In shortwave radios, the function of filters is to eliminate unwanted signals, allowing the receiver to only capture the signals of interest.
Filters are broadly classified into three categories, as also encountered in the Amateur Radio Class B exam: high-pass, band-pass, and low-pass filters. Their meanings are as follows:
- High-pass (HIGH PASS): Allows signals above a certain frequency to pass through, filtering out the rest.
- Band-pass (BAND PASS): Permits signals within a specific frequency range to pass through, filtering out others.
- Low-pass (LOW PASS): Allows signals below a certain frequency to pass through, filtering out higher frequencies.
While these concepts are straightforward, the filters we encounter in practice often seem more complex. Examples include:
- Front-end Interference Filter
- Intermediate Frequency Filter (IF FILTER)
- Image Rejection Filter (IMAGE FILTER)
- Roofing Filter
- Notch Filter
- etc.
In essence, these filters operate based on the same three principles (high-pass, band-pass, low-pass), with variations in their application.
Let’s delve into a few commonly used filters. Taking the description of filters in the Yaesu FT-DX10 radio as an example:
- 15 Independent Band-pass Filters
- 3 Optional Roofing Filters (Roofing filter)
Their respective positions within the radio are as follows:
So, what are their respective functions?
The 15 independent band-pass filters, located at the front end of the radio, are designed to combat interference. Their filtering range is relatively wide, automatically adapting to the selected frequency range of the radio and applying the corresponding filter for out-of-band signal filtering. Additionally, among these 15 filters, 10 are designed for amateur radio frequency bands, while the remaining 5 are intended for general frequency bands.
As for the 3 optional roofing filters, labeled as ‘R.FIL’ on the radio interface after startup, they can be selected at 500Hz, 3kHz, and 12kHz. The roofing filter is the first intermediate frequency filter after mixing.
The intermediate frequency filter is also a band-pass filter. What sets it apart from the previously mentioned 15 front-end filters is its narrower filtering range, making it a narrow-band band-pass filter. (In contrast, the earlier 15 filters are considered wide-band band-pass filters.) The reason for having three optional filters is straightforward—different modes such as SSB, CW, RTTY have varying bandwidths. Therefore, we naturally select the corresponding bandwidth filter based on the specific bandwidth requirement.
To summarize the filtering process: first, use one of the 15 wide-band filters at the front end to eliminate interference outside the operating frequency band. Subsequently, we employ the optional narrow-band intermediate frequency filter (roofing filter) to filter out other signal interferences within the frequency range outside of our desired signal.