Setting up SDR to get rid of FM broadcast ghosts...again? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: October 13, 2018, 01:42:32 am
content: I have a new computer and so new copy of SDRUno. Been a while since I did this but can vaguely remember messing with the offset of the local oscillator to filter out FM or AM broadcast signals. I have a really good homemade 2m yagi that works for all vhf to uhf, better then some commercial ones. But I was listening to some airport stuff around 375mhz and every signal has just a ghost of FM radio complete with those side carriers that look like bart simpson heads at 1/2 the intensity just abouve and below the FM signal. I'm guessing these are the digital signals one being "HD1" and "HD2"? I don't listen to commercials, I mean broadcast FM, so I don't know. Its really BS that you can't even decode the digital making digital radio fail hard. When it first came out I asked the kid at best buy if they sold digital radio receivers or tuners he tried to sell me satellite radio because it was "digital". Uh no. Fun trick to make a satellite radio work at your house take the lid off an old VCR stick the magnet to it and throw it out your window into the bushes. Gets perfect signal as good as the home antennas they sell. Careful shutting the window on the wire the coax isn't flexible.This is a super annoying problem that I didn't have living a few miles from here with a crappy indoor antenna in the basement apt. Seems everything I found so far just has FM broadcast on it.Why does it do this so high up? Is it the SDR itself? This is an SDRPlay which is the best in the 300$ range and only costs $110 too! So not some shitty dongle with the repurposed Rafael chip has discreet components for the radio stages.I used to be 1 mile from an AM station that wreaked havok on SW but from building a better antenna with a balun it was manageable at night. While there are FM antennas on the next highest hill they are north of the city. I'm west and I know this town is on the hill 10 miles from the city. We have a NEXRAD or TWDR 2 miles down the street so even though it looks flat I know its higher then the surrounding towns as sealevel is 12 miles from hear. We might be line of sight with the big towers but I can't find a map or google over lay. But this seems to do it with many stations.The VHF and UHF band seems useless so far. Would an actual notch filter on the antenna work? Can you make a LC T or Pi filter using hand made toroid's? This would have to made point to point or on etched copper?I did make a LC filter one time out of old CRT parts but it seemed to give you AM and cut out everything above the opposite of what I was expecting.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: October 13, 2018, 03:13:12 am
content: General coverage receivers, like dongles, often compromise on the front end.So, yes some form of front-end filter would be desirable.If it's a specific frequency you're interested in then a bandpass filter.Or if you still want to tune a wide range then a low pass filter (eg a pi network) would help.A pi network for 300 MHz will have much better attenuation at 100 MHz than one made for 120 MHz.And because we're talking VHF frequencies our values of capacitance and inductance are small.That means you wouldn't use a toroid for the inductor. Options include traces on a printed circuit board appropriately designed or (easier for experimenting) air wound coils comprising a few turns of wire.Small disc ceramics should be OK for the capacitors, with their value depending on the cut off frequency and impedance you want your filter for.

author: Beamin
author_rank: Super Contributor
timestamp: October 13, 2018, 02:54:48 pm
content: Quote from: vk3yedotcom on October 13, 2018, 03:13:12 amHow could I make this? On my old computer I had a site that told you how to make coils or even figure out values just by counting turns. They need to make a way to make your book marks portable on SD card or file you can email.So this is my problem I like to just play around and listen I don't really want a particular band or anything specific I just like hearing the signals and trying to identify them I often start at 1 MHz and listen up to 2000mhz over an evening researching them as I go. Any sites come to mind where you can put in values or frequencies? I have magnet wire and PCBs that I can mess with.Perhaps a trip to the pool store to make HCl into ghetto ferric chloride boosted with grocery store H2O2. Masking tape for the traces. Wonder if hot glue would work? Or acrylic paint for masking.Best course of action would be a pi net work that blocks 90-110 MHz? I don't ever remember having this problem when I lived in this town. Frustrating because while its near a city you get ALOT of signals but tune in a and hear FM on all of them. Even the airport when they transmit you hear FM broadcast then when they stop the signal goes back into the noise.When the SDRPlay tunes over 2ghz what are you hearing; it says its range stops but the software goes up higher.

author: radioactive
author_rank: Regular Contributor
timestamp: October 13, 2018, 03:33:35 pm
content: If you want to design your own, you might try Qucs. Look under Tools/Filter Synthesis and design a "band stop" filter. That should get you started. If you just want to buy something that works, there is someone selling FM notch filters with SMA connectors on Ebay (GPIO labs) that are a good design. >66 dB insertion loss @ 97 MHz.