# What is a reference frequency for electrical length?

There is an option for the transmission lines in ADS which says "Reference Frequency for Electrical Length"

What is that supposed to be? In the transmission line equations, I don't know of any reference frequency. Is it $\beta$ or is it $l$?

The electrical length is $\frac{\beta{}l}{2\pi}360^\circ$ (assuming ADS wants this parameter given in degrees, a detail I don't recall). For example, if the transmission line length is equal to one wavelength long, you'd specify the electrical length as $360^\circ$.

But ADS will assume the physical length of the line is not magically changing depending what signal is sent through it, meaning that the electrical length changes in proportion to the frequency. Therefore you must also tell it at which frequency you are specifying the electrical length, so that it knows how to adjust this parameter when simulating at any frequency.

For example, say you know the electrical length is 360 degrees at 100 MHz. You specify in the model that the electrical length is 360 degrees and the reference frequency is 100 MHz. Then ADS knows that that means 180 degrees at 50 MHz or 720 degrees at 200 MHz.

• I see, so you're saying $\beta = 2\pi/\lambda$ and lambda is fixed by a reference frequency right? – StackOverflowOfficial Feb 19 '17 at 22:06
• Yes, $\beta$ depends on frequency. – The Photon Feb 19 '17 at 22:07
• Is there anyway to change the electrical length with frequency? That's actually what I'm trying to do. – StackOverflowOfficial Feb 19 '17 at 22:08
• As the answers to your earlier question tried to tell you, ADS assumes the electrical length changes with frequency. It assumes the physical length is fixed so that the electrical length is proportional to the frequency. Is that what you want, or do you want some other dependence between electrical length and frequency? – The Photon Feb 19 '17 at 22:10
• Yes, that is what I want. Perhaps I misunderstood those earlier answers. Then what would I put down as the reference frequency if we don't let the electrical length to be fixed to one value? – StackOverflowOfficial Feb 19 '17 at 22:12

"Electrical Length" is typically measured in wavelengths of the signal to be transported over the transmission line.

And the wavelength is a function of the frequency. So this is a necessary thing to define. The frequency is neither $\beta$ nor $l$ – it's typically called $f$ in transmission line equations... But I'm not sure where you're taking these symbols from.

• The electrical length = $\beta *l$, so that's where it's from. But I don't understand what you mean by it's necessary to define. – StackOverflowOfficial Feb 19 '17 at 22:04
• "electrical length" only makes sense if you define it for a specific frequency. That easy. You must define the frequency. – Marcus Müller Feb 19 '17 at 22:05
• $\beta=\frac{2\pi}{\lambda}$, so for a physical line whose length doesn't magically change depending on the signal that's sent through it, the electrical length depends on the frequency. Therefore you have to specify an electrical length, and which frequency you were working with when you determined the electrical length. – The Photon Feb 19 '17 at 22:06
• That makes sense now. Btw, do you know how I would change the electrical length vs frequency in ADS? – StackOverflowOfficial Feb 19 '17 at 22:08
• If you specify 360 degrees at 100 MHz, ADS knows that that means 180 degrees at 50 MHz or 720 degrees at 200 MHz, or whatever. – The Photon Feb 19 '17 at 22:11