Author Topic: Question about Unterminated lines on RF Board  (Read 3214 times)

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Offline GeorgiTopic starter

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Question about Unterminated lines on RF Board
« on: July 31, 2016, 07:26:43 pm »
So I was watching this video https://www.youtube.com/watch?v=6jrVZu7eqiw&t=1137s, and the guy in it said multiple times, that every unterminated trace introduces capacitance and inductance. If you forward to 31:43 it is even written on one of the slides. As far as I can remember from my high frequency courses at the university every trace has capacitance and inductance, they are physical properties. But the difference between terminated and unterminated line is that a signal that goes through unterminated line will also experience reflection. Are the capacitance and inductance way to model the reflections or am I missing something? :-//.

Georgi
 

Offline rfbroadband

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Re: Question about Unterminated lines on RF Board
« Reply #1 on: August 05, 2016, 05:30:49 am »
every transmission line can be characterized by C-L-C PI networks but that doesn't tell you anything about the characteristic impedance of the TRL. If you have a 50 Ohm TRL terminated with 100 Ohm you still get lots of reflections. The key is to terminate the TRL with it's conjugate complex impedance to avoid reflections.
 

Offline T3sl4co1l

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Re: Question about Unterminated lines on RF Board
« Reply #2 on: August 05, 2016, 09:18:17 am »
Capacitance and inductance are nonphysical models we use to express real systems (that exhibit true delay, not just frequency-dependent phase shift) with simple rational expressions (RLC networks, which have transfer functions that can be decomposed into poles and zeroes).

That is, a transmission line system (which is physical) is most simply expressed with time-delay events; in the frequency domain, you get all kinds of trig functions involved, computing the phase of an incident+reflected wave.

The justification for L and C is that, for a single frequency, AC steady state analysis, as long as a transmission line segment is shorter than 1/8th the wavelength of the analysis frequency, the segment will behave inductive (characteristic impedance Zo > circuit impedance Z) or capacitive (Zo < Z).

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Offline rwgast_lowlevellogicdesin

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Re: Question about Unterminated lines on RF Board
« Reply #3 on: August 06, 2016, 10:03:48 pm »
Sorry not trying to HiJack or Change subject... I have wondered something for a long time though and it has to do with doing inter board connections using coax transmission lines.

So first off assume there are two board lets say one is a module which is just a 500mhz oscillator, the second board has just a mixer on it. Assuming the coax is longer than 1/4 wave of 500 so transmission rules apply if you just solder the coax braid to the ground plane and the signal line to the mixers pin, is this considered un terminated and will cause reflection? I would assume this is so since you always hear people say make sure to put connectors on everything so it terminates at 50 ohms. What confuses me is there is a SUPER nice hombrew spectrum analyzer project out there I dont remember the name its built by a guy named scotty if you interested. The thing is he built it in a modular form just like you would see in a nice commercial spec analyzer, where each board has a function and there is a wall of shielding separating every PCB module. I nuts to me he would go through all this work and then allow his coax to have reflection

I do a ton of coax interconnects between board, I also have a lot of SMA launch connector a soldier ends so I can term properly if need be. If I can I try to keep the coax run smaller than a quarter wave length so I dont have to properly terminate, this is out of cheapness I may have 20 launch connectors and 20 soldier on ends but there around 7 bucks a piece! Now if I needed to run a connection that was say a foot between board and it carried a 1ghz signal, obviously a term needs to be made, instead of wasint connectors on a connection what wil never need to be disconnected and will also be inside and enclosure so it will never be seen, can I just use a 52 ohm carbon resistor from the coax signal to the ground plane, basically wire like a pull down or shut resistor and this will give me a proper termination with out wasting SMA or BNC connectors?

Offline rwgast_lowlevellogicdesin

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Re: Question about Unterminated lines on RF Board
« Reply #4 on: August 08, 2016, 07:50:42 am »
:) Thanks for the answer it cleared up something I have been confused about for a long time, the last two paragraphs pulled it together for me! Ive heard different things from different people who are probably just confused, that is I have heard you ALWAYS want to use a connector and I have also heard of using 50ohm resistors instead of a connector!

Maybe you could clear this up for me too I had a bit of termination confusion while building a 7 pole bandpass lumped element filter, I built the filter to clean up a TCXO which I was using as a local oscillator and it was putting out an unexceptable amount have harmonics causing spurs in the radio. First off I just molded the filter as 50 in 50 out using a program called Elsie, After buiding thw filter it helped a bit but the harmonics had not dropped anywhere near what the software said, my 4th harmonic should have been <120db but I was only cleaning about 10 to 15 db per decade.

Well I realized I was running a piece of LMR400 from the TCXO to the transceiver chip frequency input. Then I started thinking wait my transmission line is 50 ohms... but i dont know what impeadence the TCXO or chips xtal input were and this wasnt listed in data sheets. So I used the old trick of sticking a pot right after the TCXO and then adjusted it until the VPP on my scope was exacty 50% down from the original measurement. Next I did this at the input of the chip. I measured that my TCXO had 1k impeadence out and the chip had 400ohm in. So next I went and remodled the filter with the correct impeadences and fiddled with it until it work as well at those values as 50 in 50 out. Unfourtantly I had an accident and ruined the TCXO (now im using an si5351). I did not add a 1k or 400 ohm resistor I figured I had no need to since that was the actual resistance of the parts.

When you find yourslelf running a trans line inbetween two odd as in not 50 or 75 ohm parts is better to model the filter like did using the termination values of the parts, or should I have done something like model 5k in and out and then ad a 5k resistor on the input and output of the coax?

Offline T3sl4co1l

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Re: Question about Unterminated lines on RF Board
« Reply #5 on: August 08, 2016, 12:11:29 pm »
There can be a few good reasons to do things less directly:
1. High impedance filters require very large inductors (with very little parasitic capacitance) and very small capacitors; or for transmission line filters, impractically thin traces.  A buffer can supply the low source impedance needed.
2. The device's output impedance may not be constant.  Typically, a chip's output characteristic will be a follower (low Ri BJT, or modest Ri MOS), or an open-collector (>>10kohms Ri, in parallel with some pF pin capacitance), among other kinds more likely found at lower frequencies (like an op-amp's output, which can be inductive over a wide range).  All of these parameters can vary with bias, or with output level.
3. The device may not function properly, if loaded with too much or too little impedance.  The datasheet will give a typical application.  An example reason would be: too high of a load resistance, on a constant-current type output, develops too much voltage, so that the output device saturates, causing distortion and limiting available output amplitude.
4. The device may not function properly, due to parasitics.  Semiconductor capacitances are nonlinear, so a large voltage swing means a large capacitance swing as well.

For these reasons, you'll often find e.g. a DAC or DDS chip, followed by a buffer: if the source is constant-current type, then the amplifier is usually a TIA type amplifier.  This gives a low input impedance, minimizing errors (at the expense of adding some noise to the signal path). The amplifier's output is easily matched into the filter's impedance, which can be any modest value (say 50-200 ohms).

If SNR isn't a big deal, the chip's output might be terminated into a resistor, and the filter attached to that.  Or instead of just a resistor, a resistor divider (attenuator) might be used, to achieve even more separation between the filter and the IC pins.

Usually, an amplifier follows the filter, necessitating another termination resistor.

If the chip pins are a known, and stable, impedance, the filter can be designed around that, regardless of how high or low the source impedance actually is -- there are filter designs tables for singly-terminated (one side Zo, the other side open (CCS source) or shorted (CVS source)) filters, of all the usual types.

This is the method you were going down before, which works fine if all those assumptions work out (the quality of the output pins, of the inductors and capacitors in the filter, of the filter's load..).  Accordingly, if you change any one of them (like, the oscillator!), you'll need to adjust component values to keep things in order.

There's nothing wrong with an oddball system impedance (i.e., other than 50 or 75 ohms), but you need to make sure that at least one end of each network is matched properly.  If you're using a significant length of 50 ohm transmission line in a 400 ohm system, you're incurring a lot of capacitance (if the line is short), and at the very least, you'll need to reduce the value of whatever capacitor is nearby to account for it.

If the line is long, you'll have unavoidable peaks and dips, due to the strong reflections from each end being poorly matched.  If you can't get/make a line with a low enough impedance, you need matching transformers at each end to convert from line impedance up to system impedance.

Tim
« Last Edit: August 08, 2016, 12:13:33 pm by T3sl4co1l »
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