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References: <E8d711BBdibNFwVq@g4dmp.fsnet.co.uk> <80823E80F3B94DCEB2B95EA9A57C72C4@JimPC>
Date: Wed, 2 Mar 2011 16:20:48 +0000 (GMT)
From: M0FMT <m0fmt@yahoo.co.uk>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <80823E80F3B94DCEB2B95EA9A57C72C4@JimPC>
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Subject: Re: LF: Re: WRC-2012 proposal for 'near' 500kHz band(s)
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Wonderfull! and=A0maybe you can help me grind all my crystals lower in fre=
quency=20
as well Jim.
=A073 es GL Pete M0FMT IO91UX=20




________________________________
From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Wed, 2 March, 2011 15:43:25
Subject: LF: Re: WRC-2012 proposal for 'near' 500kHz band(s)

Dear David, LF Group,

Having poked around inside quite a few radios over the years, I think 465/=
470kHz=20
IFs were common in the days of discrete transistors and tuned IF transform=
ers,=20
but are mostly ancient history now. Most of the more modern broadcast radi=
os use=20
either 455kHz or 450kHz - the former being the default centre frequency fo=
r=20
ceramic filters, the latter especially for radios with PLL synthesisers (i=
t=20
works out quite nicely with a 4.500MHz reference crystal and 9kHz or 10kHz=
=20
channel spacings). At least in theory, the more modern cheap broadcast rad=
ios=20
should have better IF rejection, since these single-chip receivers have do=
uble=20
balanced mixers, rather than the old single-ended, self-oscillating, discr=
ete=20
designs.

The trouble with trying to avoid broadcast RX IF frequencies is that there=
 is=20
now a wide range to be avoided. With f0 possibly of 450, 455, 465 or 470kH=
z, +/-=20
tolerance, and a bandwidth typically around 9kHz, a big chunk of frequenci=
es=20
from say 440 - 480kHz would have to be excluded on these grounds. It would=
 be a=20
great waste of spectrum to say these frequencies must be off-limits for al=
l=20
eternity for the sake of the inadequacies of cheap receivers used by an=20
ever-decreasing number of listeners. Anyway, these frequencies always have=
 been=20
used by high power maritime telegraphy, NDBs, etc.

Another consideration is that the proposed maritime "super-Navtex" that mi=
ght be=20
centerd on 500kHz will inevitably have big powerful transmitters, using OF=
DM=20
modulation with several kHz bandwidth. Although the out-of band emissions=
 for=20
this type of modulation is in principle very small, there will inevitably=
 be=20
some "spectral re-growth" due to imperfections in transmitter linearity.=
 The=20
high power levels compared to amateur signals, and wide coverage areas wil=
l mean=20
this would be a major problem for amateurs if we have to operate on adjace=
nt=20
frequencies (not unlike the problems with utilities on 136k). There would=
 be no=20
way of convincing the maritime users they should take remedial action - th=
ey=20
would just reply that their transmitters were fully within spec, which the=
y=20
would be. So if the maritime proposals go ahead, a large spacing between=
 any=20
amateur band and 500kHz would be a good thing. I volunteer to help struggl=
ing=20
experienced non-appliance operators to wind a few more turns on their VFO=
 coils=20
if this happens ;-)

Cheers, Jim Moritz
73 de M0BMU


     =20

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<html><head><style type=3D"text/css"><!-- DIV {margin:0px;} --></style></h=
ead><body><div style=3D"font-family:arial, helvetica, sans-serif;font-size=
:14pt"><DIV></DIV>
<DIV>&nbsp;</DIV>
<DIV>Wonderfull! and&nbsp;maybe you can help me grind all my crystals lowe=
r in frequency as well Jim.<BR>&nbsp;</DIV>73 es GL Pete M0FMT IO91UX=20
<DIV><BR></DIV>
<DIV style=3D"FONT-FAMILY: arial, helvetica, sans-serif; FONT-SIZE: 14pt">=
<BR>
<DIV style=3D"FONT-FAMILY: arial, helvetica, sans-serif; FONT-SIZE: 13px">=
<FONT size=3D2 face=3DTahoma>
<HR SIZE=3D1>
<B><SPAN style=3D"FONT-WEIGHT: bold">From:</SPAN></B> James Moritz &lt;jam=
es.moritz@btopenworld.com&gt;<BR><B><SPAN style=3D"FONT-WEIGHT: bold">To:<=
/SPAN></B> rsgb_lf_group@blacksheep.org<BR><B><SPAN style=3D"FONT-WEIGHT:=
 bold">Sent:</SPAN></B> Wed, 2 March, 2011 15:43:25<BR><B><SPAN style=3D"F=
ONT-WEIGHT: bold">Subject:</SPAN></B> LF: Re: WRC-2012 proposal for 'near'=
 500kHz band(s)<BR></FONT><BR>Dear David, LF Group,<BR><BR>Having poked ar=
ound inside quite a few radios over the years, I think 465/470kHz IFs were=
 common in the days of discrete transistors and tuned IF transformers, but=
 are mostly ancient history now. Most of the more modern broadcast radios=
 use either 455kHz or 450kHz - the former being the default centre frequen=
cy for ceramic filters, the latter especially for radios with PLL synthesi=
sers (it works out quite nicely with a 4.500MHz reference crystal and 9kHz=
 or 10kHz channel spacings). At least in theory, the more modern cheap bro=
adcast radios
 should have better IF rejection, since these single-chip receivers have=
 double balanced mixers, rather than the old single-ended, self-oscillatin=
g, discrete designs.<BR><BR>The trouble with trying to avoid broadcast RX=
 IF frequencies is that there is now a wide range to be avoided. With f0=
 possibly of 450, 455, 465 or 470kHz, +/- tolerance, and a bandwidth typic=
ally around 9kHz, a big chunk of frequencies from say 440 - 480kHz would=
 have to be excluded on these grounds. It would be a great waste of spectr=
um to say these frequencies must be off-limits for all eternity for the sa=
ke of the inadequacies of cheap receivers used by an ever-decreasing numbe=
r of listeners. Anyway, these frequencies always have been used by high po=
wer maritime telegraphy, NDBs, etc.<BR><BR>Another consideration is that=
 the proposed maritime "super-Navtex" that might be centerd on 500kHz will=
 inevitably have big powerful transmitters, using OFDM modulation with sev=
eral kHz
 bandwidth. Although the out-of band emissions for this type of modulation=
 is in principle very small, there will inevitably be some "spectral re-gr=
owth" due to imperfections in transmitter linearity. The high power levels=
 compared to amateur signals, and wide coverage areas will mean this would=
 be a major problem for amateurs if we have to operate on adjacent frequen=
cies (not unlike the problems with utilities on 136k). There would be no=
 way of convincing the maritime users they should take remedial action -=
 they would just reply that their transmitters were fully within spec, whi=
ch they would be. So if the maritime proposals go ahead, a large spacing=
 between any amateur band and 500kHz would be a good thing. I volunteer to=
 help struggling experienced non-appliance operators to wind a few more tu=
rns on their VFO coils if this happens ;-)<BR><BR>Cheers, Jim Moritz<BR>73=
 de M0BMU<BR><BR><BR></DIV></DIV></div><br>


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