An Urban Legend Disproved

Discussion in 'Ham Radio Discussions' started by N2EY, Jan 1, 2015.

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  1. N2EY

    N2EY XML Subscriber QRZ Page

    Since the 1940s, radio amateurs using SSB on the HF amateur bands have generally followed a standard or tradition about which sideband to use. The almost-universal practice is that lower sideband (LSB) is used on 160, 75 and 40, while upper sideband (USB) is used on 20, 17, 15, 12 and 10.


    And almost as long as that, hams have asked: Where did this practice/tradition originate?


    First, a bit of history.

    Back in the late 1940s and early 1950s the HF amateur 'phone bands available to US hams were somewhat different than today:


    160 had been a popular pre-WW2 band, but was full of LORAN and was only available to US hams in a very limited way.

    40 was all-CW - no phone operation permitted to US hams at all.

    15 did not become a ham band until about 1954.

    30, 17 and 12 meters did not become ham bands until 1979 or later.

    This left 75, 20, 11 and 10 meters for 'phone operation. (11 was a ham band in the USA until 1958). Of those, 75 and 20 were the most popular for 'phone operation - and the most crowded.


    So it was on 75 and 20 that amateurs first began using SSB in considerable numbers, and there were various schemes used to generate SSB on those two bands.

    One popular method was to generate the SSB at about 9 MHz, and then mix the 9 MHZ SSB signal with the output of a VFO in the 5.0 to 5.5 MHz range. Subtractive mixing would produce 75 meter output, while additive mixing would give 20 meter output. Like this:

    For 20 meters:

    9000 kHz + 5000 kHz = 14000 kHz
    9000 kHz + 5500 kHz = 14500 kHz


    For 75/80 meters:

    9000 kHz - 5000 kHz = 4000 kHz
    9000 kHz - 5500 kHz = 3500 kHz

    The 9 MHz SSB could be generated by the filter or phasing method, with a highly-stable VFO in the 5 MHz range, and the only thing you needed to do to switch bands was to change the tuned circuits in the output of the transmitting mixer and following RF amplifier stages. The VFO tunes "backwards" on 75, but that's no big deal if you mark the dial.


    Now for the urban legend:

    Some folks say that the above system inverts the sideband on one band but not the other, and so gave rise to the LSB-on-75/USB-on-20 tradition. They point to many old rigs which use the scheme as proof. Sometimes a specific rig is named as the origin.

    It's a nice story but it's not true - because it cannot be true. Heterodyning doesn't work that way. In order to invert the sideband, the local oscillator - the one used for conversion - must be higher in frequency than either the input or output signals.


    Here's a simple explanation of the mixing scheme:

    For 20 meters, 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. No sideband inversion.

    For 75 meters, 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.

    Not convinced? Here it is in more detail:


    Suppose we generate a USB signal that is 3 kHz wide, with the carrier at 9000 kHz. The signal will then consist of the (suppressed) carrier at 9000 kHz, and the upper sideband extending from 9000 to 9003 kHz.

    To put the signal on 20 meters, we add the output of a VFO at, say, 5000 kHz, to the SSB signal.

    9000 + 5000 = 14000 (suppressed carrier and one end of sideband)
    9003 + 5000 = 14003 (other end of sideband)

    What we have is an upper sideband signal - the suppressed carrier is at 14000 kHz and the sideband is above that frequency, extending to 14003 kHz. USB in gives USB out.

    To put the signal on 75 meters, we use subtractive mixing:

    9000 - 5000 = 4000 (suppressed carrier and one end of sideband)
    9003 - 5000 = 4003 (other end of sideband)

    What we have is still an upper sideband signal - the suppressed carrier is at 4000 kHz and the sideband is above that frequency, extending to 4003 kHz.

    USB in gives USB out - on both bands! No fancy math, just plain old addition and subtraction. Try it with LSB - you get the same sideband out as you started with. No inversion.


    So....where did the tradition come from?

    It turns out that, at the dawn of SSB use by amateurs, there were rigs built which used an SSB generator at around 5 MHz and a VFO around 9 MHz - the opposite of the above system. Because the VFO is the highest frequency on 75 but not 20, the sideband inverts on 75 but not 20. I have been told, but have not confirmed, that this derived from early commercial use of SSB.

    Here's proof:


    Suppose we generate a USB signal that is 3 kHz wide, with the carrier at 5000 kHz. The signal will then consist of the (suppressed) carrier at 5000 kHz, and the upper sideband extending from 5000 to 5003 kHz.


    To put the signal on 20 meters, we add the output of a VFO at, say, 9000 kHz, to the SSB signal.

    9000 + 5000 = 14000 (suppressed carrier and one end of sideband)
    9000 + 5003 = 14003 (other end of sideband)

    What we have is an upper sideband signal - the suppressed carrier is at 14000 kHz and the sideband is above that frequency, extending to 14003 kHz. USB in gives USB out.


    To put the signal on 75 meters, we do subtractive mixing instead of additive:.

    9000 - 5000 = 4000 (suppressed carrier and one end of sideband)
    9000 - 5003 = 3997 (other end of sideband)

    What we have is a lower sideband signal - the suppressed carrier is at 4000 kHz and the sideband is below that frequency, extending to 3997 kHz. USB in gives USB out on 20, but LSB out on 75!

    No fancy math, just plain old addition and subtraction.
     
    K2NCC likes this.
  2. W3WN

    W3WN Ham Member QRZ Page

    Jim,

    OK if I use this as a newsletter article?
     
  3. N2EY

    N2EY XML Subscriber QRZ Page

    Sure! With attribution, of course.

    I can dress it up more if you want.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I use whatever the other guy is using.:eek:

    On CW, I use "CW," that seems to work.
     
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    A summary for those who need it:

    -Jim debunks one myth and replaces it with an alternate myth he can't confirm.

    -From a technical perspective, Jim tells us that sideband selection customs aren't related to mixing, then tells us that it is.

    So much for Jim's annual spleen venting on sideband selection! Same time next year, folks. :) bill
     
    Last edited: Jan 1, 2015
  6. NC5P

    NC5P Ham Member QRZ Page

    When I was in MARS most of our operations were below 10 MHz and USB. So we often had trouble with some new people attempting to check in on the wrong sideband. Some modern radios automatically select LSB for lower frequencies, adding to the mistakes.
     
  7. K5GHS

    K5GHS Ham Member QRZ Page

    My FT857 automatically does it as well. It will also prevent transmission out of band, but only in the sense that, for example, I can TX on 14349.99 USB, but it will error on 14350.00.

    You do still have to watch what you are doing, but they do default to the "convention" as well. I don't think you can override those conventions either.
     
  8. WJ4U

    WJ4U Subscriber QRZ Page

    Sometimes I have to use the WC. :cool:
     
  9. W3WN

    W3WN Ham Member QRZ Page

    Of course. That should go without saying, I'm a firm believer in giving credit (or blame) where credit is due.

    No need, but if you want to, I won't turn you down.
     
  10. N2EY

    N2EY XML Subscriber QRZ Page

    What myth?

    The use of an SSB generator at 5 MHz and a 9 MHz local oscillator is documented in a QST article from June, 1948, pages 36 to 42. Don Norgaard, W2KUJ, was an early adopter and promoter of SSB.


    No, I don't.

    What I wrote was that the sideband only inverts if the local oscillator frequency is higher than both the input and output frequencies.

    Hey, I was asked to explain how it worked.
     
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