lmh2120umx National Semiconductor Corporation, lmh2120umx Datasheet - Page 19

• Manufacturer Part Number: lmh2120umx
• Description: Lmh2120 6 Ghz Linear Rms Power Detector With 40 Db Dynamic Range
• Manufacturer: National Semiconductor Corporation

For measuring the RMS (power) level of a signal, the time
average of the squared signal needs to be measured as de-
scribed in section
This is implemented in the LMH2120 by means of a multiplier
and a low-pass filter in a negative-feedback loop. A simplified
block diagram of the LMH2120 is depicted in
core of the loop is a multiplier. The two inputs of the multiplier
are fed by (i
in which i
of the RF detector and i
input signal. The output of the multiplier (i
of these two current and equals:
in which I
output of the multiplier the DC term of this current is isolated
and integrated. The input of the amplifier A acts as the nulling
point of the negative feedback loop, yielding:
which implies that the average power content of the current
related to the output voltage of the LMH2120 is made equal
to the average power content of the current related to the RF
input signal.
For a negative-feedback system, the transfer function is given
by the inverse function of the feedback block. Therefore, to
have a linear transfer for this RF detector, the feedback net-
0
LF
is a normalizing current. By a low-pass filter at the
1
is a current depending on the DC output voltage
, i
2
):
ACCURATE POWER
i
i
1
1
= i
= i
RF
LF
LF
is a current depending on the RF
+ i
- i
RF
RF
OUT
MEASUREMENT.
) is the product
Figure
FIGURE 4. Block Diagram
4. The
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work implements a linear function as well resulting in an
overall transfer function for the LMH2120 of:
in which k is the conversion gain. Note that as a result of the
feedback loop a square root is also implemented, yielding the
RMS function.
Given this architecture for the RF detector, the high perfor-
mance of the LMH2120 can be understood. In theory the
accuracy of the linear transfer is set by:
•
•
The RMS functionality is inherent to the feedback loop and
the use of a multiplier. Thus, a very accurate LIN-RMS RF
power detector is obtained.
To guarantee a low dependency on the supply voltage, the
internal detector circuitry is supplied via a low drop-out (LDO)
regulator. This enables the usage of a wide range of supply
voltage (2.7V to 5V) in combination with a low sensitivity of
the output signal for the external supply voltage.
RF Input
RF systems typically use a characteristic impedance of 50Ω;
the LMH2120 is no exception to this. The RF input pin of the
LMH2120 has an input impedance of 50Ω. It enables an easy,
direct connection to a directional coupler without the need for
additional components
measurement the input power range of the LMH2120 needs
to be aligned with the output power range of the power am-
The linear feedback network, which basically needs to
process a DC signal only.
A high loop gain for the feedback loop, which is
guaranteed by the amplifier gain A.
(Figure
1). For an accurate power
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