Transmission line terminating at high impedances--suggestions?
Transmission line terminating at high impedances--suggestions?
(OP)
Hi,
I'm working on an RF (~500 MHz) impedance transformation circuit. On the chip side of the circuit, I have a 3cm microstrip transmission line connecting the 50k ohm-impedance chip to the 50k ohm-impedance side of the transformation network. The characteristic of impedance of the line is (within a factor of two or so) bound to 150 ohms.
My question is this:
Are there any tricks that I can use to make the transmission line look less like a transmission line or otherwise improve the signal coupling to the chip?
My limitations:
1) The chip is provided by another group, and cannot be changed. I'm stuck with the high impedance and small package.
2) I can't shorten the transmission line by moving the components closer, because the chip is too small to fit eight inductors and other components.
Thanks in advance for any help that you can provide!
-TMassey
I'm working on an RF (~500 MHz) impedance transformation circuit. On the chip side of the circuit, I have a 3cm microstrip transmission line connecting the 50k ohm-impedance chip to the 50k ohm-impedance side of the transformation network. The characteristic of impedance of the line is (within a factor of two or so) bound to 150 ohms.
My question is this:
Are there any tricks that I can use to make the transmission line look less like a transmission line or otherwise improve the signal coupling to the chip?
My limitations:
1) The chip is provided by another group, and cannot be changed. I'm stuck with the high impedance and small package.
2) I can't shorten the transmission line by moving the components closer, because the chip is too small to fit eight inductors and other components.
Thanks in advance for any help that you can provide!
-TMassey





RE: Transmission line terminating at high impedances--suggestions?
Can you provide more data? Do you have any info on its complex (real/imaginary) or Smith chart input data?
RE: Transmission line terminating at high impedances--suggestions?
-TMassey
RE: Transmission line terminating at high impedances--suggestions?
As you probably found, it's hard to create a transmission line with an impedance above about 180 Ohms. Impedance of free space is about 377 Ohms. You've made me wonder when dealing with RF, just what is the maximum impedance you can ever expect.
RE: Transmission line terminating at high impedances--suggestions?
(Yes, I know these values sound outrageous; they are.)
RE: Transmission line terminating at high impedances--suggestions?
A quarter wave transmission line transformer will transform low impedance to high impedance as required, but you have to use the exact length to suit the frequency of operation.
RE: Transmission line terminating at high impedances--suggestions?
Just taking the area of the silicon from a copper pour (had it not been removed), you would have been about right on the capacitance. 3.5pF through 62 mil FR4 (er=4.35), neglecting fringing (a bad approximation at this thickness, but oh well).
I'm not trying to say that I think 50k is reasonable, but I haven't been convinced yet why it can't be on the order of 50k. Until then I have to assume the worst case.
RE: Transmission line terminating at high impedances--suggestions?
A parasitic capacitance will be a lot more lossy than that.
RE: Transmission line terminating at high impedances--suggestions?
Thank you both for your suggestions, Comcokid and logbook, and we can now consider this discussion to be closed.
RE: Transmission line terminating at high impedances--suggestions?
https://w
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RE: Transmission line terminating at high impedances--suggestions?
RE: Transmission line terminating at high impedances--suggestions?
It sounds like you have the standard problem of using a fixed impedance source, like a 50 ohm output amplifier, into a high impedance load. In that case, you do not have to do anything to match the input to the source. The source, with a high impedance terminating load, will act like a voltage source and give you the best output voltage swing at your load.
RE: Transmission line terminating at high impedances--suggestions?
-- but how can we discuss impedance transformation
without knowing the bandwitdh ?
How much loss can you afford? If not critical, you can
just terminate the line with Zo and neglect the parallel
50k (?) .
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Transmission line terminating at high impedances--suggestions?
The bandwidth is unknown, but I do know at the moment that the devices I'm working with resonate at 400 to 500 MHz, and later devices may resonate at higher or lower frequencies. I'm designing primarily for these frequencies but trying not to completely ignore the possibility of others, as well.
How much loss can I afford? No one knows. If I knew, I would tell you. I am confident, however, that the instruments will pick up whatever comes through.
Quite frankly, I find it rude that you're telling me to read the "how to ask a meaningful question" thread. I have read it. I gave you all of the information available, and added new information as it was aquired.
Furthermore, the content of your post indicates that you did not read the previous posts. If you had, you would have seen that the length/lambda was already discussed, as well as the issue of loss and the fact that I've requested that this thread be closed because the board's off being fabricatd.
RE: Transmission line terminating at high impedances--suggestions?
-- and most of then need it...
You haven't gave us all the info you have -- what
is the signal, what does the device do, how is
the information -- if any -- encoded, S/N ratio,
required error rate -- if applies etc. etc..
and I could list pages of guestions which may
be applicable but we don't know enough do tell
if it is.
E.g. if the bandwith is narrow you can make an
impedance xformer with concentrated LC elements
or capacitivelly loaded shotrened xmission line.
Finally, speaking about rudeness, the number of people
who try to help you should be allowed to expect
all of the relevant information without a tedious
20 question game.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Transmission line terminating at high impedances--suggestions?
I've been busted by your auto-added thread a few times when I didn't pay attention to who was replying. Maybe adding in a couple of blank lines and a line of "-----------------" directly above the link will help people separate the signature from the actual message.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Transmission line terminating at high impedances--suggestions?
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Transmission line terminating at high impedances--suggestions?
----------------------------
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Transmission line terminating at high impedances--suggestions?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Transmission line terminating at high impedances--suggestions?
Anyway, you're right; I didn't tell you what the device is. It's a new type of resonator being researched. I could have mentioned this, but it's not important; it might as well be a black box (in fact, it is being presented as a black box to me). The question was specifically in reference to the transmission line. I've found that extraneous information tends to send people on tangents, so I prefer to omit information that is not applicable to the problem at hand.
The applied signal will be a sinusoid swept through a wide band of frequencies in order to characterize the frequency response of the device. Of course, I'm only interested in how it performs at its resonant frequency--approximately 400-500 MHz. There is no encoding of information, and no required SNR as long as we can clearly make out the signal.
The transformation network is basically a tapped capacitor resonator with high-Q components.
Hopefully this answered some of the questions that were bothering you. Unfortunately, the problem wasn't as well defined as it would have been if this was a development task. This is why I tried to phrase my original question as a general electromagnetic theory question as much as possible.
RE: Transmission line terminating at high impedances--suggestions?
something else:
Increase the power and terminate the line with Zo.
If the level has to be measured across the input,
make a resistive divider (or "T") with Zo input impedance
and the effect of the loading on the response can be calculated.
The sweep is evidently wide band, which makes the
proper termination difficult. If this is only for measurement an other solution would be to terminate
the line w. Zo, and add a buffer amplifier NEXT to the
DUT.
By the way in my 30+ yrs I learned that when a problem
is difficult, perhaps you can step back and you
may find an other way, so you don't have to solve it
at all...
----------------------------
Please read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>