Abstract: This application note, written by Maxim Integrated Products, presents the design of a GPS IF filter at 183.6MHz for use with the MAX2538. Simulated and measured response and performance data are provided. The filter has a second order Butterworth response with capacitor coupling. The insertion loss is 3.2dB. The GPS cascaded gain is 28.9dB, and the noise figure is 1.8dB.
General Description of MAX2538
The MAX2538 LNAs (low noise amplifiers) and mixers are optimized for CDMA (code division multiple access) applications in the cellular and PCS bands. The MAX2538 addresses triple-band, quad-mode applications by providing switched signal paths for AMPS, digital cellular, and digital PCS. The mixers provide switched IF ports that can be used interchangeably with either mixer input port. In addition, all versions in the family provide an LNA/Mixer/LO (local oscillator) path for GPS (global positioning system) down conversion.
For simplicity and design speed, filter design software is used. In this example, Filter Solutions 8.1* was used.
The Filter design parameters are:
2nd order Bandpass Butterworth, capacitor-coupled filter
FC = 183.6MHz
BW = 15MHz
ZIN = ZOUT = 1kΩ
Inductor Q = 35–50
Capacitor Q = 350–500
The circuit below (Figure 2) and the frequency response (Figure 3) are the results obtained with Filter Solutions software. The light-blue, upper traces shown in Figure 3 are for a filter with lossless components. The red trace depicts the expected performance with lossy components.
Use the MAX2538 GPS Mixer model to design the matching network between the MAX2538 GPS differential mixer outputs (pin 20 and 21) and the 1kΩ IF filter.
Use a High Frequency Circuit Simulator to simulate the frequency response of the GPS IF filter matched to the GPS IF filter. Agilent ADS 2002 was used for this example. To reduce the number of passive components and reduce the insertion loss, integrate one resonant tank (the resonator on the input side) with the matching network required to match the mixer's output impedance to the 1kΩ filter.
Figure 6 is the frequency response of the GPS IF Filter matched to the MAX2538 GPS Mixer.
After implementing the circuit obtained in Agilent ADS, the cascaded measurement of the MAX2538 from the GPS LNA input to the GPS IF filter output was performed.
VCC = 2.85V
FRF = 1575.42MHz, PINRF = -40dBm
FLO = 2087.73MHz, PINLO= -7dBm
GPS LNA gain = 17.5dB
GPS mixer gain = 10dB
GPS RF SAW filter loss = -1.8dB
The 1kΩ resistor at the output is used for measurement purposes only, since the output of the filter is 1kΩs. To calculate cascaded gain, add the measured gain to the attenuation introduced by the mismatch between 50Ωs and 1050Ωs.
Attenuation = 0.5 × |20log(50 / 1050)| = 13.22dB
Cascaded gain = measured gain + 0.5 × |20log(50 / 1050)| from LNA input to 1kΩ output (GPS IF output).
Cascaded gain = 15.7dB + 13.22dB = 28.9dB
Cascaded gain = LNA gain + RF filter loss + mixer gain + IF filter loss = 28.9dB
Discrete GPS IF filter loss = +28.9dB - 17.5dB + 1.8dB - 10.0dB = 3.2dB
Cascaded NF is measured from LNA input to 1kΩ output (GPS IF output).
Cascaded NF = 1.8dB
C1 = 1.6pF ±0.1pFGRM36COG1R6B050 Murata (0402)
C2 = 1.1pF ±0.1pFGRM36COG1R1B050 Murata (0402)
C3 = 11pF ±0.1pFGJ61555C1H110JB01B Murata (0402)
L1, L2 = 100nH0603CS-R10XJB CoilCraft (0603)
L3 = 56nH ±5%0603CS-56NXJB CoilCraft (0603)
R1 = 5.6kΩ ±1%
*FilterSolutions by Nuhertz Technologies, L.L.C., www.nuhertz.com.