It would be helpful to give some specific info about the circuit and
regulators used for these experiments, such as operating voltage, load
current, and regulator type. I may have missed some points in the
discussion on this, but I think the following things are the case:
1. The circuit exhibits excessive internal interference only when using
certain regulator ICs, but works fine with others of the same type.
2. No apparent oscillation or excessive output noise can be found with
scopes and SAs to explain it.
3. Various battery voltages have been tried in place of the regulator to
run the circuit and seem OK.
4. The receiver carrier is 467 kHz (not MHz?).
5. The regulators in question are LM78XX, which use a band-gap reference.
6. Changing or adding various filtering caps seem to have no effect on a
"bad" regulator.
Without additional info, I would suspect that the bad parts have a
low-level oscillation somewhere near (or harmonically near) the carrier,
the LO, or the IF, that is too small to see above the PS noise floor,
but big enough to cause problems. It is likely these frequencies are in
the range of where a linear regulator could oscillate. I doubt that one
could oscillate in the VHF or microwave region, out of reach of your
SA's span and sensitivity (presumably, depending on what you have). If
the regulator could oscillate up there, it could certainly leak through
or around any typical near-band filtering and decoupling in the
receiver, and cause problems. Looking in the time domain with a scope,
there will be less sensitivity and dynamic range, so there could be
something below the floor, and maybe below the SA's bottom frequency
range too. The SA's low-end can be compromised by the need to safely
AC-couple the PS voltage into the 50 ohm input. A reasonably-sized
coupling cap may put it too far up to see a small but important signal
in the kHz range.
If it's a low frequency or in-band oscillation, you may want to look at
the receiver circuits for internal susceptibility and PSRR - you may
have discovered an unanticipated weakness, separate from the regulator
issue.
Another thing to consider is that there may be a specific supply voltage
that causes the problem - like a marginal circuit in the receiver front
or LO going unstable. The battery test, I presume, was at certain
discrete voltages. It may be worth running it on a variable PS over a
continuous range. It's possible that the bad regulators just happen to
land at a "bad" voltage. With a fairly wide tolerance spec, they could
be all over the place. One way to eliminate this is to measure as
precisely as possible the output voltage of a bad regulator, then
replicate it with a variable supply.
If the receiver circuit works fine throughout its supply range, then the
regulator is again the prime suspect. Also consider what the load
current is versus its max rating - if it's anywhere close, it could be
on the verge of current-limiting, and all sorts of strange things can
happen. If this is the case, adding a helper resistor from input to
output should get it back into the normal range.
If you haven't already, before all kinds of experiments and analysis,
try the good old heat and cool methods - blast a bad regulator with
freeze spray or a heat gun and see what happens. And of course, do the
same to a "good" one. And maybe also the LO.
This is an interesting case, and I think we all would like some more
info on the particulars.
Ed
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