The benefits of using a panadapter are endless. You can get an idea of where activity is on your shortwave radio with a wider view of the radio spectrum. For example, you can tune to 6.060 MHz on the radio and see up to 2.8 MHz of the radio spectrum above and below that frequency. If you are chasing DX or following ham radio operators, then you can find out where the bands are active. Other benefits include digital mode decoding for DRM, RTTY, WeFax, JS8Call, Morse Code, PSK, and more.
A panadapter allows greater flexibility with digital signal processing giving the listener more filter options, decoders, and plugins. Many high end receivers and ham radio transceivers now offer an easy way to connect panadapters or rig interfaces to your favorite computer. You can also use the panadapter as a second audio processor/receiver by listening on two frequencies at the same time.
I recommend the RTL-SDR Blog V3 which includes direct sampling of HF frequencies below 24 MHZ if you choose to use it without the ATS-818/DX-390. These units will cost about $22-$25 or you can buy a complete kit with antenna for $30 from https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles You can also use the $8 DVB TV tuners to interface to your radio found on eBay. I tested both HDSDR and SDR# on my laptop and found that HDSDR works a bit better for this application because you can tune below the radio frequency. HDSDR can be found at www.hdsdr.de
What you need to perform this mod..
- SDR as mentioned above
- Necessary cables to connect the SDR to your radio (I used a pigtail cable with a female SMA connector and SMA male/male adapter) available on Amazon.
- 1 k ohm resistor
- 1 nf to 2.2 nf non polarized capacitor (I used a 2.2 nf film capacitor)
- Drill/hand tools/soldering iron/heat shrink tubing
The DX-390 is a dual conversion, superhetrodyne receiver design. One thing that makes this radio great is hardware filtering. Looking at LW/AM/SW circuit in particular, there is a Murata crystal filter in the first IF stage and two Murata ceramic filters at the 2nd IF stage. The first IF is 55.845 MHz and the second IF is 450 kHz. We want to use the 1st IF stage for our panadapter.
For some reason, the second tuning coil (T2) was omitted in final production. We can use this for our IF tap.
Connecting your SDR directly to the radio could alter the IF stage and cause poor reception. It could also transfer unwanted DC voltage between the radio and your SDR. This is why we use a small value capacitor for blocking harmful voltage and resistor to prevent receiver sensitivity drain. Connect them in series to your radio.
Finding the correct tap point for a strong IF signal in this radio is fairly simple on the Sangean. We could tap between T1 andT3 or the output of Q3 and Q4 and receive a great signal. I decided to use the omitted T2 circuit where it connects to R39.
After cutting the cable to the correct length, I stripped and soldered the center conductor of the coax to the capacitor/resistor. I also cut the braid back since it isn't needed for the signal path and could induce noise into the IF output. You could install a ferrite core choke on the cable as well.
I decided to drill a hole in the side of the radio for easy access to the IF stage. A 1/2 inch bit was sufficient to get the connector through the radio body. It lines up perfectly with the other connectors.
Time to attach my cable to the radio.
Reassemble the radio and connect the SDR units! Both work fine for this application, however, the temp controlled oscillator is superior in the RTL-SDR V3 unit to prevent drift.
Fire up the software. Use HDSDR and install the correct ExtIO for your SDR. Files and instructions are available at www.hdsdr.de/hardware.html
Configure the bandwidth and AGC settings as shown in HDSDR. Choose 2400 or 2800 kbps for the sample rate.
Setup your receiver in the RF Front End configuration menu. Set your SDR to receive the IF output and type 55845000 in the IF frequency box. Check the "mirror RF Spectrum in general" box.
After connecting all cables and running software, turn on the ATS-818/DX-390 and adjust the RF gain as appropriate. Tone, Volume, BFO, Narrow/Wide controls don't affect the signal to HDSDR. You can set those to your preference. Now set a frequency on the radio in LW/AM/SW mode. An unused frequency is preferred somewhere within the band you would like to listen to. You will get 2.4 to 2.8 MHz of bandwidth to HDSDR that you can independently tune and listen. In HDSDR, you want to set your LO as the same frequency displayed on your radio. Right click to lock the LO frequency to prevent it from accidentally changing as you tune in the application.