Re: Matching a monolithic xtal filter

Phil Hobbs wrote:

Joerg wrote:

Tom Bruhns wrote:

On Sep 11, 1:51 pm, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:

Tom Bruhns wrote:

On Sep 11, 11:15 am, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:


Tom Bruhns wrote:


...


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.


Ouch! Not around my designs, thank you. :-(


[For the uninitiated: just stay away from them.]


What made you gun-shy here? Got hurt by them? Nowadays you can create
nice gain controlled amps and in most of my cases this is under full
computer-control. That was way different when I started as a teenage
hobbyist where the price tag of an Apple II would make you cringe. Now
you can buy a uC for a buck.

--
Regards, Joerg

http://www.analogconsultants.com



Prove to me you can add one and maintain +55dBm IIP3 and I might think
about it. Well, heck, prove to me that you even NEED it in front of a
GOOD crystal filter, too.

Cheers,
Tom


Just as an example which you can still occasionally buy but someone would almost have to die first because they tend not to part with it, scroll down to the Drake 2B:

http://home.earthlink.net/~kf6gk/

One of the finest receivers ever made, have used it myself. For something like $40 extra you could upgrade to a Q-multiplier inside the speaker cabinet (early 60's pricing, gets me drooling). This puppy has a dynamic range from here to the Klondike.


Besides the nonlinearity problem, Q-multipliers aren't the quietest things on the planet either. That isn't usually a big problem at HF, but (repeat after me) Negative Resistors Don't Have Imaginary Noise. ;)


Sure. However, Q-multipliers are usually placed after there has already been some gain. Also, often noise is less important than selectivity. It's like pulling a trailer up a hill where your car won't run very quiet and not the cleanest but you've got to get up there ;-)

--
Regards, Joerg

http://www.analogconsultants.com
.

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