>"Can you, or anyone, recommend a good scope available for a decent price?
Are any of the Chinese ones good? I would love one that gets well beyond
100MHz. Just two channels is fine."
Look for a used Agilent MSO-X-3024A. It's 200 MHz but higher bandwidths are
available in that family of DSOs. I hacked mine to open up all options,
including 16 channel MSO and RF function generator. If I had to purchase a
good 100 MHz DSO today, it would be the Rigol DS1104Z Plus. Price is
roughly USD $670 and has a 16 channel MSO logic option. Both the Agilent
and Rigol models have variable-level gradient intensity that does a
reasonably good job of creating pseudo-phosphor persistence - a feature
that's missing on almost all older DSOs. The 4-channel Rigol is just a bit
more $$ than the 2 channel version. Having access to 4 channels is nice
when you're measuring time-sequenced events like QSK sequencing and
observing Rx audio in between T/R switching.
>"As I recall. Lissajous figures only make sense if the two frequencies have
an integer relationship such as 2:1, 3:1, etc."
Yes, for instructional purposes but not for many practical applications.
Example: some of us broadcast engineering veterans will recall using pink
noise for 2-channel stereo audio alignment on magnetic tape decks.
First, a lab-grade calibration reference tape is run while observing both
audio channels in X-Y mode. If the playback tape head azimuth is not
perfectly perpendicular to the tape, a fuzzy Lissajous pattern results on
the scope's display. The goal during such alignment is to rock head azimuth
until a fuzzy ball collapses into a perfect straight line at 45 degs. At
that moment, all audio frequencies between two stereo channels are in-phase.
Why pink noise? It represents equal energy per audio octave. If single
tones are used during azimuth adjustment, it's very easy to observe a
straight line at one frequency, yet be way out of alignment at another
frequency. Pink noise avoids this inconsistency since the entire audio
spectrum is almost instantaneously represented in time. The most pronounced
effect of tape head misalignment is audio comb-filtering when listening in
mono. Small amounts of misalignment result in high frequency roll-off in
mono.
Once the playback head is aligned, the procedure is essentially repeated for
recording while applying equal-amplitude, equal-phase pink noise to both
audio input channels. Observe the playback head output while recording and
adjust the record head azimuth for straight line equal phase. In large
broadcast facilities, Lissajous alignment was often completed every few days
-- or even every day on playback-only decks, back in the days when
automation was popular.
Paul, W9AC
(Ex RKO Radio - Chicago)
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