GPS World, June 2009

SYSTEM DESIGN TEST Galileo p FIGURE 6 Spectrum of a direct converted signal p FIGURE 7 Heterodyne receiver principle fundamental issues and made wide application possible We set out to answer here whether GNSS receivers can also benefit from the economic advantages of direct conversion receivers Direct conversions main advantage is better monolithic integration single circuit including low pass filters and baseband amplifiers Eventually this could lead to a single chip receiver Another advantage is the absence of a band pass filter at IF stages The architecture does have however some unique drawbacks and challenges including even order distortion oscillator leakage flicker noise and direct current DC offset noise all specific to direct conversion and determining receiver performance Oscillator Leakage This affects other receivers in close proximity to the direct conversion receiver As the local oscillator runs on the RF frequency any leakage in the mixer will lead to radiation from the antenna of the carrier frequency which nearby receivers can then pick up As heterodyne receivers local oscillators do not run on the RF frequency they face no such prevalent problem It can also be reduced by a good circuit design furthermore a leaking oscillator would appear as a very weak continuous wave CW jammer to a nearby GNSS receiver Such a jammer is usually not a problem as the correlation suppresses this jamming signal DC Offset Noise The center of the spectrum of the desired signal is converted down to DC as in Figure 6 However several other sources may result in a DC offset voltage which can dominate the received signal and potentially saturate the receivers ADC The signal is on the order of micro volts but the DC offset could be on the order of volts requiring the ADC to have a very high dynamic range The DC offset noise comes from leakage of the local oscillator through the mixer This leaking frequency is then reflected back to the mixer by the low noise amplifier LNA or the antenna or even from objects outside the antenna FIGURE 8 This leads to a DC offset as the local oscillator frequency down converts itself to DC This offset is not constant therefore cannot easily be removed by the circuit designer Back reflection from objects outside the receiver can lead to a rapidly changing DC offset Several solutions can compensate for this effect One method uses periods with no signal present to measure the offset and then remove it prior to digital conversion This can be used when the signal is periodically absent as in a time division multiplexed structure of mobile telephone networks Unfortunately this does not work for GNSS where a signal is present continuously Another method applies AC coupling this works best if the signals do not have energy at DC as presented subsequently Flicker Noise An intrinsic noise of metal oxide components this is also called 1 f noise or pink noise The noise p FIGURE 8 DC offset sources GPS Signal Distribution even in adverse environments High Performance Amplifiers Splitters Combiners Repeater Kits Antennas Fiber Optic Links and More Custom solutions with prompt delivery Expert GPS and RF technical support Regulatory compliant designs Providing friendly service and personalized technical support Toll Free Phone 866 289 4777 www gpssource com GPS World June 2009 www gpsworld com 14