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AN4855Application Note
■ A small frequency error from nominal (>0.3ppm) at start-up will cause the system to start searching for satellite
codes at the wrong doppler-offset frequencies, even when the ephemeris (orbital) data is up-to-date; this will
extend the search time and impact the Time To First Fix (TTFF).
■ A large frequency error from nominal (>50ppm) will cause the GP2010/GP2015 to impose a significant
frequency shift inthe down-converted L1 signal. This can result in a significant part of the ±1.023MHz GPS
wanted signal being removed by the tight frequency response of the Dynex DW9255 SAW filter.
■ Abrupt frequency changes, of much less than 0.1ppm, can either inhibit satellite-code lock or cause the system
to lose satellite lock. Digital temperature compensation schemes can experience this problem.
■ Modulation of the reference frequency across a very wide frequency range (10s of Hertz up to 10s of MHz),
will translate into spurious signals which can distort the down-conversion of the wanted GPS signal through
the RF front-end, and can also mix out-of-band signals into the GPS wanted signal band that can degrade the
signal-to-noise of the correlated satellite signals. This can result from noise on the power-supply line to the
TCXO, and appropriate supply filtering can alleviate this problem (Refer to “Supply Filtering for the TCXO”).
Although the standard GPS Orion software will fail to acquire GPS signals which are offset by >2.5ppm, the software
can be adjusted to look for satellite signals across a very wide doppler frequency range (Refer to GP2021 data sheet).
The TCXO selected for the GPS Orion is from a range of sub-miniature 2.5ppm units from either NDK (5111A-ANL50-
A @10MHz) or Oscillatek (T-1115@10MHz or OSC-3AO-AUK @ 10MHz or Rakon (TXO200B@ 10MHz). The
output level from each of these TCXOs is a 1.0V peak-to-peak minimum clipped sinewave.
If the amplitude of the signal from the TCXO into a GP2010/GP2015 is greater than 1.2V peak-to-peak, a spurious
output will appear on the IFOUT signal due to harmonics of the 10.000MHz PLL reference. These can interfere with
the 3rd stage mixer and produce interference spurs on the signal at IFOUT( e.g. Third harmonic (30MHz) will produce
a spur at 1.111MHz). For this reason, the TCXO output level is attenuated by 6dB using a potential divider network
to produce a 600mV peak-to-peak amplitude, and being AC coupled into the REF 2 input (pin 27) of the GP2010/
GP2015
Supply Filtering of the TCXO
The components on the GPS Orion Receiver are operated from a common power-supply - Vdd. This means that all
the ICs, the active antenna and the TCXO share the same power supply.
The GPS Orion runs software at a typical bus speed of 20MHz (not including wait-states). Consequently, there is
a large amount of digital interference produced over a wide frequency range, which can be modulated onto the Vdd
power-supply. The GP2010/GP2015 is a RF linear IC, which requires careful placement with respect to all the digital
components, in order to reduce the risk of digital interference into the RF receive chain. Combined with this is the
need to ensure that the 10.000MHz TCXO has a very clean power-supply. The GP2010/GP2015 PLL is referenced
to the TCXO signal - the cleaner the TCXO signal, the cleaner the 1.400GHz VCO signal produced, and hence the
cleaner ALL the local-oscillator frequencies and digital clocks will be.
Figure 7 shows the difference in the GP2010/GP2015 1.400GHz VCO quality, between having no low-frequency
supply filtering and having optimised low-frequency supply filtering on the TCXO.
Note the dramatic difference in VCO signal quality. When the software is running, and the TCXO has no low-
frequency decoupling, there are large digital interference spurs, which are up to -20dBc in amplitude. These can have
a detrimental effect on the quality of the down-converted L1 signal, by mixing out-of-band signals into band, and also
smearing the L1 spread-spectrum signal. The optimised low-frequency supply filtering removes all detectable low-
frequency interference modulation on the 1.400GHz signal. This means that the out-of-band interference rejection
of the GP2010/GP2015 is greatly improved, and the amount of jitter in any digital clock is greatly reduced.