On Tue, 18 Mar 2014, Geoffrey S. Mendelson wrote:

matt weber wrote:
All that your experience says is that SDR probably has poor front end
selectivity.

Almost none. The dongle was designed to pick up megawatt digital TV stations
with short antennas (about 4-6 inches) antennas. Using it as an SDR is
a happy accident.

To be fair, it's not related to an SDR, but to a specific and cheap
implemtnation of the idea. One can argue that a lot of low end wide band
receivers don't have good front end selectivity, either.

If there's a conversion to a high IF, image rejection can be done with a
low pass filter. That's true with any receiver that converts to a high
enough first IF. I seem to recall from a description that the Racal
receiver that used the Wadley Loop had both a good low pass filter and a
traditional LC front end, and one could switch between the two. The low
pass was good enough for the image rejection, but of course meant all the
signals from DC to 30MHz or wherever the LPF cutoff was seen by the first
tube, so it had to handle all of that. Not necessarily a good thing.

Michael

The ones sold as DVB-T dongles come with software for decoding digital
TV broadcasts, some come with digital audio broadcast decoding software,
most don't. The most popular E4000 receiver dongle did not come with FM
broadcast reception software.

Geoff.

--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379