AD834 Data Sheet
Rev. F | Page 14 of 20
POWER MEASUREMENT (MEAN-SQUARE AND RMS)
The AD834 is well-suited to measurement of average power in
high frequency applications, connected either as a multiplier for
the determination of the V × I product, or as a squarer for use
with a single input. In these applications, the multiplier is followed
by a low-pass filter to extract the long-term average value. Where
the bandwidth extends to several hundred megahertz, the first
pole of this filter should be formed by grounded capacitors
placed directly at the output pins, W1 and W2. This pole can
be at a few kilohertz. The effective multiplication or squaring
bandwidth is then limited solely by the AD834, because the active
circuitry that follows the multiplier is required to process only
low frequency signals. Using the device as a squarer, like the
circuit shown in Figure 8, the wideband output in response to a
sinusoidal stimulus is a raised cosine.
sin
2
ωt = (1 − cos 2 ωt)/2
Recall that the full-scale output current (when full-scale input
voltages of 1 V are applied to both X and Y) is 4 mA. In a 50 Ω
system, a sinusoid power of +10 dBm has a peak value of 1 V.
Thus, at this drive level, the peak output voltage across the
differential 50 Ω load in the absence of the filter capacitors is
400 mV (that is, 4 mA × 50 Ω × 2), whereas the average value of
the raised cosine is only 200 mV. The averaging configuration is
useful in evaluating the bandwidth of the AD834, because a dc
voltage is easier to measure than a wideband differential output.
In fact, the squaring mode is an even more critical test than the
direct measurement of the bandwidth of either channel taken
independently (with a dc input on the nonsignal channel),
because the phase relationship between the two channels also
affects the average output. For example, a time delay difference
of only 250 ps between the X and Y channels results in zero
output when the input frequency is 1 GHz, at which frequency
the phase angle is 90 degrees and the intrinsic product is now
between a sine and cosine function, which has zero average value.
The physical construction of the circuitry around the IC is
critical to realizing the bandwidth potential of the device. The
input is supplied from an HP 8656A signal generator (100 kHz
to 990 MHz) via an SMA connector and terminated by an
HP 436A power meter using an HP 8482A sensor head
connected via a second SMA connector. Because neither the
generator nor the sensor provide a dc path to ground, a lossy
1 μH inductor, L1, formed by a 22-gauge wire passing through
a ferrite bead (Fair-Rite Type 2743001112) is included. This
provides adequate impedance down to about 30 MHz. The IC
socket is mounted on a ground plane with a clear area in the
rectangle formed by the pins. This is important because significant
transformer action can arise if the pins pass through individual
holes in the board; it can cause an oscillation at 1.3 GHz in
improperly constructed test jigs. The filter capacitors must be
connected directly to the same point on the ground plane via the
shortest possible leads. Parallel combinations of large and small
capacitors are used to minimize the impedance over the full
frequency range. Refer to Figure 4 for mean-square response for
the AD834 in a CERDIP package, using the configuration of
Figure 8.
To provide a square root response and thus generate the rms
value at the output, a second AD834, also connected as a
squarer, can be used, as shown in Figure 20. Note that an
attenuator is inserted both in the signal input and in the feed-
back path to the second AD834. This increases the maximum
input capability to +15 dBm and improves the response flatness
by damping some of the resonances. The overall gain is unity;
that is, the output voltage is exactly equal to the rms value of the
input signal. The offset potentiometer at the AD834 outputs
extends the dynamic range and is adjusted for a dc output of
125.7 mV when a 1 MHz sinusoidal input at −5 dBm is applied.
Additional filtering is provided; the time constants were chosen
to allow operation down to frequencies as low as 1 kHz and to
provide a critically damped envelope response, which settles
typically within 10 ms for a full-scale input (and proportionally
slower for smaller inputs). The 5 μF and 0.1 μF capacitors can
be scaled down to reduce response time if accurate rms opera-
tion at low frequencies is not required. The output op amp must
be specified to accept a common-mode input near its supply.
Note that the output polarity can be inverted by replacing the
NPN transistor with a PNP type.
