band pass filter

4 Articles

A Bandpass Filter Pulls In The Signals

September 20, 2026 by 8 Comments

It’s an unfortunate side effect of proximity to a large transmitter that the received signal can overload a receiver’s front end even when tuned to other frequencies. [Rfrht]’s had this problem with nearby FM broadcast transmissions overloading the 2 metre and 70 centimetre amateur bands. The solution?  Design and build a bandpass filter. This allows the signals you want to pass through while rejecting or attenuating out-of-band frequencies. The resulting PCB is very nice indeed.

On board, aside from the filters themselves, are a low-noise preamplifier and relays to switch between receive and transmit. Everything is controlled by logic-level signals. All components are surface-mount, and the PCB layout clearly takes special care with RF routing.

Continue reading “A Bandpass Filter Pulls In The Signals” →

The Right Circuit Turns Doppler Module Into A Sensor

March 31, 2017 by 22 Comments

Can you buy a working radar module for $12? As it turns out, you can. But can you make it output useful information? According to [Mathieu], the answer is also yes, but only if you ignore the datasheet circuit and build this amplification circuit for your dirt cheap Doppler module.

The module in question is a CDM324 24-GHz board that’s currently listing for $12 on Amazon. It’s the K-band cousin of the X-band HB100 used by [Mathieu] in a project we covered a few years back, but thanks to the shorter wavelength the module is much smaller — just an inch square. [Mathieu] discovered that the new module suffered from the same misleading amplifier circuit in the datasheet. After making some adjustments, a two-stage amp was designed and executed on a board that piggybacks on the module with a 3D-printed bracket.

Frequency output is proportional to the velocity of the detected object; the maximum speed for the sensor is only 14.5 mph (22.7 km/h), so don’t expect to be tracking anything too fast. Nevertheless, this could be a handy sensor, and it’s definitely a solid lesson in design. Still, if your tastes run more toward using this module on the 1.25-cm ham band, have a look at this HB100-based 3-cm band radio.

Continue reading “The Right Circuit Turns Doppler Module Into A Sensor” →

Tiny Raspberry Pi Shield For High-Quality RF Signals

March 16, 2016 by 18 Comments

Among its many tricks, the Raspberry Pi is capable of putting clock signals signal out on its GPIO pins, and that turns out to be just the thing for synthesizing RF signals in the amateur radio bands. What [Zoltan] realized, though, is that the resulting signals are pretty dirty, so he came up with a clever Pi shield for RF signal conditioning that turns a Pi into a quality low-power transmitter.

[Zoltan] stuffed a bandpass filter for broadband noise, a low-pass filter for harmonics, and a power amplifier to beef up the signal a bit into a tiny shield that is cleverly engineered to fit any version of the Pi. Even with the power amplifier, the resulting transmitter is still squarely in the realm of QRP, and the shield is optimized for use as a WSPR beacon on the 20-meter band. But there’s plenty of Pi software available to let hams try other modes, including CW, FM, SSB, and even SSTV, and other signal conditioning hardware for different bands.

Yes, these are commercially available products, but even if you’re not in the market for a shield like this, or if you want to roll your own, there’s a lot to learn from [Zoltan]’s presentation at the 2015 TAPR Digital Communications Conference (long video below). He discusses the difficulties encountered getting a low-profile shield to be compatible with every version of the Pi, and the design constraints that led to the decision to use SMT components.

Continue reading “Tiny Raspberry Pi Shield For High-Quality RF Signals” →

Retrotechtacular: The Voder From Bell Labs

August 12, 2014 by 36 Comments

This is the under-the-hood view of the keyboard for the Voder (Voice Operating Demonstrator), the first electronic device capable of generating continuous human speech. It accomplishes this feat through a series of keys that generate the syllables, plosives, and affricatives normally produced by the human larynx and shaped by the throat and tongue. This week’s film is a picture montage paired with the audio from the demonstration of the Voder at the 1939 World’s Fair.

The Voder was created by one [Homer Dudley] at Bell Laboratories. He did so in conjunction with the Vocoder, which analyzes human-generated speech for encrypted transfer and re-synthesizes it on the other end. [Dudley] spent over 40 years researching speech at Bell Laboratories. His development of both the Voder and the Vocoder were instrumental in the SIGSALY project which aimed to deliver encrypted voice communication to the theatres of WWII.

Continue reading “Retrotechtacular: The Voder From Bell Labs” →