The Micro908 reflectometer upgrade project (generally called "REFL") is an improved measurement engine for the Antenna Analyst AA-908. The original reflectometer design in the Micro908 has some limitations that result in measurements that are low-resolution, noise-prone and bandwidth-limited. These measurements could be viewed as being acceptable for a lower-cost HF antenna analyzer, but not for a more professional instrument operating up into the VHF spectrum and beyond.
The performance areas looking to be improved with the REFL design include ...
- frequency-dependent components used in the reflectometer bridge (diodes and capacitors)
- resolution (bit-depth) of the A-to-D converter (8 bits may not be enough);
- DDS reference signal level drop-off (sinx/x, video amp response);
- pc board traces (too long); and
- computation limitations (fixed integer, non-scientific functions).
The REFL board is designed to be easily added to the original Micro908 with minimal modification, enabling the user to immediately experience improved performance over the full 60 MHz range of the instrument once the new software is installed. The REFL card plugs into the existing DDS-60 socket and is soldered directly to the Micro908's BNC RF jack. In turn, the DDS-60 plugs into the REFL card, along with a new cable conntecting several control and data paths from the Micro908 motherboard.
Two design prototypes have been under study since May of this year ... "REFL-1" and "REFL-2".
REFL-1 ... is a "log amp" approach to getting better stability, frequency range and measurement resolution of the Micro908. We kept the the same basic Wheatstone bridge configuration for the measurement engine. An Automatic Level Control (ALC) circuit enables the DDS-60 daughtercard to produce a constant level into the Bridge. An AD8310 logarithmic amplifier is used to detect/measure each component signal Vr, Va and Vz in the Bridge. A broadband video amplifier (LMC6484) is used to boost the log amp output signals in order to maximize the usable range of the Analog-to-Digital converter (ADC) in the next stage. The ADC selected (AD7998) has 12-bit resolution and is controlled by the Micro908's HC908 processor via the 2-wire I2C bus and protocol. The Micro908 processor also controls a powerful math chip (Micromega uM-FPU) via the same I2C bus in order to offload specific advanced and scientific computations from the main processor, as well as to perform anti-log function on the measured data. See all REFL-1 details here.
REFL-2 ... is a "compensated linear" reflectometer redesign approach, a linear version of the original reflectometer design but now with diodes fully matched and compensated using the Grebenkemper method ("Calibrating Diode Detectors", Aug 1990 QEX)). As with the REFL-1 approach, we have the ALC in there to keep the DDS signal level at a contant level, and the math chip is still on the board to facilitate complex computations with the advanced coax features. See all REFL-2 details here.
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Page last updated: Nov 2, 2007