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[aprssig] APRS Bandwidth

Steve Noskowicz noskosteve at yahoo.com
Wed Jun 24 01:42:26 UTC 2009




I never considered that this can be analyzed either as FM or AM.
What Steve S. has is completely correct except for the fact that the modulation won't be square waves limited by the transceiver.  At least it shouldn't be or rather needn't be.  Only the first set if sidebands should be needed (and are in the calculation).  
The SSB transmitter would allow more (>500 Hz.) if a real square wave is used.

Thinking about it, I don't think FSK teletype does any band limiting (at least it didn't when I was doing it), but just switches between the two frequencies, therefore taking more BW than ncessary.

I wonder what modern transceivers do that have FSK.

73, Steve, K9DCI

--- On Tue, 6/23/09, Stephen H. Smith <wa8lmf2 at aol.com> wrote:
> 
> 
>   
>   
> 
>  
> SEE BELOW.....
> 
> 
> 
> 
> 
> Frank or Barbara Rossi wrote:
> 
>   
>   
> Looks like IARU-Region 1 Band plan is way to restrictive Click  
>   http://www.ari.rc.it/Download%20vario/bandplan.pdf
> 
> With all the restrictions, there will be no APRS on 30
> Mtrs.
> 
> So much for a world wide band plan.
> 
> N3FLR - Frank
> 
>   
> 
> On 6/23/2009 6:37 AM, G0JXN Jim wrote:
>   
>     
>     
>     
>     
>     Hi Guys
>      
>     The UK Amateur Radio
> license no
> longer allows the operation of an APRS digipeater
> without special
> permission (Notice of Variation) from the regulator
> (Ofcom). It being
> filtered first through the RSGB.
>      
>     I wish to run a
> digipeater on
> 10.151MHz (KAM) but am advised the it would probably not be
> recommended
> by the RSGB because the bandwidth of 300bd would exceed the
> 500Hz laid
> down in the IARU bandplan for 30m. The implication being
> that we should
> not operate APRS on 30m at all.
>      
>     Monitoring signals on
> 30m I find
> that they all seem to be within 400Hz but I will need to
> prove that on
> paper.
>      
>     Can anyone please tell
> me what the
> bandwidth will be for a properly adjusted 300bd station and
> how this
> might be calculated?
>      
>     73
>      
>     Jim,
> G0JXN
>     
>   
> 
> 
> 
> 
> 
> See my previous response to this thread under the heading:
> 
>     "Re: [aprssig] APRS Bandwidth -
> Carson's Rule Recalculated"
> 
> below.
> 
> 
> 
> 300 baud / 200 Hz shift AX.25 packet is perfectly capable
> of fitting
> into a 500 Hz occupied bandwidth.   Aside from
> the issue of "unattended
> operation" mentioned in the bandplan document
> mentioned above, I don't
> see any problems. HF APRS is certainly far less disruptive
> than the
> Pactor III being used with the AirMail mail box system on
> HF.  
> 
> 
> 
> 
> 
> 
> 
> Keith VE7GDH wrote:
> 
> 
> 
> 
> Regarding 500 Hz bandwidth... aren't 300 baud tones
> just 200 Hz apart?
>   
> 
> I viewed the reply from Chris G4HYG about Carson's
> rule...
>   
> 
>   http://en.wikipedia.org/wiki/Carson_bandwidth_rule.
> Doesn't that apply
>   
> 
> to FM? See
>   
> 
> 
> 
> 
> You are partly correct.  Carson's rule applies to
> modulating audio or
> data frequencies applied to an FM transmitter. 
> 
> For SSB, I would have thought the
> 200 Hz
> difference between
>   
> 
> the two tones would set the bandwidth.
>   
> 
>   
> 
> 
> 
> 
> Sending FSK data with an SSB transceiver is more like FM
> than SSB. 
> 
> Applying a single audio tone to the mic jack of an SSB rig
> creates a
> single RF frequency offset from the (suppressed) carrier
> frequency by
> the frequency of the audio tone. 
> 
> 
> 
>   
> 
> Applying an audio tone 200 Hz higher to to the SSB rig will
> create a
> single RF frequency 200 Hz farther away from the
> (suppressed) carrier
> frequency.   
> 
> 
> 
> 
> 
> The net effect is exactly the same as if you have a single
> key-down CW
> carrier and shift it 200 Hz periodically (i.e. direct FSK
> with a 200 Hz
> shift)       
> 
>      [Some, mostly higher end, HF
> transceivers actually offer this
> direct FSK mode.  Rather than having feed alternating
> audio tones into
> the mic jack, you feed a TTL-logic-level 300 baud (for
> packet) or 45
> baud (for RTTY)  data stream directly into a special
> FSK input on the
> radio.] 
> 
> 
> 
> 
> 
> The bottom line is that you essentially have a FM (i.e.
> constant
> envelope power) transmitter modulated by 150 bps square
> waves rather
> than rather than higher-frequency audio sine waves.  
> 
> 
> 
> 
> 
> 
> [Note that "300 baud" (300 bits / second)
> corresponds to only 150 Hz
> square waves.  Assuming the worst case of alternating
> "1"s and "0"s,
> each square wave cycle will require TWO bits to complete --
> not one
> since one bit has to be the high half of the square and
> next bit the
> low half. Any bit pattern that  has adjacent
> "1"s or adjacent "0"s will
> have a lower effective "squarewave
> frequency".  ]
> 
> 
> 
> 
> In either case, the occupied bandwidth is greater than just
> the the
> spacing between the two tones (a.k.a. "mark" and
> "space" frequencies). 
> This is due to the transients created during the
> instantaneous shift
> from one frequency to the other.    [Even CW
> transmission has a
> bandwidth greater than a single frequency when you start
> keying at a
> rate higher than zero!]
> 
> 
> 
> 
> The original poster that applied Carson's rule for
> approximating
> occupied FM bandwidth forgot that the base band modulating
> "frequency"
> would be 150, not 300 Hz.  Recalculating the occupied
> bandwidth,
> 
> 
> 
> 
>      2 (100 Hz deviation + 150 bps)
> =  500 Hz.    
> 
> 
> 
> 
> 
> Considering that  the actual data
> "frequency" is actually lower than
> 150 Hz since runs of 2 or 3  ones or zeros DO
> frequently occur, the
> actual average occupied bandwidth is actually lower. 
> 
> 
> 
> 
> [Note that the occupied bandwidth created by true
> square-wave
> modulation would be greater than for sine waves. However,
> the finite
> bandwidth of the SSB modulator and IF filters "rounds
> off" the corners
> of the square waves and increases the rise/fall time to be
> more
> trapezoidal than square, resulting in a bandwidth nearly
> the same as
> with sine waves.]
> 
> 
> 
> 
> I can attest to the fact that this is "REAL"
> since I have no difficulty
> receiving 200 Hz HF packet through an 500 Hz bandwidth CW
> filter on HF
> transceivers.
> 
> 
> 
> 
> 
> 
> ------------------------------------------------------------------------
> 
> 
> 
> 
> --
> 
> 
> 
> 
> Stephen H. Smith    wa8lmf (at) aol.com
> 
> 
> EchoLink Node:      WA8LMF 
> or 14400    [Think bottom of the 2M band]
> 
> 
> Skype:        WA8LMF
> 
> 
> Home
> Page:         
> http://wa8lmf.net
> 
> 
> 
> 
> JavAPRS Filter Port 14580 Guide
> 
> 
>  http://wa8lmf.net/aprs/JAVaprsFilters.htm
> 
> 
> 
> 
> "APRS 101"  Explanation of APRS Path
> Selection & Digipeating
> 
> 
>  http://wa8lmf.net/DigiPaths
> 
> 
> 
> 
> Updated "Rev H"
> APRS           
> http://wa8lmf.net/aprs
> 
> 
> Symbols Set for UI-View,
> 
> 
> UIpoint and APRSplus:
> 
> 
> 
> 
>  
> 
> 
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> 
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