Measured results of RF LNA junk box power supply filter - Page 1
Forum section: electronics / testgear

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2018, 02:22:41 am
content: HiThis is another by-product of learning about my Siglent SSA.I brought a dirt cheap RF preamp on a bare PC. I don’t like bare exposed RF circuits. Aside from all the technical issues, they look as cheap as they are and aren’t user friendly.I found a nice cheap extruded enclosure that is a good size for the hand and the rf amp. I want to use a cheap mains-power pack to supply this thing. I wanted to ensure that I minimised the risk of RFI from either the crappy power pack electronics or pickup from the power lead. I decided to build a scrap bin RFI filter. Before anyone jumps in and says I should use a battery pack, I chose not to in this case.The RF LNA uses the ubiquitous and obsolete INA-02186. The datasheet doesn’t provide a value for PSRR (Power Supply Rejection Ratio) but the reference circuit does include an RFC so at least some power supply filtering is desirable, if not essential.Designing and building a broadband RF power filter is not a trivial process. It requires proper engineering design, analysis and development to optimise the performance. I didn’t bother. I just did a quick trawl through the scrap bin to cobble something together.I designed a 4 stage RFI filter. The aim being to filter different bands of potential interference. It starts with common mode toroid filter with a twisted pair carrying the + and – supply. This blocks external RF pickup coming down the cable. This was followed by a low frequency stage consisting of a single coil toroid (mounted on the underside of the PCB) and a 47uF capacitor. This combination is only good for blocking 50Hz/100Hz hum. The toroid by itself is a signficant inductance that blocks sub-500MHz down to the spec-an noise floor.Then followed a ferrite bead with 2 turns to block the sub-1GHz RF. Two 100nF caps were connected in parallel before a wondrous and seldom mentioned 3 terminal capacitor that provided the final filtering. The device I had was a 1995 vintage RS 209-005 leaded part that provides 20dB attenuation at 1GHz. Murata now make surface mounted versions that perform much better.I could have just connected the RF amp at this stage and hoped it all worked. I am a firm believer in measuring stuff to see how well (or badly) it works. I ran some tests using my shiny new Siglent SSA.For these initial tests I temporarily soldered on SMA connectors to where the power supply and load would normally be connected. The common mode toroid hasn’t been fitted yet. I fitted a 10dB attenuator between the tracking generator output and power supply input.The results are plotted below.The first plot is of the bare filter. As expected, each filter stage has a self-resonant frequency.The 2nd plot is with the filter partially covered by the aluminium extrusion. It wasn’t possible with this setup to fully enclose the PCB. Note the significant change in the plot resulting from the proximity of the enclosure.The last plots 3 and 4 illustrate the effects of placing ferrites at the openings of the enclosure basically to soak up stray RF energy.I have also produced a plot with Excel from a saved CSV file (I can't attach it). This is a really nice feature for a number of reasons. The CSV file saves all of the instrument settings to make it easy to come back and reproduce the exact setup without having to write it all down. Having numerical data in a spreadsheet opens up the possibility of doing sophisticated analysis that can’t be done inside the instrument. For example, I could plot the data sheet frequency response of the Murata 3 terminal capacitor in a spreadsheet, then subtract this from the measured data to find the filtering contribution of the other stages of the filter. A Spec An can't plot data sheets or do really complex maths that can be achieved in MS Excel.So the data shows that the filter works quite well for a junk box dive. Basically it provides at least 30dB attenuation all the way up to 3GHz. This is way past the frequency the LNA works to.The data also shows that the enclosure does and will have a significant effect on the filter performance. Mostly for the good.More to follow as I progress the design/build/test.

author: tautech
author_rank: Super Contributor
timestamp: July 02, 2018, 03:10:10 am
content: Nice.Dazz, some tips if I may.Bottom right button, Marker table will show the values of all active markers on the display and condense the main display vertically some but still large enough for us to see what's happening.CSV files can be attached (within forum SW size limits) by adding another accepted file extension to the filename, eg.****.csv.txt

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2018, 04:58:17 am
content: HiI found the marker table but it does scrunch up the plot.Attached is the csv => xlsx => xlsx.txt file with a simple chart. Just a hint of the potential.This automatically includes all of the markers.Dazz