Monday, February 2, 2015

RF Level Control Board (Prj135)

While waiting for the fab of an updated high(er) speed ADC board I returned to another project in progress. The goal of that effort is to develop a board that provides some amount of level control as well as coarse power measurement.  Out of this came the L board or Prj135.  The idea is to include two stages of amplification with a digital step attenuator in between.  At the final output a power meter samples the output. Control of the board is a SPI like interface using the I board from previous work. The block diagram is shown below.
The component selection was straight forward.  I wanted to stay within a 3.3V supply and had used the AD8353/8354 successfully in the past across a wide bandwidth.  For the digital step attenuator I wanted to try a CMOS based part such as the PE4302 to see if I could avoid some of the problems encountered in the past. For the output power assessment I did not need a large dynamic range so decided to go with an RMS power meter.  The LTC5587 is nice as it includes an integrated ADC.  Since I wanted to be able to use the board at low frequencies I went with a power divider/resistive coupling between the output and the detector. The schematic for the board is shown below.
Level Control Board Schematic.  Control and power via 10 pin header from a Beagle Bone Black interface board.
The last time I worked with a digital step attenuator there were two issues encountered.  First, there was low frequency impulse noise (and here).  Second, at a board level the higher levels of attenuation were not achieved (assumed to be the switches and overall layout and proximity of input and output ports).  The current approach allows plenty of isolation between the RF ports and shielding.  I decided to proceed slowly and first evaluate the attenuator.  Shown below is a picture of a partially assembled board.  Only the attenuator in the center is populated, with the amplifers at left and right unpopulated and jumpered over.

RF Level Control Board. AD8353 amplifiers at left and right.  PE4302 digital attenuator in the middle.  LTC5587 power meter in lower right.  Control and power lower center. The form factor and bolt holes match the BBB.
A couple of quick tests showed most of the attenuation range achievable and a fraction of a dB in insertion loss at sub 1GHz frequencies.  Higher frequencies showed some degradation in both, however, with the jumpers and no shielding I expected this.  The best part was no impulse or other unexpected noise to at least -90dBm (my measurement limit). The next step is to evaluate the RMS power meter and output coupling to it.

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