26 August, 2022

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.
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5 comments:

  1. Anonymous29 August, 2022 08:52

    Thanks for the info. I am just right before to build my second QDX. QDX Rev.2 is operational, now I have to start with the QDX Rev.3. Due to the absence of any 80m antenna, I am in the same situation. Good luck for further mods and please let us know your success with 17 and 15m. 73s Wolfgang, OE1MWW

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  2. Sverre Holm29 August, 2022 08:59

    These bandpass filters puzzle me as they seem to have multiple peaks. In the plot above you see the response rising to a peak around 30 MHz. I am puzzled as to whether I should use the lowest peak (as i do now) or the second one. Also, the 14 MHz bo filter works well in 18 MHz also

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  3. Anonymous29 August, 2022 13:09

    guess the first peak should be OK. 73s OE1MWW

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  4. K8VFO29 August, 2022 22:30

    Using ELSIE, I designed a LPF for 15M/17M that seems to fit the requirements in simulation. Here are the values: C18=220pF, C12=390pF, C5=220pF, L6=430nH (T37-6:12T) and L2=360nH (T37-6:11T), C9=18pF. I haven't built it yet. As for the BPF, the values I came up with when tacking it onto the LPF are: L12 tapped at 16 turns and C30=62pF, though it has little effect because the shape of the LPF dominates. The LPF reduces second harmonics 30dB, third harmonics are reduced >60dB. When I get time, I'll make the modifications and try it out.

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    Replies
    1. Sverre Holm29 August, 2022 23:51

      Very nice. It's an alternative to the LP filter designed by Ken, g4apb https://groups.io/g/QRPLabs/message/90650

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