RF filter block

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Since I’ve got my radio amateur license I’ve reached the limits on the band pass filters integrated in the RSP1A receiver.

My location is near some high power transmitters and they where constantly overloading the receiver’s front-end. That created a lot of false signals and a high noise base line that made HF reception difficult / impossible on some days.

To fix it, I had to create a set of band pass filters that can be attached in front of the receiver. Another requirement was the ability to electronically switch them, to reduce  the wear on the receiver SMA inputs.

For that I’ve chosen the PE4259 SPDT RF switches. They seem to work down to 3.5MHz – as seen in the VNA sweep.

The filters where designed with LC Filter Design Tool from Marki Microwave.
I chose a 3’rd order band pass butterworth topology.
I used fixed value capacitor and adjustable TOKO 10K coils bought from aliexpress. Their Al varies from 8.5nH/N^2 with the core removed, to 22nH/N^2 with the core fully inserted.

The board contains 5 filter blocks that are controlled by a set of 2 SPDT switches. Each set has it’s own ENABLE signal brought to a pin header on the PCB edge.
There’s also a pin header for the power supply. As an afterthought, I’ve tacked a reverse polarity protection diode.

The board is a 4 layer design chosen to simplify routing and to allow for better ground plane coverage.

The board I’ve build has the following filters:

Band L C1 C2 C3 C4 C5 C6 C7 Notes
3.5MHz – 4.2MHz 510nH 1.5nF 2.7nF 470pF 2.7nF 470pF 2.7nF 1.5nF 5 turns
6.8MHz – 7.6MHz 510nH 220pF 680pF 56pF 820pF 56pF 680pF 220pF 5 turns
13.4MHz – 14.5MHz 330nH 82pF 270pF 22pF 330pF 22pF 270pF 82pF 4 turns
20MHz – 22MHz 330nH 56pF 120pF 18pF 150pF 18pF 120pF 56pF 4 turns
27MHz – 30.2MHz 330nH 39pF 56pF 8.2pF 82pF 8.2pF 56pF 39pF 4 turns

 

The assembly was simple – solder all smd components and then add the coils

Each filter was aligned with a calibrated NanoVNA. For each filter I’ve measured transmission and reflection coefficients:

3.5MHz – 4.2MHz:

6.8MHz – 7.6MHz:

13.4MHz – 14.5MHz:

20MHz – 22MHz:

27MHz – 30.2MHz:

The isolation is quite good when no filter is selected:

All the documentation is available on github: https://github.com/ealex/hfBandPassFilterBlock

Category: Electronica, RF | Tags:
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