Hello Andre, I believe the SDR bandwidth is dependent on antenna impedance. Also, depending on how you have arranged your op-amps, the gain may be dependent on antenna impedance as well. You can remove the gain vs. antenna effect by using differential op-amps (also known as instrumentation amplifiers).
All this is explained in one of the four Youngblood papers on SDR: Youngblood's "SDR for the Masses" 4-part QEX article. http://www.arrl.org/tis/info/sdr.html Of course, using a pre-amplifier in-front of the QSD removes these affects. You can also try an antenna impedance matching circuit. I'm not sure whether a high-impedance input pre-amp would work, at least not without an isolator or circulator to mitigate mismatch effects. Speaking of which... There is an interesting type of op-amp isolator or circulator circuit that was originally realized by Wenzel. You might try putting one of these Wenzel circuits in-front of the QSD, I don't know anybody that has tried it yet; be sure to select low-noise amplifiers. The Wenzel circuit uses voltage amps, current amps typically go higher in frequency. But these days finding suitable voltage op-amps that can cover 1-30MHz should be easy. Do a Google search for "isolator circulator Wenzel" I don't have the link at-hand. What ever you do, don't forget to take into account your input band-pass or low pass filtering. Good Luck, David --- In [email protected], FRANCIS CARCIA <[EMAIL PROTECTED]> wrote: > > a 6 dB resistive pad works for me connecting to the IF of my RX > > Andre Belokon <[EMAIL PROTECTED]> wrote: Hello > > I recently studied schematic of SDRZero > http://py2wm.qsl.br/SDR/SDRZero-2.html. this sdr receiver include rf > amplifier (MMIC variation) between bandpass filter and QSD. author > wrote: "An RF amplifier follows, the main reason to use one here is to > present resistive impedance to the QSD (quadrature sampling detector)." > As I can understand connecting QSD to source with reactive impedance > will produce phase/gain imbalance. is this correct? someone research this? > > Regards, Andre >
