Questions tagged [spectrum]

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31
votes
5answers
11k views

Can I turn Radio waves into light?

Wikipedia says that the frequency of light is 300 THz. I've made a radio waves transmitter that transmits about 100 MHz. If I increase the frequency of the transmitter to 300 THz, will the antenna ...
10
votes
1answer
336 views

Do different frequencies have different capabilities?

I read that google was planning on purchasing a spectrum from the Fcc and they were going to use it to transfer data. I was wondering if the value of the frequency affects the data that is being ...
8
votes
4answers
564 views

LEDs with highly stable spectrums

We've noticed that off-the-shelf LEDs exhibit some change in the shape (and sometimes peak) of their spectrum as the amount of current flowing through them changes. For example, a UV LED we tested ...
6
votes
2answers
284 views

If I buy a spectrum analyser, how do I do an EMI test?

Maybe when I have the cash to buy one, I will. But my question is more related to how to use one once I've got it. I assume I'll need some kind of antenna to receive the EMI. Are there general rules ...
5
votes
3answers
4k views

LED lighting with incandescent spectrum?

Is there any reason why they don't produce LED lights with a spectrum closely matching that of an incandescent bulb. Incandescent bulbs produce a nice warm light, while LEDs (even warm white ones) ...
5
votes
1answer
6k 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)...
4
votes
4answers
22k views

Cool White LED bulbs: Are they “full-spectrum”?

I don't have much knowledge on LED lights and find the technology quite fascinating (yes, I know, it is not that new!) I have read that in order to create "white" light, LEDs actually need to emit ...
4
votes
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 ...
4
votes
1answer
598 views

Sampling frequency required to detect peaks

I am processing a signal in the digital domain and I have to detect "peaks" in it, e.g. by finding local maxima formed by three consecutive values. (More sophisticated criteria could be used like ...
4
votes
1answer
71 views

Why Johnson's noise is not uniform when plotted in log mode?

I have a 2 resistors of 1000 k each , I have connected them in series and biased one end with +9 V and other end with +9 V. The output is taken from connection point of two resistors and I have given ...
3
votes
3answers
141 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 ...
3
votes
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 ...
3
votes
2answers
804 views

Light detector linearity with varying wavelength

Given the same intensity of light, in varying wavelengths, which is typically more linear with their output: Cds (photo resistor) or a Photo transistor? I would assume that the junctions in the ...
3
votes
2answers
43 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 ...
3
votes
1answer
421 views

Fourier series of modified sine

I must do this task: Short translate: Picture presents modified sine signal. I must calculate F(k) using Fourier Series, next draw amplitude spectrum and phase spectrum. I have 2 problems: Is this ...
3
votes
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
5answers
20k 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 ...
2
votes
3answers
1k views

478 MHz band, allowed for hobbyist use in USA?

Somebody advised me to look at this wireless module as a lower-cost alternative to xbee. I noticed that it operates on the 478 MHz band. Is this band permitted for hobbyist use in the United States?
2
votes
2answers
443 views

Can an oscilloscope add a frequency component to my signal?

I want to check the spectrum of a DDS signal ("pure" sinewave) for undesired frequency components. The way I am doing it is by storing the waveform from a 2GS/s oscilloscope (Agilent DSO81204) and ...
2
votes
3answers
744 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 ...
2
votes
1answer
193 views

How does the energy of non-resolved spectral lines get distributed in an FFT?

For example, if my FFT has a poor resolution of 170Hz (data sequence length is small due to latency constraints) and I am trying to detect a sinusoid in the 150-250 Hz range, will this FFT be good ...
2
votes
2answers
1k views

Making a spectrum analyzer probe

I bought this dirt cheap Spectrum analyzer from a Chinese web site. It arrived and all it had was an antenna. I have very little experience with spectrum analyzers, how can I build a probe for this ...
2
votes
1answer
139 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 ...
1
vote
2answers
821 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 ...
1
vote
2answers
82 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 ...
1
vote
1answer
56 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 ...
1
vote
1answer
44 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 ...
1
vote
3answers
98 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 ...
1
vote
1answer
63 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 ...
1
vote
1answer
86 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: ...
1
vote
1answer
91 views

Effect of Quantisation of a Signal on the Estimated Spectrum

I have been applying the Welch Estimation technique to a signal in MATLAB to give a Spectral Density plot for frequencies from 0 to 0.5 the sample rate. I have two issues: I have a peak at 0.25 for ...
1
vote
1answer
42 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 ...
1
vote
1answer
38 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}. ...
1
vote
0answers
27 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 ...
1
vote
0answers
126 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 ...
1
vote
1answer
113 views

calculating the spectrum of digital modulation methods

I have just started studying the techniques of digital modulation. The book i am reading only has mentioned the psd of bpsk, bfsk , qpsk and msk. I would like to know, how the psd is actually ...
0
votes
2answers
968 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\$?
0
votes
3answers
165 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 ...
0
votes
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 ...
0
votes
3answers
85 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/...
0
votes
1answer
266 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 ...
0
votes
1answer
595 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 (...
0
votes
1answer
138 views

How is (non-interfering) data communication possible over the tv spectrum? What is the technology used?

I've heard that FCC-US (Federal Communication Commission - United States) has approved the free usage of TV spectrum for wireless communication. How is it possible to enable such data communication ...
0
votes
1answer
58 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 ...
0
votes
1answer
288 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 ...
0
votes
2answers
159 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 ...
0
votes
2answers
160 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 ...
0
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2answers
157 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.)
0
votes
1answer
40 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 ...
0
votes
2answers
118 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 ...