Effects of diode parasitics on the performance of lattice mixers

Abstract

It has been known for a number of years that the noise figure of a simple receiving system is directly proportional to the conversion efficiency of the mixer. Any improvement in this will have great bearing on the performance of a communications system, offering the possibility of an increased range of reception. The narrow-band open - circuit lattice mixer has been shown theoretically, under certain conditions, to produce a loss approaching zero. Exploratory low frequency circuits have given conversion power losses as low as 1 dB. The increasing use of communication systems at microwave frequencies brings with it the demand for low-loss mixers at these higher frequencies. As the frequency of operation increases, the parasitic diode reactance's have a more pronounced effect on the mixer performance. The original work presented here analyses the effect of the parasitic diode package capacitance on the performance o f a narrow-band open - circuit lattice mixer. The main conclusion to be drawn from the analysis is that for practical diodes used with local oscillator powers normally encountered in microwave mixers, the conversion power-loss of the mixer will, generally, be less than 1.5 dB. Another important result from the analysis is that the optimum terminating resistances are considerably reduced when even small amounts o f package capacitance (eg. 0.1 pF) are present. This may be considered as being an advantage when matching the lattice to a 50 Ω coaxial system. An experimental mixer at L-band constructed by the author using lumped circuit components, gave a 2.8 dB conversion power loss at 20mW of local oscillator drive. This was a considerable improvement on existing commercial models. Had it been a broad-band lattice mixer, it would have given as 3.6 dB minimum conversion loss. On the other hand a narrow-band mixer would have given 3.75 dB minimum. The practical mixer circuit was a modification of the one first analysed in detail by Kulesza, in that the input transformer was resonated. This configuration solved the problem of feeding the local oscillator at the input of the lattice. The 25 MHz. 3 dB band-width of the mixer would be considered sufficiently large enough for use in a communications network. Less power at the local oscillator, frequency was required when comparing this mixer with some 2-diode; mixers; in some cases producing a power saving of 50%. Experimental verification of the analytical work on the effect of diode capacitance gave within 0.9 dB of the theoretical values for low values of capacitance and for local oscillator drive levels used in practice. Finally, the 'K' mixer parameters for the case when diode package capacitance is present in a narrow-band open-circuit lattice mixer are solved. These will be of importance to any future analytical work on lattice mixers.