#qdx Passband AF filter #qdx

Göran Gustafsson

Having some trouble with QDX. It does transmit, but there is only a few signals coming through to the WSJT-x and being decoded. The bar at the left corner of WSJT-x is red and only 10 dB.
When I run the audio filter sweep test it doesn't seem ok. The USB part (yellow) is around -50 dB from 100 to 3000Hz (should be around 0 dB, shouldn't it?)
The LSB part (red) is OK around -70 dB.
When he QDX AF Gain is set to 7000 I receive a few stations. When I increase the setting to 30,000 I get 25db received signal in WSJT-x and more stations, but the audio filter test gives the same result.
As i understand, the passband AF filter is digital. Is this a software problem? Any thoughts?


Hi Göran,

I’m most definitely an amateur when it comes to how this QDX magic works, but from looking at the schematic diagram https://www.qrp-labs.com/images/qdx/schem3.png on the RX side of things it looks like L12 (the tapped inductor) and T2 (the trifilar wound toroid) are the critical components which are user installed. So definitely worth checking these are 100%. Also worth checking the correct values for C28-31 have been used.

With regards to the schematic, I’m not sure how the RF input gets to L12 - maybe somebody could help explain that (or maybe there’s a part of the schematic missing?)



Answering my own question… :)  It looks like it comes through via Q6… 

Alan G4ZFQ

On 28/06/2022 10:11, Jim G7FRI wrote:
Answering my own question… :)  It looks like it comes through via Q6…
Yes, Clock2, R19 and the selected LPF.


Your description of the audio trace does not seem right although the figures might indicate 20dB image rejection.
Normally in a radio like this there is either good rejection or none. A small variation in opamp gain might make 20dB difference. Look carefully make sure there is nothing making a short around the opamps. Check marked resistor values.

Do you use a dummy load?
What do the RF traces look like?

73 Alan G4ZFQ


Thanks, Alan.

Something I hadn’t understood - that the low pass filters as well as the band pass filters are active during receive mode…

My audio and RF sweeps attached for reference… not sure what “good” exactly looks like. Think my audio sweeps look reasonable, but the RF sweeps for 80m & 40m don’t look like low pass filters. 20m looks ok which is good as I don’t have any “wire in the sky” for any lower bands at present.

Göran> maybe useful to post pictures of you audio/RF sweeps?


Alan G4ZFQ

Think my audio sweeps look reasonable,
Yes Jim,

but the RF sweeps for 80m & 40m don’t look like low pass filters. 20m looks ok
The LPFs will not normally affect the trace very much, their cutoff will just be beginning at the right.
Mostly it is the BPF you see. As you say 20m looks OK.
I'm not sure about the others, the pictures I've seen from v3 QDXs
often look worse than the v1 traces I've seen and in the manual.
Some have tried peaking them. The real test is how it receives, whether you see a drop in noise when the antenna is disconnected. (Not easy to see in WSJT-X).

73 Alan G4ZFQ