u/Cute_Wallaby6361

Hi All,

I have designed and built a compact bandpass filter for the 915MHz ISM band,aimed at reducing out-of-band interference in LoRa/SDR/ISM/IoT front-end applications.I'm looking for feedback from other rf design engineers on whether the measured performance and layout look reasonable.

I'm mainly trying to validate the design and understand if there are any obvious improvements before I iterate further.

Design Overview

Type: Bandpass filter module with integrated u.fl rf connectors

Centre Frequency: 915MHz (26MHz wide)

System Impedance: 50 ohms

Intended use: ISM/LoRa/SDR front-end/IoT gateway filtering/Lab interference suppression

PCB Size: 10mm by 12mm by 2.4mm (footprint) including the u.fl rf connectors

Measured Results

See plots of narrowband and wideband S21(db) filter performance below:

https://preview.redd.it/ozuwcub77xzg1.png?width=1755&format=png&auto=webp&s=8656216a27d77f8ff2852bd1ab565448faeb3987

https://preview.redd.it/awhx2or47xzg1.png?width=1755&format=png&auto=webp&s=ae912d6587cde657dd517fdc3c1f70a84984075e

Measurements where made on a VNA using a SOLT calibration.

Physical Implementation

PCB: FR4 (4Layers)

Size: 10mm by 12mm

RF Connectors: u.fl

Full 2-port S-Parameters are available for this filter module covering the frequency range (100MHz-6.1GHz).

Finally,find attached photos of the RF Filter-the 2nd photo compares the size of the RF Filter to a standard SD-Memory card.

https://preview.redd.it/o2ms9rii6xzg1.jpg?width=1536&format=pjpg&auto=webp&s=976dab554b042df27439d73d1fb74feef34cb990

https://preview.redd.it/vjjddern6xzg1.jpg?width=1265&format=pjpg&auto=webp&s=5c186e9c780e610d63d8a783084cde6482560c11

Hi-With regard to Drone Receivers being used at 915MHz, I am aware that when flying, de-sensing of the receiver can become a regular occurrence especially when Drone devices are flying close to 5G/LTE Cell Towers. Usually,most receivers have a very wide rf front-end and it is relatively easy for unwanted 5G/LTE signals to overdrive the receiver LNA into compression such that gain drops,unwanted intermods and harmonics are generated and the receiver sensitivity at 915MHz can reduce dramatically. This means that important data does not get received by the Drone,meaning you have lost control of the Drone.

I have developed a 915MHz saw filter pcba,size 10mm by 12mm and 1.2mm high and the board includes the rf connectors (u.fl).This filter board works perfect up to an receiver input level of +10dbm with a zero-drift reliability.The performance of the rf bandpass filter with regard to In-Band Insertion Loss and Out-of-Band Rejection is exceptional.This filter will safely attenuate the 5G/LTE bands just below and above 915MHz such that the LNA never becomes overloaded so the important control data always gets received correctly.This filter provides excellent rejection from 100MHz through to 6.1GHz such that all unwanted blocking signals at the input of the Receiver Antenna terminal are greatly attenuated.

If anyone is interested, I can provide photos of the pcba and VNA plots of the filter performance close in at 915MHz and also wider band over the range (100MHz-6.1GHz). I can also provide S2P files across the two bands mentioned such that you can look at the performance of the filter in RF Design Software such as ADS,Microwave Office or free software such as RFSIM99 or ATYUNE.

The main target of this rf bandpass filter was to provide a solution to Drone Unwanted Receiver Blocking problems as a re-fit to the Receiver channel.The pcba weighs less than 1 gram (it was too light for my electronics scale to measure a value,just showed 0.00 grams!!!) and can easily fit inside most sort of enclosures.I would be interested to hear feedback regarding everyone's point of view on whether lost control of a Drone due to LTE/5G blocking is a real problem and if yes do you think my hardware solution might help?