The AD6600 has two identical input attenuators, Channel A
and Channel B. These dual inputs are typically used as diversity
channels but may also process two independent IF signals. For
maximum oversampling the device is used in single channel mode;
in this case only one input channel is required. The attenuator
steps are 0 dB, –12 dB and –24 dB. The attenuator settings are
based on the decisions of the RSSI stage (see Peak Detector/
RSSI section). The outputs of the attenuators connect to an
analog multiplexer that selects either Channel A or B for subsequent
processing (see Input Mode). The selected signal drives
a dual-gain amplifier set to either 12 dB or 18 dB; the selected
gain is also determined by the RSSI stage. Therefore, based on
all possible combinations of attenuation and gain, the input
signal receives –12 dB to +18 dB of voltage gain in 6 dB steps
(Table I). Overall gain-matching is typically within 0.1 dB. With
a bandwidth of 1 GHz, the phase delay through the front-end
ranges from 0.2 degrees to 0.5 degrees, depending on input
frequency. Additionally, the input impedance does not change
with attenuator settings so there is no AM-to-PM distortion.