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Looking for a very small broadband directional coupler

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M. Spock

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I have a need for something that I can't find anywhere. I'm looking for a very broadband, very small, medium power directional coupler. Maybe it does not exist, or maybe someone here knows of such a part. Maybe someone knows of a unique or strange way to do such a thing.

My needs:
1) Operate from 225MHz to 2600MHz in a single path.
2) TX output power is from 20dBm (0.1W) to 40dBm (10W). If I can only get up to 33dBm (2W) that could work for some applications.
3) Low loss. Since the max power is 10W any loss here hurts hard.
4) I need to have a "reasonable" sense of forward or reflected power. Directivity and coupling factor are flexible.
5) This is a super small product, so size is at a premium. Must be less than 0.5" x 0.5" x 0.2", ideally much less.
6) Not a cell phone application, so we can spend some $. Of course lower is better.

Things I've considered:
1) Directional coupler. I've not found anything even close to what I need. The challenge is getting down to 225MHz.
2) Resistive "pickoff" topology. This approach is load VSWR sensitive.
3) Isolator. Use of isolators and circulators are non-starters. They are too large and don't have the bandwidth.

If anyone has some ideas I'd love to hear them.

Thanks.
 

A small pigs nose, use one hole for a current transformer, the other for a voltage transformer and phase the windings appropriately.
Make the single turn winding out of a bit of small hard line with the screen grounded only at one end to minimise E field coupling.

33:1 turns into a matched load will get you ~30dB coupling factor for either voltage or current.

It will not be very flat over that range, but little really is, calibrate it out.

The other approach (but the coupling will vary wildly), is a coupled line job printed right on the board, make the sampling loop small compared to a wavelength at 2.6GHz and maybe use a log detector so you can still see the signals down at the LF end.

Regards, Dan.
 

hi, you may check Fiberstore to see if they have the coupler you need. They have lots of such kind of optical products. Hope it helps.
 

Transformer based couplers are probably your only option for the lower end of your bandwidth, but I rarely see them used for over 2GHz. Minicircuits has some that nearly fit the bill, with space to spare: https://www.minicircuits.com/pdfs/DBTC-17-5LX+.pdf. 50-2000MHz, 2W, 0.15"x0.166"x0.155". A different core material might get you the extra bandwidth you need. If this is for a product, might be worth chatting with them about the possibility of a custom solution.
 

Thanks for replying guys. The EMC power samplers parts won't work since they are not directional couplers. Mini Circuits does have a new directional coupler that covers 50MHz to 6 GHz, the SCBD-16-63HP+. Mainline loss, directivity, and power handling, are excellent. However the coupling factor varies by 25dB and its huge (700x320x200 mils). I think this is my only (known) practical option.
 

Thanks for replying guys. The EMC power samplers parts won't work since they are not directional couplers. Mini Circuits does have a new directional coupler that covers 50MHz to 6 GHz, the SCBD-16-63HP+. Mainline loss, directivity, and power handling, are excellent. However the coupling factor varies by 25dB and its huge (700x320x200 mils). I think this is my only (known) practical option.
Good find. It's probably a compact microstrip design, hence why the coupling craters below 1GHz. For what it is, it's a very impressive part, too bad they don't make a smaller version for lower power levels. Are you using this for VSWR monitoring?
 

Yes, exactly. I need to measure FWD and REV power to protect the PA.
 

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