Wednesday, October 19, 2016

VHF 2m Low pass filter design for 146Mhz ideal for Raspberry Pi

VHF 2m Low pass filter design for 146 Mhz ideal for Raspberry Pi 

I wanted to use the Raspberry Pi SDR TX capabilities using the IO pins to generate RF.
The following was possible.
APRS
FM and SSB and CV
WSPR
SSTV....

There is a very nice library that will run on Raspberry Pi one B called rpitx witch can generate CW,SSB,FM SSTV...

The problem in using a Digital IO pin on the pi to generate RF signal would mean it will generate a squire wave signal witch has lots of harmonics. To solve the harmonic problem you will need to build a low pass filter.
Harmonic about 25 db down with now filter.




This harmonic would not be in regulation with rf transmission and would therefore need to create a low pass filter to remove the harmonics.

Here is a filter I build to get rid of the harmonics.

RFSim99 application.


Here is the Low pass filter spectrum analyzer curve on my LWT500,
The Harmonic level should be about 25 db below the carrier.

LPF Curve for 146Mhz (1 Harmonic down about 25db)


Here is some photos of the veroboard test filter I am now busy make a PCB for Raspberry Pi with a small 10db amp.


146 Mhz Low pass filter SMD components

Still need to replace the two wires with Coaxial cable.
This filter board with 7 stage low pass filter will be available soon. (60db)
3D model of 2m Filter board
I will still add a screenshot with the Filter attached later.
Boards has arrived. Assembly and testing will start. 




Here is a Design for a 145Mhz Band Pass Filter

145Mhz Band Pass Filter

I also have simulated a 7 stage low pass filter here is the details.

7 stage Low pass filter
7 stage low pass filter (svcfilter)




Tools

Simulator tool http://www.electroschematics.com/835/rfsim99-download/
Filter design tool http://tonnesoftware.com/svcfilter.html
Toroid selection tool http://kitsandparts.com/tcalc.html
RFSim99   http://www.electroschematics.com/835/rfsim99-download/