Global Positioning System Reference
In-Depth Information
FIGURE 6.5
Two arrangements of data collection.
between the mixer and the oscillator is used to improve impedance matching and
it reduces the power to the mixer. After the mixer an IF amplifier with 24 dB of
gain is used to further amplify the signal. Finally, filter 2 is used to reject spurious
frequencies generated by the mixer and limit the noise bandwidth. Filter 2 has a
center frequency of 21.25 MHz and bandwidth of 2 MHz. If filter 2 is not used all
the noise will alias into the output band and be digitized by the ADC as shown in
Figure 6.3. The overall gain from the four amplifiers is 110 dB
(
26
+
24
)
. Subtracting the insertion losses from the filters, bias
T
, and mixer, the gain is
slightly over 100 dB. There is no filter after the mixer because it is not available
in our laboratory. In general, the local oscillator and the digitizing clock are phase
locked. In our special design, the local oscillator is a crystal based commercial
oscillator and the digitizing clock is a signal generator and they are not phase
locked. In a later design, the local oscillator and the digitizing clock are phase
locked.
+
30
+
30
6.9 DIRECT DIGITIZATION FOR REAL DATA COLLECTION
(
8,9
)
Direct digitization at RF is a straightforward approach. The only components
required are amplifiers and two filters. The amplifiers must provide the desired
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