Crystal filters are often used for IF filters in receivers, where the bandwidth of this filter largely determines the selectivity of the receiver. The unique feature of the filter described here is that the bandwidth has been made adjustable.
The configuration is a so-called ladder filter with three crystals of the same frequency. Because the crystals should actually be identical, we recommend that you buy three from the same production batch, which is generally the case of your order/buy them all at the same time.
Varicap diodes are usually specified from Ur= 0.5 V. The measuring result for 0 V is shown nevertheless. With a range in Ur = 0 to 12 V, the bandwidth is adjustable from 2 to 6 kHz, which is suitable for the range of CW/SSB to standard AM.
The ripple of the filter is determined by the input and output impedances Zin and Zout.
With smaller values of Zin and Zout the ripple will increase, but the roll-off will be steeper. A compromise is Zin = Zout = 330 Ω resulting in a ripple of <3 dB. It is expected that the characteristics at other IFs such as 10.7 or 9 MHz will be much the same.