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AN043 small size 2.4GHz antenna questions

Matt Shira
Expert 2180 points
Other Parts Discussed in Thread: CC2511

Chatto,

CC2511 USB-Dongle Reference Design 1.3 (Rev. C) (swrc062c) readme file gives copper thickness of 35um, whereas on our board the top layer copper thickness is about 17um (0.5oz copper). We are trying out best not to alter this thickness as this has been proven to work with other high-speed components by providing the right impedance. New change to the existing copper thickness would be risky and expensive.

You mentioned in http://e2e.ti.com/support/low_power_rf/f/155/t/106382.aspx#375099 that balun and filter network has to be changed if the circuit is different from the reference design. We have some questions:

1. Does antenna itself has to be changed?

SWRA117D page 4 said:

Information about the  PCB  can  be  found  in  a  separate  readme  file  included  in  the  reference  design.  To compensate for a thicker/thinner PCB the antenna could be made slightly shorter/longer.

However the expression “slightly short/longer” is vague and is not instructive since designer exact information to do the adjustment. How exactly should the shorter/longer mean? Among the various antenna dimension figures, which one do we need to change?

 

  

2. AN068 shows a cascading of 1) DC blocking capacitors + 2) balun + 3) Pi filter network. On the other hand, if we use Johanson chip balun 2450BM15B0003, the datasheet shows no DC blocking capacitor nor Pi filter network. So are they all integrated into the chip balun? Please refer to figure below.

Again page 3 of AN043:

SWRA117D page 3 said:

The IFA was designed to match an impedance of 50 ohm at 2.45 GHz. Thus no additional matching components are necessary.

What are the “unnecessary matching components” here? We are considering to use a combination of AN043 antenna + Johanson 2450BM15B0003, so would the circuit be as simple as just a Johanson chip balun, preceded by an Johanson L-07C10NJV6 or L-14C10NJV6 inductor, and the directly followed by AN043 antenna?

  

Thanks and regards,

Matt

  • 0
    TI__Genius 12545 points

    1. By longer or shorter, the note means change the length of L1 to compensate for the frequency shift that the change in thickness of the board causes. You can iteratively shave off the L1 on the open end to effectively increase the frequency.

    2. Yes. The entire balun and matching filter is combined into the chip balun. Simplifying life as we know it :-). You do not have to add any other components except for the balun, the inductor across the inputs. If using the meandered inverted F antenna, add a series and shunt component to the antenna feed point. This is provisionally added just in case the impedance of the antenna does not match that of the output of the balun (50 ohms)

  • 0 in reply to Chatto
    Expert 2180 points

    Chatto,

    1. The 2450BM15B0003 datasheet shows 8x40 mils transmission. Does 8 refer to track width, and 40 its length? I measured in the document and actually the length to width ratio in each of the four segments is NOT 40:8=5. For the upper left red segment before the inductor, the ratio is over 9. What does the 8x40 mean then? And does the Johnson datasheet drawing assumes 1oz copper?

    2. Is there a reference to see how the “series and shunt” component should be inserted?

    3. By “shaving off”, do you mean to cut off L1 by small pieces using like a shaver, or an XACTO knife?

     

    Matt

  • 0 in reply to Matt Shira
    TI__Genius 12545 points

    1. The thickness of the RF output lines should be 8mil, while the length is 40.

    2. The output of the balun is connected to a series 0 ohm resistor. The other end of this resistor is connected to the antenna feed point. The same end of the 0 ohm resistor that is connected to antenna feed point should also be connected to a cap (do not mount) the other end of which is connected to the ground (shunt component).

    3. XACTO knife will do :-). During testing, you will have to remove a little bit of L1's length to compensate for the thickness of the board, and how it offsets the frequency. Once you have the exact  L1 dimensions. The production boards should use the revised L1 length for the antenna.

  • 0 in reply to Chatto
    Expert 2180 points

    Chatto,

    Chatto said:

    2. The output of the balun is connected to a series 0 ohm resistor. The other end of this resistor is connected to the antenna feed point. The same end of the 0 ohm resistor that is connected to antenna feed point should also be connected to a cap (do not mount) the other end of which is connected to the ground (shunt component).

    Do you mean like this?

    If we got it wrong, could you provide a reference drawing?

     

    Matt

  • 0 in reply to Matt Shira
    TI__Genius 12545 points

    The diagram shown above is correct. The cap should not be mounted though.

    Again, if the antenna impedance matches that of the balun output, the additional components and provisions for them can be dropped in the subsequent boards.

  • 0 in reply to Chatto
    Expert 2180 points

    This clarified the questions. Thanks.

  • 0 in reply to Chatto
    Expert 2180 points

    Chatto,

    I guess we were wrong in the drawing above.

    In your answer to anothe post (http://e2e.ti.com/support/low_power_rf/f/155/p/84657/292023.aspx#292023), R10 R_0402 is a resistor, while on our drawing (http://e2e.ti.com/support/low_power_rf/f/155/p/200230/714078.aspx#713036) we have placed a capacitor.

    Do we differnetiate between the function of R9 and R10?

    1. For the 0ohm R9 which is in series between the balun output and Antenna, it purpose is for turning the impedance to reach the 50ohm requirement, if the impedance at the balun output is not such value.

    2. For R10, should it actually be a resistor or capacitor? This is a bit puzzling because capacitor shunts (filters) signal at high frequency, and 2.4GHz is certainly high enough for capacitor shunting to work; if we use a resistor at the location of R10, I am not certain of the exact role this resistor would play since current would always go through this resistor to the ground.

    Could you give a bit mroe explanation on this?

     

    Matt

  • 0 in reply to Matt Shira
    TI__Genius 12545 points

    Matt,

    The cap or resistor in shunt is a place holder, so is the 0 ohm resistor in series. If you see no need for it during prototype testing, you can get rid of them in production build.

    So, for R9 and R10 you are putting down a 0402 component pad essentially. That will either be populated with 0 ohms and nothing, respectively or get some inductance or cap values depending on how you need to match the impedance of the antenna

  • 0 in reply to Chatto
    Expert 2180 points

    Chatto,

    We now get it, thank you.

    Could you also have a look at our impedance questions (link 1, link 2)? We have finished routing, but are just unsure of how to control the impedacne of several critical tracks, because we choose to use JTI balun rather than copying the reference design directly.

     

    Matt