with Cin and Lgrd. Since a series resonant circuit produces a
dead short across its terminals (in an ideal world), this puts a
tremendous load on the probed circuit and greatly reduces
signal amplitude. Oddly enough, since each leg (L or C) has
maximum current and impedance at resonance, the voltage
will soar across these components causing a very high
displayed screen level when in reality, the circuit’s signal
level is at its lowest point.
In my experience, most 10X probes will resonate in the
70 to 110 MHz region and will be somewhat subjective to
the signal source R and C values. Due to those values, the
resonant point can narrow, broaden, and or have a major or
minor effect on signal amplitude. The probe’s reactance can
also add horrendous overshoot and ringing, along with
slower rise times on displayed square waves that otherwise
are not present. Three things happen when a probe is
attached to a test circuit:
1) The circuit’s signal is altered.
2) The signal is divided and distorted at the junction of
Cin and Lgrd.
3) The signal sent to the scope’s vertical input will not
be an exact reproduction of the signal present at the test
circuit’s signal in normal operation.
46 November 2010
The magnitude of these effects are dependant on the
probe, the circuit under test, and the signal’s frequency
content, making measurements quite unpredictable!
Now that we are armed with this new information, let’s
go back and review the ‘problems’ we had earlier with
those prototypes. First, the wide band amplifier is a simple
circuit with a 300 ohm collector load. When operating at
150 MHz, the probe has a loading effect of <100 ohms Xc
across that resistor. This load drops the amplifier output to
30% of what was really there. When operating with a 90
MHz signal, the probe is deep into resonance producing a
much higher displayed signal than expected. Again, not
really there in normal operation. Similar results are falsely
displayed on subsequent stages of that proto.
In the oscillator circuit, the first point probed was in a
critical feedback leg and the probe’s Cin to ground totally
killed oscillations. The second point in that circuit was less
critical and oscillations survived albeit with a large shift in
frequency. The fast rise square waves have been slowed by
the probe’s loading capacitance and ring at the probe’s
resonant frequency. So, it’s bad enough that the displayed
wave forms may not always follow what the probe is seeing
but also, the connected probe is altering the test circuit
operation. Worse yet, the results are unpredictable.
What You Can Do
About the Problems
The first thing is to remove the clip tip to reduce series
inductance, and use the shortest ground lead possible for
the same reason. If possible, try to probe lower impedance
points in the circuit to reduce loading effects. As far as
probe resonances, they are most prominent at source
impedances below 50 ohms, having lesser effects with
source impedances above several hundred ohms. For the
low Z sources, a low ohm value tip resistor can be added to
dampen the resonant point. A 1/4 watt resistor wrapped
around the tip and with the other lead cut short will suffice.
This resistor now becomes the ‘new’ tip. Generally, this
value will be in the 50-100 ohm range. Lastly, I do a
thorough study of the circuit under test to try and
predetermine the results of adding a probe to that circuit.
These suggestions are a rule of thumb rather than an ideal
There are two types of probes that will overcome most
of these problems. One is the low Z passive probe shown in
Figure 2A and the other is the active probe shown in Figure
2B. The first one is cheap and easy for home construction. It
is merely a 1/4 watt carbon resistor soldered to the end of a
50 ohm coaxial transmission line with a mating BNC
connector at the opposite end. The resistor leads should be
cut short and act as the probe tip. The coax ground lead at
that point should also be kept relatively short. These types
of probes are much more immune to ground leads effects
than other probes, but when making very critical
measurements, that shield should be temporarily tack
soldered to the circuit’s ground.
The upside of this probe is that it is “king of the hill” for
high speed probing, and has superior fidelity, very fast rise