All, I have just been reading a review on the SDR-1000 by Peter Hart, G3SJX, in the June 2006 issue of RadCom.
He is generally upbeat, but he has a concern about image rejection. He is talking about v1.6.0 of the software, so maybe things have improved since then. He says: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A potential weakness of the SDR-1000 architecture is that there is a receive image 44kHz above the tune frequency. The only protection against spurious responses is the balance achieved in the image rejection mixer. This really needs to be better than about 70dB, or higher than 90dB for a top flight receiver. As received, without further calibration, the SDR-1000 [under review] achieved about 60dB image rejection at 1.8MHz, reducing to around 40dB at 30MHz and 32dB at 50MHz. By tuning in a strong signal on the image frequency, or more conveniently by using a signal generator, the image signal can be nulled down to better than 80dB. However, the nulling does not hold across different bands or for substantial changes in frequency. Nulling to a depth of 80dB on 14.2MHz was reduced to 65dB at 14.0 and 14.4MHz and about 50dB at 1.8 and 30MHz. Image nulling can be performed automatically but it is fairly slow and not that accurate. Manual null adjustment is usually quicker and more accurate. There is room for improvement of the nulling algorithm and possibly also a means of storing null settings for different frequencies and bands in a look-up table for automatic recall. Similarly on transmit there is also an image. This can be separately nulled but a second receiver or preferably a spectrum analyser is needed. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I don't have any SDR hardware (yet), so I'm not able to see these defects. However, it does raise some questions in my mind: 1. Is it really a matter of having to automatically/manually null the image every time you change band or move through a large frequency increment? 2. Do you really need a second receiver/spectrum analyser to make sure you're not transmitting an unacceptably high-level image? Do you have to keep checking every time you change transmit frequency? 3. Is there any plan to have a frequency-based look-up table for optimum rejection, both on receive and transmit? (Similar, I guess, to automatic ATU tuning). 73 Ian, G3NRW _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
