PT9120 Datasheet, PDF (7/18 Pages) Princeton Technology Corp – GPS Receiver RF Front End IC

GPS Receiver RF Front End IC
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Preliminary
PT9120
If desired, the antenna switch may be bypassed by connecting the active antenna directly to the ISNS
pin. Furthermore, the antenna detector may be bypassed by shorting PVDD to ISNS (AOK will always
be set to logic LOW). In both these cases, the short circuit current-limiting protection circuit is disabled.
If no active antenna is used, PVDD must be connected to ground, while AON and ISNS may be
connected to ground or left open. In this case, the AOK pin will always be set to logic HIGH when the
PT9120 is in fully active mode, and will be set to logic LOW in all other modes.
EXTERNAL LNA
As shown in the application circuit, an off-chip cascade LNA (15dB gain and 1.5dB NF) may be used to
amplify the 1575.42MHz L1 GPS RF input signal prior to sending it to the RF input of the PT9120. The
input and output impedances for the LNA are nominally 50Ω at 1575.42MHz.
RF LNA
Impedance matching at the PT9120’s integrated LNA’s input and output is required. At the LNA input,
an optimum noise match is required for best sensitivity performance. At the LNA output, a match to the
50Ω impedance of the SAW filter is required. Typical matching topologies and component values are
shown in Typical PT9120 RF application circuit schematic. Note that the layout of the application PCB
may affect these component values.
RF MIXER
The RF mixer down-converts the GPS signal band to a 1st IF near 20MHz (depending upon the chosen
crystal reference frequency as specified in Supported frequency plans). The RFIN input of the mixer is
on-chip matched to 50Ω and is internally biased near ground potential (AVSS) and should not receive
any external dc biasing.
IF FILTER AND AGC
The PT9120 also requires 2 external IF filters at the mixer output for channel selection and to reject
image frequency noise at the input of the sub-sampling 2-bit A/D converter. These filters should have a
bandwidth of at least 2MHz, centered at the 1st IF corresponding to the frequency plan chosen (see
Supported frequency plans), and should also provide a low impedance path to ground at the local
oscillator frequency.
A typical GPS receiver application may include the 4th order L-C band-pass filter connected between
the IF1P/IF1N and IF2P/IF2N pins as shown in the application circuit of Typical PT9120 RF application
circuit schematic. With the component values shown, the filter is centered at 20.46MHz and has a
bandwidth of roughly 4MHz to accommodate component tolerances of ±5%. On the PCB, the IF filter
components should be placed far away from digital signals.
PT9120 PRE1.0
-7-
August, 2007