Anyone who
has to reduce the amplitudes of RF signals in a controlled manner
needs an attenuator. Linearly adjustable attenuation networks using
special PIN diodes are available for this, but they require quite
intricate control circuitry. A simpler solution is to use an integrated
attenuator that can be switched in steps. The RF 2420 is an IC built
using gallium-arsenide (GaAs) technology, which works in the frequency
range between 1 MHz and 950 MHz. It can thus be used as an attenuator
for cable television signals, for example. The attenuation can be set
between 0 and 44 dB in 2-dB steps. An insertion loss of 4 dB must also
be taken into account. This base attenuation can be measured in the 0-dB
setting, and it forms the reference point for switchable attenuation
networks that provide 2, 4, 8, 10 and 20 dB of attenuation.
These
are all controlled by a set of 5 TTL inputs. The control signals must
have Low levels below 0.3 V and High levels of at least +2.5 V. The RF
2420 works with a supply voltage between +3 V and +6 V, with a typical
current consumption of 4 mA. A power-down mode, in which the current
consumption drops to 0.8 mA, can be activated by removing power from the
bussed VDD- pins. The sample circuit diagram for the RF 2420 shows
that the only external components that are needed are decoupling
capacitors. The coupling capacitors at the input and output determine
the lower operating frequency limit. The table lists possible capacitor
values. The input and output are matched to 50-ohm operation, but they
can also be used with 75-ohm cables with a small increase in
reflections. The RF 2420 is available in a 16-pin SOP-16 SMD package.
Its data sheet can be found at ww.rfmd.com.
