Re: [Discuss-gnuradio] QT GUI Waterfall Sink: frequency mismatch

[Ellie White]

Thank you very much for the helpful suggestions; my setup is working very nicely now! I changed the sample rate to 2MS/s, and the bandwidth to samp_rate instead of bw as you suggested, and tested it again with the function generator at 20 kHz. This time, the frequencies match exactly where they should be. I’m so glad everything is working smoothly; now I can move forward with my project. Thanks again for your input!

 

Best wishes,

 

Ellie

 

From: Discuss-gnuradio [mailto:discuss-gnuradio-bounces+address@hidden On Behalf Of Kyeong Su Shin
Sent: Thursday, March 30, 2017 11:53 PM
To: GNURadio Discussion List <address@hidden>
Subject: Re: [Discuss-gnuradio] QT GUI Waterfall Sink: frequency mismatch

 

To whom it may concern:

First, as Marcus Müller mentioned in an another e-mail thread, you cannot sample at 300MS/s with an RTL-SDR. The maximum effective sampling rate (=actual ADC sampling rate / decimation ratio) that RTL-SDR can achieve is somewhere around 2.6MS/s - 3.2MS/s. Your RTL-SDR must be operating in an different sampling rate in reality.

Also, I believe that the 'bandwidth' parameter of the QT waterfall sink & QT frequency sink must be samp_rate, instead of bw in this case, since the frequency range of the periodogram is determined by the sampling rate of the ADC, not by the filter bandwidth.

I never used RTL-SDR in direct sampling mode or with a down mixer, so I am not sure how you should configure your parameters to listen to VLF channels, but in many cases, it is sufficient to set sample rate = bandwidth. In fact, I am not sure if it is truly configurable in this case, as some SDR transceivers simply ignore that field (as well as some gain parameters).

Regards,

Kyeong Su Shin

 

On Thu, Mar 30, 2017 at 3:03 PM, Ellie White <address@hidden> wrote:

Hello,

 

I have constructed a small loop antenna with the intent to monitor the signal strengths of VLF submarine communication stations in order to detect sudden ionospheric disturbances (caused by solar activity). I have this antenna connected to an upconverter and RTL-SDR dongle, and have been testing it with a function generator using two different softwares: SDR# and GNU Radio. The problem I am having is that when I transmit a 24 kHz test tone, it shows up as ~24 kHz above the LO frequency on my SDR# spectrogram plot, but when I transmit the same 24 kHz test tone, on the GNU Radio waterfall plot it appears to be only 5 kHz above the LO frequency – I am wondering why that might be? (I attached my grc flowgraph if that would be of any use). I am very new to this, so any advice would be much appreciated! If any more info is needed please let me know.

 

Thanks in advance!

 

Ellie

 

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