Gee guys, this already exists. The LP-100 microprocessor wattmeter (http://telepostinc.com/ )now has the capability to produce a Smith Chart. Couple it with the SDR-1K as the source, and you have a very powerful combination. The usual disclaimers apply. I am only a very satisfied owner of an LP-100, along with a couple of the SDR-1K's. Ain't technology wonderful? ;o)
73, Army -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Lux Sent: Thursday, June 28, 2007 9:26 AM To: Gerald Youngblood; [EMAIL PROTECTED]; 'Dudley Hurry'; 'elan paim'; [email protected] Subject: Re: [Flexradio] antenna Smith Chart can this be ad to program ? At 05:00 PM 6/27/2007, Gerald Youngblood wrote: >It should be possible to use the FLEX-5000 for impedance measurement since >it is full duplex capable. It is not possible to do this with the SDR-1000 >because of its half duplex architecture. On the 5000, we should be able to >use the XVTX as the signal source (~0 dBm max) and XVRX as the detector. >All that is needed is a directional coupler and the software to do the >analysis. Effectively it would be a two port vector network analyzer (VNA). >This is something we would like to do as time permits. > This is moderately straightforward.. after all, all a VNA really consists of is a clean source and a calibrated measuring receiver with the ability to measure phase. And, in fact, I've done basically this between two SDR1000s (i.e. use one as the source and another as the receiver). A couple gotchas: 1) While the calibration math is fairly straightforward, it's non trivial to implement. You have to take (uncertain) measurements of your cal standards and then turn those into calibration parameters. 2) A "real VNA" has well defined measurement uncertainties. This is what separates the MFJ antenna analyzer from a 1 port reflectometer. If you read a particular number off the MFJ, you don't really know what the tolerance on the measurement is (is it good to 1%, 5%, 20%?). With a VNA, you do. In both cases, the measurement uncertainty varies with what you're measuring. 3) Ideally, you'd want the transmit frequency offset from the receive frequency, so you're not fighting the DC offset, LF sound card idiosyncracies, and LF noise in the audio. But then, you need to figure out how to measure phase. 4) One has to be careful about spurs, unless you're happy with 20-30dB of dynamic range. Consider measuring the stop band attenuation of a filter with 60 dB of rejection, when you have a -40dB spur. 5) Your calibration process can't just look at S21. You also have to look at the effects of the source's S11 and the receiver's S22, which is tricky with a single in and single out system. (However, there's a fair amount of literature on calibration using only "load" and "thru" ). Example.. Say you have a device which reflects 1/3 of the power, passes 1/3 of the power, and absorbs 1/3 of the power. A single S21 (thru) measurement can only measure one of those. And, if the reflection is due to a mismatch on input and/or output, you need to deembed the source and receiver contributions.. (unless you don't care about accuracy.. then, you could probably just rely on a brute force "pad the heck out of it" approach to control the port impedances.. but then, you have to calibrate the pad) 6) As with anything else the hard part isn't necessarily making the measurement and calibrating it.. it's providing a useful user interface. You can control a SDR1000 with a command line interface, but it's not real pleasant or convenient. The original post wanted a Smith Chart and sooner or later you're going to want things like log mag, phase, etc. What would be a boon is if there were some "gnuplot"-ish package out there oriented towards display of RF data. Feed it a S2P file or similar, and it plots, scales, etc. 7) Integration (or not) with PowerSDR... When I was doing my network analyzer stuff with SDR1000s, I didn't use PowerSDR (or its progenitor) because the kinds of things you want to do for VNAing are different than for radioing. Given all the math you want to do for calibration, etc., my own preference is that you have a simple hardware control application that makes the raw measurements and another application that does the calculations and display. Write the latter one in Matlab or Octave, which gives you the arrays of complex numbers, statistical functions, and plotting stuff you need. That all said, I'd be interested in working on such a thing. James Lux, P.E. Spacecraft Radio Frequency Subsystems Group Flight Communications Systems Section Jet Propulsion Laboratory, Mail Stop 161-213 4800 Oak Grove Drive Pasadena CA 91109 tel: (818)354-2075 fax: (818)393-6875 _______________________________________________ FlexRadio mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Knowledge Base: http://kb.flex-radio.com/ FlexRadio Homepage: http://www.flex-radio.com/ No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.9.10/875 - Release Date: 6/27/2007 9:08 PM No virus found in this outgoing message. Checked by AVG Free Edition. 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