OPTIONAL BANDPASS FILTER
NOTE: We do
not
recommend using the filter unless it is required by the application. Any tuned circuit will
produce phase shifts that vary with temperature, and this creates an additional source of instability.
Also note that the earlier version of this document had some erroneous information about component values
and toroid winding. That has been partially corrected in the assembly instructions, but the frequency
response plot below is from the original filter testing and does not reflect the slightly changed values
recommended now. And, the schematic has
not
been updated to show the changed values.
The optional bandpass filter (“BPF”) is laid out on the circuit board as a 2
nd
order series-input filter. The
component values suggested in the schematic (and shown in the assembly instructions below) implement a
Chebycheff filter with a center frequency of 10 MHz, though other designs can be used. Its rejection is
greater than 30 dB at 5MHz (one-half the fundamental), and is 35 dB or more at 20 MHz (the second
harmonic) and above. See the frequency response plot below for more performance details.
This plot is based on a 270 ohm value at R9, which terminates the filter. The value of this resistor has an
impact on both the output level and the frequency response of the filter. Using a 50 ohm resistor will result in
as much as 3 dB better stopband attenuation, but increases the insertion loss by almost 6 dB.
The assembly instructions include component values for center frequencies of both 5 MHz and 10 MHz.
