Hi Alfred, > CW we'll get to in a moment, but I don't consider AM or FM to be weak > signal modes. Agree:-) Nevertheless I found it very useful to use high power and enhanced detect methods in VHF contesting. With a kW transmitter I could work dozens of small stations in Helsinki from my QTH in Stockholm. With a standard FM radio it would not have been possible...
AM is another thing, but there are MW DXers who want to copy weak signals... > They certainly present challenges for detection in the > presence of noise, but even though I use both of them I really don't > care too much. They are just for local chatting. But others may want to push the limits:-) > >> A way of approaching it is to match the detection method with the known > >> characteristics of the signal. For CW that means selecting the proper > >> bandwidth, and the PSDR does a really good job with it's narrow filters. > > ?????????????? > OK. I've snipped a bunch of your following comments as basically I am > aggressively agreeing with you. > PSDR has some very good filter topologies, that run rings around my > conventional radios. Is it the the best possible? No, of course not, but > it is very good. "of course not" ???????? Surely PSDR could give you the best possible, but of course that would not be commercially significant for them. I do think, however that allowing a much larger DSP buffer would be trivial and I think it would work without problems on modern computers. The delay would become large of course, but that would not be any problem at all in weak signal VHF communication. It would be enough to allow a 8 times bigger buffer (32768) > I don't have Linrad on my F3K or SDR1K boxes but I have > experimented with Winrad and have found exactly what you state. > Everything here is locked to my Z3801 GPSDO, so I am within a fraction > of a Hz at 10Gigs. Fine! Working stations with the same stability should be fairly uncomplicated. The challenge is to work "small stations" however. They tend to have less good frequency stability.... > Piping the audio from PSDR via VAC to Winrad I can > get a significant improvement in CW copyability, at the expense of a > significant delay. If I had control over the sampling rates it would be > even better. PSDR, and the whole FLEX system, is intended for a mass > audience. The thing could be modified to get much better performance YES:-) > but would there be a market for it? I think NO:-( You can use Linrad with your Flex hardware. It would give you a CW performance close to the theoretical limit. > > This filter will do an excellent job on crowded HF bands > > but it will not improve the detection of really weak CW > > signals in a background of white noise for the simple reason > > that the complete signal path also involves the human > > brain that has a filtering that is similar or even a bit > > narrower (for a well trained weak signal operator.) > I agree, but cutting out the superfluous hiss can be good to reduce > fatigue. Absolutely - but we can do much better!! > 48 hours of listening to noise puts me into an odd mindset. > (cf: William Hurt/ Altered States). > The usual HF operator does not understand the VHF weak-signal attraction. True. In Linrad you have an option to add a dynamic range expander. It does not improve the CW detection, but it is very helpful in reducing fatigue. It made it possible for me to work full moon passes at optimum sensitivity. Without it I could not work more than 15 minutes before loosing my sensitivity. > > Such a narrow filter must be exactly centered on the carrier > > and therefore an AFC (automatic frequency control) is needed. > You have to find the signal first, before AFC can be used. That is trivial. Signals appear on the waterfall at levels well below what can be decoded. (PSDR could be greatly improved in this respect.) > > On HF where the frequency response of antennas and of the > > interference source become important Linrad is not so good. > > > But that is not it's intended application. The HF crud is very different > than that we find on the VHF+ bands. Hmmm, "Wait half an hour for better conditions." Maybe good on HF, bu ton VHF it is surely not a good idea. > Unfortunately, it seems to be > creeping upwards. There will come a time that we have to move into the > countryside to be able to hear anything. Well, I hope you are wrong here:-) Pretty soon we will have two channel SDRs. Flex already has it, but from my horizon it is not easy enough to support. Linrad was originally developed for two RF channels. With a two channel hardware one can null out any interference source completely. With several interference sources it becomes more copmplicated, but new things will be possible:-) I think that 10 years from now we will have low cost hardwares that allow us to use e.g. 12 ferrite rods to give our receiver the x, y and z components of the magnetic field at 4 locations around our QTH. (separated by 0.25 wavelengths or more.) We could use all the information to get rid of interference and to get a substantial antenna gain. 73 Leif / SM56BSZ _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
