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Circulators and isolators are the non-reciprocal parts that protect PA outputs, separate transmit and receive on a single antenna and clean up the match between stages. They are also easy to misapply because the datasheet numbers assume a perfect world.
1. How they work, briefly
A magnetised ferrite disc makes the junction non-reciprocal: power entering port 1 leaves at port 2, power from port 2 goes to port 3, and so on. An isolator is a circulator with port 3 terminated in a load, so reflected power from the output is absorbed instead of returning to the source.
2. Reading the key specifications
- Insertion loss: typically 0.2 to 0.5 dB for drop-in parts at sub-6 GHz. It adds directly to PA output loss and receiver noise figure.
- Isolation: often 18 to 23 dB over the band, better in a narrow centre region. Check the full temperature range; ferrite properties drift and the band moves.
- Return loss / VSWR at each port: matters because the circulator also sets the match seen by the PA.
- Bandwidth: wide-band parts trade isolation for coverage.
3. Power handling is about the load
For an isolator, the forward power rating and the reverse (reflected) power rating are different. If the antenna can present a full open or short, all the forward power ends up in the termination. Size the load resistor and its heatsinking for that case, not for the nominal VSWR. Also check peak power for high PAPR signals.
4. Temperature and magnets
- Ferrite saturation magnetisation drops with temperature, so performance shifts at hot and cold extremes. Industrial and outdoor radios should be specified across the full range.
- The bias magnet creates a static field around the part. Keep steel screws, shields and other ferrite parts away, as they can shift the band. Check the vendor's keep-out recommendation.
- Do not place two circulators side by side without checking the vendor's spacing guidance.
5. Surface-mount and microstrip types
SMT and drop-in parts need a solid ground directly underneath with plenty of vias; a poor ground raises insertion loss and lowers isolation. Microstrip circulators on ferrite substrates are compact but need careful wire-bonding or ribbon transitions at higher frequencies.
6. Alternatives
For low power, active isolators or a well-matched pad may be cheaper. For high power with antenna mismatch, a circulator plus a load is still hard to beat.
What isolation do you budget in front of a GaN PA for the worst-case antenna VSWR in your application?
1. How they work, briefly
A magnetised ferrite disc makes the junction non-reciprocal: power entering port 1 leaves at port 2, power from port 2 goes to port 3, and so on. An isolator is a circulator with port 3 terminated in a load, so reflected power from the output is absorbed instead of returning to the source.
2. Reading the key specifications
- Insertion loss: typically 0.2 to 0.5 dB for drop-in parts at sub-6 GHz. It adds directly to PA output loss and receiver noise figure.
- Isolation: often 18 to 23 dB over the band, better in a narrow centre region. Check the full temperature range; ferrite properties drift and the band moves.
- Return loss / VSWR at each port: matters because the circulator also sets the match seen by the PA.
- Bandwidth: wide-band parts trade isolation for coverage.
3. Power handling is about the load
For an isolator, the forward power rating and the reverse (reflected) power rating are different. If the antenna can present a full open or short, all the forward power ends up in the termination. Size the load resistor and its heatsinking for that case, not for the nominal VSWR. Also check peak power for high PAPR signals.
4. Temperature and magnets
- Ferrite saturation magnetisation drops with temperature, so performance shifts at hot and cold extremes. Industrial and outdoor radios should be specified across the full range.
- The bias magnet creates a static field around the part. Keep steel screws, shields and other ferrite parts away, as they can shift the band. Check the vendor's keep-out recommendation.
- Do not place two circulators side by side without checking the vendor's spacing guidance.
5. Surface-mount and microstrip types
SMT and drop-in parts need a solid ground directly underneath with plenty of vias; a poor ground raises insertion loss and lowers isolation. Microstrip circulators on ferrite substrates are compact but need careful wire-bonding or ribbon transitions at higher frequencies.
6. Alternatives
For low power, active isolators or a well-matched pad may be cheaper. For high power with antenna mismatch, a circulator plus a load is still hard to beat.
What isolation do you budget in front of a GaN PA for the worst-case antenna VSWR in your application?