Digital Step Attenuators
Precision TTL-controlled step attenuators, accurate over 10 to
1000 MHz and -55o to +100o C, are available
from Mini-Circuits in hermetically-sealed TO-8 cases and SMA
connectorized packages. In each package style, five models are
offered, each with three internal discrete attenuators switchable
to provide eight discrete and accurate levels up to 35 dB. Models
can be cascaded to provide increased attenuation because of the
excellent input/output return loss of each model; the final value
of attenuation will equal the sum of the attenuations of the
individual models. The accuracy of these step attenuators is
comparable to precision fixed attenuators. Attenuation values are
flat over the full frequency range and the full - 55o
to +100o C temperature range.
Three TTL lines are provided, one to correspond with each step of
attenuation. To achieve 6-bit attenuation, merely cascade two
models. Each step is switched on or off by a TTL signal, with the
difference in insertion loss between the on and off state being
the step attenuation. The 50 ohm TOAT- and ZFAT-series perform
with 6 
sec switching
speed and can handle power levels up to +10 dBm above a few
hundred MHz, and 0 dBm at 10 MHz.
Modern Definition of Terms
digital attenuator/electronic
step attenuator
an attenuator whose attenuation can be varied by digital control
signals. As digital signals have finite states, a digitally
controlled attenuator has a corresponding number of finite
attenuation states.
thru loss
insertion loss of the step-attenuation in "000" state
of the digital control signal.
primary attenuation step
Mini-Circuits' step attenuators have 3 digital control lines.
When one of the signals is in the "high" state and the
other two are in the "low" state, one step of the
attenuator is activated, which is called a primary attenuation
step.
step accuracy
is the deviation of the actual attenuation step from the
specified value over the specified frequency range.
rise time
time required for the attenuator to change from 10% to 90% of its
final value when one of the steps is switched on. The digital
control signal should have a much lower rise time (approximately
1/10 of the attenuator) to make a good measurement.
delay time
is the time difference between 50% of the TTL signal to 90% of
the final attenuation step value when the attenuator is turned
"ON" or 10% of the step value when it is turned
"OFF."
maximum attenuation
the sum total of the primary attenuation steps. It is the maximum
achievable step attenuation.
Most Often Asked Questions About Digital Step Attenuators
Q. Suppose I don't work at less
than the specified 1dB compression of a step attenuator, what are
the consequences?
A. Should you exceed the specified
compression, typically given at 0dBm input, harmonics will be
generated which may affect system operation. Also, when you
measure the fundamental for attenuation, an error will appear
because some of the fundamental's power has been transferred to
the harmonic products.
Q. What is meant by the
"zero" value of a step attenuator?
A. Step attenuators are referenced to the
minimum insertion loss of the switch. If the switch in
its"zero" position has an insertion loss of 3dB at
10MHz, it might be 3.5dB at 1GHz. All attenuation values are
given relative to this "zero" reference.
Q. Why do Mini-Circuits' ads on
TTL controlled step attenuators emphasize the word
"precision"?
A. Most step attenuators on the market have
a relatively wide variation of attenuation over the specified
frequency band; for example, a 3dB step could be off by as much
as ldB so that attenuation is anywhere from 2 to 4dB. With
Mini-Circuits' step attenuators, the difference would be 0.3dB or
less. The same situation occurs for temperature variation.
Further, when switching internal resistors to increase attenuation from say 3dB to 6dB, the sum may be off by as much as 2dB if return loss within the unit is poor. With Mini-Circuits' precision step attenuators, designed with 24dB return loss, the sum will be within 0.6dB of nominal.
Q. What makes Mini-Circuits' step
attenuators so precise?
A. Along with the 24dB return loss criterion,
extremely short lead lengths, resistors trimmed with high
accuracy, and switches with low parasitics yield precision
performance.
Q. What happens when step
attenuators are cascaded?
A. Attenuation will be arithmetically additive
provided return loss of each unit is extremely good, 20dB or
better. Otherwise, the sum may be other than the total of the
units. Mini-circuits' attenuators can be cascaded without loss of
accuracy.
Q. How fast is attenuation
switched from one value to another?
A. TTL switching from one value to another is
specified by delay time, and is 6 s max. Delay is defined as the time to achieve 90% of
the resulting RF amplitude change.
Q. If I exceed the maximum input
level, will I burn out the step attenuator?
A. It's not recommended, but you can exceed the
input power rating by 10dB without damage. The penalty would be
the presence of distortion products and a decrease in attenuation
accuracy.
Q. What is the consequence of
feeding pulses with excessive peak voltage into a step
attenuator?
A. Excessive voltage peaks will widen the pulse
width.
Q. Please sketch the configuration
of a typical TTL-controlled step attenuator.
A.
Each of the three steps is configured with two
single-pole, double-throw switches and a fixed attenuator; see
figure. Step 1 is shown with B1 and B1' contacting Attn #1, Step
2 with B2 and B2' contacting Attn #2, and Step 3 with B3 and B3'
contacting Attn #3. Thus, all steps are connected to their
individual attenuators and are in cascade to provide maximum
attenuation.
Q. What are the TTL requirements
for proper operation and how much latitude is allowable?
A. For MCL attenuators, TTL"0"
corresponds to 0 to 0.8V and TTL "1" corresponds to 2
to 5V.
Q. Must the TTL input be 50-ohms?
What if it is 75-ohms?
A. TTL interfaces are defined in terms of
voltages and currents, rather than impedance.