[ARC5] Visual alignment of BC-454

[J. Forster]

> 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".

You need a RF probe or envelope detector. (diode and LPF)

> 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".

You need an envelope detector (diode and LPF)

> 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
>
>