[Photo] Installation and Improvement of "Shoes" Simple Micropower Telegraph Transceiver

Installation and Improvement of "Shoes" Simple Micropower Telegraph Transceiver

Basic index power supply: 7V-12V (recommended 9V laminated battery)
Circuit board: 56mm x 41 mm
Antenna: 50 ohms, unbalanced, BNC / Q9 interface LO leakage: about 1mW (on a 50 ohm dummy load)
Frequency range: 7.060-7.064MHz (50p trimmer capacitor connected in series with 7.060M crystal)
receive:
Current: less than 10mA (at 9V power supply)
Headphones: low-resistance headphones (recommended SONY, aiwa and other highly sensitive Walkman headphones)
emission:
Power: about 200mW
Current: about 50mA (at 9V power supply)
Spurious (harmonic) suppression: -20dB

Subjective evaluation of receiving sensitivity and selectivity is poor and susceptible to broadcast interference (BCI). Good frequency stability and high intelligibility when listening to SSB signals. The noise floor of the circuit is small. The switch sound is loud when switching between sending and receiving, which may easily lead to wrong code.

The circuit introduction is shown in Figure 1. Q1 and the surrounding components form a typical Kobitz oscillator and keep oscillating (so there is about 1mW of oscillation signal leakage during reception). The signal is directly coupled to Q2 through the 82pF capacitor. Press the key), Q2 acts as a Class C amplifier, the amplified signal is coupled to a π-type low-pass filter through a 0.01uF capacitor, and then sent to the antenna for transmission; in the receiving state (the key is released), Q1 and the surrounding components form a beat Oscillator (BFO), Q2 is biased in the nonlinear region (think of it, the triode is nothing more than two diodes back-to-back!), The signal received by the antenna is mixed with the BFO signal, and the resulting audio is mixed The signal is coupled to the audio power amplifier formed by LM386 through a 0.1uF capacitor coupling, and the amplified audio signal is sent to the earphone after the LM386 pin 5 is separated by a 10uF capacitor. The electric key not only controls the on and off of the LM386 power supply, but also switches the bias of Q2 to make it work in different states.
Figure 1 Electromechanical schematic diagram of "shoes" 200mW micropower constant amplitude telegraph transceiver

Component selection

All inductors choose color ring inductors, of which L3 uses 2.2uH at 80 meters. C6 and C7 use 820pF at 80 meters. Transistors Q1 and Q2 have no strict regulations. Silicon NPN transistors with magnification between 100 and 200 can be used normally. For example, 9011, 9013, 9018, 8050, 2N2222A, 2N3904, etc. It is recommended to use 9013 or Q1 for both Q1 and Q2. Both use 2N3904. The crystal must be a fundamental frequency crystal. Both 7.060M and 7.042M crystals are available for sale at the antenna. It is recommended to use sockets on the crystal and L3, C6, C7 on the circuit board to switch the band or frequency. In order to increase the transmission power, you can use a 12V power supply, but you need to increase C10 to about 100u.

Debugging method

Check if there is a short circuit at the end of welding. If not, add a 9V laminated battery, connect the headphones, and do not connect the antenna. Under normal circumstances, you should hear a small "rustle" sound, connect the antenna to increase the noise or you can hear some signals , The current of the whole machine is below 10mA. If you hear a loud howling or excessive current, it means that the circuit is self-excited. The solution is to connect a 103 ceramic capacitor at both ends of "SPEAKER". If it is invalid, connect it in parallel at the 10uF capacitor of the LM386 power filter. If a 103 ceramic capacitor is still invalid, connect a 103 ceramic capacitor in parallel with the 9V power input. The reception should be basically normal by now.

Figure 2 Schematic diagram of the high-frequency power meter with 1W 50 ohm dummy load

Then connect the high-frequency power meter with dummy load (Figure 2 shows the reference circuit diagram), short-circuit the two ends of "KEY", the headset should be silent, and the high-frequency power meter has a certain output. The current of the whole machine is 40-100mA under the launching state. Place a short-wave / medium-wave radio 0.5 meters away from the transmitter to check all the received frequency ranges. Except for the carrier frequency and octave frequency, no other signals generated by "leather shoes" should be heard. If there are other signals (especially howling), there is high-frequency self-excitation. Cut the power line between Q1 and Q2, use 100uH inductance and 100 ohm resistor in parallel and then connect in series to effectively eliminate high-frequency self-excitation. The attached table gives the DC reference voltages of the main components in the transmitting and receiving states.