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,

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.

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.

Hi Frank,

I'm really dominating the Q&A on this topic; starting to get embarrassed, but I need to clarify something - to be sure my understand is on the right track. Using your example from rftools, signals will only be attenuated starting below 90 and above 110? Meaning, as in my demo, if I transmit a signal at 102.9 and there is a foreign signal at 103.9, they can still have an affect each other? Or is it that everything within 90 & 110 is attenuated from each other? I'm beginning to doubt what I've learned and when I do I freeze from moving forward :-/

HI Nathan,

No worries, I am pleased to try and help...  Filters are usually specified at the -3dB point and a Chebyshev filter is considered to have a sharp response.  So, everything below 90 MHz in the design I posted is greatly attenuated as is everything above 110 MHz.  The band in between passes most of the signal although Chebyshev have more ripple and are not quite as flat as some of the other filter types.  This ripple is more easily seen in the design that Shabaz posted although mine would have it as well.  For this particular crude application I think we can ignore that however.

As Shabaz has noted, there is variance in the components we are using and so it is necessary to have some kind of a plateau either side of the design point (102.9 in your case).  I had a look in my text which says Chebyshev are somewhat more sensitive to this but I think the design by Shabaz is sufficiently broad to cover that.  By the way, if I ever write something that seems to conflict with Shabaz, always go with what Shabaz says :-).  You could explore this by varying component value by their stated tolerance in the design tool if interested in the influence.

Now, how much should we worry about other stuff sneaking into what is being transmitted?  And it is important to remember this is transmission, not reception.  I haven't really investigated the radio here but it is bit banging the output if I remember correctly.  That means the Raspberry Pi is generating rectangular output over time varying between 0V and 3.3V.  It gets rounded off but it is still a really ugly signal.  If we look at it in the frequency domain then there will be lots of harmonics at other frequencies.  This is of course undesired in transmission because it causes interference across the spectrum.  But how bad is it?  First of all it is really low powered so it isn't going to transmit too far anyway.  Nonetheless, it is interesting to try and attenuate those harmonics and get rid of undesired energy away from the design point.

To give an idea of what happens, the FFT below is from a post I made a while back on entry level digital storage oscilloscopes with FFT capability.  It is a 49 kHz signal from a clock with a strong 148 kHz third harmonic.  The spikes that can be seen would be equivalent to the undesired transmission we want to get rid of.  Note that the 3rd harmonic and the primary signal are quite a ways apart (49 kHz Vs. 148 kHz).

Your signal won't look like this, just giving you an idea of what is going on.  If it was a perfect sine wave there would be no harmonics.  Your signal will have harmonics spaced some distance from the desired 102.9 MHz just like my FFT.  The idea with the filter is to make sure the radio transmission is inside the band pass and the harmonics or at least most of the undesired energy are outside of it.  Make sense?

Frank

Yes this is great Frank, a lot of good points here! One might accuse me of overthinking this project, but I have an OCD nature and you understand me with "it is interesting to try and attenuate those harmonics and get rid of undesired energy away from the design point". This is really about exploring what I can and cannot do. I obviously got the project working I just want to dabble and experiment with the filter side of things; clean it up if you will. Thanks again and until my next 'point of confusion'...

nk85  Forgive me if someone else has already mentioned this and I missed it but... you're using wirewound resistors (aka inductors).  You would have gotten better filtering if you would have used carbon film.  This little adaption cleaned up my pirate radio output.

As fmilburn states, the Pi is bit banging the signal out so I opted for a simple lowpass filter to attenuate the harmonics.

Thank you for bringing this back up.  I've been thinking about digging back into trying to make this work at Amateur Radio frequencies.  Wow.  I hope I didn't try this a year ago with the lowpass filter in...

Good point.  And the breadboard added unknown capacitance as well. 

Good point.  And the breadboard added unknown capacitance as well.