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RF Tuning...Success! | Print |
Written by Akiba   
Friday, 24 July 2009

Spent the past two days working on tuning the RF portion of my circuit. I wasn't sure how it would turn out because it's the first time I tried to make a 2-layer RF board. If you haven't noticed, most of the reference designs are on four layers since the impedance is easier to control that way. It was also good because it gave me time to get to know my network analyzer and me and her are becoming best buds.

When I first started tuning, I was totally in left field. I had used the online transmission line impedance calculators and for a 70-mil microstrip on FR-4 with a 59 mil dielectric, I was supposed to be getting about 63 ohms for characteristic impedance. After tuning the circuit, I calculated the impedance of the receive path to be around 35 + j8 ohms. The imaginary component implies that its an inductive impedance, probably due to the length of the trace. 

It was really bizarre because the real part of the impedance was about half what I expected it to be. It was a good lesson for me because I learned that you can't really trust math when it comes to real world performance of RF circuits. For the online microstrip calculators, they assume that you have a trace on the top layer and an infinite ground plane on the bottom layer. However in the case of my board, it was untrue. I actually had a ground plane on both top and bottom layers. I think the top layer ground plane, which was separated from the trace by just 8 mils, affected the impedance pretty drastically. Also, there were contributions from the balun, DC blocking caps, and the differential RF lines coming out of the chip. I suspect that's why my initial tuning values for my matching network were so far off.

It took me about twelve tries to converge on a good impedance match. It actually took me about 8 tries to see the LC resonant dip for the return loss and then an additional 4 tries to center it on 2.4 GHz. The final outcome was that the minimum of the return loss dip was centered at 2.46 GHz (channel 22) with a return loss of -40 dB or about 99.99% power transmitted. My worst return loss value was at 2.405 GHz (channel 11) which was at -16.1 dB or about 97% power transmitted. Still, not too shabby. Word to the wise, those RF inductor and capacitor kits from DigiKey really came in handy.

So this little experience took me pretty deep into the realm of RF design. I can see that RF calculations can only give you a very, very ballpark understanding of how your circuit will behave and that you actually have to measure it to know the real performance. In short, RF design is messy. Luckily, it seems like I survived the ordeal.

Here's some screen captures of the return loss testing with my network analyzer:

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Ground planes
written by Michael, July 25, 2009
I see from your pics of the RF boards you connected your top and bottom ground planes via the 4 posts of the SMA connector. I also noticed many holes throughout the ground planes. Not being a fabbed board, I doubt those holes are PTH. Did you fill them with solder to make a connection? Wouldn't the more uniform ground vias help alter the impedance of the top ground plane? I'm a dummy when it comes to RF, so I'm just wondering.
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written by Akiba, July 25, 2009
Not sure if uniform ground vias would change the impedance. They are mostly for EMI suppression, where you space the vias less than a quarter wavelength. That way, you can zero out any peaks if the high frequency signal gets into the ground plane since the minimum distance for a peak to occur is a quarter wavelength. I used jumper wires to connect the top and bottom planes through the vias. It's tough to see through the pictures, though.

The impedance is largely determined by distance from the ground plane. You can imagine the dielectric between the trace and the ground plane as slightly conductive. As the distance decreases between the ground plane and trace, the impedance of the trace will decrease as well. The further away from the ground plane you get, the impedance will increase. Of course the dielectric isn't really conductive, but the principle is similar. That's why I think that the top layer ground which was very close to the trace affected the impedance.
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design
written by wasim, July 29, 2009
hi akiba
i would like to know ur contact mail.
i am also having a 2 layer card and need to tune it for the impedance.
please help me in tunning
regards
Wasim
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written by Akiba, July 29, 2009
I won't be of much help since you would really need a network analyzer and some RF knowledge to design the matching network. Some people are hesitant to spend the money on a network analyzer so the other alternative is to take a chance and just design the board. It's unlikely that you'll get a 0% transmission and a roughly 50-60% transmission power is fairly easy to achieve. You can probably do that tuning the circuit based on received RSSI at a remote node. However as you can see, the resonant peak (trough) is narrow so it would be very difficult to hit it without an analyzer. That'd be similar to shooting arrows at a target when you're blindfolded.
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