Setting Meter Output Impedance; Filter Testing - Ramsey Electronics SG560 User Manual

DETAILED INFORMATION

Setting Meter Output Impedance

The default output impedance of the SG560 is 50 ohms, but it is possible to change this set-
ting through the use of internal jumpers. The alternate settings are 0 ohms, and 25 ohms out-
put impedance.
A 0 ohm impedance simply means that the output of the SG560 would be coming directly
from a powerful opamp inside the SG560. With the unit set for 0 ohms output impedance
there are no protection resistors between the jacks and the opamp. The opamp is robust but
cannot handle heavy loads that may pull more than 2.5W without the potential for permanent
damage. A direct short will cause the part to shut down. It can recover from this, but a high
voltage present on the output pins may damage it permanently. The part is expensive and
not easy to replace as it is surface mount with a soldered thermal tab underneath. Therefore
extreme caution should be used when setting the SG560 up for 0 ohms output impedance.
Also, with a 0 ohm output impedance no resistors are in series the opamp and it is possible
for the opamp to become unstable (meaning self-oscillate) with a capacitive load of more
than 50pF across the leads. This can lead to unusually heavy current draws which can over-
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heat the output part over a period of time. An overheated part will simply shut down, then
turn back on once it has cooled.
Jumpers J8 and J7 bypass the large 24.9 ohm surface mount resistors. By adding jumpers
across these headers, you effectively short out the resistors. One jumper will give you an out-
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put impedance of 25 ohms, and two will be 0 ohms. The default is to have no jumpers and a
50 ohm output impedance.
By running in 0 ohm configuration, you are able to have confidence in that the front panel
setting of the SG560 is the same right at the jacks no matter what the load is. Normally with a
known output impedance from a signal generator that impedance must be taken into account
as a series resistor in a circuit under test. If the circuit has a 50 ohm input impedance the out-
put of the SG560 divided in half. For example, with the SG560 output impedance on the 50
ohm setting and the output set to 1V peak to peak, when testing a circuit with a 50 ohm load
the effective output of the SG560 is 0.5V peak to peak.
This is found using ohm's law of V = I x R and the behavior of resistors in series.

Filter Testing

It is interesting to note how a 0 ohm output generator can interact with a complex load such
as a passive filter. When a filter is designed, it typically has a known input and output imped-
ance. With the 0 ohm capability of the SG560 it is possible to put the resistor value of the in-
put impedance in series with the circuit under test to get a good representation of how that
circuit would work in larger design. We are no longer limited by the impedance of the gen-
erator.
To illustrate this point, the circuit below represents a filter with a 330 ohm input and output
impedance being driven by the SG560 with a 0 ohm output impedance. The top circuit in-
cludes a 330 ohm series resistor to match impedances properly. The lower circuit does not
include this resistor (0 ohms).
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