Implementation of a CW Filter in ham radio setups using a PSoC

Meng He, Cypress Semiconductor

December 18, 2013

Meng He, Cypress SemiconductorDecember 18, 2013

Editor’s Note: In this Product How To article, Meng He of Cypress lays out the steps to implement a Continuous Wave (CW) filter in a typical ham radio setup by using a Cypress programmable SoC rather than placing a bandpass filter component in the RF tuning portion.

Amateur radio – or ‘ham’ radio - was the first technology-based social network, but it has more serious utility as well, and used to provide emergency communications during natural disasters, such as tornados and hurricanes, long before today’s Internet and satellite communications. Ham radio operators maintain finely-tuned antenna systems that allow them to send/receive good quality Morse code with ultra low power. Some hams even use old tube-based equipment and maintain them in good shape. In fact, amateur radio is the foundation of hundreds of advances in radio/communication theories.

Most amateur digital modes are transmitted by inserting audio into the microphone input of a radio and using an analog scheme. The Continuous Wave (CW) filter is a crucial part of this implementation, as it works on the audio from the radio set by narrowing the bandwidth. A good CW rig would do this with the received RF by placing a bandpass filter on the RF tuning portion. However, cheap receivers don't do this. This leaves engineers with two options: modifying the RF circuitry in the receiver or playing around with the audio.

In this article we describe an alternative design approach based on the Programmable System-on-Chip (PSoC), which provides a simple mechanism to work with the audio.

The CW tone heard through the speaker is generally from 600 to 800 Hz, so the filters we used were bandpass filters that would allow only that tone to come through the speaker. By placing a selective audio filter in the path, you can keep a certain set of frequencies and suppress unwanted ones. A 600 Hz bandpass filter centered on 600 Hz passes tones from 300 to 900HZ while rejecting others. This is helpful when there is a lot of noise on the received frequency.

In this specific design, we implemented four bandpass filters with a central frequency of 250 Hz, 500 Hz, 750 Hz, and 1K Hz on a single chip that allows users to alter the filter channel in real time by using the dynamic re-configuration feature of PSoC (Figure 1). These four filters and two PGAs are all implemented with one CY8C27443 device using switched capacitor analog blocks.

Figure 1: CW filter design with four center frequency selections

As shown in Figure 2, only four out of the 12 analog blocks are used in the design. Two blocks are dedicated PGAs for signal conditioning before and after the filtering, while the other two provide the hardware platform for the four bandpass filters.

Figure 2: Chip view of the design of four filters and two PGAs

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