Hobbyist Forums banner
1 - 2 of 2 Posts

·
Registered
Joined
·
94 Posts
Discussion Starter · #1 · (Edited)
Output characteristics of a receiver

Does anyone know what the output of the receiver's signal pin for each channel looks like on an oscilloscope or can someone describe the signal to me? I would do it on my own, but I'm home from school and I don't have access to an Oscilloscope and I need to know it to build a circuit around it. All I have is a DMM and all I can tell is that it isn't straight DC with an offset changing depending on the input on the radio like I assumed.

Thoughts?

John Tortorice
 

·
Premium Member
Joined
·
15,151 Posts
A google search on servo signal would have found:

The servo signal is a simple digital pulse. It spends most of its time at a logic low (0 V). About every 20mS it goes logic high (3 to 6 VDC) and then quickly goes low again. It is this tiny window of logic high time, called the pulse width, that gets the attention of the servo.

Please refer to the drawing. The period labeled "A" is called the frame rate. In the example it is repeated every 20mS (50 times per second), which is quite typical for most radio systems.

Modern servos define center as a 1.5mS pulse width, as shown by detail "B" in the drawing. Full servo rotation to one side would require that this pulse width be reduced to 1.0mS. Full rotation to the other side would require the pulse width to increase to 2.0mS. Any pulse width value between 1.0mS and 2.0mS creates a proportional servo wheel position within the two extremes. The frame rate does not need to change and is usually kept constant.
The servo will not move to its final destination with just one pulse. The servo amp designers had brilliantly considered that multiple pulses should be used to complete the journey. This little trick reduces servo motor current draw and it helps minimize erratic behavior when an occasional corrupt signal is received. To move the servo, you must repeat the pulse every few milliseconds, at the chosen frame rate. Modern R/C systems use a 40Hz - 60Hz frame rate, but the exact timing is not critical. If your frame rate is too slow, your servo's movement will become rough. If the rate is too fast the servo may become very confused.
 
1 - 2 of 2 Posts
Top