Why do we use LSB below 10Mhz and USB above 10Mhz when operating SSB HF?

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یکی از سوال های اکثر رادیو آماتورها  علت انتخاب LSB در فرکانس کمتر از 10مگا هرتز و استفاده از مود USB  برای فرکانس بالاتر از 10 مگاهرتز در باند HF می باشد

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The changing of ISB sideband positions at 10 MHz actually has an engineering background.

In the earliest ISB exciters, it was found appropriate to change the final mixer scheme from subtraction to addition mixing at around 10 MHz due to spurious suppression concerns. This sometimes caused interoperability problems in international point-to-point ISB circuits.

To overcome this, a practice was formalized in 1959 by the ITU/CCIR as the Recommendation 249, which prescribed that the ISB sideband positions could either automatically or manually be interchanged when the output frequency went through 10 MHz. An often quoted reference where ISB exciter design considerations are handled with German thoroughness is W Kleische: "Fernbedienbarer Steuervorsatz für Kurzwellen-Nachrichtensender" in "Telefunken-Zeitung" December 1962.

Later advances in exciter design made this Recommendation obsolete, and later generations of exciters only had this facility as an option, and current production ISB equipment lacks it entirely.

http://ham.stackexchange.com/questions/1336/why-do-we-use-lsb-below-10mhz-and-usb-above-10mhz-when-operating-ssb-hf

 

 

AB1GA Ham Member QRZ Page

The standard operating practice for SSB is, as 'KLI says, to use LSB below 10 MHz, and USB above 10 MHz. This is not a regulation, nothing forbids you from using the "other" sideband, but then who would you talk to?

The convention is, so I've heard, largely historical, based on the design of an early SSB rig. The IF was 9 MHz, which is 5 MHz below the bottom of 20 meters and 5 MHz above the top of 80 meters. When mixed with a 5-5.5 MHz VFO, the sum frequency was at 14.0-14.5 MHz, which covered 20 meters, and the difference frequency was at 4.0-3.5 MHz, which covered 80 meters.

I wrote the 80 meter frequency range backward to indicate that with this approach the higher the VFO frequency, the lower the mixer output frequency. This also meant that if you started with an upper sideband signal on 9 MHz, you still had a USB signal on 20, but an LSB signal on 80, and thus the convention was born.

Now, with the digital modes, all bets are off. I've heard old RTTY ops say that RTTY is LSB everywhere, and new PSK ops say that PSK is USB everywhere, and that if things don't work to just use the software "invert" switch. I'll leave it to someone else to enlighten us on that part.

73,

 

Back in the late 1940s and early 1950s, a great number of SSB generator circuits used 9 MHz (mainly "filter" designs) because if you used a 5 MHz VFO frequency to "mix" with the signal, you could get both the 75 meter and the 20 meter bands. There were a lot of BC457 and BC458 ARC-5 transmitters around that would work very well as a 5 MHz VFO. However, due to the "mixing" scheme, lower sideband was produced when operating on 75 meters and upper sideband was produced when operating on 20 meters.

http://forums.qrz.com/index.php?threads/usb-lsb-hf-band-plan.6997/