Bandwidth and SDR Reception

Discussion in 'Amplitude Modulation - AM Fans' started by KC3EPA, Mar 18, 2019.

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  1. KC3EPA

    KC3EPA Ham Member QRZ Page

    I often listen to the AM stations on 80 /75meters.
    Great listening.
    I understand the bandwidth is greater for AM do to the nature of the beast.
    I understand each online SDR site will have its own quirks.
    I mostly use Websdr.org


    Not all operators of AM but many around 3875 have bandwidths of easily 20khz and sometimes more.
    The Websdr will demodulate them in AM mode or Ssb mode which could be do to the way SDR works.
    I also used my mobile and their signal seems to be wide and so strong that I can not tell their exact frequency even when turning gain all the way down.

    Could be me and my unit and or the SDR site.


    It is not every station and some do and some do not whether high power or not many seem to really have a wide bandwidth.

    It does not bother me at all just noticing.
    It seems to be more frequent on the ones using and stating they are using Class E .

    What am I doing wrong
     
  2. N2DTS

    N2DTS Ham Member QRZ Page

    I don't think you are doing anything wrong.
    And there is nothing wrong with wide signals on a band that has the space.
    If an AM signal is 8 KHz wide (4Kc audio) you can listen to it in 6 KHz, 8 KHz, or 10 KHz bandwidths.
    In 6 KHz you will be removing some of the highs, in 8 it will be just right, and in 10 you will just get more noise.
    You can go even wider but it makes no sense to go wider then the signal is.
    You can zero beat by selecting ssb and tuning till it sounds normal.
    Some stations (modern equipment) will be spot on, other may be 2 KHz off frequency (they don't know it).

    As stations get stronger, any low level high frequency stuff will get stronger.
    Really strong stations will seem really wide in some cases.
    Others (sdr) can cut the frequency like a knife and NOT be wide even when really strong.

    Some stations will transmit a very wide signal even though no one would open the bandwidth up that wide as you just pick up other qso's, AM and ssb.
    Ssb can get very close to an AM signal and not even notice it while the AM stations can hear the ssb VERY well...

    A good sdr receiver can allow you to select the passband exactly to fit the signal listened to, or even just one side of it.
    That helps when the QRM is on one side of the signal.
     
  3. K4KYV

    K4KYV Subscriber QRZ Page

    Some AMers as well as SSB ops have complained about intentionally wide AM signals from class-E rigs from a couple of stations near that frequency. It appears perfectly legal but still a matter of good operating practice, since there are no specific enumerated bandwidth limits to signals in the amateur rules, and that's a good thing. Best to just ignore it.
     
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  4. W4KJG

    W4KJG Subscriber QRZ Page

    One of the great things about an SDR is the waterfall, and in many other cases an "IF" spectrum that includes the lines showing the filter bandwidth while also showing the signal bandwidth.

    I love this feature, especially for 160/80/40 meter AM, but also for the very high quality SSB stations on 160 -20 meters who are using very high quality modulation through bandwidth filters wider than 2.5-2.7 kHz.

    I expect I'll take some grief for this. But the bands are nowhere near as crowded as they were 40-60 years ago, so a few extra kHz of bandwidth can be tolerated. I'm happy to hear a near in--person sounding audio signal. This is very much the opposite of nasty modulated/over-modulated garbage that has found it's way to the ham bands from 11 meters, that often occupies far more bandwidth with distortion, splatter, and intermods.
     
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  5. WA3VJB

    WA3VJB Ham Member QRZ Page

    Ken I agree with your observations against "nasty" audio that may have descended from 11 meters. I'll point out that in the past 20 years or so, we have discredited the ARRL's "spectrum conservation" schtick that they unsuccessfully tried to promote to curry favor with FCC regulators. Somewhere in the distant past, someone in Newington came up with the idea to hold SSB aloft as proof radio hobbyists were wisely using our allocations. But no one at the FCC really cared, given we are an unchannelized, mixed-mode venue of non-essential communications. I actually spoke with a long-retired FCC staffer to whom the "conservation" pitch was attributed, and he himself praised the sound of AM, cited experiences with an old National receiver when he was a youth, and essentially gave a pat on the back to those of us who favor the mode and activity on the shortwave ham bands.

    Carry on. There's little risk anymore you will catch grief for scaling your passband to available conditions.
     
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  6. N2DTS

    N2DTS Ham Member QRZ Page

    Another great thing about sdr's is you can do the same on TX!
    Many AM stations on can not change the passband of the transmit audio to fit the space between two other QSO's.
    Most sdr's will go out as wide as you set and not a bit over, no mater what you do with the microphone.
    The downside is they are all 25 watt carrier output....

    Also, there are a lot of broadcast transmitters on 80 meters (and 160) and they will tend to be strong and wide.
    A really strong signal will SEEM wider then one that is not as strong.

    I got a new little sdr receiver (R928) from China and the big signal guys overload it.
    Good antenna's at both ends and plenty of power, plus the right distance from the stations gives 50 over signals, I can listen to them on the dummy load in the basement!
    Only an SDR TX is going to maintain its bandwidth at those signal levels.

    Also, if someone runs a bc610, or a Globe King 500, I accept the signal they put out.
    Like a vintage car that does not have the safety equipment and emission controls, they should get a pass.



     
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  7. W2WDX

    W2WDX Subscriber QRZ Page

    I have always viewed the passband used as relative. When compared to SSB most AM stations seem to have vastly wide bandwidth. But it is relative. If SSB was not so limited (say the norm was 6KHz) would anyone be complaining? I do think good amateur practice is the way to go though. Being 20 or 25 KHz wide with some of the signal being nothing more than harmonic distortion, overload distortion and even IMD from the audio chain above 10KHz, makes no sense. I have heard this often on the air. More often with Class-E transmitters since these types can transmit these bad audio artifacts due to their inherently wide nature. It's not the transmitters, it's the operation of the audio gear preceding.

    I'm not saying there is anything wrong with these transmitters. On the contrary. However, they do offer challenges to operators in terms of how audio is employed pre-transmitter. Most hams couldn't even describe "unity gain" competently, let alone proper gain structure in an audio chain. This always amazed me in ham radio. My favorite is when I hear, "... looks good on the scope". Really?! You can see that on your oscilloscope? Really?! I use an SA to view my spectrum. However, I also know my audio chain has the correct gain structure.

    Narrowing the passband on receive on an SDR can help smooth out these operators. They may know a bunch about transmitters and those transmitters may be "clean" unto themselves, but the operators know little about how to minimize distortion in their audio chains, which also gets transmitted. They may seem wide, but in reality what is being transmitted at those extreme ends of the passband is usually just artifacts of improper audio levels and/or low quality hardware employed. No ... Alesis or Mackie (or other budget pro-audio brands) is not high quality, especially in the hands of audio amateurs.
     
    Last edited: Mar 19, 2019
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  8. K4KYV

    K4KYV Subscriber QRZ Page

    Or even 5 - 10 years ago. There has been a noticeable dwindling of activity from last year to this.
     
  9. WA1QIX

    WA1QIX Ham Member QRZ Page

    If I can ever get retired, I am going to complete a series of real-time, adjustable bandpass, zero-phase-shift, hardware based (i.e. no computer needed) audio filters. These will be available at a very reasonably cost. The DSP work is reasonably intense (at least for me, who is reasonable with math, but was not a math major). That's lots of math involved with DSP.

    When running remote control (which I do on a daily basis), and where DSP is obviously involved, it is very nice to have, essentially, a brick wall filter that does not start to introduce phase shift as the cutoff point is approached. This is a problem with any sort of analog audio filter (phase shift), but can be avoided with DSP.

    If a station had a high end of, say, 7.5kHz or 8kHz (daytime conditions) with a flat frequency response, and the passband were brick walled above this, fidelity would be excellent (all other things being equal) and the signal would be nicely contained. At night, or when the band is otherwise crowded, this could be brought down to something like 6kHz (frequency response not bandwidth), and things would still sound good.

    Obviously, the transmitter involved would need to be clean and not introduce appreciable distortion, otherwise the bandwidth containment measures would be largely mitigated.

    It's really amazing how much working at a job interferes with great projects like this one ;)
     
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  10. N1BCG

    N1BCG Ham Member QRZ Page

    Funny how this topic should come up. I'm alpha testing a variable bandwidth processor:

    [​IMG]

    Rick (W8KHK) and I decided to collaborate since I had an audio processor design that needed an output filter and he had a great filter circuit that allows continuous adjustment from 3 to 8 kHz (6 to 16 kHz bandwidth). I'm looking forward to the on-air sea trials!

    The LEDS on the left show NORM (yel) and PEAK (red) input levels as well as POS (grn) and NEG (blu) prevailing audio polarity.
     
    Last edited: Mar 20, 2019
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