50 ohm cable and a non-50ohm source - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: January 13, 2020, 02:39:36 pm
content: Hi!Can I use coax cable with a non-50ohm source? If yes, why?I always thought that a transmission line should have a proper source, and a proper termination, or signal will be distorted due to reflections. But in this video Dave has only termination, or only 50 ohm resistor at source:. And it seems to be working, which puzzles me.

author: TimFox
author_rank: Super Contributor
timestamp: January 13, 2020, 03:47:26 pm
content: The three ideal cases above give frequency-independent amplitude and delay from source to load.If the frequencies of interest are very low (compared to the reciprocal of the cable’s delay time), with arbitrary source and load resistances, then the cable acts as a capacitance in parallel with the load resistance, and produces a R-C low pass filter governed by that capacitance and the parallel combination of the source and load resistances. For example, 3 feet of RG-58/U 50 ohm cable has approximately 5 nsec delay and 75 pF capacitance. At 20 kHz audio, with 600R source and high resistance load, the signal sees a filter down by 3 dB at 3.5 MHz.

author: David Hess
author_rank: Super Contributor
timestamp: January 14, 2020, 05:18:26 am
content: Quote from: blueskull on January 13, 2020, 02:49:20 pmThat also works if the source impedance is very high which is the case with a current output. At high frequencies the current sees the 50 ohm transmission line impedance and at low frequencies, the current sees the 50 ohm load resistance.The cable only needs to be terminated at one end assuming that only the ends see the signal. At midpoint however the reflection will be apparent so termination is required on both ends in party line applications.

author: hsn93
author_rank: Regular Contributor
timestamp: January 14, 2020, 08:15:32 am
content: Quote from: blueskull on January 13, 2020, 02:49:20 pm50R cable - 0R source - 50R inline terminationamplitude 50% for first 2 exmaplesamplitude 33.3% for last one ?what is my mistake?

author: Dave
author_rank: Super Contributor
timestamp: January 14, 2020, 08:22:55 am
content: You mistake is assuming that a 50R cable has 50 ohms of series resistance.Cable impedance has to do withcharacteristic impedance, which is a ratio between the series inductance per unit length and parallel capacitance per unit length.\$Z_0 \approx \sqrt{\frac{L}{C}} \$

author: magic
author_rank: Super Contributor
timestamp: January 14, 2020, 08:50:33 am
content: I prefer to define it as:The apparent resistance you see when you apply voltage to the cable and wait for its capacitance to charge up, until the charge reaches the other end and all hell breaks looseOr we can describe "characteristic admittance" as the product of capacitance per meter times propagation velocity. I think it makes sense and shows what's going on.editOr maybe not completely so, by ignoring current

author: hsn93
author_rank: Regular Contributor
timestamp: January 14, 2020, 09:46:53 am
content: Quote from: Dave on January 14, 2020, 08:22:55 amno, i assumed the cable impedance = ZL .. 50ohm parralel..50ohm series is the termination resistor