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Below 50MHz - RF Cafe Forums

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scwallac

Post subject: Below 50MHz Posted: Fri Aug 04, 2006 1:32 am

Lieutenant

Joined: Fri Aug 04, 2006 1:22 am

Posts: 3

Location: Alexandria, VA

Hi,

I am working on a PCB layout comprised entirely of MMIC components. I have a silly question regarding IF circuitry below 50MHz....

Given the long wavelengths at these low IF frequencies, is it really necessary to bother with microstrips for the IF portions of my circuitry? A quick calculation shows that 1/20th the wavelength of 50MHz is something like 16", which is well below the total size of my circuit board or any possible trace lengths.

So, is microstrip a waste of time for IF circuitry?

-Sean

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IR

Post subject: Posted: Fri Aug 04, 2006 4:38 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello Sean,

Microstrip lines are not a bad idea at all even at low frequency, because they are located in the top latyer of the PCB - This helps to debug problems with test equipment.

In general, with whatever kind of transmission line you will use, it is important to keep the right dimesnions of the line to match the characteristic impedance.

*) Another note: Of course that the lines needs to be as short as possible even for these lwo frequencies, because as someone mentioned if you have lines which are connected to the IF port fo a Mixer they transfer relatively high power levels which can radiate a lot.

Hope this helps!

_________________

Best regards,

- IR

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yendori

Post subject: Posted: Fri Aug 04, 2006 1:16 pm

General

Joined: Thu Sep 25, 2003 1:19 am

Posts: 50

Location: texarcana

Hi,

Immediatly after a mixer for example, the LO feedthrough signal may be high in power, greater than 0dBm at times.

Eventhough your desired signal is below 50 MHz, it it s important to be aware of you spurious signals. If you traces are high in impedance you may cause unwanted radiation.

Rod

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languer

Post subject: Posted: Fri Aug 04, 2006 3:55 pm

Captain

Joined: Sun May 14, 2006 8:53 pm

Posts: 17

Location: Earth

I am not sure I fully understand the question, but let's see.

At 10MHz, 100MH, 1000MHz the microstrip width for a specific impedance is all the same (i.e. Zo is not related to frequency). At higher frequencies where the with may be comparable to the wavelength (say 1/8) you want to adjust H or Er to avoid this, but at lower frequencies it's a non issue. So for impedance purposes, it's all the same.

So why would you want to provide microstrip connections? To minimize reflections, power loss, radiation (for the later it may even be worth considering CPW).

You could use stripline, but why would you? This may require additional layers (save for suspended stripline) and you have no access to the traces (it may help in radiation though).

Whatever you do there is one thing you most definetly want to have, a return path for the signal. So if you do not have differential lines (unlikely from your description), microstrip or stripline would guarantee this. And microstrip is pretty much "free", two-sided circuit with ground on bottom.

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scwallac

Post subject: Posted: Fri Aug 04, 2006 4:40 pm

Lieutenant

Joined: Fri Aug 04, 2006 1:22 am

Posts: 3

Location: Alexandria, VA

Quote:

Eventhough your desired signal is below 50 MHz, it it s important to be aware of you spurious signals. If you traces are high in impedance you may cause unwanted radiation.

yendori - Interesting point. Thanks.

Quote:

I am not sure I fully understand the question, but let's see

languer - Let me phrase the question a little bit differently:

If I am working exclusively with frequencies below 50MHz, and my trace lengths are all less than 12", then by "textbook" theory my traces are not acting like transmission lines at these frequencies anyway. If the traces are not acting like transmission lines, then why not just pretend this is an audio signal and route the signal around without controlling the trace impedance at all?

(BTW, I'm not arguing that this is actually true - rather I am wondering if this is true and soliciting your opinion!)

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languer

Post subject: Posted: Fri Aug 04, 2006 6:29 pm

Captain

Joined: Sun May 14, 2006 8:53 pm

Posts: 17

Location: Earth

Quote:

If I am working exclusively with frequencies below 50MHz, and my trace lengths are all less than 12", then by "textbook" theory my traces are not acting like transmission lines at these frequencies anyway.

I guess what I tried to clarify is that no matter what the frequency they are transmission lines, even a wire is a transmission line (by "textbook").

Quote:

If the traces are not acting like transmission lines, then why not just pretend this is an audio signal and route the signal around without controlling the trace impedance at all?

Using microstrip does not imply controlled impedance. Whenever you use a trace-dielectric-ground you have by definition a microstrip. If you decided to control its impedance or not, that is a different matter.

Ground "flooding" and other similar RF techniques were extensively used years ago when double-sided PCBs were leading edge technology. This was for IF frequencies of 10-20MHz. My point is, you do not have to use microstrip, stripline or anything similar. But whatever you use you will be using transmission lines, the key thing is to ensure the proper return path. You do mention the use of MMICs, if this refers to MMIC amplifiers and such then careful layout consideration is important because most devices now have gain up to several GHz. Meaning that unless you suppress this gain, you have to make sure how the device will react at these frequencies.

All this of course is quite important if one is making some sort of commercial product, for some hobby-type endeavor one could take many exceptions.

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scwallac

Post subject: Posted: Mon Aug 07, 2006 11:52 am

Lieutenant

Joined: Fri Aug 04, 2006 1:22 am

Posts: 3

Location: Alexandria, VA

Thanks for the insight everyone!

Posted  11/12/2012

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