Notes: Maximum i-f bandwidth is about 16 kHz @ -6 dB.
Description of the system: R-390A i-f output > 15 dB attenuator > 455 kHz SoftRock > Delta 44 sound card > P4/2800 MHz/1G Ram > Delta 44 sound card > Hafler DH-110 preamplifier > Hafler P3000 amplifier.
The R-390A is run in the 16 kHz BW position and operated in manual gain control (MGC) mode with the rf gain control set to 8 - 1/2. This level was previously determined to be 'optimum' for this particular receiver. Info on this can be found at an AMfone thread:
Power SDR software was used for the test and set for 16 kHz BW, slow AGC and both standard AM and synchronous AM detectors were tested. On rare instances there was at most 1 dB difference between the two detectors - the vast majority of the time the measurements were identical.
It's possible that the distortion numbers are even better as we're likely at the limit of the test equipment in use.