LMH2100 Datasheet, PDF (40/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
10 Layout
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10.1 Layout Guidelines
As with any other RF device, careful attention must be paid to the board layout. If the board layout is not properly
designed, unwanted signals can easily be detected or interference will be picked up. This section gives
guidelines for proper board layout for the LMH2100.
Electrical signals (voltages and currents) need a finite time to travel through a trace or transmission line. RF
voltage levels at the generator side and at the detector side can therefore be different. This is not only true for
the RF strip line, but for all traces on the PCB. Signals at different locations or traces on the PCB will be in a
different phase of the RF frequency cycle. Phase differences in, for example, the voltage across neighboring
lines, may result in crosstalk between lines due to parasitic capacitive or inductive coupling. This crosstalk is
further enhanced by the fact that all traces on the PCB are susceptible to resonance. The resonance frequency
depends on the trace geometry. Traces are particularly sensitive to interference when the length of the trace
corresponds to a quarter of the wavelength of the interfering signal or a multiple thereof.
10.1.1 Supply Lines
Because the PSRR of the LMH2100 is finite, variations of the supply can result in some variation at the output.
This can be caused among others by RF injection from other parts of the circuitry or the on/off switching of the
PA.
10.1.1.1 Positive Supply (V_DD)
In order to minimize the injection of RF interference into the LMH2100 through the supply lines, the phase
difference between the PCB traces connecting to VDD and GND should be minimized. A suitable way to achieve
this is to short both connections for RF. This can be done by placing a small decoupling capacitor between the
VDD and GND. It should be placed as close as possible to the VDD and GND pins of the LMH2100 as indicated
in Figure 91. Be aware that the resonance frequency of the capacitor itself should be above the highest RF
frequency used in the application, because the capacitor acts as an inductor above its resonance frequency.
Low frequency supply voltage variations due to PA switching might result in a ripple at the output voltage. The
LMH2100 has a PSRR of 60 dB for low frequencies.
10.1.1.2 Ground (GND)
The LMH2100 needs a ground plane free of noise and other disturbing signals. It is important to separate the RF
ground return path from the other grounds. This is due to the fact that the RF input handles large voltage swings.
A power level of 0 dBm will cause a voltage swing larger than 0.6 V_PP, over the internal 50-Ω input resistor. This
will result in a significant RF return current toward the source. It is therefore recommended that the RF ground
return path not be used for other circuits in the design. The RF path should be routed directly back to the source
without loops.
10.1.2 RF Input Interface
The LMH2100 is designed to be used in RF applications, having a characteristic impedance of 50Ω. To achieve
this impedance, the input of the LMH2100 needs to be connected via a 50Ω transmission line. Transmission lines
can be easily created on PCBs using microstrip or (grounded) coplanar waveguide (GCPW) configurations. This
section will discuss both configurations in a general way. For more details about designing microstrip or GCPW
transmission lines, a microwave designer handbook is recommended.
10.1.3 Microstrip Configuration
One way to create a transmission line is to use a microstrip configuration. A cross section of the configuration is
shown in Figure 89, assuming a two-layer PCB.
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