Pulse Width Generation

MikeML

Joined Oct 2, 2009
5,444
We fly close to Bozeman when we visit my wife's mother near Calgary...

combover61

Joined Oct 16, 2015
18
Let me know if you plan to visit here ... a worthy stop ...
Enjoy your plane, I have not flown since high school ... actually got my student license and soloed prior to getting my auto license at 16.
Flying was big fun ... what was it ... I think a Piper Cherokee?
Landing in a cross wind for first solo flight ... sliding the plane in side ways ... what were they thinking of letting a kid fly their planes???

combover61

Joined Oct 16, 2015
18
So this output above ... that looks so good now ... needs to feed an instrument that is labeled 50 Ohm input.

However, when I physically measure the input (power off), it measures 1000 Ohms to ground.

Is it possible in the power up state that it is actually 50 Ohm?

Sounds like a stupid question, but sometimes I don't know what I don't know ;-)

This is important, since I can easily drive the 1k Ohm load with the 74HCT11, but not drive the 50 Ohm load.

MikeML

Joined Oct 2, 2009
5,444
To check the input impedance with the instrument powered up, drive the input with a signal generator that has a known 50Ω output impedance. While watching the output voltage level from the generator with a Hi-z scope using a BNC T, connect the coax to the unknown input. If before connecting, the voltage was say 1Vpp, and as you connect it, the voltage drops to ~1/2V, then the input is truly 50Ω. If after connecting it, the voltage stays near 1Vpp, then the input impedance is much, much higher...

combover61

Joined Oct 16, 2015
18
Mike ML, thank you ... hooked it up ... and did your test above ... the voltage level only dropped 10mV ... from 5V to 4.9V.

So it would appear the this gate input is much higher than the 50 Ohm noted in the operator's manual.

Based on this, do you expect that the circuit (of yours) would drive this gate input OK?

The final two output IC's are the 74HC&08 & 74HCT11.
They publish and output drive current of 25mA max.
5V/25mA = 200 Ohms.
I measured an gate input impedance of 1k Ohms when the unit was powered off.

MikeML

Joined Oct 2, 2009
5,444
I think the HC will drive it. Again, put in a BNC tee, and see how much the voltage drops when you hook it to the input.