Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 115

6.8 DOWN-CONVERTED RF FRONT END FOR REAL DATA
COLLECTION ( 8-10 )

In this section a down-converted approach to digitize the signal will be discussed.
The IF and sampling frequency will be determined, followed by some general discus-
sion. A set of hardware to collect data for user location calculation will be presented.
In this approach the input signal is down converted to an IF, then digitized
by an ADC. In Equation (6.6) there are three unknowns: n , f i ,and f s ; therefore,
the solutions are not unique. Many possible solutions can be selected to build
a receiver. In the hardware design, the sampling frequency of f s = 5MHz is

selected. From Equation (6.6) f i = IF = 5 n + 1 . 25 MHz, where n is an integer.

The value of n = 4 is arbitrarily selected and the corresponding IF = 21 . 25 MHz,

which can be digitized by an ADC.
Of course, one can choose n = 0 and down convert the input frequency to
1.25 MHz directly. In this approach the mixer generates more spurious frequen-
cies. The input signal is down-converted from 0.25 to 2.25 MHz, which covers
more than an octave bandwidth. An octave bandwidth means that the highest
frequency in the band is equal to twice the lowest frequency in the band. A
common practice in receiver design is to keep the IF bandwidth under an octave
to avoid generation of in-band second harmonics.
There are many different ways to build an RF front end. The two important
factors are the total gain and filter installations. Filters can be used to reject
out-of-band signals and limit the noise bandwidth, but they add insertion loss. If
multiple channels are used, such as in the I-Q channels, filters may increase the
difficulty of amplitude and phase balancing. The locations of filters in a receiver
affect the performance of the RF front end.
The personal computer - based ADC card discussed in Section 6.3 is used as
the ADC. It requires about 100 mv input voltage or

10 dBm to activate all the
bits. A net gain of 101 dB is required to achieve this level. If a digital scope is
used as the ADC ( 8 , 9 ) because of the built-in amplifiers in the scope, it can digitize
a rather weak signal. In this kind of arrangement, only about 90 dB gain is used.
Two RF front-end arrangements are shown in Figure 6.5. The major difference
between Figures 6.5a and b is in the amplifiers. In Figure 6.5a amplifiers 2, 3,
and 4 operate at IF, which costs less than amplifiers operating at RF. Filter 1 is
used to limit the input bandwidth. Filter 2 is used to limit the spurious frequencies
generated by the mixer, and filter 3 is used to limit noise generated by the three
amplifiers. Although Figure 6.5a is the preferred approach, in actual laboratory
experiments Figure 6.5b is used because of the availability of amplifiers.
In Figure 6.5b, the M/A COM ANP-C-114-5 antenna with amplifier is used.
Amplifier 1 is an integrated part of the antenna with a 26 dB gain and a 2.5 dB noise
figure. The bias T is used to supply 5-volt dc to the amplifier at the antenna. Filter
1 is centered at 1575.42 MHz with a 3 dB bandwidth of 3.4 MHz, which is wider
than the desired value of 2 MHz. Amplifiers 2 and 3 provide a total of 60 dB gain.
The frequency of the local oscillator is at 1554.17 MHz. The mixer-down converts
the input frequency from 1575.42 to 21.25 MHz. In this frequency conversion,
high input frequency transforms to high output frequency. The attenuator placed

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