Matching a monolithic xtal filter

Thanks Tom. As you know rf is not my bag so I'm an interested lurker on these threads, trying to follow the sums..... and where the sums produce something that may not be practical to implement. That was the reason for the question.

A 1:4 transformer could be used as a 1+4 auto of course.

AFAIR from the data sheet, 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.

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

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.

Cheers, Tom

Let's see here...if I didn't miss anything, to begin with, the loss will be quite a bit more than 6dB. The filter load/source is supposed to be 1200 ohms in parallel with 1.8pF. I believe Joerg left off the

Cheers, Tom

That wasn't the point, Tom. You are absolutely correct, the total loss will be a great deal more than 6 dB; my ORIGINAL point said "...right off the crack of the bat..." with resistive matching. If I had carried it through to a logical conclusion it would have been much worse.

No, my point to the person who said that if you lose 6 dB in voltage you have lost 3 dB in power was the ultimate decibel freshman student blunder. I'm sure you will agree that if you lose 6 dB, you lose 6 dB measured in voltage (2:1) or power (4:1).

Jim

-- "If you think you can, or think you can't, you're right." --Henry Ford

I actually had a homework question that asked you to describe why resistive matching was generally undesirable for low-level, RF signals. It almost seemed insulting by then, as this was a senior-level RF class...

Of course I also had a freshman or sophomore calculus question asked you to painfully describe why, if a cop times you traversing a mile in, say, 40 seconds, he knows you've exceeded the speed limit of 60Mph... aiee....

Thomas wrote he had an active mixer in there ;-)

If it's got enough gain like >8dB or so you are usually fine with a passive scheme plus a follower at the output.

The old saga of voltage dBs and power dBs. Or the perpetuum mobile :-)

Once when someone "higher up" brought up power dBs in a seminar back in college I asked him whether power dBs get sick more often than voltage dBs. I got kicked out of the room ...

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.

Yep.

He should at least have a trap for the mixer LO feedthrough, that will be the biggest undesirable in the mix and catches up with most designs. Of course if it's tunable, he has problems.

Yeah, well no person said that. I did say that half-power is 3dB. You have a problem with that?

...

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

Cheers, Tom

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.

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

No, I have no problem with that. What I have a problem with is when you said...

Jim said: > "Except that method loses you 6 dB right off the crack of the bat in a > place where loss adds directly to noise figure."

Fred said: "That would be 3dB in power and who says it has any effect on total NF at this stage of IF processing anyway."

If there is any way of interpreting that NOT to mean that you said a 6 dB voltage loss is a 3 dB power loss, then I will stand corrected.

Jim

Should not be a problem. It's all a matter of supply voltage and how far you have to go in BW reduction ;-)

I've never said that, only responded with a suggestion because someone wrote it might be needed. It's a habit of my trade, if a client absolutely wants BW reduction then the client gets the BW reduction.

Baaackk iiinnnn theeee oolldddd ddaaaaysss wwhhhhheenn IIII wwwwwwwaaaaaaaaasssssss yyoouuuunnnggg, I had the bright idea to add negative resistance to improve the Q of an otherwise passive bandpass circuit, since the coil was limiting the performance due to its Q. An experienced engineer told me not to bother trying for commercial grade equipment that had to operate across a wide temperature range. I never revisited it.

So you've had luck with this technique at RF? Seems more a ham/garage thing to me.

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:

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.