The main reason for this is that I didn't have a reliable receiver in the low frequency range. The uSDX has a sensitivity of approximately -80dBm at 136kHz, and the RTL-SDR performs similarly poorly. The LCD screen also introduces significant interference in the low frequency range.
Regarding replicating, the core SOP-8 package, SA602, is no longer in production and can be difficult to find; they typically cost over 20 yuan. However, there should be plenty of used DIP-8 packaged NE602 chips available, which you can use with a new package. The NE602, NE612, SA602, and SA612 can all be used interchangeably here.
Source of design principle diagram
The schematic for this design was based on a direct frequency modulation (FM) receiver kit from icas.to.
Official Website Link: 136 kHz Band DC Receiver
The original introduction was as follows:
This is a DC (Direct Conversion) receiver kit specifically designed for the 136 kHz band, utilizing SDR software.
Assembled using only DIP components, and with a low part count, it can be easily assembled in a short amount of time.
Conventional DC receivers produced images that were largely impractical and unusable; however, this device utilizes a BPF (band-pass filter).
By implementing this, you can suppress the image and use a Windows PC's sound card for restoration.
By using it, you can increase the resolution accuracy to approximately 0.1 Hz.
You can use it as a main receiver or as a secondary monitor.
■ Details
Receive frequency: 101 ~ 149 kHz
(When using a 48 kHz sound card)
Center frequency: 125 kHz, 8 MHz with a 64-minute period.
Power: External DC 9V ~ 13.8V, approximately 30mA
Dimensions: Main board size 77 x 43 mm
The original kit has been discontinued, and the only remaining resource is the circuit diagram included with the installation instructions. Based on this circuit diagram, I created the following circuit and PCB using GaLaad, making some minor modifications.
The receiving principle remains largely consistent: the signal first enters the MMBFJ310 as a buffer follower, and then passes through three yellow center frequencies (acting as variable inductors) to form a bandpass filter. The NE5534 operational amplifier is used to amplify the bandpass loss. Finally, the SA602 converts the frequency to a level that can be received by the sound card.
The intermediate frequency (IF) uses an 8 MHz active crystal oscillator. After dividing the frequency using a CD4060, it outputs a stable 125 kHz IF signal. It is possible to convert a 136 kHz signal down to 11 kHz.
The following modifications have been made compared to the original version:
- The resistance of the main power line is too high, causing a voltage drop of 100Ω. Adjust it to 10Ω or 0Ω. The 1N4004 diode can also be removed if needed.
- Replace the passive crystal with an 8 MHz active crystal and add some components to reduce interference.When using the CD4060, only a 100nF capacitor with 4 pins needs to be soldered.The CD4060 itself has weak drive capability, and when used in an RC oscillator circuit, the maximum frequency cannot exceed 650 kHz. If an external crystal is connected, a typical input at 5V is 7 MHz, but it can operate at 8 MHz. The 74HC4060 can replace the CD4060 and has better performance. When using the 74HC4060, note that the pin arrangement of the 74HC4060 differs from that of the CD4060.Requires a 100nF capacitor with 6 leads, not soldered with 4 leads.If a crystal oscillator is directly connected to the SA602 mixer, it will be difficult to maintain both frequency accuracy and stability for receiving long-duration QRSS signals.
- Here, the feedback resistor potentiometer for the NE5534 operational amplifier uses a 50K value and is compatible with both the common 3296W and 3362P potentiometers.
- A 50Ω resistor is added to match high-impedance antennas, along with a switch. This resistor appears to only reduce signal strength, and is usually not necessary.
- The LED, when using a 1k current-limiting resistor, consumed 8mA of current. However, the entire board only consumed 32mA. Therefore, I switched to a 10k resistor, which maintained similar brightness, and the total current consumption dropped to 24mA. This is more suitable for continuous operation (7*24 hours).
Welding and Debugging
The finished product is shown in the figure. When designing the board, I considered fitting it inside an aluminum alloy box.
Soldering is complete. I used a NanoVNA to measure the system gain of S21. While adjusting the center frequency, I also added a 30dB attenuator for calibration to protect the components. This measurement was performed after adding the attenuator, considering that the minimum output power of the NanoVNA is -11dbm. It's also possible to adjust the center frequency without soldering SA602.
The capacitance needs to be adjusted initially. In this case, "mid-week" refers to a variable capacitor. Please use a non-magnetic screwdriver to adjust it.
The adjusted S21 frequency response curve is shown in the figure, with a bandwidth of approximately 4 kHz. For more precise adjustment, the scale on this curve can be reduced, and the reference level can be appropriately adjusted.
Adjust the potentiometer to adjust the gain of the NE5534 operational amplifier. The actual gain can be adjusted to over ten dB, but it is not necessary to have such a high gain. Leave some margin for gain adjustment.
Reception effect:
I am using the HDSDR software and FSKviewer to try to receive DFCW-3. The transmitter is located at home, using a small loop antenna (approximately 1 meter high with a 50Ω resistor to absorb energy) for transmission, and I am receiving it outdoors from the back of the house.
The reason for using HDSDR is that it uses fewer resources compared to SDR#, and SDR++ cannot recognize the external CM108B sound card. Additionally, HDSDR allows for very convenient setting of the LO (Local Oscillator) and displays information in a more user-friendly way.
The sensitivity measured was approximately -120 dBm (12 dB signal, CW, 500 Hz to 137000 Hz frequency offset of about 1 Hz).
The receiving device consists of a circuit board enclosed in an aluminum alloy casing, with a battery attached. The antenna was a gift from BG5ACF (thank you!).
Schematic diagrams / PCB / Component purchase links are now publicly available.
Lichuang EDA Professional Version Engineering FilesClick to download: 136 kHz receiver engineering documents
TPJ701 Yellow Mid-Week LinkReady-to-ship FM receivers, complete set for all seven days of the week (TP701 / TP702 / TPJ701), with capacitor – available on Taobao.
54 x 23 x 80 external casing - link54*23 Aluminum alloy outer shell, aluminum profile outer shell, aluminum box, aluminum case, housing, instrument housing, power box - on Taobao.
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