Hi Ed, There is more to this story. The FLEX-6000 series radios have two modes of TR switching: QSK and non-QSK. For QSK, we use reed relays for switching the RX paths to each SCU and a PIN diode to switch the TX path. This eliminates distortion on the RX path and improves isolation. In non-QSK modes such as SSB and AM, we use traditional relay switching on the TX path. This eliminates any IMD that would be caused by a PIN diode in the TX path on the linear modes where it counts. On CW, you don't really care about IMD since it is a single tone.
The SCUs are capable of duplex operation so you can receive while transmitting it you provide appropriate antenna isolation. Gerald Gerald Youngblood, K5SDR President and CEO FlexRadio Systems(TM) Email: [email protected] Web: www.flexradio.com <http://www.flex-radio.com/> Tune In Excitement (TM) PowerSDR(TM) is a trademark of FlexRadio Systems On Mon, May 21, 2012 at 10:35 AM, Edward H Russell <[email protected]> wrote: > Hi Ray, > > Thanks for the information, which is of course very general. I'm meaning > specifically what sort of practical impact does the use of PIN diode > switching have on receive and/or transmit performance, especially when the > rig is used for CW or FSK operation. > > Besides possible (theoretical?) introduction of IMD and noise as a > downside, > there are practical considerations that come into play for QSK switching, > especially at medium to high speeds. Two of these are switching time and > noise (relay chatter). > > I'm not sure the IMD or noise generated by the diodes would be significant, > or even measurable, in an operational context. But if they were, that small > effect would have to be balanced against the improved and quieter operation > of diode switching. > > Also, there is another approach that might achieve the same result. If the > radio operation is full duplex on separate antennas, it might not be > necessary to actuate the relay in order to switch to transmit. Possibly T/R > switching in full duplex could be a software operation. In that case, the > PIN diode T/R switching could be done externally, as it is now in my power > amplifier. > > Thanks & 73, > Ed W2RF > > -----Original Message----- > From: Ray, K9DUR [mailto:[email protected]] > Sent: Monday, May 21, 2012 11:18 AM > To: 'Edward H Russell'; 'Tim Ellison' > Cc: [email protected] > Subject: RE: [FlexEdge] 6000 series T/R switching > > Ed, > > You wrote, "Where can I get more information about what sort of IMD is > caused by PIN diodes?" > > Any diode is an inherently non-linear device. By definition, non-linear > devices generate distortion. > > Even when biased into the "linear" region of the conducting state, they are > still not perfectly linear. The farther the deviation of the current from > the bias point, the more distortion is introduced. > > A typical voltage vs current curve for a given device should be available > from the device manufacturer. > > I heard it said that another reason that relays were chosen over > solid-state > switching was to avoid the noise introduced by solid-state devices. > > 73, Ray, K9DUR > http://k9dur.info > > > > _______________________________________________ > Flexedge mailing list > [email protected] > http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz > This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is > used for posting topics related to SDR software development and > experimentalist who are using beta versions of the software. > _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
