[Yaesu] FM Receive on FT-757GX

[Glen Zook]

Although the commercial designation for +/- 5 KHz
deviation has been called "narrow band", if you look
back at the amateur FM operation from the 1940s and
1950s, you will definitely see the designation NBFM
and the designation Narrow Band Frequency Modulation. 
Many receiver manufacturers of the era made NBFM
adapters for their receivers (i.e. Collins Radio with
the 148C-1 NBFM Adaptor Unit for the 75A2 and 75A3
receivers from 1950 onward).  Also, quite a number of
transmitters, or frequency oscillators (VFOs, etc.)
came with NBFM capabilities (i.e. Hallicrafters
HT-18).

Frankly, I have been involved with FM commercial
communications since 1965, and with amateur FM since
1959, and I have never heard of the term "Sliver
Band"!  I have heard the term "Belly Band" for +/- 7.5
KHz which a number of amateurs ran in the mid to late
1960s when the trend was towards +/- 5 KHz deviation
(or "narrow band" per commercial specifications) which
allowed sufficient audio recovery for the wide band
receivers and yet was narrow enough not to "chop out"
in the narrow band receiver of that era.  Also, I was
the first FM Editor of CQ Magazine from 1971 to 1973
and owned the Motorola reconditioned equipment center
for the south-central US from 1970 until Motorola went
out of that end of the business in late 1979, and I
never heard the term therein as well!

You are correct about the modulation index being not
to exceed 1.  However, the vast majority of equipment
being manufactured these days for operation below 30
MHz has audio limited so that the maximum frequency
does not exceed 3 KHz.  This is due to the filters,
etc., limiting the audio frequency bandpass for SSB
operation.  Thus, with a 3 KHz maximum audio
frequency, a deviation of +/- 3 KHz is the maximum
allowed.

Of course, commercial FM audio requirements have
resulted in narrower signals as well.  Even on those
frequencies where +/- 5 KHz deviation is still
allowed, any transmitters that have been type-accepted
for quite a number of years are now restricted to 16F3
type of emission.  This equates to a maximum deviation
of +/- 5 KHz but a maximum audio frequency of 3 KHz. 
The older equipment (which most people consider to be
better "sounding") was type accepted for 20F3 which is
+/- 5 KHz deviation with a maximum audio frequency of
5 KHz.

Now days, on the 900 MHz band, etc., maximum deviation
is considerably reduced, as is the frequency spacing
between channels.  Also, the stability now required is
something that was completely un-heard-of twenty years
ago!

As a practical matter, I have found the best way to
set deviation to +/- 5 KHz (or to any other deviation
as well) is with a calibrated oscilloscope output
(i.e. on a service monitor) while voice modulating. 
By setting the "IDC" (instantaneous deviation control)
to +/- 4.7 KHz (or similar if the deviation is less),
this insures that the transmitter will not "over
deviate" and get you in trouble with the FCC.  Also,
many two-way technicians say the number "five" over
and over when setting deviation.  This particular word
seems to have more "energy" in it than any other
common word.  I have seen quite a number of
transmitters that were set using tone modulation
instead of voice modulation actually "over deviate"
when voice was applied.  This happened even though all
of the "proper" procedures were followed using a tone
(i.e. 1 volt, 1000 Hz tone, etc.).  Thus, whenever I
set deviation (or, when I employed a number of
technicians I also insisted that this be done), I
always use voice modulation.  However, there are
definitely people in the business that use tones for
setting of the deviation level.

By the way, until 1957 with the first "splitting" of
the FM channels, very wide deviation was allowed (i.e.
Motorola FMTRU 30D transmitters).  This was well in
excess of +/- 15 KHz.  When the FCC first introduced
the 60 KHz channel spacing (on "high band" - 40 KHz
channel spacing on "low band") effective 1957, quite a
number of the older units lost type acceptance and had
to be removed from service.  This was a real "boone"
to the amateur radio ranks with the influx of Motorola
FMTRU 30D transmitters, older General Electric, Link,
etc. units.  A lot of these went on 147.300 MHz. 
Previous to 1957 the channel spacing was 120 KHz on
"high band" and 80 KHz on "low band".

When the channels were again reduced in spacing in
1965, more of the radios had type acceptance revoked
(i.e. Motorola 5V series).  This again was a "boone"
to amateur radio.  The channel spacing was reduced to
30 KHz on "high band" and 20 KHz on "low band".  Of
course, the 450 MHz channels came from 200 KHz to 100
KHz to 50 KHz to 25 KHz over the years as well.

Anyway, I have never heard the term "sliver band"
applied to what the amateurs of the 1940s, 1950s, and
1960s called NBFM!

Glen, K9STH


--- "Christopher (Chris) W. Boone" 

No, actually the rules allow a Modulation Index of 1
at the highest freq of modulation, so IF you modulate
the xmtr with a 3kHz tone and set it for 3 kHz dev,
you're legal...but with voice, or a 1 kHz tone
standard, you'll find thats different...
 
Also its called SLIVER BAND; has been for
decades......
5 kHz dev is NBFM...15 kHz was and is still (though
noone uses it) WideBandFM.....


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