Re: Newbie RF frontend protection Qs
Posted: Thu May 24, 2018 2:28 am
The diodes will clamp the RF input to less than 1 volt zero to peak (app. +10 dBm) unless they are damaged due to excess current. The 1n4148 is rated for 300 ma continuous maximum forward current and a repetitive peak forward current of 500 ma with surge up to 2 amps. The tungsten bulb acts like a fuse because it is only rated at 150 ma and will burn out at currents two to three times that amount. An incandescent tungsten bulb has a cold to hot resistance ratio of 10-15 to 1. When cold the bulb will have a resistance of less than 4 ohms and when operating at the rated current (150 ma) the 6V bulb will have a hot resistance of 40 ohms. If a strong received signal of S9+60 dB (-13 dBm) is applied to the input this will be 50 millivolts RMS, the bulb resistance will be less than 4 ohms and the diodes will not be clipping the signal (however they will generate weak intermodulation products). If a very high +30 dBm signal (7 volts RMS) is applied to the input the bulb will have 6.3 volts RMS across it and be lit with an on resistance of 40 ohms. The diodes will now be clamping the input to the receiver at +10 dBm and protecting it (RSP's are rated at 0 dBm continuous, +10dBm intermittent). If the voltage were to increase to 20 volts RMS (39 dBm) the current would be around 500 ma. and the bulb will quickly burn out and no signal will reach the receiver.
The bulb in this design is the weak link in the chain and if it burns out before the maximum current and power handling of the diodes is exceeded the receiver front end is protected.
On a side note the RSP1 used BAV99 surface mount diodes and because of their size they cannot dissipate much heat. I do not know if they have used this same device in the RSP1A, RSP2 and RSPduo. In any event the small physical size of thee devices means you need an external RF protector. The diodes are mainly there for ESD protection and not strong RF signals.
I would also add a GDT device for static and distant lightning protection if I were building a front end protection device.
The bulb in this design is the weak link in the chain and if it burns out before the maximum current and power handling of the diodes is exceeded the receiver front end is protected.
On a side note the RSP1 used BAV99 surface mount diodes and because of their size they cannot dissipate much heat. I do not know if they have used this same device in the RSP1A, RSP2 and RSPduo. In any event the small physical size of thee devices means you need an external RF protector. The diodes are mainly there for ESD protection and not strong RF signals.
I would also add a GDT device for static and distant lightning protection if I were building a front end protection device.