Implement CW filter in ham radio setup using PSoC
Most amateur digital modes are transmitted by inserting audio into the microphone input of a radio and using an analogue 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 800Hz, 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 600Hz bandpass filter centred on 600Hz 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 250Hz, 500Hz, 750Hz, 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 analogue blocks.
Figure 1: CW filter design with four centre frequency selections.
As shown in figure 2, only four out of the 12 analogue 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.
There is a two-pole bandpass filter User Module in PSoC Designer that features:
User programmable centre frequency, Q, and Gain
No external components required
Centre frequency 20Hz to 200kHz
Automated design using wizard
User-selected over-sample ratio (OSR), a ratio of sample frequency to corner frequency
Built-in polarity control
Built-in comparator output for use in full-wave detection and communication applications
Built-in modulator for use in frequency translation and signal generation