I have been reviewing the schematics of LoRaWAN Gateway designed by Microchip (Page 48). The output of the LNA has been connected to voltage source via inductor.

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According to me, the LNA module doesn't have power input pin and it requires the power to operate. This particular LNA Module requires power to be applied to the output and the signal will be modulated upon dc voltage. The Inductor makes sure the frequency won't get pass to the power plane.

I found another schematics and a different LNA module (SAFEA1G58KA0F00). The LNA module again doesn't seem to have power pin and module has been connected in similar configuration (voltage source has been connected at the output via Inductor). See the picture below: (The picture is of SIM7600I reference design and the picture below shows GPS part of the design)

enter image description here

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    \$\begingroup\$ You might gain some insight by reading about bias-T or bias-tee. \$\endgroup\$ – glen_geek Oct 14 '18 at 17:58

You have only two schematics, but three cases.

The first schematic is one case. The second schematic describes two ways the circuit can be assembled.

To the first schematic:

This is a simple case of providing DC current to an RF amplifier. As you suspected, L9 is there to separate the RF from the DC supply. Without out it, operation of the amplifier would cause a short circuit from Vcc to ground - destroying the amplifier. With it, the amplifier "sees" a high impedance to Vcc - no short circuit happens.

To the second schematic:

This describes two ways to connect a GNSS antenna.

The simplest requires an active GNSS antenna. Such active antennas require a source of DC power - and they also require the same kind of coupling as the amplifier in the first circuit.

So, you have L505 to provide power to an antenna with an amplifier. You also have C510 to keep the DC out of the rest of the signal path.

In this case, U501and U503 aren't needed, and jumpers are placed to route the signal around U501 and U503.

The other alternative is to have a passive antenna (no amplifier.)

In this case, you leave out L505 since the antenna doesn't need power.

Since the antenna doesn't have an antenna, you need the amplifier formed from U501 and U503.

U503 doesn't need a power source. It is a passive surface acoustic wave (SAW) filter.

R506 has a value given in nH, and is presumably used to remove any higher frequency stuff that managed to get through the filter. I'm honestly not real sure what is going on there, though.

U501 has a more conventional connection to the DC power supply.

You can select whether to use a passive antenna or an active antenna by installing various jumpers as described in the text to option 1 and option 2. They refer to resistors that aren't shown in the section of the diagram you posted. They call them resistors, but I expect they are zero ohm jumpers that can be placed as resistors.

  • \$\begingroup\$ So, Schematics 2 operating with a passive antenna doesn't require L505 and power supply at it's output? \$\endgroup\$ – abhiarora Oct 15 '18 at 10:28
  • \$\begingroup\$ Is SAFEA1G58KA0F00 passive and doesn't require power at a particular pin or output? I can't see the word "passive" in the datasheet. \$\endgroup\$ – abhiarora Oct 15 '18 at 10:41
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    \$\begingroup\$ That is correct. \$\endgroup\$ – JRE Oct 15 '18 at 11:31
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    \$\begingroup\$ SAW filters are in general passive. \$\endgroup\$ – JRE Oct 15 '18 at 11:32
  • \$\begingroup\$ but in schematics 2, it is not mentioned to NM L505 in case of passive antenna. \$\endgroup\$ – abhiarora Oct 15 '18 at 11:39

The SPF 3043 is a GaAsFet. The inductor acts as a drain load, in this case equal to the impedance of the inductor at the operating frequency. At DC it has a value close to zero; much higher at the operating frequency which will increase the AC gain.


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