Saturday, May 24, 2014

R Board Results

Each of the channels on the board was briefly investigated.

Channel 0

Channel 0 was selected with no carrier module.  This provides a basic sanity check on the input/output muxes and board noise environment.  The following is a 0-1000MHz sweep of the R board output with no input and channel 0 selected.
The small signals at ~-80dBm in the 300MHz - 400MHz are expected local environmental noise.

Channel 1

Channel 1 had a -30dBm 100MHz square wave input signal with the following results (0-1000MHz sweep).
The signal attenuation is as expected, however, the noise hump was not expected.  The noise is actually a set of 4.7MHz harmonics (shown below centered on 100MHz).  These are independent of the input signal (level and frequency) and present with no input.  This is unique to this channel (more later).

Channel 2

Channel 2 was initially populated as a 150MHz common base amplifier using the following components.

Component
Value
Function
C114
DNI
CC Input Bypass
C103
1nF
CB Input Bypass
C116
DNI
CC RF ground
C126
1nF
CB RF ground
R106
2.2k
Bias upper
R107
2.2k
Bias lower
R105
33
Collector current control
L104
1uH
Output tune
C115
10pF
Output tune
This was anticipated to produce no more than 17dB of gain.  Large signal gain measurements (i.e. -30dBm input) show power gains dropping to -9dB as expected and centered slightly above 150MHz (following figure).

The output of this channel was used with an A-B stack as a one stage SDR receiver on the local weather service narrow band FM channel at 162.45MHz.  The following captures those results - left is channel 0 at 0dB attenuation, right is channel 2 selected. The level difference is 15dBm which aligns nicely with expected results.  

Channel 3

The amplifier used in channel 3 is an AD8352 with a 3.3V supply.  The following is from the datasheet.
Channel 3 gain was evaluated by using a -30dBm source and the 7L12.  Since the source is home made and the 7L12 hasn't been professionally calibrated in a very long time, the overall error is at least 1 dB.  The following figure captures the measured gain to 1800MHz (7L12 limit).

We would expect to see a reduction in level and gain for each of the RF muxes.  The switch datasheet indicates an average insertion loss of 0.65dB across the 0 – 2000MHz range (nominal) and maximum of 0.95dB.  Using 2x worst case (for in and out mux) we would expect to see no more than 2dB of insertion loss due to the muxes.  This would place the measured gain for channel 3 around 18.5 to 17.5dB in the sub 2GHz region.  The measurements are slightly lower than this and have more high frequency roll of than indicated by the datasheet, however, the limitations of the test equipment at this resolution of level measurement come into play above 1GHz.  In short, for this stage of characterization I am content with the initial results.

Which leaves the attenuator issue...  My first thoughts on the spurious noise were directed at an error in the QFN mounting.  I have seen multiple instances where a loose solder pad, that was not visible with a 30x loop or under the device, caused odd behavior.  Generally, these instances all involved the spurious responses changing with changes in signal level and/or frequency.  I'm left with the following options relating to the 4.7MHz harmonics: a) intrinsic to the device, b) ground pad under the device solder joint problems, or c) something in that channel's power supply/regulator.

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