Retuning RF antenna boosters

Discussion in 'Wireless & RF Design' started by Paul Reedy, Aug 31, 2019.

  1. Paul Reedy

    Thread Starter New Member

    Aug 31, 2019
    4
    1
    Hello everyone, first post here. I didn't find anything like this in search so here goes...

    I have a Sennheiser AB 2-C antenna booster which is rated for 740-778 Mhz. I would like to use it in the 470-608 spectrum, or even 470-558 range.
    I'm not sure exactly the preamp used on this one, it's marked L02. All parts on this model are fixed, the inductors look like a circus.

    I also have an AB 3-A amplifier, which is one generation newer which is in the actual range I need, and I notice that those have variable capacitors in those for tuning. The amps used in these cover dc to 6ghz so I know the active components can operate where I need it to.

    Back to the AB 2, I have tried to trace the schematic and I think I have it down. I know the value of a FEW inductors. Other than that, I only know the range it's supposed to be tuned to. I know on the input side, there are 3 capacitors going to ground with 2 inductors between them, with the inductors in series. The 2nd one is a 1uh. On the output side, there are a TON of series capacitors with inductors going to ground and toward the end a bunch of inductors in series.

    Not sure if someone could help with this. My RF knowledge is limited to work I did when I was younger in Ham Radio, I really haven't been in the game in a while. I've not been able to find any schematics on these, it seems they must be locked up at Sennheiser, and these are several generations old units.
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
    2,279
    687
    I think it is very unlikely retune the amplifier that far from it's design frequency. I think it is likely that all of the frequency dependent components will consist of striplines. You have only provided one fact which is that there is a 1 uH inductor in the device. This is most likely being used as an RF choke as it has a reactance of about 4700 ohms at 750 Mhz. (For it to resonate at 750 Mhz it would require a capacitor of about 0.045 pF It would probably have more than this as self capacitance.) If you post the schematic you have traced out and good pictures of the device there is a VERY SLIGHT chance that someone can help.

    Les.
     
    carloc likes this.
  3. Paul Reedy

    Thread Starter New Member

    Aug 31, 2019
    4
    1
    I’ll have to post the schematic later. Here’s pics of the circuit. I don’t think there’s any strip lines on this board.
    A00F4802-38CA-4B09-ACDE-370C3A7C695F.jpeg DC08B3C9-0995-4C62-9AC7-213A9ACD7BF6.jpeg
     
  4. bertus

    Administrator

    Apr 5, 2008
    19,795
    4,105
    Hello,

    I see a lot of coils on the back of the PCB.
    They likely make the sharp filtering.
    There are other models of the amplifier with other ranges:

    SENNHEISER AB 2 MANUAL.png

    Bertus
     
  5. Paul Reedy

    Thread Starter New Member

    Aug 31, 2019
    4
    1
    Yes I know there are other models, but it's a shame for all these to be wasted, they're still good amps, and probably not worth shipping to another country if the freq range is legal there. The AB2 is a discontinued item, and has been replaced by the AB3. It would be nice if the schematics were available from Sennheiser, but I'm sure they just want everyone to replace them with new units.
     
    hendersonnoel likes this.
  6. SLK001

    Senior Member

    Nov 29, 2011
    1,406
    574
    Quite possibly, the amplifier itself is fairly wide band. The filter "circus" is probably a band pass to limit the amplification of out-of-band signals. You could probably strip out the BP filter and use it on the other bands, but with the resulting cross talk, interference and IMD that would result.

    Sit down and trace out a schematic on your own.
     
  7. n6ac

    New Member

    Apr 29, 2011
    3
    0
    [Sit down and trace out a schematic on your own.[/QUOTE]
    And then find a version of Spice circuit simulator to run on your computer, and put the schematic there, and put in your best estimate of present component values, and tweak until the passband looks right for as-is.
    Them you will be ready to change values to the new frequencies.
    About 60 percent lower, so maybe can do by putting inductors with 2.5x the number of turns ... won't be perfect but a start.
     
    Last edited: Sep 10, 2019 at 3:16 PM
  8. SLK001

    Senior Member

    Nov 29, 2011
    1,406
    574
    Without a network analyzer or equivalent, putting in inductors with no idea of their values would be a waste of your time. If you look at the inductors in the present circuit, it is obvious that they were manually tuned by some operator.
     
  9. Paul Reedy

    Thread Starter New Member

    Aug 31, 2019
    4
    1
    Ok, I think this is it. None of the values here are known.
     
Loading...