One interesting aspect of the BCD-996 and similar radios is the digital command and status protocol. In digging into the software, I learned that there is a RSSI (Relative Signal Strength Indicator) included, which delivers a numeric value relative to the signal strength of a received signal.
Given that I could command the radio to a fixed frequency on the fly, this got me thinking – could I use this to generate a plot of frequency vs. amplitude, effectively turning the scanner into a “poor man’s spectrum analyzer”. I used LabView to write the program, as it provides a very easy to customize interface. In addition, I could add features like a digital signal strength meter, and bandwidth adjustments (resolution bandwidths based on receive mode – its rough, but better than nothing).
One of the challenges was how I could determine what a displayed value really equated to – could I come up with a “real” signal level based on the RSSI?
The following test setup was implemented:
The computer was configured to select a frequency, and then would step from -35 dBuV to 120 dBuV, and record the RSSI at each step. I was hoping for some form of direct correlation, but one was not noted. I collected data at a number of frequencies from 30 MHz to 900 MHz, to try and map out performance across frequency. I did note some trends up to about 30 dBuV in all bands. Above 30 dBuV, I wonder if the scanner has some AGC behavior (I did NOT use the internal attenuator). Realistically, this is how the scanner would behave when exposed to a true strong signal. There is some definite deviation between frequency bands.
Realistically, about the best I can use this data for is a lookup table to more closely approximate real data, but I cannot deliver “fully calibrated” data with the scanner without a much larger data set. While I would normally expect cable loss to create some of the deviation as frequency increases, the deviation appears to correspond more to power level than it does with frequency.
The figure below shows the correlation between RSSI and dBuV signal strength.