odd results working at 250ksp/s

David Jacobowitz david.jacobowitz at gmail.com
Tue Oct 22 00:28:54 UTC 2013


Hi,

I've been working on a project the involves tuning VOR signals. They are
narrow band signals. > 25 kHz. For that, the wide bandwidth of the RTL
dongles isn't that helpful to me. (and actually a hindrance when two
signals are inside the sample range but widely different strengths)

Anyway, the lowest I can get my dongles (R820T) to go is 250ksps, which is
fine. I've been working at that frequency for awhile, but getting some
strange results. For example, if I tune in to 116.8 MHz, which should be
the OAK VOR, it works fine. But if I tune to 115.8 MHz, which should be the
SFO VOR I get ... the OAK VOR. That is, there is a perfect copy of the OAK
signal 1 MHz shifted down.

If I switch the sample rate to 1.024Msps or 900ksps, I don't get this
problem.

I don't understand what is happening here. 1 MHz is an even multiple of
250kHz, so maybe I'm getting an image of OAK overpowering the relatively
weaker SFO signal. But should there not be filters that manage this?

I guess I was expecting that if the device is set to 250ksps, then it would
"close down" filters appropriately to reject signals out of that band. But
maybe the filters don't work properly below a certain sample rate? Like the
rtl2832 can sample down to 250ksps, but the 820T tuner was not designed for
it?

Or perhaps I'm doing something wrong? Can I control the filters directly?

I'm glad I find this issue because I was going nuts thinking my software
had some mysterious bug. But I can reproduce this issue just with SDR# or
whatever. :-)


Regards.
Dave J
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