Easy way to create hairpin filters? - Page 1
Forum section: electronics / rf-microwave

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 06, 2016, 03:37:16 pm
content: Ive been working on a 0-4ghz transverter for RTL sticks for quite a while. Ive finally got most of the harmonics and spurious noise the dongle creates cleaned up whith a few tricks I plan to release later a long with results. Part of this project is also adding a pre selection filter. I think using a bunch of home etched hairpin filters may be a fairly easy route to go. Is there any software out there that work about as simple as elsie but designs hairpin/lumped element pcb based filters by entering a few parameters? Something thats not 1000 of dollar either

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: June 06, 2016, 03:44:34 pm
content: I would be interested in that mods, to get rid of the harmonics and birdies out of the dongle.I've seen some mods, of replacing the crystal by a better one and remove some other components of the board.I think there was something from Anasoft for that or HP, for designing, not sure if they are pay.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 06, 2016, 04:37:46 pm
content: well i plan on sharing everything in a blog. Basically im working on a 2 though 4ghz transciever based around the dongle as the heart. Right now i plan to remote control it via wifi and pi from a remote location eventually id like to turn it in to a full on tranciver running gnu radio with a 7 inch screen.As far as noise mods im not done. I still need to add the new power supply, the txco with lp filter and sheilding. When i get time ill post a picture of what ive done with a spectrum scan from 27mhz to 1700mhz. Ive managed to knock out over half the spurs without even filtering the LO yet

author: KJDS
author_rank: Super Contributor
timestamp: June 06, 2016, 05:01:46 pm
content: The filter wizard in AWR suite will do what you need, to a reasonable first guess approximation. It's also not $1,000s, but that is because, added in with the rest of the suite it's $10,000sAlternatively you could spend a few months calculating them from first principles, checking them with Sonnet lite and going from there, but it will be hard work.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: June 06, 2016, 05:05:54 pm
content: That looks good.Something's that I've seem include, replacing the 1.2V supply of the RTL2832U, for an low noise one with LT3042, replace the 3.3V supply from the USB for the dongle with 2xLT3042, removing the 28.800 crystal by an better one with decoupling filter, adding USB common mode filter, etc.Did your experiments improve the noise figure in the circuit?How is the thermal drift?Is the dynamic range improve?Will wait for your news on your blog.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 06, 2016, 08:12:18 pm
content: Sorry was in the drs office. All I have done in these scans is use a stock dongle vs a dongle which has a heatsink on the A2D it is also soldered to a sheilded USB cable with the shielding disconnected from the dongle ground. There are 4 clip on ferrites down the cable. Everything else is stock. The antenna ports have terminations, RTL gain is set at 49.6/MAX. Obviously top is the semi tamed dongle, which still contains all the harmonics.I am currently switching everything over to a TI tps70445 dual regulator 3.3v/1.2v. These are quiete and meant for low noise RF applications. once the dongle is running off of the regulator I will hook up my TXCO pcb, This will have a 29mhz Low Pass to elminate harmonics as best as possible, I will also lower the current from oscillator to rtl chip to as little as possible to help knock down the harmonics even more.The best thing you can do for these dongles is, do all the known tips then add two nice LNAs in front with adjustable attenuation. The less you rely on the RTL sticks gain the better off you are!

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: June 06, 2016, 09:01:03 pm
content: Still many harmonics present, lets see how one can eliminate that.Ok on the heatsink on the A/D and also good on the shielding.On ebay you can get very good 28.800 crystals with 0.5ppm.For LNA maybe you can use the MGA30889, in cascade, 2xMGA30889.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 07, 2016, 12:07:05 am
content: No im not sheilded yet, I have a $20 through hole TCXO that is all set up on its own board i just need to attenuate its current and add the 29mhz lp filter. I havent set the TCXO up yet becuase im still dealing with the power supply which will power it and the board. I also want to experment with filtering the USB clock signal. -The noise floor of the stick stinks to begin with you get about 48db of dynamic range, so with no aliasing or harmonics you still have a noisy receiver due to the 8 bit d to a. This is why I will use an LNA4ALL right in front of the RTL stick, it helps bring the weak signals past the RTL's noise floor, then I will be using an LNA at the base of every antenna, there are homemade from mini circuits parts, done ugly style.I have a metal box just right I got from spark fun, the USB cable will run inside of it, the USB sheild will get connected to the box through a cap and ferrite bead. The box will have three walls which will be sealed and plugged tight from the others. The first section will provide some chokes around the USB power cables along with common mode rejection stuff, it will power the "Uber Hi End Low Noise" LDO, The next compartment will contain the TCXO board along with the proper LO low pass filtering, and finally the last compartment will have the RTL running from new power with a stable clock, and will have a single hole letting an MCX or SMA connector protrude.This system will keep the LNA sheilded from the oscillator and the radio, making sure the radio is in an anmeic chamber the only thing that has the ability to carry noise to the reciver besides the antenna jack will be the USB data lines (I have some ideas to fix this).So basically this will be just the reciver module alone there will also be a Transverter 0-4ghz, ans a preselctor block. Finally there will be the rasberry pi either set up to bean rtl tcp through wifi off the top of a mountain, or maybe just running GNU Radio itself so it can operate as a stand alone radio. Once thats completed it will be transmitter time!

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: June 07, 2016, 06:49:21 am
content: Ok I just etched this hairpin tonight, its for 1-1.2ghz use. My ham it up 1.3 arrived too, it has the broadband noise source included on it, Im hoping I will be able to test the filter that way. In the mean time can anyone tell me if I did to awfull of a job etching? The copper is not as nice as I would have liked, what if I tin it with solder will that throw off the filter?1worse than the tracks is that I did it on Radio Shack Ckad which is 63mm as required by filter but im not sure if its fr4 and if its not what it is