RadioBanter - "Ham speak"

On May 13, 11:44�am, "Don" wrote:
On Sun, 13 May 2007 12:09:21 +0000 (UTC), Don wrote:
On 12 May 2007 19:04:44 -0700, wrote:

Many hams did use SSB rigs with that heterodyne scheme. But it does
not result in sideband inversion on either additive or subtractive
mixing.

Actually, that was precisely the reason. *I know; I was there when the
first ssb rigs became popular during the middle 1950's. *Simple
modulation theory will tell you that subtractive and additive mixing do
make a difference.

I thought so too - and then I worked out the numbers.

Think more carefully about the process of mixing two signals: *You get
both the sum and difference frequencies as sidebands, but the sidebands
are mirror images of one another because of the way that mixers work.

Well, sort of.

The rule to remember is this:

In any heterodyne scheme there are three frequencies of interest - the
input signal frequency, the output signal frequency, and the local
oscillator frequency.

You only get sideband inversion if the local oscillator frequency is
higher than both the input signal frequency and the output signal
frequency.

[snip]

I apparently should have followed my own advice. *Upon further
reflection, I decided that n2ey's explanation is correct. *While that was
indeed the way that some Central Electronics 10A users described the
process, it seems that they were wrong. *They must have flipped the
sideband switch when moving from 75M to 20M, because low side injection
(where the carrier at 5 MHz is lower in frequency than the signal at 9
MHz), results in two inversions of the difference sideband - one about
the carrier frequency, and another about DC. *The net effect is that the
difference sideband is not inverted.

Exactly.

Where this idea came from may be the fact that in the 5 MHz VFO/9 MHz
SSB generator system, the *tuning direction* inverts between 75 and
20. On 20, increasing the VFO frequency makes the output signal
frequency increase, but on 75, increasing the VFO frequency makes the
output signal frequency *decrease*.

But the sideband does not invert.

I never used that mixing scheme in my own home brew exciters (I used
on-frequency phasing), so that's probably why I accepted that explanation
for many years.

It shows up from time to time, and on the surface looks very
believable. But it's still wrong.

The standard for LSB below 10 MHz and USB above is older than the
mid-1950s, too. As far as I can tell, it goes back at least to the
late 1940s in amateur radio, almost a decade before "Cheap And Easy
SSB" and the CE exciters (10A, 10B, 20A, etc.)

However, note this:

If you use an SSB generator in the 5 MHz frequency range (typically
5.2 MHz), and a VFO in the 9 MHz fequency range (typically 8.7 - 9.2
MHz) then you *do* get the desired sideband inversion without doing
anything to the SSB generator. (the SSB generator is set to USB for
this to work out.) Some SSB rigs like the National NCX-3 used this
scheme successfully. They then switched the VFO to 12.2 - 12.5 to
cover 40 meters.

*Live and learn, and my apologies to Jim

No biggie, it's an urban legend that has fooled many hams, myself
included, until we bothered to sit down and work out the numbers.

The problem is that now that you know it's an urban legend, you have
to disprove it when you hear or see it put forth as fact......;-)

73 de Jim, N2EY