Suppose I wanted to filter a Raspberry Pi's (or other similar) GPIO digital output (running pifm or rpitx or similar) to be clean enough to not splatter RF harmonics outside of FCC Part 15 (or equivalent for other countries) unlicensed RF broadcast allowances. What might be some suitable FM broadcast band low-pass filter or band-pass filter designs that could be constructed out of the easiest-to-procure and manipulate components and materials (e.g. something suitable for a kid's student project, nothing tiny surface mount, etc.) Is this possible?
Electronic – easy to construct FM low-pass or band-pass filter
filterraspberry piRF
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You should maximise purity of the signal fed to your amplifier to start, to make your job easier.
Depending on your output turned circuit and amplifier device topology you may have to deal with 2nd harmonic on upor 3rd on up. This is very standard fare at this sort of power level and you should have no trouble [tm] implementing a single filter.
The output matching stage of an RF amplifier is usually also a low pass filter by design, whether a traditional pi coupler (C-L-C) or LC circuit or other, or strip line or resonator functional equivalent. As you are usually concerned with harmonics (2x, 3x etc the desired fundamental) the requirements are usually not severe.
You'll find relatively few RF output stages with complex explicit low pass filtering.
Around 500 MHz is a bit of an in-between range where lumped element (RC) or waveguide / resonant trough etc type tuning is appropriate.
A good start is to look at Amateur Radio amplifiers that work around that frequency and see what sort of output tanks they use. Common amateur bands of relevance are at 432 MHz (70 cm) and 1296 MHz (23 cm). There is also the "2 metre" Ham band at about 144 MHz but that will be leaning more towards lumped element tanks and filters.
In 1990 Motorola published application note AR347 A compact 1 kW 2-50 MHz solid-state linear amplifier. While this is below your frequency range of interest, this amplifier has become a workhorse starting point for a zillion spinoffs and much can be learned by looking up examples.
Amateur amplifier designs up to 1 kiloWatt are common enough so something that works at 432 MHz and 1 kW should be scaleable to your use in the 500-600 MHz range.
Here's a page with Many amateur RF amplfier designs - some very close to your application.
This 432 MHz PA useuses steam power aka a power triode but shows you how simple a design is deeemed acceptable. Plumbing skills will be handy. An antenna tuner may follow this stage - but reading a number of amateur design articles will rapidly introduce you to the subject.
Steam power design from here:
Nice 432 MHz 1500 Watt amplifier pictures only here but you can follow it up if it looks useful
Design using older tech Russian made vacuum tube but useful for its output stage comments article here
They note:
- The output circuit: The output uses a λ/2 75 Ohm stripline with both tuning and loading at the open end, it is constructed with silvered cooper 1mm thick with 125mm width and 220mm overall length. The line has a 25mm collar to reduce the spacing to ground while the finger stock contacts the tube anode on the lower cooper ring of the anode cooler. The line is fixed at 45mm from ground by ceramic or Teflon insulators (Teflon insulators are recommended). The output on a 7/16 connector (or a good quality N connector) is connected directly to the loading flapper. This flapper is 15mm by 30mm at a distance from 10 to 30mm from the end of the line. A choque connects the output to ground to avoid the presence of high voltage in the case of a flash over at the output flapper. The movable tuning flapper "C1" is 76mm wide by 45mm high, and tunes at 432MHz aprox. at 16mm distance from the line. The fixed flapper "C2" is 76mm wide by 35mm high, and is about 15mm from the inner side of the end of the plate line. The use of a kapton sheet between the flappers and the stripline has eliminated arcing from the flappers to the stripline
One kW at 432 mHz useful discussion and pictures
1st stage is a 1.9995 kHz bandpass filter using a standard multiple-feedback topology. It has a Q of about 20 and a gain of about 40 dB.
2nd stage is a 2.009 kHz BP filter with a Q of about 25 and a gain of about 40 dB.
Total gain of first stage is 40 dB which makes a 10nV 2 kHz signal into 1 microvolt.
Bandwidth of 1st stage is \$\dfrac{f_C}{Q}\$ = 100Hz and for the purposes of noise calculation you can assume the bandwidth is 1.6x greater at 160 Hz.
\$E_{NOISE}\$ from ADA4004 is 1.8nV /\$\sqrt{Hz}\$. This means noise in a 160 Hz bandwidth is 22.8 nV
This is bigger than your signal (10nV) therefore this isn't going to be a great design.
Even if you took into account the Q of the 2nd stage, the bandwidth would only be halved i.e. a half power point would become a quarter-power point. Noise voltage would be about 16 nV and still a significantly bigger value than your signal. This is made worse by the 2nd stage being 9.5 Hz different to the 1st stage.
Best Answer
If you dont care about bulky and messy construction, a DIY filter made with hand wound coils and copper clad is the way to go. Get yourself some enamel wire and a ceramic capacitor kit from ebay.
However, in my opinion do not bulid the doubly tuned band pass filter suggested by one of the answers, atleast not the form shown. The filter shown, couples the resonators using capacitors only, this introduces a "zero" in the frequency response. Simply put, the filter roll off for frequencies greater than the resonant frequency is terrible, so it is not a sensible choice for harmonic suppression. Furthermore, aligning it without variable (expensive!) capacitors is difficult. (The filter is however an excellent choice for an IF filter)
Build a 7th or 9th order Butterworth Low Pass filter using similar techniques, they are much more well behaved and easier to construct.
Here a 9th order LPF I built. The bandwidth is 100Mhz and at 200Mhz I get > 70dB attenuation.
Use the calculator found here: http://www.wa4dsy.net/filter/hp_lp_filter.html
EDIT: How to wind the inductors:
At these frequencies the inductors are small enough to be air core and hand wound. So start by some enamel wire, wind a few turns around something like a pen and measure the inductance produced. This will give you a standard to refer to in terms of inductance per turn for a certain diameter. Now take it from there. You do not need to be spot on, just get close enough and then tweak the inductance by varying the coil separation. This is how I did it.
You might not have a LCR meter capable of measuring such small inductances (I didnt have one either). So simply, build a fast edge square wave (or use the TTL output of your cheap signal generator) and feed it into a test circuit with a known capacitor and unknown inductor. Probe it using a X10 probe and measure the ringing frequency and calculate the inductance. Careful about the length of coax you use in the above setup as that is going to have comparable capacitance to your test circuit.
EDIT: The wire I've used has a 0.6mm diameter and the coil diameter is about 7mm. So this is a good starting point for you to use for building inductors for this frequency range.