Because applying DC to these piezoelectric devices causes electromigration, and therefore the filter destruction. Manufacturers warns about applying DC to the filters, but manufacturers have been ignoring this for decades.
A transceiver with this problem can be easily repaired by just replacing all the damaged filters, and to prevent this problem to appear again, a pair of DC blocking capacitors must be inserted at the filter input/output pins. In fact some manufacturers have modified their PCB in latest revisions to add these capacitors. But are these capacitors enough to keep DC out of the filters?
The DC resistance of these filters is very high, greater than 40 megaohms, so for any practical purpose, we can consider them open circuits in DC. Let's analyze the circuit in DC with the blocking capacitor in line:
Because the filter is a DC open circuit, there is no current through C, and therefore, no charge is developed in it. With no charge in C, its voltage between plates is zero, so all DC applied voltage is present at the filter pin. This capacitor does not prevent DC to be applied to the filter.
For a capacitor to work as a decoupling capacitor, it must be charged up to the difference in voltage between stages. and the only way to get a capacitor to be charged is to permit current to flow through it. How can this be accomplished with a IF filter? Simply add a resistor:
In this way, the capacitor can be charged / discharged and DC voltage applied to the filter fill be zero ( obviously except for transients ).
The filter before modification (left) and after modification (right)
What is the appropriate value for the resistor? Well, these filters have impedance around 1500 - 2000 ohms, so something 10 to 20 times these values will do: in practice anything between 20 and 50 kohms will be fine.
Manufacturers are using 10nF capacitors for 455kHz filters, and some modifications are using 100 nF ones, so 10-100nF is ok, even for 455 kHz.
Replacing damaged filters is ok, adding DC blocking capacitors is better, but add DC capacitors and resistors to close the DC circuit is just perfect.
Miguel A. Vallejo, EA4EOZ