On Sat, Jan 1, 2011 at 9:12 AM, ikjtel
<address@hidden> wrote:
> Anybody else had any success in tapping the IQ in a receiver
> and feeding that into GNURadio?
Yes - haven't gotten into the 9-10 MHz range IF's, but I have built converters for 455 KHz IF's - overall they work quite well although there are some slight compromises in the overall system; for example I've found that the AGC action in the host receiver may interfere somewhat with some types of signals although it's not usually a severe problem.
After building several I/Q converters it was discovered that (for our application) it wasn't necessary to perform quadrature sampling in hardware - that is, it was sufficient simply to downconvert the signal to the center of the soundcard range and sample that conventionally using a single soundcard input channel. The signal is then "converted" to the complex format entirely in software (setting the real components to the input values and stuffing zeros into the imaginary (Q) components). This has the advantages of simplifying the hardware and of freeing up one of the two soundcard input channels.
It would no doubt be straightforward to extend these ideas to the 9-10 MHz IF's. One thing though is that there's perhaps 50-60 dB more signal available to work with after the 455 KHz IF amp chain has done its work. So if you tap into the 9-10 MHz IF you may be looking at building up a relatively significant amount of RF gain yourself...
Here are links to a couple of my pages containing schematic and other diagrams, and application notes:
http://www.lightlink.com/mhp/lsm/
http://www.lightlink.com/mhp/iq/
Good luck
Max
Max, Marcus, thank you very much! A lot of this is over my head right now but I'm reading up and hope to put to work some or all of your suggestions. I especially like the prospect of having a cheap but effective USB SDR. Although if I had a USB SDR I probably wouldn't even need to perform surgery on my HF allmode rig.
Also, a little more reading revealed that my rig has a third IF that operates at 455Khz and is utilized in FM and WFM modes, which may be just what I need, and what you were describing, Max.
BH