LMH2100 Datasheet, PDF (20/32 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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(a)
(b)
FIGURE 1. Convex Detector Transfer Function (a) and Linear Transfer Function (b)
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Figure 1 shows two different representations of the detector
transfer function. In both graphs the input power along the
horizontal axis is displayed in dBm, since most applications
specify power accuracy requirements in dBm (or dB). The
figure on the left shows a convex detector transfer function,
while the transfer function on the right hand side is linear (in
dB). The slope of the detector transfer function — i.e. the de-
tector conversion gain – is of key importance for the impact
of the quantization error on the total measurement error. If the
detector transfer function slope is low, a change, ΔP, in the
input power results only in a small change of the detector out-
put voltage, such that the quantization error will be relatively
large. On the other hand, if the detector transfer function slope
is high, the output voltage change for the same input power
change will be large, such that the quantization error is small.
The transfer function on the left has a very low slope at low
input power levels, resulting in a relatively large quantization
error. Therefore, to achieve accurate power measurement in
this region, a high-resolution ADC is required. On the other
hand, for high input power levels the quantization error will be
very small due to the steep slope of the curve in this region.
For accurate power measurement in this region, a much lower
ADC resolution is sufficient. The curve on the right has a con-
stant slope over the power range of interest, such that the
required ADC resolution for a certain measurement accuracy
is constant. For this reason, the LOG-linear curve on the right
will generally lead to the lowest ADC resolution requirements
for certain power measurement accuracy.
1.1.2 Types of RF Power Detectors
Three different detector types are distinguished based on the
four characteristics previously discussed:
• Diode Detector
• (Root) Mean Square Detector
• Logarithmic Detector
These three types of detectors are discussed in the following
sections. Advantages and disadvantages will be presented
for each type.
Diode Detector
A diode is one of the simplest types of RF detectors. As de-
picted in Figure 2, the diode converts the RF input voltage into
a rectified current. This unidirectional current charges the ca-
pacitor. The RC time constant of the resistor and the capacitor
determines the amount of filtering applied to the rectified (de-
tected) signal.
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FIGURE 2. Diode Detector
The advantages and disadvantages can be summarized as
follows:
• The temperature stability of the diode detectors is
generally very good, since they contain only one
semiconductor device that operates at RF frequencies.
• The dynamic range of diode detectors is poor. The
conversion gain from the RF input power to the output
voltage quickly drops to very low levels when the input
power decreases. Typically a dynamic range of 20 – 25 dB
can be realized with this type of detector.
• The response of diode detectors is waveform dependent.
As a consequence of this dependency for example its
output voltage for a 0 dBm WCDMA signal is different than
for a 0 dBm unmodulated carrier. This is due to the fact
that the diode measures peak power instead of average
power. The relation between peak power and average
power is dependent on the wave shape.
• The transfer shape of diode detectors puts high
requirements on the resolution of the ADC that reads their
output voltage. Especially at low input power levels a very
high ADC resolution is required to achieve sufficient power
measurement accuracy (See Figure 1, left side).
(Root) Mean Square Detector
This type of detector is particularly suited for the power mea-
surements of RF modulated signals that exhibits large peak
to average power ratio variations. This is because its opera-
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