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

The Powerboost 1000 is a nice little piece of kit that combines a boost converter and regulator from the LiPo's 3.3V to the requisite 5V for the Pi as well as providing a convenient charge and protection circuit for the battery. It's not required if you're using a different power source. Mine was a self-contained, mobile unit to capture that "third act of Pump Up The Volume" aesthetic

Cheers, and happy broadcasting!

Considering my inspiration, I just realized that I probably should've closed that comment with

TALK HARD

I did a quick look.  Check your filter calculations. Matthew used 1 Mega ohm resistors? One resistor I noticed in your filter is 0.01 ohm?  That is the same  order as what the leads, connections, and such will have.

edit:  this topic is outside my area of expertise but it would seem a LC filter might be better for an antenna

Hi Nathan,

As Frank says, a LC filter is better. Here's one that is better than nothing (there will still be radiation from the board since it is unshielded):

To calculate this, free 'Elsie' software was used. Response:

Unfortunately it is hard to find appropriate inductors in through-hole packages, so they are surface-mount, but I've listed ones large enough to easily solder using a normal iron, if you hold the part using tweezers etc.

560nH inductor560nH inductor (qty 2 needed)

27nH inductor27nH inductor (qty 1 needed)

4.7pF capacitor4.7pF capacitor (qty 2 needed)

100pF capacitor100pF capacitor (qty 1 needed)

Thank you both for getting back to me. Where does the calculator (as seen in my video) fall short? I had to special order those resistor types lol. And why only a low-pass filter? Need I not concern myself with the high-pass?

Hi Nathan,

I am a mechanical engineer, and mechanical engineers should be barred from commenting on filters in antenna design :-) .  Hopefully shabaz will correct any major errors and add to what I say and we will both learn.

I didn't check your design calculation but noticed the use of a 100 milliohm shunt resistor right away.  This is tiny - as noted above the Dupont jumpers, gunk on the resistor and  capacitor leads, connections, breadboard, etc. could easily add up to 100 milliohms.  For your design you would want these values to be miniscule compared to the resistor.

From what I have observed, LC circuits are generally used in antenna design.  They have a sharper cutoff and most designs seem to be for 50 ohm loads.  The design that Shabaz chose was probably for simplicity and reasonable performance with common components.  It is a bandpass filter as can be seen from the figure he posted.  I get something similar in this design tool although I specified different cutoff points.

Hi Nathan,

I only had a look at the video now, I too agree with Frank's comments.

Using online calculators alone will give unrealistic values unfortunately, although the math works, it doesn't know what your input signal is, and in practice you can't put a 0.01 ohm resistor across your signal (when you're working at high frequencies, the 100nF capacitor almost looks like a short circuit) and therefore cannot expect much output; the reason is your input signal has it's own resistance in the order of ohms or more, and a potential divider calculation can be used to see what the effect of that is.

Also, nothing is ideal, a real-world component is different to a pure resistance, capacitance or inductance. A real resistor can be approximated to be a pure resistance in parallel with a capacitance and in series with an inductance! The reason is, because wires are inductors, and a resistor has two wires. And two plates are a capacitor, and a resistor has two plates at the ends (as well as millions of tiny ones inside the resistive material perhaps).

Also, levels matter. The Pi outputs several volts, but a receiver can pick up 0.000001V. Some of the signal picked up by the receiving antenna will be from before your filter and get inside the receiver that way.

Also, in the spectrum view, it will be hard to see a measurable difference between 103MHz and 102 MHz, because the one-stage filter isn't as sharp as this.

This is another example where the online calculator doesn't care what inputs you give it, it will output stuff that isn't practical. It is impossible to have an RC circuit with such a narrow band as this. An RC filter has a slope, it isn't a sharp vertical line. Similarly even with an LC circuit, you'd need to start looking at a different topology, known as a resonant circuit for such a narrow band. Notice the filter designs pasted above from Frank and me are using a much wider bandwidth, and you can see the slope on the sides too, and the x-axis markings in MHz shows that the slope is significant. Another reason for the wider bandwidth is component tolerance, because it was anticipated in those designs that components won't be ideal.

Another thing is that power is needed out of the circuit to drive an antenna, and the RC filter calculators don't care about what load (the antenna is the load) you've got on the output.

The Pi transmitter is a great demonstration, but trying to eliminate harmonics will be hard without putting it into a metal box. If the output is very low and cannot be picked up with a receiver beyond a few meters, personally I wouldn't try to expend too much effort on the filter. If you intend to use it beyond the scope of a simple demo (e.g. like amplifying the output or trying to broadcast beyond say 10 meters would be outside the scope) then that's another matter (and there's the legalities of transmitting at particular levels and frequencies too).

Yes, thank you both again very much for taking the time to spell things out for me. I obviously have much to learn here, but I'm having fun trying my hand at the this filter. I did realize that I wouldn't be able to 'scrub' all of the harmonics out, but I thought at least (before completing the project in a box for shielding) the tool would show a measurable difference between having the filter plugged in and not - then even more so once in a box. I'll try my hand at the 3rd order bandpass filter, although for such a 'simple' project I might be pushing the budget; knowledge learned by doing knows no bounds thought right lol? Thank again. If get anywhere I'll post another video.

Shabaz, for a lack of not knowing much better I'm going to stick true to your shopping list. Maybe I'm not reading into it right, but does this filter schematic dictate the FM frequency to be used? In previous tutorials and forums I was lead to believe that the specs of your components does.

Shabaz, for a lack of not knowing much better I'm going to stick true to your shopping list. Maybe I'm not reading into it right, but does this filter schematic dictate the FM frequency to be used? In previous tutorials and forums I was lead to believe that the specs of your components does.

Hi Nathan,

You're right, the values dictate the frequencies, and the schematic provides the overall shape. The values in either of the filters (the one in Frank's post, or mine) provide a reasonable bandwidth for this particular schematic. If you wanted much sharper cut-off, the circuit would need to change (it would need more components) and then it is harder to build because each part needs to be reasonably accurate. As it is, a few pF difference will make an impact to the filter, so the components listed are slightly more accurate than normal, and are also suitable for high frequency work.