Quick Summary
A DAB notch filter is used to suppress strong Digital Audio Broadcast signals in the VHF Band.
These signals can cause radio receivers to saturate thereby preventing reception of signals in neighboring frequency bands. A DAB notch filter, when inserted at the input to a software-defined radio receiver will allow it to receive and process signals in adjacent bands – interference-free!
Table of Contents
What is Digital Audio Broadcast?
Digital Audio Broadcast (DAB) is a digital radio broadcasting standard designed to deliver high-quality audio to listeners. Unlike traditional analog radio, which uses frequency modulation (FM) or amplitude modulation (AM) to transmit audio signals, DAB uses digital signals. This digital transmission method offers several advantages over analog, including improved audio quality, more efficient use of the radio spectrum, and the ability to carry additional data alongside audio content.
What is the Frequency Range of DAB?
The frequency range for Digital Audio Broadcasting (DAB) varies by region, but generally, DAB operates within specific VHF (Very High Frequency) and L-band (microwave frequency range) allocations.
Here are the common frequency ranges used for DAB:
VHF Band III
- 174 to 240 MHz: This is the most commonly used frequency range for DAB broadcasts. It’s divided into several smaller frequency blocks or channels, typically numbered 5A to 13F, each occupying about 1.75 MHz of bandwidth.
L-Band
- 1452 to 1492 MHz: L-band frequencies are less commonly used compared to VHF Band III but are utilized in some countries and regions, especially where VHF Band III is already crowded with other services.
Detailed Breakdown of VHF Band III
The VHF Band III is further divided into the following blocks:
| Block | Frequency Range (MHz) |
|---|---|
| 5A | 174.928 |
| 5B | 176.640 |
| 5C | 178.352 |
| 5D | 180.064 |
| 6A | 181.936 |
| 6B | 183.648 |
| 6C | 185.360 |
| 6D | 187.072 |
| 7A | 188.928 |
| 7B | 190.640 |
| 7C | 192.352 |
| 7D | 194.064 |
| 8A | 195.936 |
| 8B | 197.648 |
| 8C | 199.360 |
| 8D | 201.072 |
| 9A | 202.928 |
| 9B | 204.640 |
| 9C | 206.352 |
| 9D | 208.064 |
| 10A | 209.936 |
| 10B | 211.648 |
| 10C | 213.360 |
| 10D | 215.072 |
| 11A | 216.928 |
| 11B | 218.640 |
| 11C | 220.352 |
| 11D | 222.064 |
| 12A | 223.936 |
| 12B | 225.648 |
| 12C | 227.360 |
| 12D | 229.072 |
| 13A | 230.784 |
| 13B | 232.496 |
| 13C | 234.208 |
| 13D | 235.776 |
| 13E | 237.488 |
| 13F | 239.200 |
What is the Frequency Range of DAB in the United Kingdom?
In the UK, DAB frequencies vary between 174.928 MHz and 229.072 MHz.
The Problem with DAB
DAB signals are very strong (just like FM signals) and they can overwhelm radio receivers designed for other applications. For instance, a DAB signal can saturate a software-defined radio and prevent it from listening to signals at 400 MHz.
What are Notch Filters?
Notch filters, also known as band-stop or band-reject filters, are electronic filters designed to eliminate specific frequencies from a signal. Unlike band-pass filters that allow a range of frequencies to pass through, notch filters are engineered to reject a narrow band of frequencies, effectively creating a “notch” in the frequency spectrum.
In the context of digital audio broadcasting, notch filters are used to remove strong DAB signals that can cause interference and prevent the reception of signals in other bands.
DAB Notch Filter
The filter shown below will suppress strong DAB signals in the band
The filter rejection is summarized in the table below
SPECIFICATIONS
| Frequency (MHz) | Attenuation (dB) |
|---|---|
| 130 | 1 |
| 140 | 2 |
| 150 | 2.7 |
| 160 | 5 |
| 170 | 19 |
| 174 | 35 |
| 180 | 60 |
| 185 | 75 |
| 190 | 75 |
| 195 | 75 |
| 200 | 70 |
| 205 | 70 |
| 210 | 70 |
| 215 | 60 |
| 220 | 45 |
| 225 | 30 |
| 230 | 20 |
| 235 | 8 |
| 240 | 5 |
| 250 | 3 |
| 270 | 1.6 |
The insertion loss from 300 MHz to 3 GHz < 1.5 dB.
Note that the maximum signal rejection occurs between 170 MHz and 230 MHz. While the band extends to 240 MHz, this is the region of the spectrum where practical signals are encountered in the UK for instance.