Recently I have build a small APRS modem based on the MicroAPRS that has been developed by Mark Qvist. Sending frames worked quite well but I observed that my setup had problems with decoding APRS messages. I am using a Yaesu FT-7800 as 2m transceiver connected to my modem. The FT-7800 provides an audio output of max 500 mVpp (peak-to-peak) on its TNC connector. It looks like that many received APRS data frames produced only 100-200 mVpp - obviously not sufficient for the APRS modem.
In the MicroAPRS blog Klaus DJ7OO proposed to put an audio bandpass filter in between the transceiver and the MicroAPRS modem. He provided a link to the Airgate Wiki which describes such a filter. I modified that filter design by adding an additional amplifier stage and an input level limiter:
The input signal is decoupled by a 1 uF capacitor. Two diodes 1N4148 will limit the audio signal for the following bandpass filter to a level of about 1.2 Vpp. The filter design is copied from the Wiki page mentioned before. It ihas a pass-band of about 1200 Hz - 2200 Hz. The filtered signal goes finally to a small amplifier stage. The amplification factor can be adjusted by modifying the 47k resistor.
For the audio filter I used a Quad-OpAmp TL074. It has a low noise figure and a very low signal distortion. Here is the datasheet tl074.pdf (1.81 mb).
For testing I assembled the whole thing on a breadboard and connected it to my transceiver and terminal program. The decoding works now excellent. I can also decode APRS frames from my Yaesu VX-8G which is known for its low APRS modulation. The combination of the MicroAPRS based modem and the audio filter now has an equal or better performance than my old TNC2S.
Thanks again to Mark OZ7TMD for developing the MicroAPRS and Josef DC9RK for the filter design!