Gemini:   A 2400 MHz to 28 MHz converter

This dual conversion receive converter is currently undergoing final testing. It is a joint project between Sam Jewell G4DDK and David Bowman G0MRF.

The converter is a dual injection design with a first I.F. around 502 MHz and a second at 28MHz. The choice of a high first IF gives excellent image performance while the 28 MHz output allows the use of HF receivers which frequently have better filtering (DSP and variable IF filters) than their VHF equivalents.

                           Gemini SMD side


The first 3 prototypes of the Gemini converter used an Agilent Technologies ATF36077 PHEMT and a GALI-3 to achieve a noise figure of around 0.85dB. A later modification to the circuit reduced the noise figure to 0.55dB with 37dB of conversion gain.

Design Concepts
The local oscillator is a 118.600 5th overtone crystal followed by  x4 and x2 multipliers. The LO stages are simplified considerably by using a harmonic mixer which only requires an input of around -1dBm at 948 MHz from the LO chain. The LO multipliers and the first IF filtering use commercial helical filters for frequency selection. The use of these filters and not tuned lines significantly reduces spurious signal generation and enhances stability. Finally, the filters come from the manufacturer aligned to a center frequency very close to that required by the converter. When assembly is complete, the converter will require only minimal alignment to reach its design specification.

The Gemini converter can be used with inputs at 2304 2320 2400 or 2450 MHz depending on the crystal used in the local oscillator.
Experimental work is continuing with a version of the design with a 50MHz output. To-date 2400 to 50MHz is possible with minor helical filter realignment. Work continues on 2304 / 2320 versions. 

Presentations on the Gemini converter have been made at the Dayton Hamvention 2002 and the Amsat-UK 17th annual colloquium held at the University of Surrey.

Click here for 7/2002 Amsat UK presentation in Word Format. 116k 


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