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Re: In Line Resistive Terminators for Z Measurement? ?BITX Crystal Filter

 

Thanks again, sir.? These filters are curious beasties.? I'm having too much fun playing with them.

DISHAL predicted a 100¦¸ impedance for my second build (100pF caps).? I ran the numbers for an LC match and checked it.? The first image is with the 4:1 balun, the 2nd is with no match (50¦¸ cables in and out) and the third with the LC match.? Still a bit wide at about 3.4KHz but still...


^ with 4:1 balun (assuming 200¦¸)


^ same filter with no match - 50¦¸ in and out


^ same filter with DISHAL-predicted/calculated 100¦¸ LC network in and out.

I found a nugget in the RSGB 11th Handbook.? It recommends placing the higher frequency crystals in the center of the filter and those more on-frequency at the ends.? I'll build filter number three tomorrow.?

The Sweeperino.? Good heavens, man - I'm sweeping just about everything I can get my hands on.? If I could get something to stick to her tail I'd probably have scanned the cat by now.? LOL

Thanks for the radios and tools - too much fun!
Andy, KG5RKP


On Sat, Dec 16, 2017 at 08:12 pm, Ashhar Farhan wrote:
andy,?
this a terribly confusing topic. let me attempt it as best as i can from the phone.
a filter can be driven and terminated with any impedance without the rf police turning up at your doorstep.
the question, really, is what do you expect the filter to do. at every termination, it will show a different ripple and passband. here is a quick summary :
higher the value if caps, smaller the bandwidth. lower the impedance, lower the ripple. low ripple also means, worse skirt.
search out w7zoi paper on ladder filters.
- f
?

On 17 Dec 2017 8:51 am, "AndyH" <ahecker@...> wrote:
Greetings RF Gurus - a noob has ascended to bother the masters. ;)

Would someone please point me to a schematic or other resource that would help me determine the correct resistor values so that I may construct a pair of resistive terminators?? I'm trying to figure out how to measure the impedance of the two crystal filters I've constructed.? VK2SJA has an excellent blog post, but I'm missing a vital piece of tribal knowledge, apparently.? (Ok, I'm missing a number of pieces of tribal and non tribal knowledge.? hi hi)? He used a pair of terminators with pots to determine his termination needs.


I built a pair of 8 crystal filters for a ?BITX, and assuming the 4:1 match, wound a pair of center-tapped toroidal transformers.? My new Sweeperino (a magical device that makes the invisible visible) showed me that my impedance matching guess was incorrect....like 10 dB of ripple's worth of incorrect.? :D

Iterating DISHAL suggests the filters are 75 ¦¸.? I'd like to measure them to know for sure.

Thanks all
Andy, KG5RKP




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