Andy,
250 kHz is the magic number. Sweeps that start below this cannot use the preamplifier, which raises the noise floor.
The TG sweeps now use two sweeps to build the final result. One covers the low loss region (approx. 0 to -40 dB for a passive device, or -20 to 20 for an active device). The other covers the high loss region (approx. -40 to -80 dB or -20 to -60 dB for an active device).
When the start frequency is below 250 kHz, the software selects the active device range since it cannot use the preamplifier.
