Quote:> The distortion you are referring to is a symptom of a problem with the
> oscillator, and probably means the signal has excessive sideband noise
> that will affect its operation more significantly than its harmonic
There are two distinct issues here: harmonic content and phase noise.
I believe the original poster was talking about harmonic content. In
a crystal oscillator, the close in phase noise is normally determined
by the intrinsic noise of the crystal, and the far out noise floor is
normally determined by the buffer amplifier following the oscillator.
Hence, oscillator design has little effect on phase noise in general,
and in any event, even the worst crystal oscillator still has phase
noise that is quite adequate for a 20 meter radio. It is far more likely
that if there was a phase noise problem, it would be due to noise
on the tuning voltage of the varactor used to pull the oscillator.
Harmonic content in oscillators can frequently be minimized by
taking the output through the crystal. For example, if the oscillator
is designed so that one terminal of the crystal is grounded, then the
connection from the crystal to ground can be replaced by a grounded
base amplifier, which has a few ohms of input impedance. The crystal
then acts as a crystal filter.
Quote:> Beyond the sideband noise issue, any mixer will generate a lot of
> harmonics on its own as part of the switching/mixing process, so the
> harmonic content is not as much of an issue as sideband noise
Here is a frequent misconception we see on the net. For optimum
performance, a mixer such as the SBL-1 should driven with a sine
wave LO. Any even order harmonics in the LO unbalance the mixer
and degrade IMD performance. Mixers don't generate even harmonics,
so the argument that "any mixer will generate a lot of harmonics" doesn't
apply. So why not drive the mixer with a square wave? Well, first
of all, you have to guarantee that the square wave is exactly "square"
ie, it doesn't contain even harmonics. Even if that constraint is met,
you then have the problem of getting the square wave to propagate
through the balun transformer in the mixer without distortion. The
phase shifts at different harmonic frequencies will in general be different,
so the diodes will not see a square wave, but rather a distorted mess.
Quote:> If nothing else, a low pass filter can be used to clean a harmonic problem
> up especially if the signal has a symmetry problems.
Now here is some good advice. Simply put a low pass filter
between the VXO and mixer. You will find that the mixer
works best if filter element nearest the mixer is a series
inductor, rather than the more common shunt capacitor.
A little known fact.
> - Dan, N7VE
> > Ive built a 20 meter VXO an am using it to drive a sbl-1 LO to produce
> > DSB. The VXO output waveform is not the best, in spite of all my
> > efforts. The question is, how important is LO purity into these diode
> > modulators. Should I lowpass filter the VXO output before feeding the
> > sbl-1. Does it matter since the diodes switch on and off anyway?
> > Gary
> > W4AF