# Tag Info

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### Large spike at the center frequency when using Ettus X310

The problem that I have is that I always have a big spike (10-15 dB) directly on the center frequency (no matter what frequency I set). I am relatively new to all this so I would appreciate any ...
• 26.6k
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### Sampling frequency lower than half of sampled signal frequency?

The channel frequency controls the local oscillator on your SDR which is the frequency about which it covers. So you are receiving signals from 16KHz below 107.5MHz to 16KHz above. The concept you ...
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### What kind of modulation is this?

Ok I did some signal forensics on the data capture and believe the modulation is a form of FSK. The FSK modulation was +/- 20 KHz with a data rate of 38 KHz. UPDATE: The OP discovery that this is ...
• 37.7k
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### FFT-based coarse carrier recovery for QPSK

A square operation creates an unmodulated tone for a BPSK signal at 2x the carrier frequency (a pure tone for the case that the signal was unfiltered or rectangular pulses with perfect phase and ...
• 37.7k
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That is above the Nyquist rate, so why is the signal degraded? It's not degraded in any way form or shape. The perceived degradation is purely cosmetic but not functional. See for example: How is ...
• 32.6k
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### How do I extract data from a gnuradio flowgraph now that the message sinks have been deprecated?

So, the important takeaway from your introduction is that you have an application which needs to get chunks of items out of the flow graph repeatedly. Which means you're in the streaming case. (for ...
• 26.6k
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I also read this from a response to a USRP user's question about RSSI measurements: [The] Received Signal Strength [Indicator is] always relative to some signal model, incorporating ...
• 26.6k
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### Sample rates, Samples per Symbol, and Digital Pulse Shaping

Yes the OP is correct in that you can implement pulse shaping in less than 2 samples per symbol for exactly the reasons that was outlined. However importantly we must also keep in mind having excess ...
• 37.7k
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### Is Bluetooth Low Energy jamming possible with an SDR like the HackRF on GNURadio?

I am not revealing any big secrets here on jamming and anti-jamming techniques, nor would I condone creating any such interference. What I am about to say is quite simplistic and well known, but ...
• 37.7k
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### Clock recovery using Mueller and Muller adds noise affecting EVM or SNR (Two cases - GNU Radio & Python code)

Unlike the Gardner Loop, the M&M synchronizer should be performed after the RRC filter in the receiver for best performance. With cases of high RRC alpha, the M&M won't work as expected ...
• 37.7k
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### am I recording LTE uplink signals?

There's a lot of different domains of knowledge coming together here, so I'll split my answer into multiple sections, each answering an implicit question that you raise in your explicit question. Hope ...
• 26.6k
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### Pulse shaping with RRC : Number of taps

See pages 5 and 6 and the plots on the following pages specific to number of samples per symbol in this very helpful reference by Ken Gentile on designing RRC pulse shape filters: http://www.analog....
• 37.7k
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### PLL phase and frequency characteristics

Before I address your questions, you should understand: a. the integral branch of the loop filter maintains a average phase increment in units of radians/sample. It is not a frequency value, though ...
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### GNU Radio - PSK Mod block - unexpected constellation diagram

What you are seeing is the transitions from one constellation point to another. In order to reduce the signal bandwidth, the baseband signal is low-pass filtered. This causes the transitions to not ...
• 11.8k
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### OFDM symbol acquisition block in GNU Radio gives rotating constellation even if there is no carrier offset. How is this possible?

Got me at that one! The "OFDM symbol acquisition" block is in fact not from gr-digital (where your other OFDM blocks come frome), but from gr-dtv, where it is used ...
• 26.6k
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### GNU Radio | Packet Encoder/Decoder | Unwanted bytes added to the transmitted stream

Packet Encoder and Decoder are broken; they drop data. That's why they are in the deprecated category (for years now!). We've removed them, because as a project, GNU Radio has not been able to fix ...
• 26.6k
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### Cosine of amplitude 1 but GNU Radio shows FFT has amplitude > 30,000

Consider the formula for the DFT (which the FFT efficiently computes as an algorithm): $$X(k) = \sum_{n=0}^{N-1}x(n)e^{-j2\pi nk/N}$$ Notice that it is a summation over $N$ samples total. Also note ...
• 37.7k
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### Denoising effect in GnuRadio OFDM Serializer block

Further inspection shows indicates that the serializer block only does remove non data carriers. It just probably so happens that anything that is non data is super noisy, and data carriers are not ...
• 26.6k
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### Better way to mute fundamental baseband frequency?

To mute the fundamental frequency, consider using a notch filter such as the one shown in the figure below, where $\omega_n =2\pi 60/f_s$, where $f_s$ is your sampling rate, and $\alpha$ is chosen ...
• 37.7k
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### How does transition width of a low-pass filter affect a message which is well within the pass-band?

For an FIR filter there are 3 main components that determine the filter length for equiripple designs: Passband ripple Stopband attenuation level Transition width (Width from the edge of the passband ...
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### Why am I seeing a negative frequency on FFT graph in Gnu Radio Companion?

The stock FFT (in GNU radio?) is a complex-to-complex transform. Thus any positive frequency peak you see represents a complex signal (phasor) that can include both real and imaginary components. ...
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### Sampling frequency lower than half of sampled signal frequency?

I believe that what you are looking for is Bandpass Sampling. What Nyquist theorem says is that your sampling frequency must twice the bandwidth of your signal - not carrier frequency of it. Hence in ...
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### GNU Radio loop bandwidth normalization

I can only answer your second question: "How can the loop bandwidth in GNU Radio synchronization be configured as a percentage of the symbol rate?" The tracking loop in the symbol synchronizer block ...
• 2,545
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### How can an SDR recover a high-frequency signal?

The high-frequency (RF) section of an SDR is all analog. Typically, the analog receiver downconverts the RF signal to an intermediate frequency that is within the Nyquist range of the ADC. As Stanley ...
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### Communication between two USRP : mismatched spectra between transmission and reception

A few (hopefully useful) comments and ideas: The HackRF One is not a USRP. If you're receiving with an NI 2921 USRP, you should be using UHD to interface with it, not osmocom. Use a standalone ...
• 13.8k
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Let's say you want to filter a signal $x[n]$ through a Gaussian filter with impulse response $h_g[n]$ and a Moving Average (the "sqwave") filter with impulse response $h_s[n]$. Then the resulting ...
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### How to adjust receiver gains to avoid saturation and quantization noise to optimise post digital processing?

This is a great question and comes down to the AGC design and optimizing the available dynamic range on the ADC, given a receiver minimum SNR, sensitivity and interference rejection requirements. I ...
• 37.7k
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### I wish to create a GNU Radio program in C++ that doesn't need GRC or Python, is there a reference or template for this?

you can create flowgraph in c++ like gqrx and i think GNU Radio Manual and C++ API Reference documents will help you. This is gnuradio c++ document for top_block if you could not use it let me know to ...
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