[ARC5] Visual alignment of BC-454

[Ian Wilson]

Hi Ken,

You will see the envelope if you are looking at the output of
an AM detector. If you are looking at the IF then you should see
a symmetrical, "filled-in" display. If you have an asymmetric
display with fill-in then you are probably looking at the output
of a not very linear detector with inadequate filtering :)

If you can access the Z axis then you can blank the retrace
and clean up the display significantly. You will still have to wait
for the transient response to die away before starting a new
sweep. It is very easy to do experiments like this if you are
messing around with PICs or whatever. It is somewhat more
challenging if you are doing this the fun way with 555's, etc.

73, ian K3IMW



On Thu, May 23, 2013 at 8:43 AM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> On 23 May 2013 at 4:02, J. Forster wrote:
>
> > Generally, fast sweep rates causes artifacts.
>
> Yes. Even a low "Q" circuit will ring if given enough "incentive". :-)
>
> > You need to sweep slowly
> > enough that the circuit is in essentially sinusoidal steady state all
> > the time.
>
> Yes.
>
> In this case, I STILL don't have the setup correct: for one thing, the
> "bump"
> on the bottom of the trace shown on the photos is caused by the "not quite
> vertical" retrace of the sawtooth.
>
> For another, the pattern shouldn't be "filled in": there should be an
> "outline"
> of the bandpass instead of a "blob".
>
> I am not quite sure what is wrong here yet, but intend to figure it out
> .....eventually.
>
> I switched the scope connections to an X-Y type last night, connecting the
> sweep to the Y input (and through a BNC "T" to the 8640B), and the output
> of the diode load resistor (R-18 on the BC-454 schematic) to the X input.
> The
> result was a very weird trace.
>
> I went to bed quite late, still very confused. This morning I came to
> think that
> perhaps the input signal level from the signal generator is much too high.
>
> In any case, the "vertical gain" control of the "Y" axis works exactly as
> it
> should: with no input signal from the 8640B, I can adjust the "vertical
> gain" to
> present a straight line across the scope as wide or narrow as I want it,
> and it
> sweeps at the exact same rate as I set on the sweep genny.
>
> The problem is the vertical, X axis. I cannot see the outline of the
> bandpass,
> but a really strange-looking "blob".
>
> It has been suggested that I should use a Demodulator Probe on the X axis
> input. Although I have one of those, I would have thought that the signal
> from
> the diode load should be correct, although perhaps upside down.
>
> I would have to use the Demod probe if I looked at the signal from much
> earlier in the IF chain, though.
>
> In my searching on the net, one person suggested using the "Z" axis input
> for something, but that was the only one who did. In any case, none of the
> other articles I have read on this method ever mention the "Z" axis.
>
> > It is easy enough to build a solid state triangle or sawtooth
> > generator, using an opamp and comparator or a timer chip.
>
> Yes. I have a couple of schematics of those. However, the B&K does output
> a decent enough sawtooth, as long as I keep its sweep rate fairly fast. As
> I
> slow the sweep rate, the horizontal line gets very rounded.
>
> By "fast", BTW, I mean something like 30 to 50 Hz. By "slow" I mean like 1
> Hz.
>
> The strange thing is that I used the B&K and a different scope to get a
> perfect single-bandpass representation of the IF of an R-1004 receiver
> several years ago, so I know it can be done. I just don't remember exactly
> how I did that.
>
> I DO remember that I had to use a very slow sweep rate. In fact, it was so
> slow that I had to turn the lights out in the shop, and turn the scope
> intensity
> way up in order to really see the representation clearly since the scope
> persistence was so short. I think the sweep rate was on the order of 1 Hz
> or
> 2 Hz.
>
> Well, back to the shop.
>
> Ken W7EKB
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