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I am designing a system on megahertz frequency bands (~ 400-1000MHz.) This device is connected to a variable antenna which does not necessarily stay in match with the circuit so there will be some reflection of the power back to the circuit. The feeding power would not be more than 0dBm at maximum. Before starting the design, I need to know:

  1. In this range of power, do I ever need to take care of the reflections at all? What could go wrong and which devices might get damaged? Don't VCOs and PLLs have any internal protection from this?

  2. What are the techniques that I can use? In this frequency range, I could not find any surface mount small-sized circulators or isolators suitable for PCB design. Should I go with lower power, or is there a specification that I should look into when browsing datasheets?

JRE
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Anonymous
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  • 0dBm (1mW) is not a big issue; the PA would be the only one suffering from that (if even). Integrated chips (PLL+VCO+PA or similar) usually have some protection, look for RPP (reverse power protection) in the datasheet – Lorenzo Marcantonio Sep 30 '21 at 06:21
  • I agree with Lorenzo, the issue I can see is if you plan to certificate your design. The returned power may affect the PA and generate undesired spurious emissions. – Marcos Sep 30 '21 at 20:08

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If you generate more power than you need then feed the output through an attenuator it will reduce the affects of any reflected power.

For example a 6dB attenuation will reduce the forward power by a factor of 4 but any reflected power will be reduced by an additional factor of 4 on the return trip giving a factor of 16 reduction in any reflected power.

These effects can normally be ignored on low power devices (VCOs, PLLs etc) that can tolerate operating into a mismatched load with no ill-effects. They don't need any special protection.

For special purpose devices the datasheet will often have any limits or recommendations to protect them.

Kevin White
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