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Narrow or Broad?

The second in our Wake Up! Time to Filter! mini-blog sequence.

What’s getting in the way? Are you trying to filter something out, or filter something in? Both the wanted signal and the uninvited guest information can be either narrowband (spectral components over a small span of frequencies) or broadband (that’ll be a large span of frequencies). That gives us four cases to consider, but the two common ones for driving filtering decisions are:

  • You’ve got a narrowband signal, but it’s being smothered by some broadband noise.
  • You’ve got a broadband signal, but it’s being drowned out by a narrow-band interferer.

A narrow-band signal tends to “contain” its useful information in the frequency domain and is usually reasonably insensitive to the time-domain behavior of a filter applied to it. So, it’s common to use fairly aggressive filtering practices that squeeze down the passband in order to provide as much stopband as possible.

A broadband signal is spread out in the frequency domain and so can express some fairly complex time domain behavior. (The more broadband a signal is, the less it looks like a sinewave — which is the ultimate low-information signal.) This can present a tough filtering challenge when a big narrow-band interloper needs to be suppressed. It’s the stopband that needs to be concentrated, with as wide a passband as possible. The filter design challenge then revolves around providing as much attenuation as you need (but no more — it’s wasteful), together with the most benign — or at least “compensatable” — behavior in the time domain. Always a challenge, simulation of the signals, the processing, and the detector are pretty essential in this case.

Untangling the behavior of two broadband signals is a challenge for “conventional” filtering, because their spectra are likely to overlap significantly. The solution in this case is not to use frequency-domain filtering at all, but process signals entirely in the time domain, encoding a symbol as a uniquely identifiable sequence of time domain events. A correlation process is then used to assess the likelihood that an incoming signal “contains” that sequence. This code-spreading method underpins most modern forms of digital communication. Filtering is still heavily involved, but only to define the boundary conditions for the frequency band in which all this time-domain activity happens.

The case of two narrowband signals is the easiest, so we solved it a long time ago. Good old-fashioned radio relies on this.

Please join us on Wednesday, October 22, at 1:00 p.m. ET (10:00 a.m. PT) for a chat session in which we will discuss “Filter Design.”

Sign up now by clicking here to join or to leave an early message! Just click “Reply” or “Post Message” when you get to the site.

12 comments on “Narrow or Broad?

  1. Davidled
    October 17, 2014

    In the wireless signal band, narrowband signal and wideband signal are used depending on bandwidth (BW). In the traditional communication system, information carried on frequency is sent via narrowband signal BW. In the spread-spectrum system, information is sent to other place by wideband signal BW.

  2. kendallcp
    October 17, 2014

    That's a good point and could come up in the chat.  We use frequency domain filtering in an attempt to mitigate the deterioration that unwanted signals cause.  But much modern communication methodology also exploits techniques for avoiding or ignoring that interference.  OFDM is one such approach; narrow-band interferers may mess up some of the channels, but the result is lower symbol rate, not lower SNR.

  3. samicksha
    October 17, 2014

    I guess most common use today for narrowband is audio spectrum to describe sounds, will be interesting to join this chat.

  4. geek
    October 31, 2014

    @DaeJ: Shouldn't the choice of broad or narrow depend on the kind of application, i.e. what kind of a transmission rate is required, what is the acceptable level of latency, what is the acceptable level of lost packets etc?

  5. yalanand
    October 31, 2014

    But much modern communication methodology also exploits techniques for avoiding or ignoring that interference.

    @kendallcp, I agree with you. I think we also use concepts such as Phase shift keying (PSK)  and Frequency shift Keying (FSK) along with other traditional methods.

  6. yalanand
    October 31, 2014

    Shouldn't the choice of broad or narrow depend on the kind of application, i.e. what kind of a transmission rate is required, what is the acceptable level of latency, what is the acceptable level of lost packets etc?

    @tzubair, ultimately the bandwidth will have an impact on other parameters such as speed etc. Hence bandwidth determines whether we have to choose narrow band or broadband.

  7. geek
    October 31, 2014

    “Hence bandwidth determines whether we have to choose narrow band or broadband.”

    @yalanand: I think the resultant bandwidth is the outcome of the type of band you use. However, you're right that the required bandwidth by the application determines the choice of the band.

  8. Davidled
    November 1, 2014

    All answers related to question might be depending on how wireless communication could be designed toward engineer intention. Broad band could provide more capability to communicate simultaneously as avoiding the overload of Network, compared with narrow band. Broad band should be handled appropriately in backend in order to meet this feature.

  9. nasimson
    November 29, 2014

    @ Perry:

    Your three part OR series blog was kind of nostalgic. I really enjoyed reading them and refresh my basics on design trade offs. Please have another series like this.

  10. kendallcp
    November 29, 2014

    Thanks ever so much for your comment!  I'm sure Steve Taranovich will back up your request.  He's always prodding me to put more content onto Planet Analog.  Time is the only confounding factor.  I havent got enough time, which means that the frequency is inadequate  (-8b

  11. nasimson
    November 29, 2014

    @ Perry:

    I am talking about these writings. And I believe these are by you? So where does Steve Taranovich comes in?

  12. SunitaT
    November 29, 2014

     Please have another series like this.

    @nasimson, I agree with you. This series was very informative. Would really like to see more such series which refreshes our basics.

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