We have variants of LPF PCB. First is LPF100 and using low-pass filter 5th order. And the second one is LPF-mini using low-pass filter 3th order. Both PCBs have a SWR meter circuit. All of filters are for 50 ohm.
The LPF100 unit, much better to use if you need attenuation for more harmonics.
Additionally, the board LPF100 have protection circuit if excess a predefined value of SWR. This protection circuit based on transistors VT1, VT2 and relay K16. Sensitivity of the circuit you can set by resistor R11. When the level exceeds the SWR Reflected, defined by resistor R11 value, the transistor VT1 will activate the relay K16. From relay K16, for example, you can cut off voltage for the input relay RX-TX in PA. Or cut off the supply voltage of the first stage PA. Protection scheme is with the "sticky" status, ie VT1 key state is hold until you will connect ESC contact with the ground or OFF/ON the main +13,8 V voltage for the protection board. Hi SWR indication LED VD14, which is connected to the contacts LED SWR and +13,8 V or in any other way, using the relay K16 contacts.The LPF-mini unit, you can use if you need attenuation up to 30db harmonics. Also, you can use this unit for coupling between TRX and amplifier. Bur needs some recalculation for filter elements if you using amplifier based on Triods or Grounded Grid Tetrode.
LPF100 toroid and winding info:
LPF-mini toroid and winding info:
Circuit, pictures for LPF100 Unit, KIT LPF100:
Circuit, pictures for PCB LPF-mini, KIT LPF-mini:
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