This device converts the GPS signal at 1,575.42 MHz to an analog IF signal at 4.309 MHz. In addition, a digital IF at 1.405 MHz is provided as sign and magnitude bits. Figure9.1shows a block diagram of the GP2015.
9.1.1 Triple Conversion to 4.039 MHz IF
Three mixer operations are used to downconvert the GPS signal to the analog 4.039 MHz IF. This analog IF output is not used by the GP2021 baseband proces-sor, rather the GP2021 uses the digital IF @1.405 MHz. Each conversion stage has a corresponding LO which is synthesized on the chip. This includes LO1, which is a phase-locked oscillator. The other oscillators are derived using frequency division of LO1. The reference frequency for the LO1 synthesizer is 10 MHz. A TCXO oscillator would typically provide the 10 MHz reference.
The IF bandpass filters are external elements that must be connected to the GP2015 with the exception of the 4.309 filter, it is on the chip. These can be SAW
D. Doberstein,Fundamentals of GPS Receivers: A Hardware Approach,
DOI 10.1007/978-1-4614-0409-5_9,#Springer Science+Business Media, LLC 2012
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FREQ 175.42MHz 1STIFBPFRFBPF @1575.42MHz
FREQ 1575.42MHz DIV BY7 DIV BY4
DIV BY5DIV BY2 DIV BY9
LOOP FILTER PHASE DETECTOR10MHz REFERENCE
1400MHz
FREQ 35.42MHz 2NDIFSAWBPF AGC 140MHz 31.111MHz
4.309MHz 2NDIFBPF A/D CONVERTER
4.309MHz ANALOG IFOUT SIGN BIT MAG BIT5.71428MHZ SAMPLE CLOCK 40MHzCLOCK OUT
FREQ GP2015FUNCTIONSAREINSIDEDASHEDBOX FUNCTIONSOUTSIDEDASHEDBOXARETYPICALFORUSINGGP2015
~2MHz ~2MHz~2MHz~2MHz DIGITALIFOUT @1.405MHz Fig.9.1ZarlinkGP2015GPSRFdownconverter(simplified)
168 9 The Zarlink 12-Channel GPS Receiver
filters or discrete filters. Typically, a SAW filter is used for the second IF at 35.42 MHz. This filter should have a 3 dB bandwidth of approximately 2 MHz as the signal is wideband through out the IF conversion process.
9.1.2 Digital Sampling Creates IF@ 1.405 MHz
The 4.039 MHz third IF is digitally sampled to create a fourth IF frequency at 1.405 MHz. This IF is a digital IF and does not have an analog output. In particular, it is a rather messy digital waveform at the output of the GP2015. The Digital output is available as a SIGN and MAGNITUDE bit. This approximates a sinewave as shown in Fig.9.2. The low number of bits used to capture the GPS signal may seem like a crude approximation. But it is important to remember that the GPS signal
MAG Positive Threshold
MAG Negative Threshold IF (CW)
into A/D
SIGN Threshold
180 Deg
54 Deg.
out of 180 54 Deg.
out of 180
Composite Digital Dashed line is Signal That would result After Analog Lowpass SIGN
MAG
Fig. 9.2 Sign and magnitude representation of sinewave
9.1 The Zarlink GP2015 RF Downconverter 169
contains only Phase Modulation. Phase information is contained only in the zero crossings of the signal and not in its amplitude. This allows a low number of bits to be used to represent the GPS Signal.
The sampling process performed at 5.71428 MHz creates the 1.405 MHz digital IF. In many respects, the Sampling process is acting like a mixer. By sampling the 4.309 MHz IF at 5.71428 MHz, the spectrum is repeated at various spots in the frequency domain. Since no filtering is provided by the GP2015, almost all of these possible spectral locations are available to process by subsequent digital methods.
For the GP2015/2021 chip set, the difference spectrum at 1.405 MHz is chosen for processing. Centered here is the GPS signal in its Uncorrelated, wideband form with a bandwidth set by the filter at 4.309 MHz (approx. 2 MHz). This is essentially the lower side band product from the “mixing” of the 4.309 MHz IF and the 5.7128 MHz sample clock.
9.1.3 GP2015/GP2021 Clock Signals and Complex Mode
Two important clock signals are used by the GP2015 and by the GP2021. First is the sample clock at 5.71428 MHz. The second clock signal is the 40 MHz clock derived from division of LO1@1,400 MHz. To be absolutely clear, all clock signals and LOs are derived from the common reference at 10 MHz through division or phase-lock processes. In this respect, the GP2015/2021 pair form a coherent receiver chip set.
The 5.71428 MHz sample clock is generated by the GP2021 by division of the applied 40 MHz-clock signal. It uses this clock for its own digital processing methods and provides it for use by the GP2015 to sample the IF at 4.309 MHz.
The 40 MHz-clock signal is used by the GP2021 for generating a multitude of internal timing signals. A short list of these is TIC, the internal DCO clocks, MICRO_CLK, etc. The 40 MHz clock is provided as non-inverted (CLK_T) and inverted (CLK_I). It is also possible to use the GP2015/2021 in a complex mode.
In this configuration, some clock frequencies are changed from those presented here.
In this discussion, we will focus on the REAL mode of operation and that set of clock frequencies. For Complex operation, the reader should consult the Zarlink data sheets.
9.1.4 The TIC Signal
The TIC signal performs the same function that the SNAP_SHOT signal does in the GPS100SC receiver. It is normally set to ~1/10 of a second. Just as in the GPS100SC, the TIC captures the state of the SV replica clocks present in the GP2021. In the GP2021 that means 12 SV replica clocks are captured every TIC signal. The structure of the SV replica clocks is slightly different compared to the GPS100SC. The interpretation of the data is exactly the same, it is an estimate of the uncorrected “time sent” information for each SV tracked.
170 9 The Zarlink 12-Channel GPS Receiver