Encased in a pencil box to keep
the point-to-point wiring
on the back
of the board from shorting to wires and things on the workbench, the
plastic case also holds the offset and gain post as well as the
input
connector and the switches
that switch on the
near-zero and near-full scale
references.
I wanted a way to measure the current in some HF power circuits and
thought it would be a 5 minute task to throw together a sufficiently
linear peak detector and calibrate it. I was wrong. Months
later, I settled on the circuit shown here.
Circuit
Overview
The blocks are (left-to right)
top: 2 MHz preamp, unity gain buffer,
and output amplifier with gain
and offset adjustment;
bottom: +12 and
+5 V regulators, 8 kHz oscillator, buffer with voltage multiplier to
supply -3.5V to the op-amp.
Diode detectors are good at changing AC into DC, even at high
frequencies. The main problem with diode detectors have inherent
nonlinearities, particularly for small signals. To minimize the effects
of the nonlinearities, a preamplifier boosts the amplitude of the input
signal by 16X, thereby reducing the the diode detector’s nonlinearity
effects by the same factor. The detector is followed by a buffer and
adjustable gain stage. This last stage compensates for DC offset that
results from bias on the diode detector and reduces the full scale
amplitude so that the output signal DC level is equal to the input
peak-to-peak value.
Power for the circuits is provided by analog regulators, and an
oscillator followed by a buffer and half wave voltage multiplier
provides -3.5 volts DC as the negative supply for the dual op-amp.
Input
coupling and Preamp
The input net work consists of AC coupling made of a 0.047 uf capacitor
and a 10 Meg resistor and transient protection. The -3 db corner of the
high pass filter is 0.34 Hz. Some degree of transient protection is
obtained by two back-to-back emitter-base junctions, which work a lot
like back-to-back Zeners. The main difference between these and Zeners
is that the reverse biased junction capacitance is lower and the knee
fairly sharp, meaning that it should have no measurable effect on a 200
kHz, 200 millivolt input signal.
The input of the preamp is the gate of the 2N5485 JFET, and since the
drain drives the forward-biased base-emitter junction of the 2N3906,
the signal on the JFET’s drain is very small, thus there is little
miller capacitance, keeping the input capacitance low.
Negative feedback thorough the 20k resistor from the collector of the
2N3906 and the source of the 2N5485 JFET stabilizes the DC operating
point of both transistors and stabilizes the gain of the stage overall.
I measured the gain of the one I built at 16X.
The 10K resistor was selected for the individual 2N5485 to set the DC
level on the output of the stage. On the one I built, the DC level at
the output is 4.7 VDC. This resistor (10k in this schematic) may
need to be changed for individual JFETs because of variation in the
pinch-off voltage.
Checking with an oscilloscope, the preamp appears to flat from below 1
Hz to past 2 MHz (-3 db point). It looks like the gain is flat to
within
a couple of percent past 400 kHz. If either of the semiconductors
are substituted the 50 pf peaking capacitor may have to be changed. To
check peaking, drive the input with a square wave and adjust the
capacitor for the most “square” edge you can get without visible
overshoot. If you see ringing, it is most likely because of a layout
problem.
P-P
Detector and Voltage Follower
The AC signal from the preamp is connected to a half-wave voltage
doubler employing Schottky diodes. The doubler is just slightly forward
biased to make it more linearl particularly for small singals, by the
voltage divider made of 2.2k resistors and the 10 Meg
resistor. The voltage divider makes 2.5 volts on the anode of D1 and
the 10 Meg resistor provides a current return to ground from the
cathode of D2. Since the 10 Meg resistor is so large, the current
through the diodes is very small, meaning that the forward voltage of
D1 and D2 is also very small. The current is about 250 nanoamps. From
this we would normally subtract the input bias current of the of-amp,
but since at room temperature the input bias current of a TL062 is a
couple hundred nanoamps, it can be neglected. It should be pointed out
that with the very large load of 10 Meg Ohms and a large bias voltag
source (2.5V), the current through the diode will not change
significantly with signal levels of a couple hundred millivolts, and I
think this contributes to the excellent linearity.
The bias improves the linearity of the detector a lot. Above is a plot
of output signal error, with the circuit output normalized to the
input, as a function of the expected input signal amplitude for both an
unbiased detector and the nearly zero biased detector used in this
circuit. The signal path includes the entire circuit from preamp
through gain control. The “expected input signal amplitude is the P-P
voltage calculated to be at each tap on a voltage divider as show in
the section below under Calibration, based on the marked values of the
resistors used and the amplitude of the square wave used to drive the
divider. The signal was measured at the DC output of the circuit and
the gain and offset adjusted using a spreadsheet with the points near 2
millivolts and 200 millivolts adjusted for accuracy by using the offset
and gain adjustments, respectively.
The chart above is the output error in millivolts as a function of the
P-P input voltage. Note that there is a preamp with a gain of 16X just
ahead of the detector, so I would expect the bare detector, without the
16X preamp, to have much larger errors.
Here is the raw data:
Ideal
250 na bias bias shorted
0.19605
0.198
0.066
0.1486
0.1495 0.0169
0.10058
0.1006 -0.0299
0.0525
0.0521 -0.0772
0.004515
0.004 -0.1177
0.00225
0.0023 -0.1186
The output of the half wave voltage doubler is filtered by the .047 uf
capacitor and 10 Meg resistor, resulting in a time constant of 0.47
seconds.
A unity gain voltage follower (op amp pins 1,2, and 3) isolates the
buffer from the output amplifier.
Output
Amplifier
The signal from the P-P Detector has a DC offset as a result of the 2.5
volt bias voltage and is not properly scaled because of the gain of the
preamp stage. The output stage’s purpose is to compensate for the
offset and gain so that the voltage measured on the DC output of the
circuit corresponds directly to the peak-to-peak input voltage of the
signal being measured. For example, if a 123 millivolt
peak-to-peak sine wave is applied to the input, the DC output voltage
should be 123 millivolts DC.
The output amplifier is connected as a differential amplifier with an
adjustable offset Adjusting R1 adds current into the inverting input of
the op-amp. A side effect of this way of adding offset is that changing
offset affects gain slightly, but it saves an op-amp. A remote or panel
mounted offset control can be connected R3. If the
A remote or panel-mounted offset trim signal can be connected through
R4. At this writing, this input has been tested but it is not being
used.
R2 adjusts the scale of the output voltage so that the output
voltage agrees with the input signal’s P-P amplitude. This results in a
output resistance of between 0 and 5k Ohms. As long as the output is
connected to a high impedance load, such as a 1 Meg Ohm DVM input, the
output resistance would not be important. But when using with much
lower resistance loads, such as when calibrating on a 1 Meg Ohm DVM
then trying to use the detector with a 1,000 Ohm per volt moving coil
meter would result in significant errors.
A remote or panel mounted gain trim adjustment can be made through R3.
This could be in the form of 1 Meg Ohm rheostat (potentiometer with
only the wiper and one leg connected) to ground. This input has been
tested but at this writing, it is not being used.
Power
supply and Reference Signal
The circuit needs +12 volts. +5 volts, and -3.5 volts in order to
operate. The positive voltages are provided by analog monolithic
regulators powered from the +15 volt input. The -3.5 volt power supply
is made with a half-wave voltage doubler that derives its power from
the +5 volt supply.
The 74HCT14 Schmitt Trigger
oscillator generates pulses at about 8 kHz. This particular chip from
Philips has a an approximately 80% duty cycle output. The output of the
oscillator connects to three parallel inverters that
drive a half-wave voltage multiplier, which makes the -3.5 volts for
the negative power supply of the op-amp.
The output of the oscillator is also fed to a voltage divider made with
a string of resistors in series; 100k, 4.7k, and 47 Ohms to ground. A
low 100 kHz pass filter formed by the first 10k resistor and 150 pf
capacitor rounds off the edges of the pulses to minimize the chance for
overshoot that could result from the point-to-point wiring of the
board. The
taps on the divider provide reference signals of about 216 mv P-P and
22
mv P-P that are used to calibrate the adapter prior to use.
Calibration
Linearity testing and calibration of the circuit was based on the the
circuit shown above. A frequency divider is driven from a CMOS
counter's 7.125 kHz output. The voltage at the taps was calculated
based on the assumption that the resistors are ideal and there was no
significant ringing on the waveform at the taps. A 100 pf capacitor
across the lower 4092 Ohms of the divider rolls the circuit off at 400
kHz to help control high frequency overshoot on the pulse edges.
The offset pot on the board were adjusted by first making sure the
internal references are not connected, then connecting a digital
voltmeter to the board's output, putting the Hi and Lo Adjust front
panel post to the center of their ranges, connecting the input of the
board to the 2.25 millivolt calibration signal and then adjusting the
offset pot to obtain a reading of 2.3 millivolts.
Then, the input of the board is connected to the 196.5 mv calibration
voltage and the gain control on the circuit board is adjusted to obtain
a reading of 196.5 millivolts.
The offset is then checked and adjusted, and then the gain pot is
checked and adjusted as above. This procedure is repeated until the
adjustment is not needed when moving the input between the 196.5
millivolt and the 2.25 millivolt calibration signals.
A built-in signal source provides high and low level reference signals
necessary to adjust the HI ADJ and LO ADJ controls on the adapter prior
to everyday use. Before they can be used, the reference signals need to
be measured. To do this, the adapter is calibrated as described above,
then the nominally 2.2 millivolt reference is switched in and the
resulting DC output is measured and recorded for later use, hen the
nominally 2.2 millivolt reference is switched out and the nominally 216
millivolt reference is switched in and the DC output is measured and
recorded for later use. For example, on the one I built, the 2.2 mv
nominal output reference measured 1.9 millivolts and the 216 millivolt
reference measured 192.6 millivolts.
To adjust the HI ADJ and LO ADJ controls, the 2.2 millivolt reference
signal is switched in and the LO ADJ control is adjusted -in the case
of the one I built, to obtain 1.9 millivolts out. Then, the 2.2
millivolt reference is switched out and the 216 millivolt reference is
switched in and the HI ADJ control is adjusted -in the case of the one
I built, to obtain 192.6 millivolts out. A second, and perhaps a third
iteration may be necessary if the adjustments were way off to start
with.
Layout
Layout is important with much of the circuit. Minimize parasitic
capacitance between the CMOS oscillator/ power supply and the analog
section, particular around the 2N5485 JFET and the input circuit. Try
to keep the decouping capacitors as close to the 74HCT14 as possible.
Keep the capacitors in the preamp circuit close to the circuit itself
and keep the leads for the three grounded capacitors in that circuit
back to a single point before connecting to other grounds. This single
point should also be the ground for the input signal and the reference
voltage divider.
The DC level at the output of the first buffer (op-amp pin 1) should be
about 2.5 VDC when the input is grounded. If it is much more than than,
look for noise getting into the preamp from the 74HCT14 or oscillation
in the preamp itself.
[Key words: Audio millivoltmeter, RF millivoltmeter, Schottky detector,
RF detector, Schottky linearity.]
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