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WHY LSB BELOW 9 MHz AND USB ABOVE
NOTICE
This
subject is highly controversial. Many agree with the synopsis below -
others disagree- take it for what is worth.
You have to see the circuitry
for early SSB transceivers to appreciate this -- but the easy
explanation is -- in the early days of SSB design, one of the common
SSB generating schemes used a 9 MHz carrier oscillator/IF. Anything
below that freq was inverted (LSB) compared to those freqs above it
(USB). So there was no USB/LSB switch, it automatically went to LSB for
frequencies below 9MHz and vice versa. The protocol has stayed with us
to this very day. But you can operate USB at 7 MHz and below if you want
and vice versa. Few do (or should) as it is a gentlepersons agreement
(not an FCC rule).
Another opinion from the
internet. Once upon a time we had 9 Mcs carrier generators for ssb. We
used surplus ARC-5 aircraft transmitters as a VFO. TWO MOST popular
bands were 75 and 20. Subtract 5 Mhz from 9 and there was 75. ADD 5
mhz to 9 and you had 20. The side bands were translated. So
there is the rest of the story why 75 was LSB and 20 was USB in general!
Another opinion from the
internet. The answer is not dependent on the ARC-5. The original rigs
generated the sideband signal at 9 MHz and either added 5 MHz to get 14
MHz or subtracted 5 MHz to get 4 MHz. The addition process preserves
the sideband (upper or lower) and the subtraction process inverts it.
Since nearly all rigs generated the 9 MHz signal as USB, we came to use
USB when adding and LSB when subtracting. Many hams used the ARC-5 as a
VFO for the mixer, but *any* 5 MHz VFO would do. ARC-5s were cheap and
easy but not required.
Someone else e-mailed me
complete with intricate math and vehemently sed that there is NO
inversion or translation --- hmmm I sed after being confused with the
math.
-------------------------------
This from Sweden
Dear Mr.
Dinkins,
I have "stumbled" across your very interesting web-site, and noticed
that you have some discussion about the reason for the amateur radio use
of LSB below 10 MHz and USB above.(As you probably know, the relevant
Radio Regulations explicitly prescribes the USB mode for all other
regulated SSB users).
In the early days of SSB, the frequency translation scheme of
contemporary SSB and ISB exciters used a signal processing IF in the
"few" MHz range (a common amateur IF was 5.2 MHz, and commercial IF's
were 2 and 3.1 MHz) which was mixed with a variable injection frequency.
You ended up with a sideband inversion when the IF was subtracted from
the injection frequency. (Example: to obtain 3.8 MHz LSB using 5.2 MHz
USB IF and 9 MHz injection). When the IF is added to the injection
frequency no inversion occurs. (Example: to obtain 14.2 MHz USB using
5.2 MHz USB IF and 9 MHz injection).
Commercial ISB exciters were often equipped with 4 independent sidebands
(often called the B2 or LLSB, B1 or LSB, A1 or USB and A2 or UUSB, their
relationships to the center frequency according to CCIR Recommendation
348-2) each containing a telephone channel or a voice frequency
telegraph system.
When international HF circuits using SSB/ISB became commonplace, it
turned out that there frequently were incompatiblity between the mixing
schemes, so there was a genuine risk that the two ends of a circuit were
using different mixing conventions, ending up in mirror-image audio
frequencies and VFT channel numbering and keying polarities.
For that reason the CCIR adopted the Recommendation 249 in 1959, in
which a provision was prescribed for inverting the ISB channel
arrangement if the operating frequency was on either side on 10 MHz.
Progress in the design of commercial receivers and exciters (for
example the Wadley loop that made IF's above the signal frequency range
practical) soon made this Recommendation obsolete, but it seems that the
radio amateur community still hang on to its provisions.
A good account of the reasoning behind the mixing schemes of that era
can be found in the first edition of "Single Sideband Principles and
Circuits" and in the articles "Die fernbedienbare Nachrichtensendeanlage
Elmshorn" and "Fernbedienbarer Steuervorsatz fur Kurzwellen-
Nachrichtensender", both in the December 1962 edition of the
"Telefunken-Zeitung".
73/ Karl-Arne Markstrom SM0AOM
Senior Radio Engineer
Maritime Networks
----------------------------------
And from Don WØPEA
Hi Rod, Here is the rest of the story........
Tony Vitale W2EWL who lived in Denille NJ
wrote an article in CQ in the early1950's entitled "Cheap and Easy
Sideband". It was a 9 Mhz USB phasing generator tweaked for optimum
suppression on LSB and an ARC-5 used as a 5 Mhz VFO. It summed the
9 Mhz USB signal and 5 Mhz VFO to work on 20 meters. Sum mixing
does not invert the USB signal. It used difference mixing to work
on 75 Meters, causing the USB signal to be inverted to LSB. At this
time commercial SSB rigs were virtually non existent. Shortly after
this article was published, Wes Schaum & Joe Batchelor formed
Central Electronics and utilized a similar design to make the 10A,
10B, & 20A. The few hams using "Ducktalk" had only the capability of
USB on 20 meters and LSB on 75 meters. Thus the convention was
set. When other rigs like the 10A, 10B, & 20A came along, they
followed the precedent that had been set by the "Cheap and Easy
Sideband" article by Tony Vitale. I met Tony in 1975 when I worked
for Cessna. Tony retired in the late '70's and died in the mid 80's.
I also met Russel Farnsworth in the 1968 when I
lived in Champaign Ill. but that is another story........ Don WØPEA
The Latest
on the controversy -- From N2EY
-------Original
Message-------
Hello,
Was just perusing your excellent website when I found the part
about why we hams use LSB on 75 and USB on 20.
Unfortunately, the myth about a 9 MHz SSB generator and 5 MHz
VFO is there. While that combo allows sum and difference mixing
to reach both bands, the sideband *is not* inverted. This isn't
a question of opinion - it's just basic math of how SSB and
mixers work. There's also a part about the W2EWL "Cheap and Easy
SSB" exciter, which has several errors in it. I'm sure they're
unintentional errors, but they're still wrong.
Here's what I found: Quoting the website
http://www.ac6v.com/73.htm#LSB
Tony
Vitale W2EWL who lived in Denille NJ wrote an article in CQ
in the early1950's entitled "Cheap and Easy Sideband".
Tony Vitale
was indeed W2EWL, but the article appeared in QST, not CQ.
March, 1956. I have the issue and can scan it if you want proof.
>It was a 9 Mhz
> USB phasing generator tweaked for optimum suppression on LSB
and an
> ARC-5 used as a 5 Mhz VFO. It summed the 9 Mhz USB signal and
5
> Mhz VFO to work on 20 meters. Sum mixing does not invert the
USB
> signal. It used difference mixing to work on 75 Meters,
causing
> the USB signal to be inverted to LSB.
NO. It doesn't work that way. The sideband does not invert on
either band using the mixing scheme W2EWL used. And there's a
sideband switch included so that the right sideband can be
chosen. The alignment procedure includes making the adjustments
so that the unwanted sideband rejection is equally good for both
positions of the sideband selector switch.
>At this time commercial SSB
> rigs were virtually non existent.
No, that's just not true.
I pulled the March 1956 QST off the shelf to be sure. In that
issue, which carried "Cheap and Easy SSB" for the first time,
the following SSB rigs are advertised:
- Collins KWS-1 and 75A4
- B&W 5100 with 51SB SSB adapter, plus 370 receiving adapter
- Hallicrafters HT-30 exciter, HT-31 linear amplifier, and
SX-100 receiver
- RME 4300 receiver with 4301 SSB receiving adapter
- Central Electronics 10B and 20A exciters, 600L *no-tune*
linear amplifier, plus Model A and Model B receiving adapters
- Eldico SSB-100A transmitter and SSB-500 linear amplifier
- P&H LA-400 linear amplifier
- Lakeshore Phasemaster II transmitter and P-400-GG linear
amplifier.
Just for the heck of it I looked at QST for March 1955 - a year
before the "Cheap & Easy SSB" article appeared. Advertised in it
are:
- Collins KWS-1 and 75A4
- B&W 5100 with 51SB SSB adapter, plus 370 receiving adapter
- Hallicrafters HT-30 exciter, HT-31 linear amplifier, and SX-96
receiver
- Central Electronics 10B and 20A exciters, plus receiving
adapter
- Lakeshore Phasemaster Junior transmitter
Now I grant that these rigs weren't inexpensive, and that there
was a far wider selection of AM gear. But there was no shortage
of SSB gear for the ham when W2EWL's article appeared.
>Shortly after this article was
> published, Wes Schaum & Joe Batchelor formed Central
Electronics
> and utilized a similar design to make the 10A, 10B, & 20A.
They used the same frequency scheme but CE was producing rigs
long before the W2EWL article appeared. In fact, the 20A is
advertised as a new rig in QST for November 1953, and the 10A
was first advertised in QST in September of 1952. CE's first rig
predates the W2EWL article by 3-1/2 years!
The few
> hams using "Ducktalk" had only the capability of USB on 20
meters
> and LSB on 75 meters. Thus the convention was set. When
other
> rigs like the 10A, 10B, & 20A came along, they followed the
> precedent that had been set by the "Cheap and Easy Sideband"
> article by Tony Vitale.
Nice story but it cannot be true. Both the W2EWL and CE exciters
can do either sideband equally well. And the CE rigs predate the
W2EWL article by *years*, as shown above. W2EWL did not invent
the LSB/USB convention at all. I don't think he ever claimed to,
either. His rig would do either sideband on either band equally
well. With phasing rigs, all it takes is a DPDT switch to
reverse the phase of one audio channel. This isn't an opinion -
it's verifiable facts.
Here's a simple explanation of the mixing scheme: If you
generate USB at 9 MHz, the carrier is at 9 MHz and the sideband
is on the upper side of 9 MHz. Add 5 MHz and the carrier will be
at 14 MHz and the sideband will be on the upper side of 14 MHz,
because all you did was add 5 MHz to every frequency in the
signal.
If you generate USB at 9 MHz, the carrier is at 9 MHz and the
sideband is on the upper side of 9 MHz. Subtract 5 MHz and the
carrier will be at 4 MHz and the sideband will be on the upper
side of 4 MHz, because all you did was subtract 5 MHz from every
frequency in the signal. That's how it works. No sideband
inversion from a 9 MHz SSB generator and 5 MHz VFO. The USB/LSB
thing came from elsewhere. The myth lives on because too many
hams repeat it without checking the math, nor actual sources of
info.
Thanks for reading. 73 de Jim, N2EY
More on the
controversy
-------Original Message-------
From:
Alan Larson
Date: 10/17/2007 11:34:51 AM
To: ac6v@arrl.net
Subject: sideband generation
The
subject came up on the QRP mailing list, and one of the
participantsquoted your web site. Going back to the web site, I
see that the actual quote was from W0PEA. Anyway, it doesn't
work. Just as you say others wrote that the math doesn't work
when you presented it above that point in the page, it doesn't
work here.
Here is
why: If you start with a USB SSB generator (of any type) at 9
MHz, and mix it with a signal around 5.2 MHz, you get 14.2
MHz. Now, let's see if it comes out USB or LSB.
To test the resulting sideband, we take a bit of the upper
sideband
signal from that
generator, and see where it lands relative to the
(hopefully
suppressed) carrier.
carrier
sideband
------- - -------
9.0 MHz 9.0001 MHz
5.2 MHz 5.2 MHzbsp;MHz
sum 14.2 MHz 14.2001 MHz
so this comes out as upper sideband.
Now, trying it for the difference
carrier
sideband
------- -
-------
9.0 MHz 9.0001 MHz
5.2 MHz 5.2 MHzbsp;MHz
diff. 3.8 MHz 3.8001 MHz
Again, the
sideband comes out above the carrier, so the result is
also upper
sideband.
In order for this
to work, it appears that the sideband generation must
be at the lower
frequency, so that the slightly higher sideband frequencies
will be subtracted
from the reference, and will give lower results.
Alan
wa6azp
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