The Agilent HP 8556A LF Section offers a frequency range of 20Hz to 300kHz. The input may be either balanced or unbalanced and measurement units may be dBv, dBm or Volts. The 8556A  is compatible with impedances normally encountered at audio frequencies. The HP 8556A  a plug-in for the 141T Spectrum Analyzer and is generally used with the Agilent HP 8552B IF Section.