BasicRx Daugterboard Frequency Wrapping

Hi,
I am hooked up to the BasicRx Daughterboard and wrote a simple
applicate
using the GRC to survey the spectrum from 1MHz to 250MHz and run the
user
tunable output through the WFM Demodulator. I can get FM radio stations
in
the 88-108MHz region just find. However, I am also getting copies of
these
stations at frequencies higher and lower than this range!!! It’s as if
the
frequency I request is out of range and the tuner wraps around. What’s
going on here? When I tune to 50MHz, I expect to be at 50MHz, not some
other frequency!

Also, why are there two Rx outputs on this daugther board? How do they
differ? Maybe my problem is here.

Thanks,
Isaac

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isaacgerg wrote:

Hi,
I am hooked up to the BasicRx Daughterboard and wrote a simple applicate
using the GRC to survey the spectrum from 1MHz to 250MHz and run the user
tunable output through the WFM Demodulator. I can get FM radio stations in
the 88-108MHz region just find. However, I am also getting copies of these
stations at frequencies higher and lower than this range!!! It’s as if the
frequency I request is out of range and the tuner wraps around. What’s
going on here? When I tune to 50MHz, I expect to be at 50MHz, not some
other frequency!

What you’re experiencing is called aliasing. The BasicRx daughterboard
has no low pass filters on it, and the USRP is sampling at
64 M-samples/sec. This allows it to unambiguously sample frequencies
up to 32 MHz. However, for any given frequency f, multiples of f
will produce either an identical set of samples as f, or an inverted
set. This produces the “wrap-around” effect you are seeing.

Check out any text on analog to digital conversion for a more detailed
description of aliasing.

@(^.^)@ Ed

isaacgerg wrote:

Hi,
I am hooked up to the BasicRx Daughterboard and wrote a simple applicate
using the GRC to survey the spectrum from 1MHz to 250MHz and run the user
tunable output through the WFM Demodulator. I can get FM radio stations in
the 88-108MHz region just find. However, I am also getting copies of these
stations at frequencies higher and lower than this range!!! It’s as if the
frequency I request is out of range and the tuner wraps around. What’s
going on here? When I tune to 50MHz, I expect to be at 50MHz, not some
other frequency!

On re-reading my first post, I realized it wasn’t very clear. So,
to be more explicit:

A 0 - 32 MHz signal will produce the expected output.

Signals from 32 - 64 MHz will get mapped onto 32 - 0 MHz
(note the frequency inversion)

64 - 96 MHz will get mapped to 0 - 32 MHz

96 - 128 MHz will get mapped to 32 - 0 MHz

… and so on, up to the limits of the board.

@(^.^)@ Ed

In the short term, how do i fix this problem? Do I just low pass filter
my
incoming signal?

Isaac

Ed Criscuolo-2 wrote:

stations at frequencies higher and lower than this range!!! It’s as if
Signals from 32 - 64 MHz will get mapped onto 32 - 0 MHz
@(^.^)@ Ed


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isaacgerg wrote:

The mapping you ask describing isnt what I am getting. According to your
scheme, with the Basic RX board, I shouldnt be able to get any FM radio
stations, but I am able to get them. Why is this?

The daughterboard software knows about aliasing, and maps accordingly
when you ask for a frequency above 32MHz. The mapping I described
folds all the spectrum into the space from 0 - 32 MHz. As a result,
you will hear the FM broadcast stations at multiple frequency tunings.
As a further note, you’ll notice that one of the “fold” points, 96 MHz,
is right in the middle of the broadcast FM band. So you should be able
to hear the same station at the same offset above and below 96 MHz.

For instance, if there’s a station at 92.1 MHz (1.9 MHz below), you
should hear that same station at 97.9 MHz (1.9 MHz above).

@(^.^)@ Ed

It looks like i need to get an RF front end to pre-filter all that stuff
out
of there…

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On Thu, May 22, 2008 at 09:34:43AM -0700, isaacgerg wrote:

It looks like i need to get an RF front end to pre-filter all that stuff out
of there…

There’s a reason Matt offers all those daughterboards :wink:
You may want to consider the TVRX d’board. It covers 50 MHz to 870 MHz.

http://www.ettus.com/custom.html

Eric

If your main use for this board is going to be broadcast FM, I would
strongly urge you to use a fixed oscillator and a simple mixer to
downconvert the received signal into a range that avoids alias. The
fact
that FM broadcast band is split by a fold when used for direct
conversion
makes it a bit challenging for such a purpose; you’re going to get alias
images no matter how much filtering you do. Since the FM broadcast band
covers 87.9 to 107.9 MHz, you need an oscillator somewhere between 75.9
and
87.9 MHz to ensure that the entire downconverted band falls below the
first
aliasing fold point. You would also want to use a bandpass filter so
you
don’t end up with aircraft transmissions creating alias images.

Or you could just use one of the other daughterboards. :slight_smile:

Kelly

The mapping you ask describing isnt what I am getting. According to
your
scheme, with the Basic RX board, I shouldnt be able to get any FM radio
stations, but I am able to get them. Why is this?

Isaac


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