On 27/12/2015 18:46, gareth wrote:
First question is, that when TV tuners are pressed into service, what is
it the comes digitised down the USB cable?

Is it chunks of spectrum downconverted in 6M5Hz chunks to suit the TV
channels, or are they capable of finer tuning than that?

Secondly, do they have a good dynamic range?

Thirdly, when narrow filtering is being synthesized in the PC, is it IF
filtering (good) or AF filtering (tends to ring for 20WPM +) ?
[]

Gareth,

The RTL devices can be tuned fairly precisely IIRC, certainly a few
hundreds of Hz and possibly tens of Hz. They are not limited to TV
channelling.

In normal mode, what comes over USB are digital TV packets, as the RF to
digital decoding is built into the RTL chip set. Amateur use is a special
debugging mode, where 8-bit I and Q packets are sent over the USB instead.

8 bits is the dynamic range, adequate for a few strong broadcast signals,
but not for weak signal work in the presence of large signals.

They are also broadband devices and may need very good front-end
filtering in RF-rich areas. If you need a good dynamic range, consider an
SDRplay, FunCube or AirSpy/SpyVerter instead which digitise to more than 8
bits.

http://www.sdrplay.com/
http://www.funcubedongle.com/
http://airspy.com/

but not the FunCube for LF work (below about 1 MHz) as it gets very
insensitive!

With PC processing, it's your choice of IF or AF filtering. Look at the
specs for the different DSP programs available - HDSDR, SDR#, SDR Console
etc.

The RTL dongles are fun to play with, and a useful introduction to SDR.