At this point it’s pretty well-known that you can tack a long wire to the Raspberry Pi’s GPIO, install some software, and you’ve got yourself the worlds easiest pirate FM radio station. We say that it’s a “pirate” station because, despite being ridiculously easy to do, broadcasting on these frequencies without a license is illegal. Even if you had a license, the Raspberry Pi with a dangling bit of wire will be spewing out all kinds of unintentional noise, making it a no-go for any legitimate purposes.
In an effort to address that issue, [Naich] has written up a couple posts on his blog which not only discuss why the Pi is such a poor transmitter, but shows how you can build a filter to help improve the situation. You’ll still be a lawless pirate if you’re transmitting on FM stations with your Pi, but you won’t be a filthy lawless pirate.
In the first post, [Naich] shows us exactly what’s coming out of the wire antenna when the Pi is broadcasting some tunes on the default 107.3 MHz, and it ain’t pretty. The Pi is blasting out signals up and down the spectrum from 50 MHz to 800 MHz, and incredibly, these harmonics are in some cases stronger than the intentional broadcast. Definitely not an ideal transmitter.
[Naich] then goes on to show how you can build a DIY filter “hat” for the Pi that not only cuts down a lot of the undesirable chatter, but even boosts the intended signal a bit. The design is surprisingly simple, only costs a few bucks in components, and conveniently is powered directly from the Pi’s GPIO. It even gives you a proper antenna jack instead of a bare wire wound around a header pin.
Cheap DC-DC converters have been a boon on the hobbyist bench for a while now, but they can wreak havoc with sensitive circuits if you’re not careful. The problem: noise generated by the switch-mode supply buried within them. Is there anything you can do about the noise?
As it turns out, yes there is, and [Shahriar] at The Signal Path walks us through a basic circuit to reduce noise from DC-DC converters. The module under the knife is a popular buck-boost converter with a wide input range, 0-32 VDC output at up to 5 amps, and a fancy controller with an LCD display. But putting the stock $32 supply on a scope reveals tons of harmonics across a 1 MHz band and overall ripple of about 66 mV. But a simple voltage follower built from a power op-amp and a Zener diode does a great job of reducing the spikes and halving the ripple. The circuit is just a prototype and is meant more as a proof of principle and launching point for further development, and as such it’s far from perfect. The main downside is the four-volt offset from the input voltage; there’s also a broad smear of noise at the high end of the spectrum that persists even with the circuit in place. Centered around 900 MHz as it is, we suspect a cell signal of some sort is getting in. 900 kHz.
If you buy an amateur transceiver cheap enough to make a reasonable grab bag gift or stocking stuffer, you get what you pay for. And if this extensive analysis of cheap radios is any indication, you get a little more than you pay for in the spurious emissions department.
Amateur radio in the United States is regulated by the FCC’s Part 97 rules with special attention given to transmitter technical specifications in Subpart D. Spurious emissions need to be well below the mean power of the fundamental frequency of the transmitter, and [Megas3300] suspected that the readily available Baofeng UV-5RA dual-band transceiver was a little off spec. He put the $20 radio through a battery of tests using equipment that easily cost two orders of magnitude more than the test subject. Power output was verified with a wattmeter, proper attenuators were selected, and the output signal scanned with a spectrum analyzer. Careful measurements showed that some or all of the Baofeng’s harmonics were well above the FCC limits. [Megas3300] tested a few other radios that turned out to be mostly compliant, but however it all turned out, the test procedure is well documented and informative, and well worth a look.
The intended market for these radios is more the unlicensed crowd than the compliant ham, so it’s not surprising that they’d be out of spec. A ham might want to bring these rigs back into compliance with a low pass filter, for which purpose the RF Biscuit might prove useful.
I became aware of harmonics and the sound of different shaped waveforms early in my electronics career (mid 1970’s) as I was an avid fan of [Emerson Lake and Palmer], [Pink Floyd], [Yes], and the list goes on. I knew every note of [Karn Evil 9] and could hear the sweeping filters and the fundamental wave shapes underneath it.
I remember coming to the understanding that a square wave, which is a collection of fundamental and (odd) harmonics frequencies, could then be used to give an indication of frequency response. If the high frequencies were missing the sharp edges of the square wave would round off. The opposite was then true, if the low frequencies were missing the square wave couldn’t “hold” its value and the top plateau would start to sag.