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Can pocket FM radio output at 50kHz?

  1. Jun 18, 2015 #1
    I've recorded the output hiss from a small battery powered radio operating inside a metal tin. From related posts, I can say that the radio is effectively shielded from receiving any FM transmission, and the resultant hiss is thermal noise in the early stages of the receiver.

    I believe that the radio can output frequencies up to 50 kHz via it's earphone socket. Can this be correct?

    My findings suggest that it can. When not constrained by handling FM audio, the input to the early receiver stages is thermal noise which (unconstrained) has an almost infinite bandwidth. The only constraint operating on it then comes from the radio's internal electronics. These will have spare capacity well above the audio range so as not to clip FM audio.

    I've attached screen grabs from Audacity. This is the recorded hiss, and visually compared to a basic 10 kHz sine wave you can deduce how the hiss might reach 50 kHz, albeit at lower amplitudes.


    I've also uploaded a spectrum analysis, and a spectrogram. These suggest Pink noise.



    And finally, I've uploaded a zipped FLAC file of 5 seconds of the hiss should anyone have a real spectrum analyser. This would help greatly :kiss:

    Is there any reason to think that a pocket radio can't have electronics capable of handling frequencies up to 50 kHz..?

    Attached Files:

  2. jcsd
  3. Jun 18, 2015 #2


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    Define handling.

    The purple spectrum shows three spurs at 30kHz or below. The presence of those spurs above the noise floor demonstrates that it is not white noise.

    The -3dB point for the spectrum is at about 5kHz. That is the bandwidth of the audio output stage.
    At 50kHz, the noise is about 55dB below the audio. That is about 0.25% of the audio band and so will be swamped by the audio and the spurs.
  4. Jun 19, 2015 #3


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    Why would you want to use a radio when the circuit I referred to in the other thread can give you this?

    Bottom: noise floor of the measurement system
    Next: noise floor with amp on and noise diode off
    Top: Noise at 10mA and 60mA of current in the noise diode respectively.
  5. Jun 21, 2015 #4
    One word: practical expediency and risk mitigation (is that more than one word?).

    1. Why build when I can use what I already have?
    2. Most if not all of the design challenges for my radio in a tin apply equally to your circuit. It will need shielding (perhaps a sweet tin?). It needs power and output leads. The signal level looks low so perhaps additional amplification might be required, and hence it's design. What's a good physical circuit layout and grounding regime?
    3. My analogue skills are minimal.
    4. If you look at similar commercial hardware generators they're very complex. They require great analogue and EMI design and manufacturing skills. They also require sophisticated equipment to verify their correct operation. I know the radio works as I can see and hear it go "Shhhh".
    5. The operational frequency of your circuit is inappropriate for my use case. (Thanks anyway.)


    I intend to consume the radio output at a maximum rate of 96 kSa/s. In accordance with sampling theory, this should cover a bandwidth sufficient to go right up to my spectrum's shoulder at approximately 40 kHz (affording at least a 100kb/s random number extraction) . That's equivalent to the pink region I've highlighted in your spectrum. Not sure what's going on there - is the circuit even operating at that frequency? If it is, it's very quiet and perhaps indistinguishable from the other noise floors. The radio only ranges 25 dB around here.
    6. The output spectrum's profile is unimportant to my use case. Any shade of noise will do.

    I'm not categorically ruling out a dedicated noise circuit, but I suspect that most of them will still carry the above risks and limitations. So for the time being, I see the radio in a tin my least risky choice. A radio works pretty well for random.org after all.[/QUOTE]
  6. Jun 21, 2015 #5
    I mean that the radio can produce an hissy output signal in the range 0 - 50kHz in the absence of any FM signal. That's what I'm taking from the spectrum, unless you tell me otherwise. I have little knowledge of sound engineering, and am relying on more experienced persons' feedback.

    I don't understand your comment distinguishing audio from noise. The spectrum/graph is all noise. The radio's in my tin.
  7. Jun 21, 2015 #6


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    That's not really a problem, you can sample the noise to produce your required output rate.

    Some data on FM radio noise.
    Last edited: Jun 21, 2015
  8. Jun 22, 2015 #7


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    To get a quick estimate of an RF filter profile I make a noise source by chaining three or four broadband MMIC amplifiers together with a 50 ohm input termination resistor to produce Johnson noise. The output goes to a spectrum analyser to check the noise profile is close to flat. Then I insert the filter between the noise source and the analyser. The change in the noise profile is the characteristics of the filter.

    The spectrum of the output of an FM radio is the spectrum of the noise generated by the phase detector, multiplied by the passband of the detector and audio amplifier stages. You have no way of identifying what is noise profile and what is filter characteristic.

    The shoulder of the audio output when used as a noise source is clearly the -3dB point. That shows the bandwidth is only 5 kHz, which is not really surprising for a cheap audio amplifier.

    If you want a greater bandwidth than 5 kHz you must use a bandpass filter to extract and level part of the spectrum. The resulting noise signal will need to be amplified during the process. The maximum bandwidth is then the frequency difference between the -3dB points of the extracted band. I would select the band between 10kHz and 30kHz because flattening the spectrum there looks possible. The best bandwidth I could then expect would be 20kHz.
  9. Jun 22, 2015 #8

    I don't recognise these numbers from my spectrum. Doesn't it show signal ranging zero to approximately 40 - 50 kHz? Let's not use the term audio as it's >> 20 kHz.

    There's a real risk that I've miss-understood the signal analysis as I'm new to it. I uploaded a .FLAC clip of the signal with my initial post. If you have the resources to analyse the spectrum yourself, I'd greatly appreciate comments.
  10. Jun 22, 2015 #9


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    40 dB down (~ -80 on the graph) means you have 100 times less voltage at that frequency than at the peak. dB = 20 log V1/V2

    Normal frequency response corners are at -3db (half power). -33dB to -36dB is -3dB.

    You have no idea what is causing the frequency slope, nor the spurs.

    You are definitely building a toy. Period.
  11. Jun 23, 2015 #10


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    The spectrum you show is the audio output from an audio amplifier. Compared with the low frequency noise present, the -3dB point is at only 5kHz.
    Even a 1 volt 100Hz audio signal has some 1GHz Johnson noise present due to in the resistance of the wires. It will probably be about 160dB down. That does not mean you can turn the 100Hz audio tone into 1Gbps random bitstream.

    Why the -3dB point? See; https://en.wikipedia.org/wiki/Bandwidth_(signal_processing)

    The problem with signals that are -3dB relative to the bigger signals is that they are hidden by those bigger signals.

    Consider an AC coupled flat noise spectrum from 10Hz to 50kHz. If you use a comparator to sense the sign of the voltage you will get a rectangular noise waveform. You can sample that at a rate of about 25kHz and get a random bitstream.

    Next consider a 10Hz sinewave oscillating between +1 and –1 volts. Now superimpose a 50kHz sinewave oscillating between +10mV and –10mV. If you feed that to the same comparator considered above you will get a 10Hz square wave output with noisy transitions every 50ms. That is not random. The 10Hz wave will take about 320us to pass through the 20mV nearest zero. Only during that very short period will about 16 cycles of the 50kHz signal be present in the random data stream. The rest will be a long line of consecutive 0s or 1s.

    That is why the band of noise you convert to a random digital bitstream must be flat. If any part of the noise spectrum is more than about 3dB down on the rest of the band, it will not contribute to the random bitstream.

    Until you understand the concept of the –3dB bandwidth you will be unable to make wise decisions about the bitrate of random noise.
  12. Jun 23, 2015 #11


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    Not only do you need to understand the -3dB bandwidth conceptually, but you need to understand anti-aliasing filters and imaging (aliasing) versus sample rate and and the concept of decimation. But, you need to understand those even with a decent noise source.
  13. Jun 23, 2015 #12


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    @ Paul Uszak, your purple spectrum.png in post #1 is plotted with a linear frequency axis. You have the technology, please plot it again with a logarithmic frequency axis from 10Hz to 100kHz, it will then be easier to see the structure of the low frequency part of the output. While you are at it, move the zero dB reference level so that the highest amplitude part of the band is at zero dB. It will then be easier to interpret the spectrum and why it has the shape that it has.
  14. Jun 23, 2015 #13
    This is the best I can do. I can either have a log scale and no 0 dB, or 0dB with no axes!

    The FLAC file of the actual noise signal I used is attached in the first post if you have access to better resources...



    Perhaps this helps a little, it's got a combination of both ...

  15. Jun 23, 2015 #14
    An AR15 and a Ferrari are toys, yet both can kill. You really shouldn't conflate "toy" with "it doesn't work."
  16. Jun 23, 2015 #15


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    LOL --- AR15 and Ferrari are precision tools, well engineered to accomplish specific functions. Engineered down to the last thread and every coating. How many engineers do you think it took to develop the paint process for a Ferrari? That people apply them in "toylike ways" is besides the fact. Trying to compare what you are doing to precision engineered machines is further indicator of how little you really understand about electronic signals. (And, you don't seem to want to listen with an open mind to some expert advice you have been given )

    Compare a ferrari (well engineering random data generator) with a tonka toy (Not understood noise source of questionable randomness). Tonka toys work fine for what they are supposed to do.

    What you are building as a noise source is a toy. Toys work fine for what's expected of them. I don't, however, think your expectations are in line with what you can actually accomplish with the tools you have limited yourself to.

    Find some way to run auto-correlation on the flac file (rather on the data you try to extract from the flac file). Some toys are better than others. Maybe you'll get lucky.

    http://www.ltrr.arizona.edu/~dmeko/notes_3.pdf [Broken]
    Last edited by a moderator: May 7, 2017
  17. Jun 23, 2015 #16


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    Attached is spectrum with a new vertical axis with zero RL.
    Notice that the -3dB point is actually close to 2kHz.
    That could give you about 1k bits per second.

    Attached Files:

  18. Jun 25, 2015 #17
    To all Naysayers, Disbelievers and Merchants of Ennui :wink:,

    I offer two totally random images. One is my (Ferrari) image extracted from FM radio static. The other is from the binary expansion of e. Can you tell them apart?

    1.png 2.png

    Some toys can work pretty well...

    (p.s. I'd have produced bigger images, but my prototype extractor only currently supports extracting 65k words. They both pass the simple ENT test, but of course that doesn't mean much with such small files. It's a promising start though.)
  19. Jun 25, 2015 #18


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    Please do not insert images into your post. Attach them so they can be downloaded if needed.
    An image.png compression is not lossless, it merges adjacent pixels.
    Your images are therefore randomised by lossy .png compression.
    How many bits per pixel did you use to construct your image?

    What data rate did you used to extract the data stream from the FM detector noise?
  20. Jun 26, 2015 #19


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    lol I can create seemingly random pixel patterns with high auto correlation from simple formulas.

    Yet another example of how little you understand what you are on about.

    as for "Some toys can work pretty well...", I agree with that. I said as much. But they are still toys. Just depends on your expectations.
  21. Jun 26, 2015 #20


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    Above: A plot of a series of 100 random numbers concealing a sine function. Below: The sine function revealed in a correlogram produced by autocorrelation.
    (from the wikipedia autocorrelation article)
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