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Copyright: 1996 - 2024
    Kirt Blattenberger,


RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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Rules of Thumb - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues - primarily having to spend time purging garbage posts from the board. At some point I might start the RF Cafe Forums again if the phpBB software gets better at filtering spam. Note: The RF Cafe Forums were reconstituted in April 2021!

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Kirt Blattenberger
 Post subject: Rules of Thumb
Posted: Wed May 24, 2006 11:34 pm 
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 451
Location: Erie, PA

This post is a compliment to my May 25, 2006, Factoid on the IP3 versus P1dB power rule of thumb. I would like to collect as many other rules of thumb for the industry as possible, and will create a separate webpage for them if enough are provided by vauled visitors (that would be you).

Thanks for your help.

- Kirt Blattenberger :smt024
RF Cafe Progenitor & Webmaster

 Post subject: Another
Posted: Thu May 25, 2006 3:12 pm 

Joined: Sun Jan 11, 2004 4:44 pm
Posts: 16
One I use all the time is for the inductance of bondwires (1 mil gold). The rule of thumb goes that the bondwire's inductance in nH is approximately equal to its length in mm.

The equivalent for length in mils (0.001 in.) is approximately 25 pH per mil (25.4 pH actually, since 1 mil = 0.0254 mm, but since we're approximating here...).

VSWR . :idea:

The Amplifier
 Post subject:
Posted: Tue May 30, 2006 8:44 am 
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Joined: Fri Jul 16, 2004 4:08 pm
Posts: 5
Hey Kirt here's another one for your list.

"A PCB trace or coaxial cable mus tbe treated as a transmission line when its length is more than 1/10 of a wavelength."

This is because at that point the reflections can set up significant standing waves along the path.

 Post subject:
Posted: Tue May 30, 2006 12:12 pm 
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Joined: Tue Jan 13, 2004 8:42 am
Posts: 7
Good idea with this list. COme on guys, let's get with it :-D

Um..... here's mine

If you're looking at a spectrum analyzer display and see a spectral line and you need to determine whether it's a discrete spur, try increasing or decreasing the resolution BW control. If the noise floor power level changes but the amplitude of the spur does not, then it's a discrete spur.

I'm amazed at the numbe rof guys I work with don't know that simple test. The reason it works is because a discrete spur has no significant content other than at its discrete frequency, so the BW of the filter does not affect its power level (as long as your signal is not on the skirt (band edge) of the filter.

Any way, that's mine. Let's have more.............

 Post subject:
Posted: Fri Jun 02, 2006 7:21 pm 
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Joined: Wed Aug 20, 2003 11:47 am
Posts: 84
Location: Santa Barbara, CA

 Post subject:
Posted: Mon Jun 05, 2006 11:33 pm 
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Joined: Fri Jul 16, 2004 4:10 pm
Posts: 8
Location: Tennessee
Hey thanks for that great list! They have a lot I've never heard of.

I'm sure I've got one to add to it, but I'm going to have to think real hard.

I'll be back...

Curtis Crow
 Post subject:
Posted: Tue Jun 06, 2006 1:51 pm 
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Joined: Wed Jan 07, 2004 1:27 pm
Posts: 10
1 meter = 300 MHz (in air)

Kirt Blattenberger
 Post subject:
Posted: Sat Jun 10, 2006 9:13 pm 
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 451
Location: Erie, PA
Greetings kpainter:

Thanks for that great list. Somebody did a lot of work scouting out all those rules of thumb.

So, does anybody know any others, or care to refute/modify any of the ones on kpainter's list?

- Kirt Blattenberger :smt024
RF Cafe Progenitor & Webmaster

Ted Cline
 Post subject: Re: Rules of Thumb
Posted: Wed Feb 18, 2009 3:48 pm 
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Joined: Fri Aug 26, 2005 8:25 am
Posts: 6
Here's one I can offer. It's more of a tip than rule of thumb. If you are using chip caps and notice a larger than expected variation in values, it might be due to the orientation of the chip cap on the PCB. The parallel plates of the cap are all oriented in one direction. If there is enought metal near the cap, the fringe fields can be enough to affect the performance. Careful PCB layout can usually prevent it, but if you are seeing larger than expected variations, check it out.

Other than using an x-ray, there is really no way to tell which way the plates are oriented internally because most RF cap packages are square. If you have time to play around, try experimenting with this. If I had more time, I'd do a study myself. For that matter, there probably already is one somewhere.

I guess I'm just lazy. :mrgreen:

 Post subject: Re: Rules of Thumb
Posted: Fri May 22, 2009 4:26 pm 
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Joined: Mon Jun 27, 2005 2:02 pm
Posts: 392
Location: Germany
Here is one of mine.

IP3 measurement is considered valid for small signal conditions. Small signal applies as long as the difference between the fundamental product to the 3rd orer product is equal or bigger than20dBc, i.e.:



P3 - 3rd order product (dBm).
P1 - fundamental power (dBm).

This rule of thumb is according to the IEEE, so it has been already checked out rigorously. :lol:

Best regards,

- IR

Posted  11/12/2012
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