[ARC5] Visual alignment of BC-454

[Kenneth G. Gordon]

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