from  http://ac6v.com/73.htm

 
 

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 WPEA

 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 WPEA


The Latest on the controversy -- From N2EY

-------Original Message-------

From: N2EY@aol.com
Date: 10/16/07 19:50:20
To: ac6v@arrl.net
Subject: LSB/USB Urban Legend

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