Getting An RF Low-Pass Filter Right

If you are in any way connected with radio, you will have encountered the low pass filter as a means to remove unwanted harmonics from the output of your transmitters. It’s a network of capacitors and inductors usually referred to as a pi-network after the rough resemblance of the schematic to a capital Greek letter Pi, and getting them right has traditionally been something of a Black Art. There are tables and formulae, but even after impressive feats of calculation the result can often not match the expectation.

The 30MHz low-pass filter, as QUCS delivered it.
The 30MHz low-pass filter, as QUCS delivered it.

Happily as with so many other fields, in recent decades the advent of affordable high-power computing has brought with it the ability to take the hard work out of filter design, Simply tell some software what the characteristics of your desired filter are, and it will do the rest. The results are good, and anyone can become a filter designer, but as is so often the case there remains a snag. The software calculates ideal inductances and capacitances for the desired cut-off and impedance, and in selecting the closest preferred values we modify the characteristics of the result and possibly even ruin our final filter. So it’s worth taking a look at the process here, and examining the effect of tweaking component values in this way.

The idealised graph produced by QUCS for our filter.
The idealised graph produced by QUCS for our filter.

The filter we’re designing is simple enough, a 5th-order Bessel filter, and the software is the easy-to-use QUCS package on an Ubuntu Linux machine. Plug in the required figures and it spits out a circuit diagram, which we can then simulate to show a nice curve with a 3dB point right on 30MHz. It’s an extremely idealised graph, and experience has taught me that real-world filters using these designs have a lower-frequency cut-off point, but for our purposes here it’s a good enough start.

As previously mentioned, the component values are not preferred ones from a commercially available series, so I can’t buy them off the shelf. I can wind my own inductors, but therein lies a whole world of pain of its own and I’d rather not go there. RS, Mouser, Digikey, Farnell et al exist to save me from such pits of electronic doom, why on earth would I do anything else but buy ready-made?

My revised filter circuit with off-the-shelf component values.
My revised filter circuit with off-the-shelf component values.

So each of the components in the above schematic needs moving up or down a little way to a preferred value. What effect will that have on the performance of my filter? Changing each value and re-running the simulation shows us the graph changing subtly each time, and it can sometimes be a challenge to adjust them without destroying the filter entirely. Particularly with the higher-order filters with more components in the network you can observe the effect of individual components on the gradient at different parts of the graph, but as a rule of thumb making values higher reduces the cut-off frequency and making them lower increases it. In my case I always pick higher values for that reason: my nearest harmonic I wish to filter is at double the frequency so I have quite some headroom to play with.

The revised curve from the filter with preferred values.
The revised curve from the filter with preferred values.

Having replaced my component values with preferred ones I can run the simulation again, and I can see from the resulting graph that I’ve been quite fortunate in not damaging its characteristics too much. As expected the cut-off frequency has shifted up a little, but the same curve shape has been preserved without any ripples appearing or it being made shallower.

If I were using this filter with a real transmitter I would ensure that I designed it with a cut-off at least a quarter higher than the transmission frequency. In practice I find the cut-off to be sharper and lower than the simulation leads one to expect, and for example, were I to use this one with a 30 MHz transmitter I’d find it attenuated the carrier by more than I’d consider acceptable. It must also be admitted that changing the component values in this way will also change the impedance of the filter from the calculated 50 ohms, however in practice this does not seem to be significant enough to cause a problem as long as the value changes are modest.

We haven’t made this filter, but in the past we’ve featured another one I did make, and by coincidence it was in the same frequency range. When I wrote a feature on automating oscilloscope readings, the example I used was the characterisation of a 7th-order 30 MHz low-pass filter. It might even be one of the ones in the header image, pulled from my random bag of filter boards for the occasion.

The VU Meter and How It Got That Way

Given its appearance in one form or another in all but the cheapest audio gear produced in the last 70 years or so, you’d be forgiven for thinking that the ubiquitous VU meter is just one of those electronic add-ons that’s more a result of marketing than engineering. After all, the seemingly arbitrary scale and the vague “volume units” label makes it seem like something a manufacturer would slap on a device just to make it look good. And while that no doubt happens, it turns out that the concept of a VU meter and its execution has some serious engineering behind that belies the really simple question it seeks to answer: How loud is this audio signal?

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Bandpass Filters from the CNC Mill

A bandpass allows a certain electrical signal to pass while filtering out undesirable frequencies. In a speaker bandpass, the mid-range speaker doesn’t receive tones meant for the tweeter or woofer. Most of the time, this filtering is done with capacitors to remove low frequencies and inductors to remove high frequencies. In radio, the same concept applies except the frequencies are usually much higher. [The Thought Emporium] is concerned with signals above 300MHz and in this range, a unique type of filter becomes an option. The microstrip filter ignores the typical installation of passive components and uses the copper planes of an unetched circuit board as the elements.

A nice analogy is drawn in the video, which can also be seen after the break, where the copper shapes are compared to the music tuning forks they resemble. The elegance of these filters is their simplicity, repeatability, and reproducability. In the video, they are formed on a CNC mill but any reliable PCB manufacturing process should yield beautiful results. At the size these are made, it would be possible to fit these filters on a business card or a conference badge.

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These Capacitors are a Cheap Gimmick

If you search through an electrical engineering textbook, you probably aren’t going to find the phrase “gimmick capacitor” but every old ham radio operator knows about them. They come in handy when you need a very small capacitor of unknown value. For example, if you are trying to balance the stray capacitance in a circuit, you might not know exactly what value you need, but you know it won’t be very much. That’s when you want a gimmick capacitor.

A gimmick capacitor is made by taking two strands of insulated wire and twisting them together; the length and the tightness of the twist determine the capacitance. Tightening or loosening the twist, or trimming some of the wire off, makes it tunable.

These are most commonly found in RF equipment or high-speed logic because of the small capacitance involved — usually about 1 to 2 pF per inch of twist or so. The thicker the insulation, the less capacitance you’ll get, so it is common to use magnet wire or something else with a thin insulating layer. You can take this one step further and decrease the spacing by stripping down one wire as long as it isn’t going to touch anything else.

Obviously, the insulation needs to be good enough for the voltage on them, an important consideration in tube circuits, for instance. But other than that, a gimmick capacitor is a straightforward tool to have in your box of design tricks. Can we take this further? Continue reading “These Capacitors are a Cheap Gimmick”

Silicon Bugs In The FTDI FT232R, And A Tidy RF VCO Project

[Scott Harden] wrote in to tell us of some success he’s having using the FT232 chip to speak SPI directly from his laptop to a AD98850 digital signal generator. At least that was his destination. But as so often in life, more than half the fun was getting there, finding some still-unsolved silicon bugs, and (after simply swapping chips for one that works) potting it with hot glue, putting it in a nice box, and putting it up on the shelf.

In principle, the FTDI FT232 series of chips has a bit-bang mode that allows you to control the individual pins from a fairly simple API on your target computer, using their drivers and without installing anything on basically any platform. We wrote this feature up way back in 2009, and [Scott] was asking himself why he doesn’t see more hacks taking advantage of bit-bang mode.

“Square” waves

Then he answered his own question the hard way, by spending hours “debugging” his code until he stumbled on the FTDI errata note (PDF), where they admit that bit-bang mode doesn’t get timings right at all on the FT232R and FT232RL parts. FTDI has made claims that they fixed the bug in subsequent chip revisions, but the community has not been able to confirm it. If you want to use bit-bang mode, which is plenty cool, steer clear of the FT232R chips — the ones found in the ever-popular FTDI cables and many adapter dongles.

The good news here is twofold. First, now you know. Second, bit-bang mode is tremendously useful and it works with other chips from the vendor. Particularly, the FT232H and FT230X chips work just fine, among others. And [Scott] got his command-line controlled digital VCO up and running. All’s well that ends well?

We’ll wrap up with questions for the comment section. Do other manufacturers’ cheap USB-serial chips have an easily accessible bit-bang mode? Are any of you using USB bit-bang anyway? If so, what for?

Understanding a MOSFET Mixer

A mixer takes two signals and mixes them together. The resulting output is usually both frequencies, plus their sum and their difference. For example, if you feed a 5 MHz signal and a 20 MHz signal, you’d get outputs at 5 MHz, 15 MHz, 20 MHz, and 25 MHz. In a balanced mixer, the original frequencies cancel out, although not all mixers do that or, at least, don’t do it perfectly. [W1GV] has a video that explains the design of a mixer with a dual gate MOSFET, that you can see below.

The dual gate MOSFET is nearly ideal for this application with two separate gates that have effectively infinite input impedance. [Stan] takes you through the basic circuit and explains the operation in whiteboard fashion.

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A Guide To Audio Amps For Radio Builders

For hams who build their own radios, mastering the black art of radio frequency electronics is a necessary first step to getting on the air. But if voice transmissions are a goal, some level of mastery of the audio frequency side of the equation is needed as well. If your signal is clipped and distorted, the ham on the other side will have trouble hearing you, and if your receive audio is poor, good luck digging a weak signal out of the weeds.

Hams often give short shrift to the audio in their homebrew transceivers, and [Vasily Ivanenko] wants to change that with this comprehensive guide to audio amplifiers for the ham. He knows whereof he speaks; one of his other hobbies is jazz guitar and amplifiers, and it really shows in the variety of amps he discusses and the theory behind them. He describes a number of amps that perform well and are easy to build. Most of them are based on discrete transistors — many, many transistors — but he does provide some op amp designs and even a design for the venerable LM386, which he generally decries as the easy way out unless it’s optimized. He also goes into a great deal of detail on building AF oscillators and good filters with low harmonics for testing amps. We especially like the tip about using the FFT function of an oscilloscope and a signal generator to estimate total harmonic distortion.

The whole article is really worth a read, and applying some of these tips will help everyone do a better job designing audio amps, not just the hams. And if building amps from discrete transistors has you baffled, start with the basics: [Jenny]’s excellent Biasing That Transistor series.

[via Dangerous Prototypes]