diff --git a/examples/test_butter.jl b/examples/test_butter.jl
index 393149b..61e13d0 100644
--- a/examples/test_butter.jl
+++ b/examples/test_butter.jl
@@ -19,12 +19,12 @@ for i in Int(N/2):N
 end
 
 # apply the filter
-buffer = zeros(N)
 results = zeros(N)
+tfilter = DF2TFilter(butter)
 for i in 1:N
-    results[i] = apply_filter(butter, measurements[i], buffer, i)
+    results[i] = apply_filter(tfilter, measurements[i], i)
 end
-@time apply_filter(butter, measurements[N], buffer, N)
+@time apply_filter(tfilter, measurements[N], N)
 
 # Plot the step response
 p = plot((1:N)*dt, [measurements, results]; xlabel="Time (s)", ylabel="Amplitude", 
diff --git a/examples/test_butter2.jl b/examples/test_butter2.jl
deleted file mode 100644
index 5e1ebcf..0000000
--- a/examples/test_butter2.jl
+++ /dev/null
@@ -1,50 +0,0 @@
-using DiscreteFilters, ControlPlots, DSP, ControlSystemsBase, Printf, LaTeXStrings
-include("plotting.jl")
-
-function create_filter2(cut_off_freq; order=4, type=:Butter, dt)
-    if type == :Butter
-        return (Filters.digitalfilter(Filters.Lowpass(cut_off_freq; fs=1/dt), Filters.Butterworth(order)))
-    elseif type == :Cheby1
-        return (Filters.digitalfilter(Filters.Lowpass(cut_off_freq; fs=1/dt), Filters.Chebyshev1(order, 0.01)))
-    end
-end
-function apply_filter2(butterF, measurement, index)
-    results = zeros(1)
-    measurements = ones(1) * measurement
-    @views filt!(results[1:1], butterF, measurements)
-    return results[1]
-end
-
-# Define the cut-off frequency in Hz
-cut_off_freq = 2.0
-
-# Define the sampling and simulation time in seconds
-dt = 0.05
-sim_time = 4.0
-N = Int(sim_time / dt)
-
-# Design the filter
-butter = create_filter2(cut_off_freq; order=4, dt)
-
-# Create an array of measurements (step signal)
-measurements = zeros(N)
-for i in Int(N/2):N
-    measurements[i] = 1.0
-end
-
-# apply the filter
-results = zeros(N)
-tfilter = DSP.Filters.DF2TFilter(butter)
-for i in 1:N
-    results[i] = apply_filter2(tfilter, measurements[i], i)
-end
-@time apply_filter2(tfilter, measurements[N], N)
-
-# Plot the step response
-p = plot((1:N)*dt, [measurements, results]; xlabel="Time (s)", ylabel="Amplitude", 
-         labels=["Input" "Output"], fig="Forth order Butterworth Filter")
-display(p)
-
-# Plot the frequency response
-bo = tf(butter, dt)
-bode_plot(bo; hz=true, from=0.5, to=1.5, title="4th order Butterworth Filter")
\ No newline at end of file
diff --git a/src/DiscreteFilters.jl b/src/DiscreteFilters.jl
index 8be82aa..27459bb 100644
--- a/src/DiscreteFilters.jl
+++ b/src/DiscreteFilters.jl
@@ -65,6 +65,23 @@ function apply_filter(butter, measurement, buffer, index)
     res = filt(butter, buffer[1:index])
     return res[index]
 end
+"""
+    apply_filter(tfilter::DF2TFilter, measurement, index)
+
+Apply the filter to the measurement.
+
+# Arguments
+- `tfilter`: The filter, created with `create_filter` and converted to a DF2TFilter
+- `measurement`: The measurement value
+- `buffer`: The buffer to store the measurements
+- `index`: The index of the measurement
+"""
+function apply_filter(tfilter::DF2TFilter, measurement, index)
+    results = zeros(1)
+    measurements = ones(1) * measurement
+    @views filt!(results[1:1], tfilter, measurements)
+    return results[1]
+end
 
 function apply_delay(measurement, buffer, index; delay=1)
     buffer[index] = measurement