Working Principle of the Mosquito TRX
The operation is fundamentally split into two modes:
1. Receive Mode (RX)
When the Key is OPEN (not pressed), the circuit is in Receive mode.
- Biasing: The Gate of the MOSFET (Q1) is biased by the voltage divider formed by RP1 (5k Potentiometer) and R2 (10k), along with C3 (10nF). The potentiometer RP1 allows for fine-tuning the operating point of the MOSFET, which is crucial for maximizing its performance as a mixer and detector.
- Local Oscillator (LO) and Mixer: The 7030 kHz crystal acts as the Local Oscillator (LO), generating a strong signal at the operating frequency. The incoming weak RF signal from the ANTENNA is routed through the output filter (L2, L3, C7) and coupled to the Drain of Q1. The MOSFET’s non-linear characteristics act as a Direct Conversion Mixer, combining the incoming RF signal with the LO signal.
- Detection: If the incoming signal is close to 7030 kHz (e.g., a CW signal), the mixing process creates an audio frequency (AF) “beat note” at the Drain.
- Audio Output: The high-frequency RF signals are blocked by the RF Choke L1 (1ouH), while the low-frequency AF signal passes to the primary of the 12V Transformer (U1). This transformer couples the AF signal to the AUDIO output (headphones/speaker) while providing necessary impedance matching and DC isolation.
2. Transmit Mode (TX)
When the Key is CLOSED (pressed, as in sending Morse code), the circuit is in Transmit mode.
- Oscillator/PA: Closing the key applies the Gate bias voltage and enables the crystal oscillator. The MOSFET (Q1) is driven into a more linear or Class-C operating region by the keying action, where it functions as a crystal-controlled Power Amplifier (PA), oscillating and amplifying the 7030 kHz signal.
- RF Output: The amplified RF signal at the Drain is routed through the series RF Choke L1 and the coupling capacitor C6 (100 pF) to the $\pi$-network output filter.
- Output Filter: The pi-network filter, composed of C6 (100 pF), L2 (5uH), L3 (0.35 mH), and C7 (1500 pF), is a Low-Pass Filter (LPF). Its purpose is to suppress harmonics generated by the PA stage, ensuring a clean, legal signal is radiated from the ANTENNA into the 50 ohm load.
The block diagram illustrates the signal flow for using the Mosquito TRX as the frontend for a Software Defined Radio (SDR) receiver, utilizing a standard computer sound card for digital processing.The block diagram illustrates the signal flow for using the Mosquito TRX as the frontend for a Software Defined Radio (SDR) receiver, utilizing a standard computer sound card for digital processing.ion and processing.
The Mosquito TRX acts as a Direct Conversion Receiver (DCR) frontend for a Software Defined Radio (SDR) system.
Here is the concise signal flow:
- RF to Audio Conversion (Mosquito TRX): The TRX mixes the incoming 7030 kHz radio signal with its Local Oscillator (LO) to directly produce a low-frequency Analog Audio Frequency (AF) signal.
- Analog Input: This AF signal is connected via a cable to the computer’s “BLUE LINE-IN” jack.
- Digitization (Sound Card): Inside the sound card, the signal passes through Anti-Aliasing Filters and is then converted into a stream of digital data samples by the Analog-to-Digital Converter (ADC).
- Digital Processing (Software): The SDR software takes this digital stream and applies complex processing, including digital filtering for selectivity, digital mixing (using an NCO) for fine-tuning, and Automatic Gain Control (AGC) for stable volume.
- Audio Output: The final processed digital signal is converted back to analog audio by the Digital-to-Analog Converter (DAC) and is sent to the “GREEN LINE-OUT” jack to drive the Speaker or headphones.
In essence, the Mosquito TRX downconverts the radio signal to an audio frequency, and the computer’s sound card and software perform all the key tuning, filtering, and demodulation functions digitally.
This setup allows a simple piece of hardware like the Mosquito TRX to serve as a low-cost RF frontend, offloading the complex, adjustable filtering and demodulation tasks to powerful and flexible computer software.
Source: https://www.qrp.cat/ea3ghs/mosquito.pdf
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