> > your IF, and therefore is a heck of a lot of power.
>
> At the filter driver it's around 15 dbm and 82 db IFDR -- both good
> enough for this type of receiver.   The corresponding noise floor is
> close to -110 dbm -- something around a microvolt at 50 ohms.   So
> in an ideal world I'd get back to the antenna with noise-free and
> perfectly linear stages giving a gain of maybe 10 db, yielding an IP3
> of 5 dbm at the antenna and a noise floor of -120 dbm -- about a
> quarter of a microvolt.   That would be an acceptable performance, on
> a par with the usual benchmark hollow state receivers.
>
> But when I do measurements at earlier stages it turns out that they
> are not noise free and perfectly linear:  IP3 at the 2nd mixer is still 13
> dbm but a noise floor of -84 dbm.   (IFDR 65db).   Going all the way
> back to the antenna, -7.5 dbm IP3 (no longer +) and an IFDR of 69
> db.   The corresponding NF is -111 dbm which is not all that great
> either -- Drake R-4C, -138, R-390, -137.
>
> The second mixer definitely contributes too much noise.   It may
> have other problems but until the noise is dealt with, they don't
> matter very much.   Probably not the mixer's fault -- it's probably
> getting 'garbage in' as noted in my earlier post.
>
> The sharp eyed reader will notice that my numbers are slightly
> inconsistent -- the IFDR can never get better as you go backwards
> through a receiver, but I claim to have measured 65 db at the 2nd
> mixer and 69 db at the antenna.   This is at the outer limit of the
> usual error in my measurements (due to reading the S-meter, ripple
> in the filter passband ...) but might also represent some cancellation
> of distortion.
>
> Basically the filter driver is fine and the filter itself probably okay --
it's
> handling a good fraction of a milliwatt without problems.  (I don't know
> what the max rated power level for those things is, but that certainly
> seems reasonable.)  Improvements in this area are only possible by
> improving the 1st IF stage noise figure and I have decided not to do
> that right now because the mixer stages are so bad.
>
> > An often made oversight is to look at the 1 dB compression point and
> > equate it to the IP3 number.
>
> I don't think I've made that error yet, but I have made several similar
> ones.   I believe I now have the calculation right and the numbers do
> track and behave correctly, for example, the calculated IP3 is
> consistent across a range of measurement power levels and the
> distortion power does behave in a third-order fashion.
>
> More to follow!
>
> Walt Hutchens
> KJ4KV
> ************************************
> Visit the HBR Receiver Web Site with over 100 pictures of receivers and
> construction notes...... via http://www.qsl.net/k5bcq/
>
> Retrieve reflector archived data via
http://mailman.qth.net/mailman/listinfo/hbr
>
>