Hi all,
I am designing a dual IF receiver and studying the Circuit From The Lab 0140 which is indeed very interesting for me. Maybe someone could explain how the two matching networks between Mixer/SAW filter/VGA have been calculated as I cannot get such values with the Analog Devices RF impedance matching calculator nor with mine.
The network between Mixer/SAW filter is a high-pass (right?) but according to my calculations those values are beyond the freq of interest (which is 153.6MHz).
Could someone shed some light on that?
Cheers,
Michele
Let me speak to each matching circuit (or filter) separately to avoid confusion.
The circuit between the ADL5356 (mixer) and the AD8376 (VGA) aims to provide impedance match from the mixer to the SAW filter to the VGA. The ADL5336 has an output impedance of 200 Ohms and needs to be matched to the SAW filter that has a natural matched impedance of 100 Ohms (the impedance is actually complex, but for the sake of simplicity, a simple real value of 100 Ohms is provided) . This is done by transforming down from 200 Ohms to 100 Ohms through a series C and shunt L configuration (18 pF and 72 nH, respectively). The output of the SAW looks to step up from 100 Ohms to 150 Ohms through a similar (yet reversed) shunt L and series C configuration (58 nH and 18 pF, respectively). The suppliers of the SAW filter generally give recommended matching networks that can be tuned to account for parasitics.
The circuit between the AD8376 (VGA) and the AD9258 (ADC) looks to provide 1) the optimal 150 Ohm load impedance for the VGA and 2) anti-aliasing filtering (AAF) before the ADC input. Let me break up the purpose of each stage for clarity. The 309 Ohm at the VGA output and the two 165 Ohm resistors at the ADC input combine to provide a 150 Ohm load for the VGA. The shunt 1 uH inductors provide dc biasing for the VGA output stage and the 470 pF capacitors provide the necessary ac-coupling (and neither significantly affect the match or the filtering). A fourth order low pass filter - the two stages of series Ls and shunt Cs (330 nH, 3.3 pF, 330 nH, and 20 pF) - is used to start to shape the AAF. The band pass shape of the AAF is determined by the tank circuit - the shunt 56 nH and the last C stage of the low pass filter, i.e. the 20 pF shunt capacitor. The process to design an AAF like this one is described in detail in the application note below:
AN-1098: Methodology for Narrow-Band Interface Design Between High Performance Differential Driver Amplifiers and ADCs
http://www.analog.com/static/imported-files/application_notes/AN-1098.pdf
The sort of matching done between the mixer, saw filter, and VGA and the VGA’s AAF design are easier when aided by a software package that models and accounts for parasitics. A simple Smith chart program or filter design tool may be helpful for a first pass, but fine tuning is most easily done in Agilent’s ADS or something similar to minimize empirical adjustment cycle time.