Frank: One of the reasons for the stage before the QSD in the "current" SDR-1000 is to provide this kind of buffering. Gerald could have gotten a higher IP3 from a good 50 ohm load by raising the operating impedance of the Van Graas/QSD and putting a large transformer in front of it for "gain". His first circuit had the naked front end and it performed poorly at higher frequencies because of aperture jitter, etc. So the overall best approach was to settle for a lower operating impedance for the sampling detector and a preamp added via an add on board. His first choice of preamp device was a mistake and the first ARRL review reveals that clearly. He made other bad decisions when he recommended change some components in the front end termination circuit for the QSD. The end result was that the out of band impedance presented to the front end was a dead short. Once this was repaired by terminating this correctly, a different device was dropped which provide a better gain match for the system and as an added bonus improved the IP3 to nearly 30 dBm IP3 and the IMD dynamic range to 100 dB. This can be seen in the second ARRL review for the modified radio and the "bundled package".
This long winded reply is to make clear that the Van Graas sampler does not perform well if it is presented with an off the wall impedance because the RC time constants in the sampling capacitors assume R=50 Ohms. Gerald's QSD, the Van Graas sampler is <<NOT>> the Tayloe detector. It is a doubly balanced design as opposed to Tayloe's singly balanced design in the NC2030 and in his articles. I wish Gerald had taken full advantage of this by presenting the possibility of driving differential outputs at a minimum. http://www.flex-radio.com/articles_files/SDRFMP4.pdf Page 2 of the article describes the prior art done by Van Graas in QEX in 1990. Bob N4HY Frank Eory wrote: >Hi, >Just wondering if some of you have worked on the stage before the >quadrature sampling downconverter. I tend to use antennae that are a lot >shorter than the proper quarter wavelength length. Since I am not >getting a nice 36 ohm impedance from the antenna, what kind of stage >should I put for good reception? Should I go for an MMIC MAR-X chip, or >just put a few common collector NPNs to get a high impedance input? What >about JFETS like the MPF102? >Frank > > > > > >Yahoo! Groups Links > > > > > > > > > > > -- Laziness is the number one inspiration for ingenuity. Guilty as charged! ------------------------ Yahoo! Groups Sponsor --------------------~--> Get fast access to your favorite Yahoo! Groups. Make Yahoo! your home page http://us.click.yahoo.com/dpRU5A/wUILAA/yQLSAA/ELTolB/TM --------------------------------------------------------------------~-> Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/soft_radio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
