difference between balun and ceramic filter?

[Admin]

Is a balun a ceramic filter? Or are they different things that are often used together? How do they differ?

All I can find is this:
http://en.wikipedia.org/wiki/Balun

but it doesn't say anything about ceramic filters . . .

When I look up Balun at mouser.com, it lists a few called 'ceramic filter' so not sure . . . its for a 2.4GHz transmitter thingy I'm working on, and I'm no RF expert for sure!

[waltr]

A Balum is and a filter are two different things but there are other considerations. Looking at the Wiki page on Balum you see that it is a transformer thus it has reactantance with AC current that has an optimal frequency to transform. Other frequencies will be attenuated. A Balum is still not considered a filter even though it has pass-band characteristics but it is also possible to combine the two into a single package or a marketing person saw the pass-band characteristics and the use of a ceramic sub-strate then gave it that name.

Here is a good guide on ceratic filters:http://www.murata.com/catalog/p11e.pdf

The Mouser catalog does make it sound as though a Balum and filter are the same. But if you look at this data sheet: http://www.mouser.com/catalog/catalogUSD/641/1019.pdf
You will see that it covers a group of components and this is what the Mouser list is showing. Just a case of who ever put the information in the catalog does not know what these components really are and so just uses the titling from the manufacturers data.

[Admin]

An application note recommends I use this balun (an obsolete part):
http://search.digikey.com/scripts/dksearch/dksus.dll?Detail?name=732-2230-1-ND
but it says "Impedance - Unbalanced/Balanced   50 / 100 Ohm"

confused . . . so do I use a 100ohm or a 50ohm impedance balun?

Is lower impedance always a good thing?

[waltr]

but it says "Impedance - Unbalanced/Balanced   50 / 100 Ohm"

confused . . . so do I use a 100ohm or a 50ohm impedance balun?

Is lower impedance always a good thing?

but it says "Impedance - Unbalanced/Balanced   50 / 100 Ohm"

confused . . . so do I use a 100ohm or a 50ohm impedance balun?

This means that the unbalanced side is 50 Ohm and the balanced side is 100 Ohm. That Balun transforms the impedance in a 1:2 ratio as well.

Is lower impedance always a good thing?

No, the impedance needs to match the driver source or the load to have the most efficient transfer of power.

[Admin]

Another question . . . where on the balun does the antenna go?

For example, this guy:
http://www.tdk.co.jp/tefe02/e8balun_hhm.pdf
has this pinout:

TERMINAL FUNCTIONS
1 Unbalanced Port
2 GND or DC feed+ RF GND
3 Balanced Port
4 Balanced Port
5 GND
6 N.C.

The attached file shows the balun, and the schematic I need to build. Not sure where stuff connects ie where the balanced/unbalanced lines go wrt the schematic.

but it says "Impedance - Unbalanced/Balanced   50 / 100 Ohm"

confused . . . so do I use a 100ohm or a 50ohm impedance balun?

This means that the unbalanced side is 50 Ohm and the balanced side is 100 Ohm. That Balun transforms the impedance in a 1:2 ratio as well.

Hmmmm mouser doesn't list impedance ratios . . . whats a good ratio for 2.4GHz? I'm guessing 2:1 will work fine . . .

[Soeren]

Hi,

Another question . . . where on the balun does the antenna go?


The attached file shows the balun, and the schematic I need to build. Not sure where stuff connects ie where the balanced/unbalanced lines go wrt the schematic.

What goes on the other (non-antenna) side of the balun?

Hmmmm mouser doesn't list impedance ratios . . . whats a good ratio for 2.4GHz? I'm guessing 2:1 will work fine . . .

Why not stick to 1:1 as your schemmy shows?
Here is a 50/50 balun

A balun can be made with capacitors. Just about any circuit transforming signals or voltage dividing can be made either inductive or capacitive (when talking HF).

[waltr]

The first Balun in the data sheet you linked to is 50 to 100 Ohm so its 1:2. The second is 50 to 150 Ohm so its 1:3.

The Balun ratio is to match the output impedance of your device (RF amplifier) to the input impedance of your antenna and has nothing to do with the frequency. It is the actual value of the components that make a Balun that are dependent on frequency.

In the linked data sheet the HHM1517 is a 50 to 50 Ohm Balun for 2200MHz, usable at 2400-2500MHz.

A good source of information can be found in various Amateur Radio handbooks. The ARRL and RSGB both have excellent books on the subject.

[Admin]

Another question . . . where on the balun does the antenna go?

The attached file shows the balun, and the schematic I need to build. Not sure where stuff connects ie where the balanced/unbalanced lines go wrt the schematic.

What goes on the other (non-antenna) side of the balun?

Oh, the two connections on the right are part of a zigbee transceiver:

RFP is the positive terminal for the bi-directional, differential RF I/O port.
RFN is the negative terminal for the bi-directional, differential RF I/O port.

The datasheet says "The 50W single-ended RF input is transformed to the 100W differential RF port impedance using Balun B1. The capacitors C1 and C2 provide AC coupling of the RF input to the RF port, capacitor C4 improves matching." It also says the transceiver has a "100ohm differential impedance". It also goes into harmonics and dB specs. And it tells me the recommended capacitor values.

The Balun ratio is to match the output impedance of your device (RF amplifier) to the input impedance of your antenna and has nothing to do with the frequency. It is the actual value of the components that make a Balun that are dependent on frequency.

So an antenna would specify its impedance in the datasheet? And my transceiver, too? So if its a 2:1 ratio, then my balun should be 1:2 to balance? So . . . I plan to just use a PCB wire trace as the antenna . . . I wouldn't have a clue on how to determine the impedance of such a trace . . . I did a lot of googling around, but I'm honestly not even sure what I'm looking for lol . . . and why are there 50ohm:50ohm baluns?

I figured I'd bother with the antenna design after I get the balun thing done, but appears now they need to be designed simultaneously . . .

For my prototype I don't really care about range, perhaps 3 feet is good enough. I can always take measurements later and experiment with different baluns. I just need something good enough now to not epic fail on my first try.


Nothing like robotics to make me feel like a noob on a daily basis!

[Admin]

Hmmmm I didn't search hard enough . . . PCB strip impedance calculator:
http://www.pcb123.com/help/calculators/

probably others, just wanted to post this before I got called out on it


edit: It appears every pcb trace impendance calculator out there used the same code - an error occurs on all of them if you make the PCB trace width greater than 188mils. That said, it looks like I can get a 50ohm impedance with a 115mil width trace, given typical PCB measurements. Perfect, as 115mil width trace fits perfectly on my PCB. That said, if I use a 28mil width trace, my impedance goes to 100ohm - would I even need a balun in that situation?

I noticed none of them cared about trace length - does length not affect impedance, or just dB at a particular frequency?

[Soeren]

Hi,

Just a quick one before I doze off...

. . . and why are there 50ohm:50ohm baluns?

To go from balanced to unbalanced (or v.v.) where both sides are 50 Ohm.

Are the Zigbees really 50W? (Sounds like a lot of power for such a device). And a true balanced in-/output?

The antenna goes on the side marked "RF" in your drawing.
Typically, a whip (which the PCB antenna comes close to, if not made into a loop/qubical) is rather high impedance.
Really old TV antennas were 300 Ohms (not the folded dipole type, but the ones looking like two whips back to back).

[Admin]

Are the Zigbees really 50W? (Sounds like a lot of power for such a device). And a true balanced in-/output?

I'm wondering if thats a typo in the datasheet, perhaps its supposed to be mW?

The antenna goes on the side marked "RF" in your drawing.

What I meant was, which pin on the balun connects to the antenna?

My best guess:
pin 1: antenna
pin 2: ground
pin 3: RFN
pin 4: RFP
pin 5: ground

. . . and why are there 50ohm:50ohm baluns?

To go from balanced to unbalanced (or v.v.) where both sides are 50 Ohm.

So does that mean a balun is *always* required? (with the exception of the cap method you mentioned earlier)

[waltr]

"The 50W single-ended RF input is transformed to the 100W differential RF port impedance using Balun B1.

This must be a typo since a passive device can not create power. Or more likely the translation program didn't know what the Omega character was a it translated to a W. I believe it should read as:

"The 50 Ohm single-ended RF input is transformed to the 100 Ohm differential RF port impedance using Balun B1."
 Which now makes perfect sense.

Admin,
  You still haven't gotten the impedance concept yet. With RF you are trying to transfer POWER from one circuit to another.
You need to use the power equation P = I*V as well as OHM's law V = I*R when working with impedance (with AC it is a complex number but can be simplified to use DC equivalents if the reactance is ignored).

So for 1 Watt (RF amplifier output) at into a 50 Ohm load the current, I = sqrt(P/R) = 141mA and the voltage, V = P/I = 7.09V. If you tried to feed a 100 Ohm load without an impedance transform the V is 7.09V but I = 70.9mA and the power in the load is 0.5 Watt which is only half the power being transfered to the load (antenna) and a 3dB loss. The other 0.5 W that is not transfered to the load is reflected (due to the complex reactance of AC) back the the RF amplifier's output. This can also over-heat the RF amplifier's output stage if the mis-match is great enough.

To determine if you need a Balun and what its ratio would be you do need to design to antenna first.

Microchip has some app notes on their 2.4GHz RF modules that includes antenna design and matching. Here is one:
http://ww1.microchip.com/downloads/en/DeviceDoc/70329b.pdf

Note that they use a Balun built with discrete components for matching modules balanced (RFN/RFP) output to the single ended (unbalanced) antenna. They use mircostrip to maintain the trace impedance.

[Admin]

"The 50W single-ended RF input is transformed to the 100W differential RF port impedance using Balun B1.

This must be a typo since a passive device can not create power. Or more likely the translation program didn't know what the Omega character was a it translated to a W. I believe it should read as:

"The 50 Ohm single-ended RF input is transformed to the 100 Ohm differential RF port impedance using Balun B1."
 Which now makes perfect sense.

Good call . . . apparently copy/paste converted the ohm symbol to a W and I didn't notice . . . It should be ohm.

So does this mean that the transceiver has 100ohm impedance, while the antenna was 50ohms, and that the balun did a 2:1 ratio to balance them?

To determine if you need a Balun and what its ratio would be you do need to design to antenna first.

Well, I can make the antenna to be whatever I want, as long as it fits on my PCB. My plan is for it to be a straight strip trace monopole antenna, with a length of probably 1/8 wavelength and width of whatever balances the impedance. Lets say I make it a 50ohm impedance antenna.

Note that they use a Balun built with discrete components for matching modules balanced (RFN/RFP) output to the single ended (unbalanced) antenna. They use mircostrip to maintain the trace impedance.

Sooooo is that a confirmation of my guessed pin connection?

There is a Positive and Negative connection (RFP/RFN), is there a difference? If I get them backwards, would I end up inverting the 0's and 1's?


(thanks for baring with my noobishness, RF design is all new to me)

[waltr]

So does this mean that the transceiver has 100ohm impedance, while the antenna was 50ohms, and that the balun did a 2:1 ratio to balance them?

Yes, it does imply that. But the Balun does not "balance" them. The Balun does two things, first the definition of a Balun is Balanced to Unbalanced. Next it is transforming the complex impedance be MATCHED to the complex impedance of the Transceiver circuit to that of the antenna.

Well, I can make the antenna to be whatever I want, as long as it fits on my PCB. My plan is for it to be a straight strip trace monopole antenna, with a length of probably 1/8 wavelength and width of whatever balances the impedance. Lets say I make it a 50ohm impedance antenna.

The characteristic impedance of an antenna is not so simple and can not be easily modeled. Even though we use the unit of Ohms for the impedance it is not R but Z the real part of a complex number. http://www.antenna-theory.com/basics/impedance.php
My advice is don't never try to design your own PCB antenna. Use an existing design from an App Note.

From Amateur Radio sources (look up books I mentioned in a post above or use google):
A Half-wave dipole antenna in Free space has a characteristic impedance of about 73 Ohm.
A 1/4 wave vertical antenna over an infinite ground plane has an impedance of about 35 Ohm.
These are the two simplest antennas and the statements "Free space" & "infinite ground plane" as well as the diameter of the antenna elements matter as to the actual impedance.

There is a Positive and Negative connection (RFP/RFN), is there a difference? If I get them backwards, would I end up inverting the 0's and 1's?

No, This is the RF envelope not the modulated data.
Think of a Push-Pull audio amplifier (Balanced output) that drives a center-tapped transformer and the transformer's secondary drive a speaker (Unbalanced load). An The audio output transformer is really just an impedance matching Balun.

Some more reading:
http://en.wikipedia.org/wiki/Antenna_(radio)
http://focus.ti.com/lit/an/swra117d/swra117d.pdf    :an app note on a 50 Ohm PCB antenna for 2.4GHz and sounds like what you are looking for.
http://www.google.com/search?hl=en&client=firefox-a&hs=90Q&rls=org.mozilla:en-US:official&q=2.4+ghz+pcb+antenna&revid=2074361666&ei=ZIyKS8TFIcGttgee67noBA&sa=X&oi=revisions_inline&resnum=0&ct=broad-revision&cd=2&ved=0CDQQ1QIoAQ

For more info go to the web sites of any manufacture that sells transmitter modules for 2.4GHz and search for app notes of reference designs.

[Admin]

I've done a lot of reading, and have an PCB antenna design I'm confident that'll do what I need.

That said, it has a calculated 100ohm impedance. Since my transceiver also has a 100ohm impedance, I assume I'll need a 100ohm:100ohm balun.

But I can't find one, only 50ohm:50ohm. Am I being dumb, and really just want a 1:1 ratio, so a 50ohm:50ohm will do the job? (I'm assuming yes)


ps - I'll write up a tutorial on how to do this when I'm done. The information online was all very disorganized, confusing, and generally lacking in usefulness. To sum it all up into a how-to would be really helpful I think.

[nottoooily]

I suppose you could wind your own. Just had a look at the 75/300 ohm balun on my TV antenna and it should be a piece of piss to make something like that, only has maybe 8 turns, depending how you count them.

Yea you're meant to match the impedances properly, so 100:100 ohm. But I bet you could get something working just by jamming them together however you like. I always found this impedance stuff to be a bit of overkill. Sure it's important for long cable runs, picking up weak signals or transferring all your power. But will your batteries really die too soon if you waste some RF power? Will the range be too short?

Not sure how you'll connect a single antenna track to the two inputs tho ?? Or even how you define impedance for such a thing. But then I always got scared off by antenna design because people say it's a 'black art' Good on you.