Re: Matching a monolithic xtal filter

Tom Bruhns wrote:

On Sep 11, 7:44 am, Tony Williams <to...@xxxxxxxxxxxxxxxxxxx> wrote:

In article <46E69E30.3070...@xxxxxxxxxx>,
Fred Bloggs <nos...@xxxxxxxxxx> wrote:


Tony Williams wrote:

AFAIR from the data ***, that filter is about 75KHz
bandwidth, but they also specify a guaranteed attenuation
out at +/- 1MHz from the centre frequency. So presumably
the source/load impedance has to be somewhere near the
req'd 1200//1.8pF over that range.

It's like all the other crystal parameters, that impedance
derives from the motional parameters near resonance and in most
applications designers use bandpass impedance matching to avoid
exciting spurious responses and non-linear mixing in the crystal.

I take that to mean that the 1200//1.8pF need
only be maintained for not much more than the
75KHz passband. Thanks Fred.

--
Tony Williams.


A bit more than that, actually, Tony. It's good to not hit a crystal
filter with out-of-band signals that are too high in amplitude,
because crystal filters (a) have spurious responses (should be pretty
low if it's a good design, and well implemented), and (b) are not
strictly linear devices and thus will allow mixing of two out-of-band
signals to produce an in-band signal. Crystal filters are generally
pretty low distortion, but people going for the ultimate in receiver
performance end up paying a lot of attention to their crystal
filters. So anyway, it helps to have some filtering in front of the
filter, to avoid those problems, though there's an obvious limit to
how much you can do. At a 45MHz center frequency, without going
really overboard with the LC filter, you probably will end up with a
3dB bandwidth at least a couple MHz wide. To do much better while
keeping the filter loss low requires coils with high Q, which get
physically large.


That's where the old concept of the Q-multiplier comes in. After that it only boils down to how good you are able to control the CF of a resonant circuit up front. But shhht, don't tell anyone. The younger lads out there don't have the foggiest idea what that is.

[For the uninitiated: No, it has nothing to do with Q-Tips or luxury cars of the Infinity brand ...]


On the other hand, in a lot of applications, the crystal filter
distortion and spurious responses are low enough that relatively
broadband coupling is not a problem. You need to look at the whole
system to decide what's appropriate.


Yep.

--
Regards, Joerg

http://www.analogconsultants.com
.

Quantcast