  We have studied the pulse-to-pulse variability in PSR J1820--0427 and its
frequency dependence using high-quality, wide-band observations made from the
upgraded Giant Metrewave Radio Telescope (uGMRT; 300-750 MHz) and the Murchison
Widefield Array ($\sim$170-200 MHz). The low-frequency data reveal a previously
unreported feature in the average profile (at 185 MHz) after accounting for the
effects of temporal broadening arising from multi-path scattering due to the
Interstellar Medium (ISM). We advance a new method for flux density calibration
of beamformed data from the uGMRT and use it to measure the single pulse flux
densities across the uGMRT band. Combined with previously published
measurements, these flux densities are best fit with a power-law spectrum with
a low-frequency turnover. We also use calibrated flux densities to explore the
relationship between pulse-to-pulse variability and the spectral index of
individual pulses. Our analysis reveals a large scatter in the single-pulse
spectral indices and a general tendency for brighter pulses to show a
steepening of the spectral index. We also examine the frequency-dependence of
the pulse-fluence distribution and its relation to the Stochastic Growth
Theory.
