EMRFD Message Archive 7736

Message Date From Subject
7736 2012-09-03 23:33:59 Ashhar Farhan A few questions on filters from chap 3
1. How are the series tuned bandpass filters designed? Fig 3.23 shows
a topology. Martein has used them in his front end too. Is it correct
to assume that the loaded Q of inductors will be lower in this
2. When experimenting with crystal filters, how does one quickly try
out different terminating impedances? I am trying with series
resistances. In which case, I get losses that are due to the
resistors. Do I do a 'ref check' by just shorting the two resistors
together and measure the loss?
Thanks, Farhan

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7737 2012-09-04 06:33:08 William Carver Re: A few questions on filters from chap 3
Hi Ashhar,

Using resistors is perfectly good way to test filters. Be sure to
include intput/output resistance of generator/detector in your
evaluation. IE, maybe add 50 ohms.

Some commercial filters (particularly old ones designed for tube rigs)
may be designed to have a terminating capacitance. 7-35 pF trimmer to
optimize shape?

Yes, just short resistors to get 0 dB reference.

Lowpass can be designed to start with a series L and shunt C's ("T"
shape), or shunt C's then series L ("pi" shape). To turn a lowpass into
a bandpass you series resonate the series parts and parallel resonate
the shunt elements at the filter center frequency.

Part values are different for the two topologies, and they act
differently out of passband (series input become open circuits, shunt
input become short circuits), but passbands are equivalent.

7738 2012-09-04 10:25:44 Ashhar Farhan Re: A few questions on filters from chap 3
thanks bill,

i was just confirming about the crystal ladder filters. but on the topic of
series tuned filters, how does one apply the coupling constants to a series
tuned filter? if i am shooting for a butterworth filter using top coupled
topology, i control the coupling with the inter-resonator capacitance. what
is the 'handle' for series tuned filters? my guess is that it would be the
mesh capacitors (going to the ground), but how does it affect the coupling?

- farhan