Raspberry Pi Pirate Radio -- Episode 355

Turns out this was a Covid project. Here is my pirate radio 2.0 (1.0 didn't include surface mount ports). Now to try and understand why certain playlist items, despite all been converted to .wav, present with much static/noise. Even merging the tracks into one file still plays with static/noise at specific points in the file. Any thoughts on cleaning this up? The 30cm micro-usb surface-mount extension also affected the quality of the LED; it's very dim, but plugged into the RPi directly it shines bright - wasn't expecting this impact by using that peripheral.

Have a look at this link https://www.element14.com/community/people/jw0752/blog/2015/11/04/i-want-to-do-electronics-where-do-i-start

This is a great visual Shabaz. You and Frank both have provided so much insight! I've archived this entire thread for future referencing. Can you refer any good academic books (with a beginner in mind) that speak to this field of topic(s)? I'd be lying if I said I understood everything you both posted since we started chatting. The more I pick your brains though the more I am further intrigued. I adopt a bazaar thirst sometimes for having to understand everything that is going on, even if only semi-related to the topic at hand.

I am away from home with only a phone - Shabaz gave a great answer though

My shopping list would be for Coilcraft surface-mount inductors, mainly because they make ones with a tight tolerance (2.5 or 5%). For capacitors it would be NP0 types which I think are only surface-mount. The reason for those is that they have a tight tolerance too. Such a narrow filter needs accuracy in the design.

Here's a sketch of what I believe is coming out of the Pi :

The triangles represent output (it won't be triangular shaped), and the orange triangle is the desired output, however the Pi will output all three triangles in the diagram (and many more, theoretically off toward infinity). This diagram assumed the Pi software is set to broadcast at 102 MHz. The content at 306 MHz, and 510 MHz and so on, needs removing.

The width of the triangle is 0.3 MHz (it won't be precisely that, but it is of that order, and again theoretically it goes off to infinity, but at vanishingly low output).

According to this sketch, the Pi will output nothing between about 102.15MHz and 305.85 MHz, so a filter that has a response such as the orange curve would work well. Steep filters cost more (need more components), but in this case with the orange curve there is almost 200 MHz of space for the curve to drop off as low as possible, so a steep edge is not needed.

Another option would be the green filter curve, and again sharp edges are not needed, because the Pi will not output anything undesired up to ~306 MHz.

The danger with the very narrow filter (purple curve) is that due to the component tolerance, it may have shifted from 102 MHz to 101 MHz, filtering off a lot of the desired orange transmission. For this reason, very sharp narrow bandpass filters are almost never made using inductors and capacitors.

I would much rather use through hole components to be sure. But as I tightened up the band, it was asking for parts that didn't come available in through hole type. Reason being, for the tighter band, was to avoid other in-use frequencies near by (above and below the 102.9). Thats been a big part of it I guess, trying not to interfere with neighbouring in-use frequencies. Also the online catalogs (like Newark, DigiKey-Key & Mouser) overwhelm me; so many choices and so many factors to each choice. I'm definitely to rookie to be left to successfully choose the parts, so I've been delegating to the tech. support from each company to satisfy the schematic. That turned into SMD parts pretty quick. I can't always get it right the first time, and I don't suspect to, but this extracurricular interest might not be for a tight budget guy. If I could only get the part selection nailed down I shouldn't have to spend and re-spend because I didn't do my home work. Knowing the reason for the tighter band, what would your shopping list look like? I'd be interested in what Shabaz puts together too!

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.

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). 

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!

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.