time base

[aibelectronics]

hello there!
does anyone know how the time scale of this circuit was divided? in the code it says 1unit=10msec, now the x/time axis is divided in step of 10pixels, which implies 1pixel represents 1msec. Why 1msec? Why not 1us or 1nano-sec?
By the way the 1 pixel represents the time lag b/w two successive sampled points.

 

[Nigel Goodwin]

hello there!
does anyone know how the time scale of this circuit was divided? in the code it says 1unit=10msec, now the x/time axis is divided in step of 10pixels, which implies 1pixel represents 1msec. Why 1msec? Why not 1us or 1nano-sec?
By the way the 1 pixel represents the time lag b/w two successive sampled points.

You haven't attached the circuit?, but it sounds rather obvious that the reason is that it uses a 1mS sample rate!.

 

[aibelectronics]

reply

oh yes i didn't... quite forgot: https://www.electro-tech-online.com/custompdfs/2005/10/circuit1_oscilloscope-2.pdf

the sampling rate was specified as 20us, rather unfortunately.

 

[Nigel Goodwin]

Re: reply

oh yes i didn't... quite forgot: https://www.electro-tech-online.com/custompdfs/2005/10/circuit1_oscilloscope-3.pdf

the sampling rate was specified as 20us, rather unfortunately.

The entire circuit, and the software, seem very poor?, it's hard to know what might be going on, and the spec of the unit is incredibly low.

20uS is probably the minimum aquisition time of the A2D?, it then has to read the value (in two nibbles, with it's 4 bit interface) and process the results.

Have you considered trying the software scopes that use the PC sound card?, those give a MUCH better specification than this one.

 

[audioguru]

The very slow parts used in the project (LM324!) allow it to display signals only of less than 1kHz. Wouldn't using your sound card as an input be much better?

 

[JimB]

The circuit could be used as a usefull learning exercise, if you had a proper oscilloscope to follow some of its internal signals and see what it was doing. :lol:

JimB

 

[Roff]

Has anyone noticed the diode in the WR* line to the ADC? This is not a good idea for CMOS.

 

[Optikon]

Has anyone noticed the diode in the WR* line to the ADC? This is not a good idea for CMOS.

Yeah and another one is on the base of T1.. What are they for? Presumably for protecting against negative voltages? T1 used as a switch should have a base resistor.

For voltage level protection, diodes should go from signal lines to +/- rails and not in series.

And why is R19 rated for 1W? Seems like much overkill.

 

[aibelectronics]

reply

Has anyone noticed the diode in the WR* line to the ADC? This is not a good idea for CMOS.

I thought the diode was placed there to give a further voltage drop across the line so as to avoid false triggering. The diode to the transistor is to drop the 1.4 VLow from the parallel port to 0.7v also from the same purpose I specified above.
:lol: that was a funny one JimB, with oscilloscopes going into Mhz this could certainly do for an experimental module.

 

[Roff]

Re: reply

Has anyone noticed the diode in the WR* line to the ADC? This is not a good idea for CMOS.

I thought the diode was placed there to give a further voltage drop across the line so as to avoid false triggering. The diode to the transistor is to drop the 1.4 VLow from the parallel port to 0.7v also from the same purpose I specified above.
:lol: that was a funny one JimB, with oscilloscopes going into Mhz this could certainly do for an experimental module.

I'm no parallel port expert, but I thought that it was basically TTL compatible. Where did you get the "1.4 VLow" spec?

 

[audioguru]

Ha, the WR* signal is floating when the computer drives it low though the diode.
Maybe the circuit uses the capacitance of the diode to drive the WR* pin low.
The WR* pin needs a pull-down resistor. The base of transistor T1 also needs a pull-down resistor.

 

[aibelectronics]

reply

The very slow parts used in the project (LM324!) allow it to display signals only of less than 1kHz. Wouldn't using your sound card as an input be much better?

The frequency limitation of the circuit is due to the minimum of 10samples/cycle requirement to adequately display the signal.
The 0804 has a maximum sampling rate of 10khz = 10,000 samples/second this results into:

1000cycles/second = 10000samples/second/ 10samples/cycle.

Unfortunately the topic of the post (time base callibration) is still at yet unanswered.

 

[audioguru]

I guess the ADC0804 is nearly as old as the LM324.
CD, MP3 and even DVD players do a pretty good job at much higher frequencies.