Packet radio demodulator plugin

Introduction

This plugin can be used to demodulate packet radio (APRS/AX.25) data packets. Received packets can be sent to the APRS Feature for decoding and display.

Interface

1: Frequency shift from center frequency of reception

Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit. Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position. Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows. Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.

2: Channel power

Average total power in dB relative to a +/- 1.0 amplitude signal received in the pass band.

3: Level meter in dB

top bar (green): average value
bottom bar (blue green): instantaneous peak value
tip vertical bar (bright green): peak hold value

4: Modulation

This specifies the baud rate and modulation that is used for the packet transmission. Currently 1200 baud AFSK is supported.

5: RF Bandwidth

This specifies the bandwidth of a LPF that is applied to the input signal to limit the RF bandwidth.

6: Frequency deviation

Adjusts the expected frequency deviation in 0.1 kHz steps from 1 to 6 kHz. Typical values are 2.5 kHz and 5 kHz.

7: Filter Packets From

Entering a regular expression in the From field displays only packets where the source address, displayed in the From column, matches the regular expression.

8: Filter Packets To

Entering a regular expression in the To field displays only packets where the destination address, displayed in the To column, matches the regular expression.

9: Filter PID No L3

Checking this option displays only packets where the PID (Protocol ID) field is 0xf0 (no L3). This value is used by APRS and BBS data packets, and helps to filter out control packets.

10: Clear Packets from table

Pressing this button clears all packets from the table.

11: UDP

When checked, received packets are forwarded to the specified UDP address (12) and port (13).

12: UDP address

IP address of the host to forward received packets to via UDP.

11: UDP port

UDP port number to forward received packets to.

Received Packets Table

The received packets table displays the contents of the packets that have been received. Only packets with valid CRCs are displayed.

From - The source address / callsign of the sender of the packet.
To - The destination address.
Via - List of addresses of repeaters the packet has passed through or directed via.
Type - The AX.25 frame type.
PID - Protocol Identifier.
Data (ASCII) - The AX.25 information field displayed as ASCII.
Data (Hex) - The AX.25 information field displayed as hexidecimal.

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