RE: Line impedance measuring & matching -- techniques

I’m not sure I understand your question.
As an example.  RG-58U coax cable has a 50 Ohm impedance.  If you attach that cable to a TV with a 75 Ohm output impedance and an antenna rated at 50 ohms you will have a mismatch and some of the signal value will be lost or reflections will affect the signal quality.

Are you asking how cable is characterized to be 50 ohms?

If you purchase Beldon Thick DeviceNet cable the blue and white wires have a characteristic impedance of 120 ohms at the frequencies that show up with CAN messaging.  To prevent reflections the ends of the transmission line need to be terminated with that same value of 120 Ohms.

The CAN bus drivers are designed to drive a load.  That load is 60 Ohms in parallel with 120 devices that each have an input impedance high enough to not exceed the driver.

For example.  Say your driver has a max output current of 70mA.  A CAN_H signal level of 4.5V would drive current out to the two 120 ohm resistors and also into each CAN receiver.  Most of the current enters the CAN_L pin which is held at about 1.0V.  So the voltage across the bus at worst is 3.5V

If the receiver has an input resistance of 100K then the parallel combination of 120 resistors, each 100K in value, is 833 ohms.  That’s in parallel with the two 120 Ohm bus termination resistors.  Total resistance seen by the driver during a dominant is then 52 Ohms or 67 mA which is within specification of the driver.  That’s the DC side.

The AC side is also interesting.  The cable design is 120 Ohms impedance.  With 120 ohm resistors at each end the transmission line is matched and all the AC energy is absorbed by the resistors.  But if this were a 45 Ohm impedance cable you now have a mismatch and you get reflections which could mean the signal is degraded or bits are even changed.  

So you think, well I’ll terminate the cable with 45 Ohm resistors to once again correctly terminate the transmission line.  The parallel combination of those two resistors now means the drivers have to supply 156mA without even considering the parallel combination of the receivers.  That’s too high.

What if you ran 300 ohm twinax cable like the stuff used for FM antennas.    Well first of all the CAN driver isn’t a balanced driver so that’s not an ideal cable even if you terminate the ends with 300 ohm resistors.  The lack of twists means the signal is susceptible to both magnetic and electrical interference.

So could you run CAN with 300 ohm cable?  Of course you could.  Might even work really well.    I’ve run 1Mbps CAN between two ends of 100’ of Z wire.  That’s the pink 4 conductor unshielded telephone cable.  It worked.  But that’s two nodes and a block of messages from one end to the other.  No idea if some parts of the cable had  ‘null’ spots where the signals might totally cancel out.  Not even sure what Z wire impedance is.  But I wouldn’t want to fly in an airplane that used fly by wire control signals that way.

John Dammeyer







From: [email protected] [mailto:[email protected]] On Behalf Of Dinesh Guleria
Sent: February-02-16 10:22 PM
To: CANLIST
Subject: [CANLIST] Line impedance measuring & matching -- techniques

Hi ,
Please can anyone on list suggest , how to do Line impedance measuring & matching. For RS485 & CAN bus cables ?
Regards,
Dinesh

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