Filter board design

[Mario Vano]

Hi all:

I've finally gotten around to publishing the filter, vswr bridge and t/r relay board design that I've been using on several successful radios for some time.

Note that I have no plans to make pcbs or completed boards available, but the design is MIT licensed so anyone who would like to do that is encouraged to do so.

Here's the Github: https://github.com/mpvano/LPF2

and here's a picture of it in use during testing:

Note that the radioberry board is usually stacked on top of the filter board. This radioberry has pass through pins on the GPIO.

enjoy!

[Mario Vano]

Here's some more info: Kicad files, Gerbers and iBOM are at the github: https://github.com/mpvano/LPF2

Mario

AE0GL

[SNAIL]

I'm not familiar with ADS1115 and looking to use it in my project to measure power/SWR. Is everything right with R4 R3 circuit? Looks like R4 is just hanging in air.

[Samuel Lourenço]

Nice board! Very well designed. Will the the inductors fit between boards? I'm guessing that those filters are for 10/12 meters, 15/17 meters, 20/30 meters, 40/60 meters and 80 meters? What requirements are you filling?

In my case, I'm thinking about doing an external band pass filter, and particularly one which uses a single LPF cell, a button to change between bands, a very low power micro-controller, and bistable relays. Something akin to the LDG Z-817 tuner. Probably, the signal produced by the Radioberry is so pure that it only really needs a low-pass filter just to remove the sampling artifacts, even after amplification.

Kind regards, Samuel Lourenço

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[Mario Vano]

Hi Samuel: Thanks for your questions - I think I forgot to add the coil winding document - it's in a separate pdf file. I'll try to upload it today.

You can install any filters you want, but I build mine like the N2ADR filter where the WARC bands are handled by using the next higher standard band - as you surmised. That way piHPSDR, QUISK and THETIS use the default n2ADR band configuration for them.

The "dangling" resistor is connected to a pass through power connection that is compatible with my other radios that use a board called the RadioHat and whose design is also published in my account on gitub. This connector would NOT be installed for use with RadioBerry.  A connector of your choice can optionally be installed here if you wish to display battery voltage from a typical 12 Volt source - and this will be displayed in the example python program included, along with the vswr.

There is also an alternate bi-pin connector for the receiver output on the bottom of the board that can be installed for a connector that directly plugs into the RadioHat card when it is stacked under the filter board. 

There is another unused input AIN3 on pin 7 of the ADC. I've connected a 1K resistor to ground to it to make it easy to "tack" another voltage divider resistor to it to introduce another measurement signal as the fourth channel on the DAC. I've never used it, myself - but it's available for any use desired.

Note also that the ADC can also be configured for 2 differential inputs if desired - that's why the pin assignments use alternating inputs for the VSWR. There's no advantage to doing that on this board, but I have other boards that use the same bridge design that run their software this way and this makes the hardware compatible.

Because the board only requires I2C signals from the PI GPIO, it can be mounted elsewhere and the four wires cabled between the GPIO connectors. I've done this in some radios I've build whose profile is too low for stacking boards...

Mechanically, the board will stack UNDER a radioberry if it is assembled carefully with an extended GPIO connector (of the sort required with hats on a pi 5) spaced about an extra 1mm away from the board. The radioberry then stacks on top and if desired a preamp board can go there as well.

The picture shows a test configuration with the filter board on top. The stack can be assembled this way, but requires that you have Radioberry boards with longer "pass-through" GPIO connectors. I build all my RadioBerrys this way - but stock boards cannot be used this way. I believe there's a little less noise picked up from the pi by the filter board if it's on the bottom, as it's got no analog component traces on its bottom side..

I'm enclosing a couple of more pictures, including one of the RadioBerry based radio I just used to do an SSB activation of POTA park  US-0067 and often run here. It's built on a generic 10 inch touch display intended for mounting a Pi. It uses a QRP Labs 10 Watt Linear amplifer (as do most of my radios).

Note that most of my radios use X.FL connectors (like a gps or wifi board) for the low power interconnects as I find it makes for cleaner cabling. I tack solder x.fl connectors onto the SMA pads of my radioberrys.

The last picture shows the more normal stacking arrangement with the LPF board on the bottom, plugged into the Pi and a "stock" Radioberry plugged in above it. Above that is one of the "Radioberry Preamp" boards I built in order to test them with my filter board. That project is still ongoing, as you know - but of low priority.

I'm very happy with the state of the Radioberry with my LPF2 board and the QRPLabs 10 watt linear and consider it a "finished" project - other than getting the new Pi5 firmware going for testing...

M

AE0GL