AD834
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 fol-
lowed 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 band-
width is then limited solely by the AD834, since the following
active circuitry is required to process only low frequency signals.
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 ex-
tends 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 may
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 may be inverted by replacing the
NPN transistor with a PNP type.
(Refer to Figure 5 test configuration.) Using the device as a
squarer the wideband output in response to a sinusoidal stimu-
lus is a raised cosine:
sin2 ωt = (1 – cos 2 ωt) /2
Recall here 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
would be 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, since 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 chan-
nels would result 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.
Figure 13. Connections for Wideband RMS Measurement
FREQUENCY DOUBLER
Figure 14 shows another squaring application. In this case, the
output filter has been removed and the wideband differential
output is converted to a single sided signal using a “balun,”
which consists of a length of 50 Ω coax cable fed through a fer-
rite core (Fair-Rite type 2677006301). No attempt is made to
reverse terminate the output. Higher load power could be
achieved by replacing the 50 Ω load resistors by ferrite bead
inductors. The same precautions should be observed with re-
gard to PC board layout as recommended above. The output
spectrum shown in Figure 15 is for an input power of +10 dBm
at a frequency of 200 MHz. The second harmonic component
at 400 MHz has an output power of –15 dBm. Some feed-
through of the fundamental occurs: it is 15 dBs below the main
output. It is believed that improvements in the design of the
balun would reduce this feedthrough. A spurious output at
600 MHz is also present, but it is 30 dBs below the main out-
put. At an input frequency of 100 MHz, the measured power
level at 200 MHz is –16 dBm, while the fundamental feed-
through is reduced to 25 dBs below the main output; at an
output of 600 MHz the power is –11 dBm and the third
harmonic at 900 MHz is 32 dBs below the main output.
The physical construction of the circuitry around the IC is criti-
cal to realizing the bandwidth potential of the device. The input is
supplied from an HP8656A signal generator (100 kHz to
990 MHz) via an SMA connector and terminated by an HP436A
power meter using an HP8482A sensor head connected via a
second SMA connector. Since 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, since significant transformer action
can arise if the pins pass through individual holes in the board;
this has been seen to cause an oscillation at 1.3 GHz in improp-
erly 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 capaci-
tors are used to minimize the impedance over the full frequency
range. (Refer to Figure 1 for mean-square response for the
AD834 in cerdip package, using the configuration of Figure 5.)
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 13. Note that an at-
tenuator is inserted both in the signal input and in the feedback
REV. C
–7–
5秒后页面跳转
PCF8574资料解读：主要参数分析、引脚说明
AD637数据手册解读：主要特性、引脚及其功能解读、电气参数
ADUM1201资料手册解读：参数分析、引脚说明、应用分析
一文带你了解压敏电阻器在直流电路中的过压保护作用
微信公众号
抖音公众号
浙公网安备 33010502006866号 浙ICP备10014259号-119
营业执照ICP证
技术参数
数据手册
如果您发现信息不准确，
