"Dave" wrote in message
...

"K7ITM" wrote in message
oups.com...
On Apr 8, 9:23 am, "Dave" wrote:
"Bob Liesenfeld" wrote in message

...



Hey Bob,

I am feeding a signal from my RF signal generator to the circuit on
my
workbench. I mention harmonics because the sinewave goes from nice
and
clean to "blurry" and looking "smeared" across the screen of my
O-scope.
I may *be* overloading it, but I thought that would result in
clipping of
the waveform. I have the signal generator set to attenuate the
signal
severely, and *thought* that would prevent overloading. Maybe not...

Back to work on it some more, and try to make sure I am not
overloading
the device.

Thanks,

Dave

Hi again Dave,
It is also possible that the circuit is oscillating. I have seen
amps
are ok with no input signal but will break into oscillation when a
certain
freq/amplitude input signal is applied (and vice versa). How did you
build this thing? Circuit board, protoboard, dead bug.....???

Bob

Hey Bob, thanks for your interest.

I built it on a piece of perfboard with point to point wiring,
clustering
the active components and their biasing resistors fairly closely. I
don't
*think* it is oscillating, although to wouldn't swear to anything just
now.

I have the signal attenuated as much as possible, to the point where it
doesn't even cause a wiggle on the scope with my X10 probes and the
scope
set to 50mV/div. With the tuner set to resonate, it shows up as about
30 or
35 mV (5 MHz) going into the circuit. The output is 70 or 80 mV, but it
is
"blurred" and "smeared" as I described. Also, I just realized that the
scope shows *2* traces, each identicle to the other (only using one
channel,
and one scope probe.) I can upload pictures of all this if desired,
probably to alt.binaries.schematics.electronic. Don't know how else to
describe what I am seeing. How could one signal become two traces, each
identicle to the other?

Thanks much,

Dave

A picture would help. This sounds to me like it _could_ be just the
scope. Seeing two traces (obviously not really identical or they
would look like a single trace...maybe you're seeing two copies of the
same thing, one shifted in time from the other?) tells me that the
scope may be triggering or syncing twice per cycle. What kind of
scope? Do you have full control over the triggering? If so, try
varying the trigger level.

If you see a sine wave that's clipped (flat-topped) on the top and/or
bottom, that's an overload of the active devices in the circuit,
assuming you fed your circuit a good sinewave to begin with. If you
see a trace that is widened out, it may well be oscillations on top of
your desired signal.

There are lots of things you can do to determine what the problem
really is. I'd guess it's unlikely that it's the resistors. I've
used 2W metal-oxide spiral film resistors out to 450MHz for a dummy
load, and the load had a very low reflection coefficient out to 2
meters, and quite usable at 450MHz, per an HP network analyzer.

In fact, carbon composition may NOT actually be better! They may show
less inductance, but greater variation in resistance, than film-type.

In addition, if the resistors are used to bias transistors, and not as
RF loads, a little self-inductance is not necessarily a bad thing. At
first I thought your circuit was an RF bridge or some such, but
apparently that's not the case. I do think you've barked up the wrong
tree, and just need to find the right one. Leave no tree unbarked??

Cheers,
Tom


Hey Tom,

I'll try to upload a jpeg of the scope trace, and possibly the schematic
and setup producing it. Would love to solve this mystery. There was a
time when I had this active antenna working wonderfully. Now it's not
much use, and I can't figure out why.

Regards,
Dave



Just posted jpegs of the scopetrace/schematic/device under test to
alt.binaries.schematics.electronic. Please let me know if I can provide
anything else for this discussion. Thanks to all...

Dave

I'm off to fiddle with the triggering level, which I meant to do earlier.