J
Jim Thompson
Jim, with all due respect, you need to think about what I wrote in my
original post. The sign of the loop gain is opposite for USB and LSB
for a given PD null, i.e. if you use an XOR or a diode mixer whose nulls
are at quadrature, if the loop wants to lock up at +pi/2 on USB, it'll
lock up at -pi/2 on LSB. With a PFD, one sideband will have the right
sign of loop gain to lock up at 0, where (as you point out) everything
is copacetic.
Aha! Now I see our miscommunication. You said, "If you use a
**frequency-phase detector** or other very asymmetric PD". An XOR is
NOT a PFD (phase-frequency detector, AKA edge-matcher), it's a PD
(phase detector, AKA MIXER). PFD's are used in synthesizers. I am
co-inventor of the first PFD, the Motorola MC4044.
The point I was making in my original post is that the other sideband
will have to try locking up at +- pi, where there's a ruddy great
cliff--its PD gain there is like Vdd /2*pi*(f_0*t_PD), i.e. something
like 500 times larger than the other null. Of course it's noisy and
possibly metastable there, but the point is that the loop gain is
_huge_, so no lock will occur there. Thus with a tiny bit of
acquisition aiding, e.g. 2 resistors and a cap in positive FB around the
loop amplifier, you can make a reliable lock to one sideband and not
the other, _without_needing_a_SSB_mixer_.
Cheers,
Phil Hobbs
Huh? The OP wanted to **generate** either an upper or lower sideband.
An XOR mixer isn't going to do that.
...Jim Thompson