Tuesday, January 14, 2020

Mods for the Radio Shack DX-390 / Sangean ATS 818 Part 4 Panadapter

What is a Panadapter?

The next mod in this series is going to show you how to add a connection to your radio, connect to a SDR receiver, and use it as a panadapter!

A panadapter is a device that gives you a visual representation of the radio spectrum in a waterfall or line display and “captures" the audio from your radio at the same time using a low cost SDR unit.

Panadapters allow greater signal processing options surpassing some physical radio limitations. You can use an array of filters, decoders, demodulators, and other plugins to enhance your listening experience. Many high end receivers and amateur radio transceivers now offer an easy way to connect panadapters to your computer. You can also use the panadapter as a second audio processor/receiver by listening to two frequencies at the same time - one on the radio and one on the computer.


The benefits of using a panadapter are endless. You can get an idea of where activity is on your shortwave radio with a visual and wider view of the radio spectrum. 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.



I recommend the RTL-SDR Blog V3 which includes a temperature controlled oscillator to prevent drift. You can also use a generic DVB TV tuner for your radio.


What You Need

  • Female SMA cable with RG316 coax for the radio available on Amazon


  • Additional cables or adapters to connect the SDR to your radio also available on Amazon.
  • 1k ohm, 5 watt resistor
  • 1 nf to 2.2 nf non-polarized capacitor (I used a 2.2nf film capacitor)
  • ¼ inch drill bit. Cordless drill
  • Soldering iron. Heat shrink tubing
  • SDR software
  • Instructions and Drivers for your RTL-SDR

 

 

The Radio


The DX-390/ATS-818 is a dual conversion, superhetrodyne receiver. One thing that makes this radio great for modification is easy access to the 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 maximum bandwidth.
The second tuning coil (T2) was omitted in final production and we can use this for our IF tap.



Connecting your SDR directly to the radio could alter the signal of the IF stages and cause poor reception. It could also transfer unwanted DC voltage between the radio and your SDR. We will use a small value capacitor for blocking spurious voltage and resistor to prevent receiver sensitivity drain. Connect them in series to your radio.


Identifying the correct tap point for a strong IF signal in this radio is fairly simple. We could tap between T1 and T3 or the output of Q3 and Q4 and receive a strong signal. I decided to use the omitted T2 circuit where it connects to R39. 

 




Connecting the Cable to the Radio

Disassemble the receiver case and remove the large circuit board (Don’t forget to unsolder the speaker leads and disconnect the two cable sockets from the display board)

Carefully drill a ¼ inch hole into the side of the radio for the cable. The hole should be about 1 inch from the top of rear case and ½ inch from the back of the case.



 

 

Solder the resistor and capacitor together


Strip the braid and center conductor of the coax then solder the resistor side to the center conductor. Don’t forget heat shrink tubing.The cable assembly should be approximately 6 3/4 inches long from the inside edge of the connector to the solder joint at R39.


The braid was not needed to pick up the IF signal. It could induce noise to the SDR. You can install a ferrite choke over the coax cable if you have issues with noise.

Solder the capacitor side of the cable to the hole next to D10 as shown. Then route the cable across the board and secure with tape.

 




Carefully reattach the board to the radio and route the coax cable through the hole in the case.

 


Reassemble the radio and test for normal operation.

 

Setup the HDSDR Software

There are many videos on YouTube and articles on RTL-SDR Blog to help you get started. I highly recommend them if you run into issues with the SDR or software.

Connect your SDR to the computer. Install and Run the HDSDR software. Select the SDR you want to use. 

Configure the bandwidth and AGC settings as shown in HDSDR. Choose 2400 kbps for the sample rate (if using a generic SDR or RTL-SDR).

 



Setup your receiver in the RF Front End configuration menu. Select SDR Hardware on Down/Up-Converter LO Frequency.. Add the IF frequency plus the frequency displayed on the radio in the box shown. Make sure to convert everything into Hertz as shown below.

For example: If your radio frequency is 4.000 MHz.

4 * 1,000,000 = 4000000

4000000 + 55845000 = 59845000

Type 59845000 in the box and click Apply. Don't forget to change this value if you change the radio frequency!



After connecting all cables, plugging in the SDR receiver, and running software; turn on the radio, set the frequency you chose above, and adjust the RF gain as appropriate. Tone, Volume, BFO, Narrow/Wide controls on the radio doesn’t affect the signal to HDSDR. You can set those to your preference. You will be able to tune anywhere from 150Hz to 29.999MHz in HDSDR.


Setup the SDR# Software

There are many videos on YouTube and articles on RTL-SDR Blog to help you get started. I highly recommend them if you run into issues with the SDR or software.

  Setup your receiver in the device configuration menu. Click the gear and make sure Quadrature Sampling is selected. Adjust the Gain as desired.



On the main screen, find the source section for your SDR. Check the Shift box. Add the IF frequency plus the frequency displayed on the radio in the box shown. Make sure to convert everything into Hertz.



For example: If your radio frequency is 4.000 MHz.

4 * 1,000,000 = 4000000

4000000 + 55845000 = 59845000

The shift frequency must be a negative value. Type -59845000 in the box and click Apply. Don't forget to change this value if you change the radio frequency!

 





73,
Jesse W9JES