high-quality 2.3 GHz Amateur Radio Transceiver

Description: The receiver is a triple superheterodyne receiver with two phase-locked loop (PLL) guided local oscillators. The first local oscillator (LO) operates at 2.1 GHz and can be adjusted in increments of 500 kHz, down-converting the received signal to a frequency range of 188 to 188.5 MHz. It features a high PLL loop bandwidth of approximately 20 kHz, resulting in low phase noise of less than -80 dBc/Hz at 1 kHz offset from the carrier, which contributes to a very high signal-to-noise ratio (S/N ratio) at the audio output, provided the incoming signal is sufficiently strong. The second local oscillator can be tuned from 177.3 to 177.9 MHz, further down-converting the signal to 10.7 MHz. The voltage-controlled oscillator (VCO) is meticulously designed, exhibiting a phase noise of -87 dBc/Hz at 1 kHz. Due to the low noise characteristics of the VCO, a small loop bandwidth of 70 Hz can be employed, allowing for a step size of 2.5 kHz. FM demodulation occurs at 455 kHz using a NE614 baseband integrated circuit (IC), which provides an excellent received signal strength indicator (RSSI) output suitable for driving the S-Meter, with a dynamic range of up to 80 dB, yielding 50 mV per dB. High-quality Murata intermediate frequency (IF) filters are utilized for channel filtering. An LM386 audio amplifier drives the loudspeaker. The first mixer is an image-reject mixer (UAA2077), eliminating the necessity for a narrow input filter to suppress the image frequency, thereby enabling tuning across a wide frequency range while maintaining constant sensitivity. Although the UAA2077 is designed for frequencies up to 2.0 GHz (suitable for the American PCS cellphone system), it still achieves 28 dB of image rejection at 2.3 GHz when implemented with careful layout and good grounding practices. Prior to the first mixer, a very low noise amplifier with a noise figure of 1.5 dB and a gain of 22 dB is included. This amplifier incorporates a mild band-pass filter to ensure maximum gain at 2.3 GHz, while frequencies below a few hundred MHz are significantly attenuated to prevent overloading subsequent stages from strong VHF broadcast signals, cellphone signals, and man-made noise. The image frequency (1.9 GHz) is attenuated by approximately 3 dB relative to the desired frequency at 2.3 GHz. Combined with the image-reject mixer, this configuration results in an overall image rejection of 31 dB.

The circuit operates on the principle of superheterodyne reception, where incoming radio frequency (RF) signals are mixed with local oscillator signals to produce intermediate frequencies (IF). The careful design of the PLL-controlled local oscillators ensures stable frequency generation with minimal phase noise, which is crucial for maintaining signal integrity in communication systems.

The first local oscillator's frequency is set to 2.1 GHz, and its ability to be tuned in 500 kHz steps allows for precise frequency selection, accommodating various communication channels. The high PLL loop bandwidth of 20 kHz minimizes the phase noise, enhancing the S/N ratio, which is vital for clear audio output.

The second local oscillator further processes the signal down to 10.7 MHz, which is a standard IF for FM demodulation. The NE614 IC is selected for its excellent performance in demodulating FM signals and providing reliable RSSI output, which is essential for monitoring signal strength.

The image-reject mixer (UAA2077) plays a critical role in the system by allowing for wideband tuning without compromising sensitivity. Its design mitigates the effects of image frequency interference, which can degrade receiver performance. The low noise amplifier preceding the mixer ensures that weak signals are amplified effectively while filtering out unwanted lower frequencies that could cause overload.

The audio amplifier (LM386) is employed to drive the loudspeaker, providing sufficient power for audio output while maintaining fidelity. The use of high-quality Murata IF filters ensures that only the desired channels are processed, further enhancing the receiver's performance.

Overall, this triple superheterodyne receiver design demonstrates a sophisticated approach to RF signal processing, integrating advanced components and techniques to achieve high performance in communication applications.The receiver is a tripple superheterodyne receiver with two PLL-guided local oscillators. The first LO (2. 1 GHz) can be set in steps of 500kHz and sets the RX signal down to 188 - 188. 5 MHz. It has a high PLL loop bandwidth of around 20kHz, therefore gives low phase noise of below -80dBc/Hz at 1kHz from the carrier. (=> very high S/N-ratio on the audio output, providing the received signal is strong enough. The second LO can be tuned from 177. 3 to 177. 9 MHz and sets the signal further down to 10. 7 MHz. The VCO is very carefully design and exhibits -87dBc/Hz at 1kHz phase noise. Due to the low noise of the VCO itself a small loop bandwitdh of 70Hz can be used and therefore a step width of 2. 5 kHz is possible. FM-demodulation is performed at 455kHz using a NE614 baseband IC that has a very good RSSI output to drive the S-Meter (up to 80dB dynamics, giving 50mV per dB).

High quality Murata IF filters are used for channel filtering. A LM386 audio amplifier is used to drive the loudspeaker. The first mixer is a image-reject mixer (UAA2077), negleting the need for a narrow input-filter to attenuate the image frequency. This makes it possible to tune over a wide frequency range with constant sensitivity. While the UAA2077 is made for frequencys up to 2. 0 GHz (the american PCS cellphone system), it still give 28dB image rejection at 2. 3 GHz if a carefull layout and good grounding is used. In front of the first mixer is a very low noise amplifier with a noise figure of 1. 5dB and a gain of 22dB. It includes a mild band-pass-filter to make sure that the maximum gain is achived at 2. 3GHz. Frequencys below a few hundred MHz are strongly attenuated to avoid overloading the next stages with strong VHF broadcast signal, cellphone signals, man made noise etc.

The image frequency (1. 9GHz) is attenuated about 3dB compared to the wanted frequency at 2. 3 GHz. Together with the image reject mixer this gives an overall image rejection of 31dB.