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J M Noeding wrote:
On Wed, 21 Jul 2004 14:08:50 -0700, Jim Thompson wrote: On Wed, 21 Jul 2004 12:52:22 -0700, Jim Thompson wrote: 36 to 63V P-P into 50 ohms ain't easy ;-) True. Most systems will extrapolate out to IP3 pretty well, so take a series of data points at lower power levels. ...Jim Thompson Another poster has the same opinion, too then, back to original question: It seems to be some problem to build equipment to measure over +35...40dBm IP3 (at receiver input), do you know how it could be made, or is it some references which might work? 7025 and 7050kHz tone frequencies would do jm --- J. M. Noeding, LA8AK, N-4623 Kristiansand http://home.online.no/~la8ak/c.htm Well, let's see. You want the 3rd-order product to be above the noise floor of the receiver. Assuming a 10dB noise figure (you _will_ do better, yes?) and a 5kHz wide filter a -120dBm signal will be just discernible -- say you want it to be at least -100dBm. With a projected intercept of 40dBm that gives you a 140dB difference; dividing that by 3 gives around 47dB down from 40dBm, which means that you need to feed about 7dBm into the receiver from each source (this is sounding pretty absurd, I should just stop now). Oh well, perseve I think the issue is that you need to have some very stout sources, and a damn good hybrid combiner. Not only do you need to make sure that each source's output won't bleed into the output stage of the other source through the radio connection and intermodulate there, you also need to make sure that note of the energy from source A gets into source B and visa versa. The "amateur" approach would be to make two sources each capable of generating well over 7dBm (1W?) so that you can protect their outputs with attenuators. If I were doing this as a garage effort I'd make two identical crystal controlled sources, well shielded and operated from batteries (rechargables, if I'm going to be producing a frigging watt each). I have no idea what to tell you about the hybrid combiner, other than you should pay attention to 3IM in the core, and test it. Assuming that you have two good sources and attenuators, and a really stout hybrid combiner, you should be able to get clean, high-level RF into your receiver. At this point I'd take your RF (at what, 10 or 20dBm?), attenuate the heck out of it, and check the 3IM products. If you get a 3IM product that tracks the attenuation linearly then your sources or your combiner are at fault. If they track 3x attenuation then congratulations -- you're actually measuring your receiver! Good luck. Maybe I'll stick to NE602's -- the measurements are easier. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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