Raspberry Pi Pirate Radio -- Episode 355

This looks like a fun project to do with my son. I wasn't able to open this 'exclusive to members' content until I became a member. When I did I was expecting some more detailed documentation on this project, but only saw a parts list which, while helpful, wasn't necessary. Is there any 'deeper look' documentation? Particularly regarding the low & high pass filter circuitry? I'll admit I may be overlooking the link, but any help I would appreciate.

Kind regards!

Hi Nathan!

This was one of the very early projects on element14 Presents, before we really got into our groove, so to speak, and I didn't get a chance to do much follow-up regarding band pass filters. However, since you've joined element14, you do have access to one of the largest communities of electronics engineers and enthusiasts to ask questions of!

Feel free to drop questions in these comments, or start a post in the RF community!

These links might be helpful to get you started:

RF (Radio Frequency)

The specified item was not found.

Software Defined Radio Lessons with GNU Radio

Hi Nathan,

Frank's right, he picked a better filter design actually, and both should be good enough for your scenario.

You had the right approach, exploring website calculators, it's just that the particular input and calculator produced something that in practice was more unrealistic.

But this is the normal way, everyone uses the website calculators or software to design filters, and exploring how things change as values are modified, or as more stages are added (for steeper roll-off). It's a cheap way of virtually experimenting without needing to buy components, but it will be only as good as the values you feed it. For instance, if you known your components have (say) 5% tolerance, then you'd have to run your formulas or simulation with the different expected values, to at least see if the effect is significant or not. Prior to online calculators, people used tables from books.

There are free simulators like LTspice for totally custom component configurations (as opposed to calculators which will only offer defined topologies), which you could use too, but for filter design and other small building-blocks it can be better to use online calculators, since they are pre-progammed with established topologies.

When you come to want to test it with test equipment and draw the frequency response, the NanoVNA is probably a good way (quite low cost too), I've not used it, but there's several blog posts by others experimenting with it and it looks ideal for this.

Hi there, I'm back at it for a bit. Some fun with paint lol - it's been ages. Can you tell if if I'm somewhat on point here? I've attached an untampered image if you want to use it to correct my mistakes. Thanks so much!

Hi Nathan,

Those proto hats are nice as they allow the breadboard to be transferred directly to a more permanent solution.  I suggest putting it on a breadboard first, make sure it works, and then solder it like it was breadboarded.  There are some issues with it at the moment:

• Rs is the supply resistance and won't be on your board
• Rl is the load resist and is the speaker - it is not on the board either
• Remember that the vertical strips on each side of the breadboard gutter are connected.  If wired as shown, it will be necessary to cut the trace which is unnecessary if wired differently.
• Use the lead wires on through hole parts to minimize the soldering and no traces need be added to the back
• The ground and +5V/+3V are already connected to the power rails
• Run the wire to the input from the inside rail - as shown it is connected to the header that plugs into the Pi

I'll try to mark up the board and post it in a bit.

Have a look at this....

I can't remember which pin is the one bit-banging on the Pi so check that.  There are lots of ways to lay this out but note that this will minimize soldering by using the leads on the components and traces on the protoboard to make the connections.  If you decide to string it out the same way as in the schematic that is OK but I like to put the wire on top of the board, and not bridge solder or put wire on the bottom as it is easier to see the circuit and I think looks better.  But that is just personal preference.

You might want to flip it 180 degrees as well depending on where the speaker wires go which would also minimize the jumper length from the Pi pin to the filter input.

Wow, I really appreciate you taking the time to highlight those points and for marking up a sketch. By 'speaker' do you mean 'antenna'? Some of the components I was needing were only available in SMD, so to over come that I was going to use an adapter, of sorts, as in the image below. Meaning that I couldn't stretch it like you set it up. I'll either have to modify my bandpass components based on availability with 'through hole' or keep the schematic as I built it and make adaptations based on what you did.

Doh!  Yes, I have been working on an audio amplifier :-)  Embarrassing sign of old age...

Which parts are the ones you can only get in SMD?  The reason I ask is that sometimes it is easy enough to solder them between 0.1" pads.

I might as well put it out there. I'm hesitant because I don't want to hear that it won't work lol. But the truth will 'set me free' - or 'not spend money where I don't need to. Tell it to me straight!

The LO201R68 inductor has a 0201 footprint - that is tiny and will be extremely difficult to hand solder.  I recommend at least 0805 if you are trying to hand solder SMD parts for the first time although 0603 aren't too bad.  The Bourns inductor has an uncommon footprint.  I see you have tightened up the band.  I suggest axial through hole where possible.  You can get most caps in SMD 0805 (your Kemet cap is 0805). 

Hi Nathan,

Great to see you're really getting into filter design. As Frank says, the larger parts (0805, or, worst-case 0603) are needed. I don't think I've ever soldered 0201 (you'll also see why if you order one of those : ).

Regarding the filter design, this looks valid, but in practice the issue you'll have is that all parts have a tolerance (maybe 2.5% or 5% etc), and if you choose to design such a narrow filter (1 MHz bandwidth at 100MHz) then there's a chance that the implemented filter could easily be off by a MHz or more. There's actually no need to implement such a narrow filter, because the output from the Pi won't (or won't have much) have anything apart from the harmonics, which are exact multiples. So, to guarantee that the filter works for your purpose, it is better to go for a more wider bandwidth filter.

On another thread, Mike mentioned that you may not even need a bandpass filter, and a lowpass may be sufficient incidentally.

Question on Harmonics and Filters

There's no harm with a bandpass (provided it is of a wider bandwidth as mentioned), but according to that thread, you will likely be fine with just a lowpass filter.

Hi Nathan,

Great to see you're really getting into filter design. As Frank says, the larger parts (0805, or, worst-case 0603) are needed. I don't think I've ever soldered 0201 (you'll also see why if you order one of those : ).

Regarding the filter design, this looks valid, but in practice the issue you'll have is that all parts have a tolerance (maybe 2.5% or 5% etc), and if you choose to design such a narrow filter (1 MHz bandwidth at 100MHz) then there's a chance that the implemented filter could easily be off by a MHz or more. There's actually no need to implement such a narrow filter, because the output from the Pi won't (or won't have much) have anything apart from the harmonics, which are exact multiples. So, to guarantee that the filter works for your purpose, it is better to go for a more wider bandwidth filter.

On another thread, Mike mentioned that you may not even need a bandpass filter, and a lowpass may be sufficient incidentally.

Question on Harmonics and Filters

There's no harm with a bandpass (provided it is of a wider bandwidth as mentioned), but according to that thread, you will likely be fine with just a lowpass filter.