  Characterization of the frequency response of coherent radiometric receivers
is a key element in estimating the flux of astrophysical emissions, since the
measured signal depends on the convolution of the source spectral emission with
the instrument band shape.
  Laboratory Radio Frequency (RF) measurements of the instrument bandpass often
require complex test setups and are subject to a number of systematic effects
driven by thermal issues and impedance matching, particularly if cryogenic
operation is involved.
  In this paper we present an approach to modeling radiometers bandpasses by
integrating simulations and RF measurements of individual components. This
method is based on QUCS (Quasi Universal Circuit Simulator), an open-source
circuit simulator, which gives the flexibility of choosing among the available
devices, implementing new analytical software models or using measured
S-parameters. Therefore an independent estimate of the instrument bandpass is
achieved using standard individual component measurements and validated
analytical simulations.
  In order to automate the process of preparing input data, running simulations
and exporting results we developed the Python package python-qucs and released
it under GNU Public License.
  We discuss, as working cases, bandpass response modeling of the COFE and
Planck Low Frequency Instrument (LFI) radiometers and compare results obtained
with QUCS and with a commercial circuit simulator software. The main purpose of
bandpass modeling in COFE is to optimize component matching, while in LFI they
represent the best estimation of frequency response, since end-to-end
measurements were strongly affected by systematic effects.
