Pulse Generator & Repitition Rate

[imagemania]

Ok, I used one of these in an experiment the other day but i did not fully understand it. I was setting up an RC circuit, and a pulse generator was connected to the circuit setting a repetition rate of 1ms. But
a) What was the role of the pulse generator
b) Is the repetition rate like inverse frequency

[This isn't homework]

I'm slightly confused what the repetition rate does, by increasing it, do only the higher frequencies pass through a high pass filter RC circuit. Or is it nothing to do with the frequency or something to do with the amplitude?

Hope you can correct me :)

 

[schip666!]

Posting a schematic of your experiment will help...

Intuiting "RC circuit" and "high pass filter" to be a capacitor feeding a resistor to ground, rather than the other way around, I think your last question is correct...only the higher frequencies pass. The purpose of the generator is to provide a signal to your circuit so you can see what it does.

And no, Repetition rate _is_ frequency. If something repeats 1000 times a second it's frequency is 1000Hz. When you say you set a "repetition rate of 1ms" you mean you set the distance between pulses to 1ms so it repeats 1000 times a second. You can think of the 1ms as a wavelength, which is the inverse of frequency (once you know the speed of the wave).

 

[imagemania]

Ok, thouh I am still confused about the amplitude. The amplitude definatly changed as the repition rate was changed from 1ms to 1μs. The amplitude being the voltage amplitude on the oscilloscope screen. Is this change to do with the low-pass / high-pass filter (depending on the position of the capacitor & resistor of course)?

Surely if the amplitude changed then the time constant has changed, seeming the capacitance and resistor hadn't been changed at the time?

 

[schip666!]

Again presuming that my intuition of the circuit in your experiment is correct, it is a High Pass Filter... What a High Pass Filter does is Pass High frequencies... So the higher the driver frequency at the input the more signal you will see at the output.

It's a little confusing when you say the repetition rate changes from 1ms to 1us. Those values are (again, presumably) times between pulses, not rates. If your think of those values as frequencies (rates) they would be 1KHz to 1MHz.

The time constant of your circuit remains the same but the time between pulses has decreased by a factor of 1000. You can think of the time constant as how long it takes the circuit to "respond". In the high-pass case, anything that is faster than that response time gets passed-through.

 

[imagemania]

Oh my bad, they're called repitition periods, which agrees with your wording :)

Here's a one of the set up we did (we did this and switched the R and C components):
http://upload.wikimedia.org/wikipedia/commons/e/e0/1st_Order_Lowpass_Filter_RC.svg

The reasoning I am struggling with is with respect to this:
http://img822.imageshack.us/img822/6101/capturekjv.png

Im not bothered about the sketches, i just want to understand why the amplitude changes on the oscilloscope.

Thanks!

 

[imagemania]

Not wanting to double post but, can anyone explain this phenomena with respect to the amplitude?

 

[schip666!]

OK, what you posted is a LOW-PASS filter, so I'm not sure if we have been talking about the opposite of that or not...

Anyway, think of a capacitor as having a frequency dependent resistance, which decreases with frequency. If you make your indicated circuit out of two regular resistors as a resistor divider, you can calculate the ratio and figure out what the output voltage will be. It won't (within limits) care what the frequency of the driving signal is. Then replace one resistor with a capacitor and calculate the cap's "reactance" (AC for resistance) at the driving frequency: 1/ (2pi * freq * capacitance), and plug that into your ratio. You should get about what you see.

If you are driving the thing with a pulse you should see the edges of the pulse "acting-funny", either sloping or over-shooting depending on the High/Low pass configuration. That's because pulses contain higher harmonics which are also affected by the filter. To see a clearer relationship between frequency and amplitude you should use a sine wave.

Was that what you were asking?