QDX on 17 and 15 m: Receiver

I got myself one of the marvellous digital transceivers from QRPLabs recently: The QDX - QRPLabs Digital Transceiver. It is set up with receiver bandpass filters and transmitter lowpass filters for 80, 40, 30, and 20 m. It has also been found that the 40 m filters work well for 60 m. 

The latest firmware, version 1_04, has a new "Band Configuration" screen where one may configure the list of supported bands. It is the intention that higher bands will eventually be supported.

I am not so interested in 80 m, so I wanted to see if I already now could get the receiver to work on 17 m and 15 m. With reference to the schematics, I modified the input filter's inductor, L12. 

It is tapped at 19, 30, 36, and 41 turns for 20, 30, 40, and 80 m respectively. It resonates with input capacitors C28-C31, with 22, 30, 56, and 220 pF respectively. 

I gave the inductor 36 turns, omitting the extra turns for 80 m, and then tapped L12 after 13 turns. I also also changed C30 to 15 pF. That give me the  bandpass characteristics shown below, as analyzed with the built-in analysis function of the QDX:

The filter peaks at about 19.5 MHz and is about 2 dB down at 18.1 MHz and about 7 dB down at 21 MHz. It seems to work well, and in my first tests, it receives FT8 well on both bands. 

Here's a new sweep with firmware 1_05_002 (beta) which uses colors which are easier to read:

And now the next thing to do is the transmitter's lowpass filter for 17/15m, which is a bit harder.