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Antenna Tuning for GSM/GPRS chip?

 
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msj121
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PostPosted: Mon Aug 29, 2011 7:30 pm    Post subject: Antenna Tuning for GSM/GPRS chip? Reply with quote

I have a simcom 900 chip which is a gsm/gprs chip. I want to hook up a 2.4Ghz antenna to it of course. Being new to the world of antennas I designed my pcb to tune for the antenna, though I am not finished. However, I recently discovered that if the transmit line is short enough (2.4ghz has a wavelength of 125mm, which means 1/10th the wavelength would be 12.5mm) that I do not need tuning capacitors etc....

Is this true? I want my pcb to be very small anyway so making the transmit line which I understand is supposed to be 50 ohm, less than 12.5mm should be easy, and apparently requires fewer components and can be connected directly from the chip to the antenna?


Thank you, ANYONE for your help.
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bigSteve
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PostPosted: Mon Aug 29, 2011 11:11 pm    Post subject: Reply with quote

Here's some info:

Why exactly do you want to hook up a 2.4 GHz antenna to a gsm chip? Doesn't that try to radiate at 900 or 1800 MHz?

If the transmission line is short, than you do not need to take into account the high frequency transmission line effects. What type of antenna are you using? If the antenna has a spec sheet that shows it has a proper impedance, you might be ok. However, some antennas require proper mounting on a ground plane (i.e. patch, pifa, ifa), and may need impedance matching to properly radiate.
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msj121
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PostPosted: Mon Aug 29, 2011 11:32 pm    Post subject: Reply with quote

Truth be told, I am impartial to any antenna. Something somewhat short that will let me get away without tuning would be fine.
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bigSteve
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PostPosted: Tue Aug 30, 2011 12:41 am    Post subject: Reply with quote

You should probably figure out what frequencies you need the antenna to operate at. If that's not defined, there's no reason to talk about antenna types or impedance matching.
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msj121
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PostPosted: Tue Aug 30, 2011 1:04 am    Post subject: Reply with quote

I am new to this, I apologize.... Well like I said the antenna is for gsm which I mistakenly thought was 2.4GHz, but is in reality 850 MHz and 1900 MHz (for North America). I understand they have quadband antennas GSM 850/900 & GSM 1800/1900 (824 - 960 MHz, 1710 - 1990 MHz).
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bigSteve
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PostPosted: Tue Aug 30, 2011 1:36 am    Post subject: Reply with quote

So what exactly are you trying to do?

That chips looks like it has an "antenna pad". I think this means that it is simply a small footprint that connects to the receiver within the chip. You'll need to design some interface then between your antenna and the chip. You'll probably need to get a transmission line away from the chip, and then go to some antenna. What your exact application (and what is the form factor or device size/material) you are trying to achieve will determine what type of antenna you need.
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msj121
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PostPosted: Tue Aug 30, 2011 9:15 pm    Post subject: Reply with quote

My main purpose is to have the chip make a few sms messages. Yes, my hope is that a few mm away from the chip I can install an sma connector to a quadband antenna.

My hope is that I can skip any capacitors and resistors for tuning as its new to me, complicated, and I want the unit to be small.

The exact size is not yet determined, but the chip is 24mmx24mm and the device will likely only be a few more millimeters longer. I am still working on the power supply, but I will try to keep the battery and ground as close to the antenna as possible, if this is necessary, which I assume it is to rid myself of the requirement of tuning.

The form factor is to be thin (so no bigger then 24mm) and the length shouldn't be too much of an issue, which should suffice most cellular antennas. The height should also not exceed too much more then 10 mm, but there is some leeway there.


As I am trying to work around the antenna you can tell that there are a few changing variables. I really thank you for your time and help.
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wolberine
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PostPosted: Thu Sep 29, 2011 7:11 pm    Post subject: Reply with quote

Do you have a datasheet for your SIM900? I couldn't really find one, but here's my two cents:

Does the module have any filtering on it? You'll probably want to put some on if your device does not have this.

Is the output from the module 50ohm? If yes, then a short trace to the antenna or SMA should work really well. If not, then you'll need to impedance match.

You probably want to keep that battery away from the antenna, depending on what antenna you buy and how low your antenna is.

Really take care to pour some good grounding around the RF sections of your module.
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msj121
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PostPosted: Sun Oct 16, 2011 2:09 am    Post subject: Reply with quote

I do have a data sheet, a little hard to find online but here is a link if you would like:
http://wm.sim.com/Sim/News/photo/2010222155735.pdf

By filtering I assume you mean power, and no it doesn't, but yes I am trying to put some filtering in.

The output from the module is 50ohm, which is why I hope I can use a simple 50ohm trace, though I need to figure out how (I am a little new to circuit design). The only problem is the document says that the output is not perfect 50ohm and so to match the circuit a p-type or t-type (resistor vs capacitor) matching is required. Do I need this? Or can I just take for granted it won't be perfect? Also once this is calculated will it work for every chip as long as it is the same antenna?


I will try to keep the battery away, thank you for the heads up.

In regard to the grounding, when you say "Really take care to pour some good grounding around the RF sections of your module", do you mean there should also be a barrier of ground around the antenna pads or just that the ground should be poured well, ie: thick or short distance from ground to pad directly?


Side question the chip is certified for RF etc... Do I need to re-certify if I don't/do use a matching circuit?

Thank you again
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bigSteve
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PostPosted: Tue Oct 18, 2011 7:02 pm    Post subject: Reply with quote

Some notes:
- A 50 Ohm trace doesn't give an impedance of 50 Ohms. It has the property that a 50 Ohm load will not be mismatched to the transmission line. So a 50 Ohm trace connected to a short or open circuit won't give 50 Ohms, for example.

In your case I would recommend no impedance matching. And any impedance matching you do with a resistor will destroy your antenna efficiency, so shouldn't even be considered.

I would recommend connecting a short transmission line directly from the radio to the antenna and testing if it works. If you don't have a network analyzer, you probably won't be able to figure out the correct impedance matching network. And wherever you are currently getting the information (p-type or t-type - ?) is not a good source.
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msj121
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PostPosted: Mon Oct 24, 2011 4:07 am    Post subject: Reply with quote

That t-type/p-type was from the technical document, it was just a label (perhaps their own) to differentiate between designing matching circuits. But I would rather have no matching circuit, so that is good news.

I think I understand the idea that the transmission line is not 50ohms that makes sense, not sure how a line would be or not be matched for a 50ohm load, but I can research that.

By the way, when you said "Really take care to pour some good grounding around the RF sections of your module", could you elaborate a little bit, RF in this fashion is new to me.

THANK YOU so much.
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wolberine
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PostPosted: Tue Nov 08, 2011 6:50 pm    Post subject: Reply with quote

The grounding comment was to suppress harmonics for ICs soldered onto your circuit board and reduce transmission line losses.

On your IC there are different voltage inputs that might supply the PA, VCO, etc. and they can produce noise at the frequencies your IC is running (and every harmonic). If there is not an easy return path for the high frequency noise to the ground of the IC, these frequencies can radiate undesirably.

Another reason has to do with your transmission path out to your antenna. The ground return path has an impedance associated with it that changes as the transmission path extends away from the IC. If this ground plane is broken (or too small), then your transmission line will lose power to radiation before the antenna. Also your shunt capacitors and inductors will have a different impedance than desired.
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