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ECEN 317
Stephen Schultz
Winter 2009
Lab Test Equipment
1.
Introduction
The Equipment discussed in this lab tutorial are (1) the DC power supplies; (2) the signal
generator; (3) the digital oscilloscope; and (4) the analog oscilloscope.
1.1
The DC Power Supplies
There are two separate adjustable power supplies enclosed in the same chassis. NEVER
CONNECT THE TERMINAL MARKED WITH THE GROUND SYMBOL TO
ANYTHING!!!! Only the terminals marked with “+” and “-” should be used. This gives you a
voltage similar to a battery. If you need a positive voltage in your circuit, connect the “-”
terminal to your circuit ground and use the “+” terminal to supply a positive voltage. If you
need a negative voltage in your circuit, connect the “+” terminal to your circuit ground and use
the “-” terminal to supply a negative voltage.
If both positive and negative voltages are needed in your circuit, as op amps will require,
connect the “+” terminal of one supply to the “-” terminal of the other supply. These terminals
should then be connected to your circuit ground. The remaining “-” terminal will supply
negative voltage to your circuit and the remaining “+” terminal will supply positive voltage to
your circuit.
1.2
The Signal Generator
The HP33120A signal generator must have been designed by idiots. In over 40 years of being
in the field, I have never seen a design as stupid as this one. (Please excuse me for being
honest.) We have requested funds to replace these and someday may be given that opportunity.
Here are the two things that seem incredibly inane. (Do you know what “inane” means?)
1)
2)
The generator has impedance setting with the options of “HI Z” and “LO Z.” This has
nothing to do with the output resistance of the generator that is 50 Ω. This setting has to
do with the input impedance of the circuit to which you are connecting the generator. If
your impedances happened to be 50 Ω, you would set the generator to the low Z setting.
You will always use much higher input impedances in this lab. Therefore, always use
the high Z setting or the meter on the generator will be of by a factor of 2!!!! (Dumb).
The smallest peak voltage that can be generated is 50 mV (100 mV p-p). In some labs,
we will want smaller input signals to keep the output from leaving the active region.
You will need to attenuate this signal with a voltage divider when a smaller value of
input is required. I would suggest using a 510 Ω resistor in series with a 51 Ω resistor as
your attenuator. The largest resistor will connect to the output of the generator and the
smaller resistor will connect to circuit ground. Take your input from the junction of the
two resistors. This will give you an attenuation of 11 and maintain output impedance
near the 50 Ω level.
1.3
Analog Oscilloscope-Hitachi
Caution: Do not use the Tektronix probe on the Hitachi scope or vice-versa! The probes only
work correctly when used on the scope for which they were designed. Generally, the probe
switch on the Hitachi scope should be set to 10X.
The Hitachi analog scope has at least two advantages over the Tektronix digital scopes: (1)
there is no sampling noise on the analog scope; and (2) the vertical sensitivity allows the analog
scope to accurately measure smaller signals than can the digital scope. The vertical scale has a
minimum value of 2 mV/ division. The vertical scale for the digital scope has a minimum value
of 100 mV/ division.
1.4
Digital Oscilloscope-Tektronix
Two advantages of the digital scope over the analog scope are; (1) the availability of hard copy;
and (2) the autoset feature that automatically sets the correct vertical and horizontal
sensitivities.
A third difference between the scopes is that you have menus for vertical horizontal (sweep),
and triggering functions on the digital scope. The analog scope has knobs or pushbuttons to
select these functions without the use of menus.
1.4.1 Triggering Scope
Triggering a scope refers to a method used to stabilize the waveform displayed on the cathode
ray tube (CRT). If the signal is not triggered properly, the team waveform will appear to drift or
multiple waveforms will appear. When triggered properly, a repetitive waveform will appear
stationary on the screen. It is generally necessary to trigger the signal to take accurate
measurements.
With a repetitive signal the trigger circuitry ensures that each sweep across the CRT starts at the
exact same voltage level. This level can be set to be a negative, zero, or positive voltage, but
once set it will only allow a sweep to begin when the signal reaches that level. It is also possible
to set the required slope of the input signal in addition to the level, but for most of our work,
simply setting a level is sufficient. Using the autotrigger function, the level is set by the scope.
Both scopes allow this level to be changed, if necessary.
The trigger circuitry can use several different sources to initiate the trigger. The most common
sources are channels 1 or 2. If channel 1 is selected on the trigger menu, the trigger will be
referenced internally to the single of channel 1. It can also be referenced to the signal of channel
2. If one of these is an input signal and one is the output signal, either can be used. In some
cases, one of these signals may be cleaner than the other and that signal should be used to
trigger the scope.
If you have a large amount of noise on your display and it appears to be periodic, you may have
60 Hz noise in the system. To check this, change the triggering source to line. This will cause
the scope to trigger on the power line signal which is 60 Hz. If the noisy signal stabilizes, this
means you have 60 Hz noise problems. Check your ground to ensure that your system has a
ground and the scope has the same ground.
In some cases, you may have a signal such as an amplitude modulated signal that requires
special triggering considerations. If you trigger on the AM signal, it will not be stable. There
are two periods of repetition in the AM waveform: the high frequency of the carrier and the
lower frequency modulation. In this situation, you should trigger on the lowest frequency
signal. This can be done by triggering externally on the modulation source or by connecting
channel 2 to the modulation source and triggering on this channel.
1.5
General Considerations
An oscilloscope need only be grounded at one point. In fact, grounding at more than one point
may introduce noise from ground loops and is considered poor practice. If you are using two
probes to measure your circuit performance, connect the ground of only one probe of the circuit
ground. The other probe ground should not be connected.
1.5.1 Probe Compensation
Attenuator probes are used to provide large input impedance to the oscilloscope. Most scopes
have a 1 MΩ input resistance to the vertical amplifier. In addition, there is also a small
capacitance in shunt with this resistance. Often this capacitance is in the 20-50 pF range. The
probe adds a 9 M Ω resistance in series with the input impedance to the vertical amp. The
circuit to be measured is now loaded with a total of 10 MΩ of resistance which leads to
minimal loading in most cases. Unfortunately, a rise time has been added to the probe-scope
circuit as a result of the added resistor. The input capacitance to the vertical amp must charge
through the probe resistor. In order to compensate for the rise time, a probe capacitor is added
in shunt with the probe resistor. The resulting probe-scope circuit is shown below. This circuit
has a frequency-dependent transfer function unless the time constant RpCp is made to equal the
time constant RACA. If these time constants are equal, the attenuation introduced by the probe is
1:10 at all frequencies. Since the value of Ca may vary from one scope to another, the value of
Cp may need to be adjusted as the probe is changed from one vertical input to another. This
capacitor is made adjustable, often with a screwdriver adjustment. The procedure for
adjustment consists of connecting the probe to the calibrator output of the scope and observing
the resulting square wave on the screen. If the waveform has a noticeable rise time or
overshoot, the capacitor must be adjusted until a perfect square wave is obtained. This
adjustment should precede any high-frequency measurement made in the lab. Once the
compensation is done, it needs to be changed until the probe is changed to another vertical amp.