I have a 75 ohm splitter/combiner (can be used in either direction) that I'd like to modify the allowed bandpass range on two of the filter or tank circuits outputs.
I've attached a PDF scan of a schematic I hand drew.
The output labeled as 470 to 900 MHz works fine and has an actual pass range of 440 to 1000+ MHz which is perfect for my needs.
The other two are the problem.
One is supposed to pass 174 to 230 MHz but using an analyzer, it is passing 162 to 250 MHz. I'd like to lower the passband on this output for scanner use and have it pass from 140 to 230 MHz +/- a few MHz either direction.
The low band VHF output is supposed to pass 47 to 90 MHz. My analyzer shows it is only passing 83 to 115 MHz.
I'd like to also lower the output range of this section to better allow reception of the typical VHF low band range of 25 to 60 MHz +/- a few MHz.
I did not read the capacitor values as that would require opening up circuits for valid readings. I did count the turns on each coil though as well as measure the coil ID diameter and the wire gauge used in inches for each coil. All the coils are of the open air type with no coil forms or tuning slugs.
I have tried spreading the coil turns open some on the two circuits I'd like to alter the passband on but I could not obtain any difference in the passband so I suspect one or more of the caps are dictating the passband.
Can anyone understand this and offer any advice how to achieve what I'm after?
I don't need specific values but rather info such as I need a coil with more or less turns at at L8 for example. Or I need a higher or lower value capacitor at C11 for example.
Or - maybe a link to a page that helps one design L/C passband circuits may help.
I know not having the current capacitor values may make this harder but I would think there is a common law that says a higher capacitance here or a coil with more turns there would lower or raise the pass band frequency. That info is what I seek!
My intentions for this thing is to hookup a scanner that just monitors our states VHF low band using the low band port. The low band bandpass filter would eliminate most of the paging tower crud as that is in VHF High and would be outside the passband that the scanner is attached too.
Then I could have scanners hooked to the mid band output but those scanners would need extra notch filters to reduce the paging transmitters power levels. I already have those notch filters.
Then the UHF output would be used on digital scanners like those made by GRE that have very crappy internal filtering and are often overloaded by nearby strong out of band signals. I have a county police transmitter on 155.565 that is only a block or two away. When that transmitter is running, its signal wipes out the GRE (and radioshack clones) front ends when they are tuned to the 800 MHz band. Having a working bandpass filter would help reduce the 150 MHz signals on the 440 to 1000 MHz output.
Of course I'd be using a wideband antenna on the input that is capable of receiving the entire range. I just want to use bandpass filtering before the scanner or whatever radio I'm using so I can eliminate front end overload or desense from out of band signals.
But... the splitter/combiner I have now does not really pass the frequency ranges I need. It was setup for OTA TV station use and the only output that will work as is, is the one marked as 470 to 900 which really allows 440 to 1000+ MHz through before it starts attenuating the out of band signals. Thanks to crappy Taiwan components, that bandpass filter section actually fell within the range I need and worked out well for typical scanner use!
I wish the other two sections would have worked out that way but I was not that lucky. The low band (47 to 90 MHz) section does not even pass the range it was intended to pass.
All three bandpass sections are wide enough to cover the bands I need, they are just passing too high of frequencies so they attenuate my desired frequency ranges.
Anyway, if anyone could suggest what components to try changing that would alter the passband to a lower frequency range, I'd really appreciate it.
The three items marked JMPx on the drawing are simple soldered jumpers on the board. I kept them in my drawing as they helped me make the drawing by using those as a reference points when I'd need to take my eyes off the board for a bit for eye rest!
I'd also imagine the four caps mounted right at each input or output are standard DC blocking caps and have no effect on the bandpass of each circuit. I think each circuit also has a blocking cap between each bandpass section that should not need to be changed in order to alter the bandpass range of the mid and low range sections.
Let me know if I need to try and determine some of the cap values in the two sections I'd like to alter and I can disconnect some things so I can use my cap sniffer and determine the true value of some of the caps within a section.
I thought about posting this in the ham radio forum as a lot of those guys used to build there own bandpass filters and would likely know what needs to be changed to raise or lower the passband of a given circuit but I'd imagine those guys that still mess with this stuff also read the electronics forum here as well.
EDIT: Sorry for the cruddy hand drawn shematic, I never was much of an artist but hopefully it is clear enough and can be understood!