#rfbiscuit – Use Case 3: Bandpass Filter

My starting point to make RF Biscuit was actually to make lowpass / bandpass filters to experiment with rpitx – using the Raspberry PI as a signal source. (thanks to F5OEO this way) – My plan was using a FilterDesigner like Elsie – and design a filter that would cut away any unwanted harmonics – the Raspberry PI is outputting a square wave so you can expect the odd harmonics – most prominently the 3rd and the 5th – to appear as unwanted spikes in the spectrum.

Although not easy to spot the completed filter looks like this:


(unfortunatly I didn’t had the DSLR with me this time and I am using an oldschool Sony 3M-Pixel camera, which lying around in the lab)

My plan was to implement a bandpass in a mesh capacitor-coupled topology – using Elsie I came up with the following part values:


btw. Is this now a 5-Order bandpass??

Implementing this Filter I found out that I took too little time reviewing the design of Rev A. – because the inductors ( Murata LQW18ANR39G00D 0603) in series with the capacitors can not be easily fitted on RF Biscuit as you can see in the following picture:


So for Rev B. I would like to avoid constructs like this for this filter topology.

Also the inductors are really sensitive devices – especially when you solder them by hand. I had to replace two of them when I tried to solder them the second time – while handling them with tweezers it is really easy to damage the windings of the inductor.

In the Simulation this Filter should perform as a 85 Mhz to 105 Mhz bandpass:


And the measurement with the VNA shows following result:




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