Problem of using UHD blocks as Tx & Rx

Hi everyone,

I made a test for UHD blocks today, and I found a problem as follows:

I take two USRP2 boards for my test. One is used as transmiter and the
other
board is set to be receiver.And I use UHD Simple Sink and UHD Simple
Source
blocks for transmiting and receiving respectively.

sampling frequency = 195.312K (which is the minimum sampling frequncy of
the
system).
gain=0
The transmitted signal is a AM modulated sine signal.
The frequency of orignal sine wave (the message signal) is 10K Hz.
The frequency of carrier signal (center frequency for modulation at
transmitter board) is 2.42 GHz, and I use the same frequency at the
receiver
board for demodulation.
Amplitude of transmitted signal is 0.05.

I connect the two boards together, and I add a scope to observe the
period
of the received demodulated signal. The problem is the period is about
0.000045, which means the frequency of the signal I received is not 10
KHz,
but about 2.2 KHz.

I have used the same external reference clock and PPS signal for both of
the
boards, so I think they are set up correctly. Why do I get a received
signal
with different frequency from what I transmit?

Does anyone meet the similar problem before? How may I solve the
problem?

Thanks so much for every help and suggestions in advance.

Best regards,
Hongliang


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On 10/27/2010 02:11 PM, Hongliang Zhang wrote:

gain=0
but about 2.2 KHz.
Hongliang

What daughterboards are you using?

Can you share your (GRC) flow-graphs with us–makes it easier to spot
problems.

I hope that you’re using an attenuator between the transmitter side and
receiver side.

My guess is that there’s a sample-rate issue somewhere on the receive
side, like the scope sink
hasn’t been told the correct sample rate, so it’s displaying
incorrectly, etc.


Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

Dear Marcus L.,

Thanks very much for your response.

I use RFX2400 daughterboards and their TX/RX ports.

A attenuator is added between the transmitter and receiver boards now.
And
to simplify the test, I use a constant source at the transmitter side.

Here is my GRC flow graph:

http://old.nabble.com/file/p30077180/flow%2Bgraph.jpg

And here are the results I’ve observed from the scope:

http://old.nabble.com/file/p30077180/Result.jpg

Why do I receive a sine wave when I transmit a constant?

Best wishes,
Hongliang

Marcus D. Leech wrote:

blocks for transmiting and receiving respectively.
board for demodulation.
the

Principal Investigator


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Why do I receive a sine wave when I transmit a constant?

Probably because the 100MHz reference clocks of your two USRP2s are
slightly different

Here are a few suggestions for being sure that you are locking to your
10MHz reference clock:

If you have not modified the generated python code from your GRC
flowgraph, then you are definitely not locked to refclock. The
easiest way to fix this would be:
Use a multi-usrp sink and source from the UHD categorey
Set the number of motherboards and channels to 1 each
Use the Sync to Unknow PPS option

I know that is a strange interface, we will put an option to use
external ref clock in an update coming soon to git repositories near
you.

Finally, be sure your refclock meets the power/amplitude requirements
shown here:
http://www.ettus.com/uhd_docs/manual/html/usrp2.html#ref-clock-10mhz
+5dBm to +20dBm (roughly 1Vpp to 5Vpp)

Jason

Dear Jason and Josh,

Thanks very much for your reply. I followed your instructions and use
the
UHD MIMO Sink and Source again. They should be synchronised now.

First, I transmitted a constant and observe the received signal. The
following figure is my flow graph of GRC:
http://old.nabble.com/file/p30086842/FlowGraph.jpg

But I got the received signal with large fluctuation (I observe the
received
signal in a long time period window):
http://old.nabble.com/file/p30086842/Result1.jpg
The fluctuation increased when I increased the amplitude of the
transmitted
signal.

Then I made a test for transmitting a 10K Hz sine wave

http://old.nabble.com/file/p30086842/FlowGraph2.jpg

The result seems better :
http://old.nabble.com/file/p30086842/Result2.jpg
However, if I watch the received signal in real time, I can still find
the
slight vibration of the period of the signal wave.

I used a signal generator and a spliter to provide two identical 100MHz
ref.
clocks for the boards.
In my opinion, the results may imply that there are still frequency
difference between transmitter and receiver. And this frequency
difference
is a random value which may goes from -100 to 100 Hz.

May I have your understandings of the results? Would you mind to provide
me
with some ideas or suggestions about them?

Best wishes,
Hongliang

Jason A. wrote:

flowgraph, then you are definitely not locked to refclock. The
shown here:


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I used a signal generator and a spliter to provide two identical 100MHz ref.
clocks for the boards.

The ref clock needs to be 10MHz rather than 100MHz

I know that is a strange interface, we will put an option to use
external ref clock in an update coming soon to git repositories near
you.

OK, i pushed that change. Update your uhd and gr-uhd.

Also, front panel led E is an indicator of external ref lock:
http://www.ettus.com/uhd_docs/manual/html/usrp2.html#front-panel-leds

-Josh

Thanks Jason,

Sorry, I told you wrong reference clock. I have used 10MHz reference
clocks
for the test again, and I still got the same results as the previous
post.
Could you please give me some suggestions about the problem?

Cheers,
Hongliang

Jason A. wrote:

http://lists.gnu.org/mailman/listinfo/discuss-gnuradio


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