More IF Filters Than You Can Poke A Stick At

They say a picture is worth 1000 words, well here are 4 pictures so that has to worth a novel or 2. Anyway no opus this post, just the meat of it. I built 2 xtals filters, in each case i grabbed 8 random xtals out of the parts bins, no matching. The xtals are ebay specials 100 for $1 and are probably rubbish that did not meet specification and was taken out of the rubbish bins at the factory. So the highest quality man cannot buy in other words.

The difference between the 2 filters comes in the matching, filter one is matched with the traditional ferrite inductor, though i used binoculars as i prefer winding those for transformers over toriods and the 2nd filter was matched using low pass filters. The filter plots speak for themselves, the low pass filter matched has a better shape but -15db less attenuation in the stop band than the ferrite matched filter. And the losses were about the same -7db. -3db is 2200hz. So with some matching the crystals i should be able to improve the flatness in the passband and probably remove some losses.

But other than that, if you can live with -50db of opposite sideband suppression, the grabbing random xtals out of the box and matching with low pass filters might actually be a rather simple and crude way of IF filtering. For now, i think i will stick to winding the transformers, or buying 4:1 transformers off Mouser.

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SMD Is The New Black

I have a need, a need for speed. Well, not really, but I have a need for filter components that are small. I am building a hand held after all for 40 and 20m and you just cannot fit large toriods into a tiny box you can carry around very easy. Well, you might get them in there with some careful design, but the mechanical durability of thin wire being bounced about is going to be suspect at best.

And what if i want to sell 100 of these things? Who wants to sit there winding a bazillion toriods? Not me, that is for certain, i do not even want to wind 2 for a prototype. HEHE. Anyway, that got me to thinking, and my thinking i mean i really did not think a lot at all. I got on the confuser and did some jazz hands on my parts suppliers website and poked my peepers at what they stocked to see if there was an off the shelf solution to my problem.

So they have RF inductors, but the problem with these is the low Q. And we all know that Q stands for Quicksand and with a low Q your signal starts to sink into the mud and you might not hear the ducks anymore. Ok, so the question is HOW LOW CAN YOU GO. So i looked and I looked and in the end i found a 1.5uH inductor that had a Q of about 25. TWENTY FIVE you might rightfully exclaim and yes you would be right, that is a bloody low number. But such tiny inductors are always doing to have a low Q much like many hams have a low brow and low I and Q when they down convert their thoughts to base band.

So how bad is it. Well not as bad as you might think. -6db loss is -5.5db loss more than the same filter i built using hand wound T50-2 toriods, which on second thought is actually massively crappy. But hey, i still gotta stick this in a handheld box and have it mechanically secure. Thankfully there is a solution.

Back in the dark old days before time began, the Motorola company or was it Bell Labs, either way, it does not matter all that much as I am just spinning a tale here of those that do, or as I like to call them N Type and those that don’t, the P type and someone added in B J and T into this alphabet soup of inter-related terminology and created the PRE AMPLIFIER.

I need to have some gain in the system anyway so if i put some gain, i don’t know lets say somewhere between 6 and 10db before the filter, i can make up for those filter losses before we have even started. BUT WAIT you might ask, that is going to cost more, and yes sirreee Bob you would be right. But i can take those 2 SMD Trimmers out as they are not going to be needed and replace them with 2 1 cent caps and that just saved me $1 on my BOM cost.

WOO HOO i can do radio design me.

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