Questions tagged [spectrum]
The spectrum tag has no usage guidance.
71
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Spectrum of FM signal?
If a carrier (\$f_c\$) is FM modulated by a single sine wave (\$f_m\$), its spectrum is composed of frequencies at \$f_c + kf_m\$ (\$k \in \mathbb{Z}\$) whose amplitudes are weighted by \$J_n(\beta)\$,...
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2answers
38 views
What is the irregular signal at the bottom of this “spectrogram”?
I am reading research papers about LoRa, and I have come across the below figure in this one.
The horizontal axis is clearly labeled frequency, but the vertical one isn't. I recognize the chirps in ...
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4answers
214 views
How can we interpret the negative side of a double-sided frequency spectrum
We know that a frequency spectrum includes both positive and negative frequencies. However, how can we explain the negative frequency physically?
For example, in a wireless communication system, ...
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101 views
DAC “aliasing” of harmonics
While reading articles and application notes regarding DACs and their output spectrum, I stumbled across aliasing of higher harmonics to the first Nyquist band. For example, when generating a single ...
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3answers
384 views
How to convert dBm to V/m, A/m^2 and W/m^2
Currently I am using a spectrum analyser(SA), Keysight N9020, with an antenna to get readings off the air.
The reading on the SA gives a reading of a unit, dBm.
However, I need to convert dBm to V/m, ...
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1answer
47 views
Power Spectral Density units
I am trying to compare power spectral density values (obtained from dsp.SpectrumAnalyzer) with PSD values obtained from hardware which has a VSA.
MATLAB's documentation says that " The power spectral ...
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1answer
64 views
How to model and understand the asymmetry of a parallel RLC circuit spectrum
I'm working with some parallel RCL circuits and I have noted that when I record a power spectrum (so \$\rm{dB}\$ vs \$\rm{Hz}\$) that the spectrum is asymmetric:
As I'm sure you all know if we ...
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1answer
46 views
Antennas- Spacing between Reflectors & Directors in Wavelengths
I’m building an antenna and I want it to connect to channel 6 on the spectrum allocation chart (2.437 GHz). I’ve calculated the lengths of the reflectors and directors but im just a bit stuck on the ...
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191 views
Covid-19: can a UV-C source be improvised from a standard mercury vapor lamp?
I suggest it because the usual professional UV-C sources are in short supply and very costly.
A common 175 W mercury lamp designed for illumination includes the quartz 'lamp' tube itself, inside an ...
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3answers
110 views
Aperiod vs Period waveform Fourier Transform: How does nature understand which is the case?
Today I reviewed the theory behind the Fourier Transform, and I asked myself a question that I couldn't answer to in the process.
Theory: A periodic waveform has a Fourier series which can be seen ...
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1answer
49 views
Noise Floor of ADC using PWELCH
While using MATLAB's Pwelch function, I am not able to match the noise floor of the "Modelled" ADC to (Delta)^2/6 {1 sided Spectrum}.
...
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3answers
91 views
Infrared Sensor for drug detection [closed]
I saw that there has been a push to make a drug detector that can identify cannabis since it has been legalized. I was looking at mouser.com and found this ir sensor: https://www.mouser.com/...
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48 views
From noise spectral density to equivalent short circuit input RMS noise
I have an acquisition system that is supposed to be low noise. It acquires from an ADC into a wav file. I read this file and process it with Matlab. Then, being "x" the array with time signal, Fs the ...
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2answers
249 views
What's the reference dB level in spectrum analyser
I am using a Spectrum analyser.
Measuring the 248kHz buck switching frequency and its harmonics in my board.
I have set the frequency range from 200kHz and 1MHz. Using a near field H-Probe
When I ...
0
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1answer
61 views
How to see Signal frequency and Sampling frequency on the frequency spectrum
If I have a 100 Hz periodic analog signal and let us say it is band limited to 1kHz and I sample it at 3 kHz. If I want to see its frequency spectrum then how to differentiate between the signal ...
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0answers
29 views
How does spectral density of a power signal change when it is multiplied by \$cos(2\pi f_ct)\$?
Suppose we have a power signal \$n(t)\$ whose power- auto-correlation function is: \$R_{n}(\tau)\$ . Now we multiply the signal by a cosine as: \$cos(2\pi f_ct) \cdot n(t)\$, whose auto-correlation is ...
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1answer
72 views
Arduino RGB spectrum problem
Over the last few days I've been working on an anode RGB LED in Arduino, I've tried getting it to do an RGB spectrum but all I would get is just the colors from red to green, then it would get stuck ...
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0answers
33 views
Doppler spectral power density
In layman terms, why doppler spectral power density has U shape?
Does this mean that frequency components with maximum doppler shift have maximum power?
I am confused with the interpretation of the ...
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1answer
67 views
Real time spectrum analyzer and Fourier Transform
I am studying spectrum analyzers and a doubt came to me about real time spectrum analyzers.
Suppose you have a signal in the time domain: in order to calculate the Fourier Transform, you need to know ...
0
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1answer
107 views
Calculate power given PSD and bandwidth?
Given the spectral recommendation for an Asymmetric Digital Subscriber Line (ADSL) below, I'd like to calculate the power in Watts that a modem would have to emit for the upstream band. I want to do ...
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1answer
161 views
Two details about phase noise have been confusing me for a long time. Hope to be resolved
Just like FLOYD M. GARDNER said in the PhaseLock Techniques, 3rd Edition: "In light of that success in spectrum analyzers, a practitioner’s answer to the question above is: The spectrum of phase noise ...
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1answer
74 views
How does the spectral response of a photodiode change with bias voltage?
Look at this data sheet, in the second page we have a graph for the spectral response.
Now, I have learned that photodiodes must be reverse-biased in order to get a decent signal from them, but then ...
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1answer
331 views
How do you visualize the frequency spectrum for a digital waveform? [closed]
How does the mathematics of a single or periodic rectangular pulse help us quickly visualize the frequency spectrum for a digital waveform? Develop the ideas behind this.
My professor posed this ...
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0answers
30 views
non-negative FFT spectrum, subtlety
definations
\$x(t)\$ is a function of time. Physically, it can be voltage, displacement, magnetization and so on. It can be real, complex, vectors or more fancy numbers.
\$C(t)\$ is the auto ...
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3answers
148 views
Help interpreting strange spectrum
I am sampling a sinusoidal signal with an oscilloscope and viewing its spectrum using the FFT mode. The signal in time domain is this:
and I am getting this spectrum:
The peak above the red line is ...
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1answer
52 views
Are there any examples on the use of the frequency spectrum between 40 and 42 GHz? [closed]
Since everyone started talking about 5G, I got curious about some parts of the frequency spectrum.
I found out that some operators in the U.S. asked FCC if they could free the range between 40 and 42 ...
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1answer
75 views
Analog Led Audio Spectrum Analyzer Simulation
I am having a bit of trouble trying to simulate a variation(different frequency ranges/gain for the BP filters) of this project: https://makezine.com/projects/audio-spectrum-analyzer/
I am not too ...
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1answer
48 views
Why averaging in Spectral magnitude domain not in complex domain to estimate spectrum of a process
Consider we need the magnitude spectrum of the signal. Signal is recorded in \$N\$ trials to a certain stimuli. Signal varies from each trial because of noise. So, to estimate the actual magnitude ...
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1answer
332 views
How DC offset affects the spectrum of a single pulse
I know that in the magnitude spectrum of a single square DC pulse (0-8V) the lobes are zero at frequencies $$\frac{1}{t},\frac{2}{t},\frac{3}{t},...etc. $$ where t=pulse duration.
If the square ...
3
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3answers
2k views
How to compare brightness of different color LEDs (not RGB)?
I wonder - let's assume one would build some user interface using green, yellow and red or just any other available colored LED (not RGB LED) - what would be the proper (aka best) way to compare the ...
2
votes
3answers
791 views
Why do we need the Fourier transforms of sin and cos? [closed]
As we know we perform Fourier transform in case of continuous and "non-periodic" functions, while sin and cosine both function is "periodic" function then why we perform Fourier transform of sin and ...
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5answers
21k views
Full Spectrum LED(400nm~840nm) vs Grow Light LED (RED+BLUE)
I'm planning to buy lights for my planted aquarium tank. After a small research I found that regular lights are not suited for growing plants.
Beacuse they need BLUE for Leaf Growth and RED for Stem ...
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0answers
157 views
License free frequency spectrum in Australia
What is the license free frequency spectrum in Australia? Is it 433Mhz or 915Mhz Or 868Mhz? I am trying to do some IOT Project using RF (LORA/ZigBee) modules. I thought it was 433Mhz. But some online ...
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2answers
163 views
Largest spectrum image sensor and processing its entire spectrum image
I was developing a digital system to process raw images stemming from an image sensor but I would like to deal with "universal" (as possible) spectrum image sensors with no filter.
I know that for ...
4
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3answers
3k views
How Is Intensity of Blue Light Measured?
As a hobby project and to help solve issues getting up early in the morning (specially given that it's winter and the sun isn't up until long after I am) I'm trying to find a bright blue light that I ...
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1answer
92 views
Op Amp adds spectral components to amplified output signal
I'm using this simple circuit to amplify a small signal:
simulate this circuit – Schematic created using CircuitLab
Input voltage is defined by: ...
3
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2answers
46 views
Spectral properties of EFM encoding
Data is written to CD/DVD using an encoding called eight-to-fourteen modulation, where each eight bits of data are encoded by fourteen bits on disk. The encoding was designed to give the encoded ...
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1answer
7k views
How to calculate total power from spectrum?
I'm trying to measure total power(dBm) from the following spectrum, the spectrum is 5MHz wide, the peak power from the spectrum is about -40 dBm,
I have an array of spectrum points(Power vs Frequency)...
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2answers
173 views
How do I tell if my device qualifies for FCC exemption
According to the FCC:
The following devices are subject only to the general conditions of operation in §§15.5 and 15.29 and are exempt from the specific technical standards and other requirements ...
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3answers
344 views
How can I reproduce ambient EM fields inside a Faraday enclosure?
I am part of a research team looking at the possible effects of EM fields on psychological function. The experiment calls for participants to perform a series of tests while inside a Faraday tent that ...
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2answers
84 views
Using CMOS to detect transmittance in sample from LEDs
I am trying to get the transmittance through various liquid samples using specific wavelength LEDs. Originally I was experimenting with photodiodes, but do not seem to be ideal.
The target color ...
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0answers
130 views
Speaker designed for ultrasound operation
I've created a transmission protocol in order to transmit information bits between a PC speaker and a smartphone, or between 2 smartphones using ultrasounds. I've implemented a QFSK system which uses ...
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2answers
880 views
RF Transmit Power in an ISM band
I was browsing through some OFCOM Frequency Allocation Tables for use on a small project I'm doing. I require the use of some unlicensed ISM bands in the UK, and found that the lowest unlicensed band ...
0
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1answer
616 views
Usefulness of VBW in spectrum analyzer
Typically in the block scheme of a spectrum analyzer a video filter, with a bandwidth VBW, is located after the envelope detector and before the screen.
As pointed out in this document, the VBW may (...
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2answers
75 views
Spectrum precision of fluorescent lamps
How precise are fluorescent lamps when mass produced? Is their emitted spectrum really identical or is it possible to detect a difference between two random lamps using appropriate filters with photo-...
0
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1answer
968 views
How to shift signal to baseband using local oscillator frequency?
If the input to a demodulator is a signal having the spectrum shown below, how would I figure out what local oscillator frequency is required to shift the signal to baseband?
Is there a formula for ...
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2answers
998 views
How to sketch the spectrum of \$m(t) = \cos(1000)t \times \cos(3000)t\$
I know how to sketch the spectrum of \$m(t) = \cos(1000)t + \cos (3000)t\$
But I stack with that....when it's multiplied? How to sketch the spectrum \$m(t) = \cos(1000)t \times \cos(3000)t\$?
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1answer
169 views
How to listen to radio communications from a transmitter?
I have two devices communicating at a frequency which I don't know. I want to listen to that communication so that I can replicate one of the devices and fool the other. Basically I want to reverse ...
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2answers
165 views
Find the power spectral density?
I don't understand how the graph of H(w) is used to find the equation for H(150) and H(200.)
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1answer
991 views
Calculating noise power spectrum
I have some signal which I have measured N different times and I want to calculate the noise power spectrum of it. Intuitively to me the way I would do this is simply calculating the power spectrum of ...
