EMRFD Message Archive 194

Message Date From Subject
194 2006-11-08 21:02:38 Gary terminating impedance
Hi all,
I have some surplus crystal filters which I would like to use in a SSB
tx/rx. I would like to determine there terminating impedance. They are
still in circuit with the support ic's but they only have house
numbers marked on them. I do have the schematic diagrams of the
modules. Does any one have any idea's

73's Gary VK2KYP
195 2006-11-09 06:43:58 Harold E. Johnson terminating impedance
Best way to do that is to remove one from the board. Terminate the filter (Both ends) with a pot of perhaps 5 K and starting from something like 200 Ohms, sweep the filter passband looking for minimum ripple. Don't mind the IL, it will be high. Increase the pot values until the ripple goes away and measure the resistance. Then wind a couple of autotransformers for the measured values to 50 Ohms. ie, if the value was 1500 Ohms, make a pair of windings on ferrite cores of 11 turns tapped at 2. 11/2 = 5.5, squared = 30.25, times 50 Ohms = 1512 Ohms. Use these to match your filter and parallel the transformer output with a pair of small trimmer caps to trim out the reactance. Perhaps 100 pF for starters, and trim them for minimum ripple. You'll wind up with a filter with minimum ripple and IL, and matched to 50 Ohms.
Any other impedance is realizeable just recalculate your transformation.
196 2006-11-09 15:51:00 Gary terminating impedance