What (cheap) resistors and capacitors to buy for RF? - Page 1
Forum section: electronics / rf-microwave

author: mio83
author_rank: Regular Contributor
timestamp: May 06, 2020, 08:25:18 pm
content: Hello,recently I built a basic RF sampler (just a voltage devider) to analyse the output of >1Watt signals with my spectrum analyzer. The schematics is just:IN --------|------- OUT|50k ohm|------------- CPL|5k ohm ---- GNDIf the CLP port is connected to a 50ohm load (e.g., a spectrum analyzer), then the 5k ohm resistor can be almost ignored and we get a 1:1000 voltage division, so CPL should carry the INput signal attenuated by -60dbm.It works kind of OK at low frequencies but the attenuation is reduced at higher frequencies (see video at min 14:00)https://youtu.be/_DgPjxWSHIk?t=835I guess this is kind of expected and unavoidable due to real-life capacitance/inductance of resistors, but I would like to ask you suggestions for:what are generally speaking the best (through-hole) type of resistors for RF? Ideally with low inductance and capacitance (at least at below 500MhzSimilar question: what are the best capacitors to be used in RF DIY projects?I would like to avoid SMT components, because I am not so good at soldering.Also, I'd like (of course) low-cost stuff.I'd like to make an order at Mouser or Digikey and buy a number of these components to have them ready for my RF projects when I need.Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: May 06, 2020, 08:45:38 pm
content: You need to see what the parasitic characteristics are of your components. If you have a nanoVNA you can measure the resistors and see where they depart from pure resistance, vs frequency.Film resistors are little inductors because they are built in a helical manner, and they resonate with their distributed capacitance. Carbon resistors are better in spite of their parasitic capacitance.If you make the entire circuit out of identical resistors, you should get wide band performance. In other words, use eleven 5k resistors in series. The load, being low impedance, will destroy that situation. But at least it should improve the performance.If you do that, pay attention to the placement of the parts so as to prevent unwanted coupling. This could be between interconnections (capacitance) or between resistors (mutual inductance).If the input impedance isn't critical, consider parallel trimmer capacitors to equalize the individual drops. Or maybe a single trimmer across the lot.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2020, 01:33:44 am
content: If you want flat response up to 500MHz, the physical construction and layout is as critical (if not more) than the component choice! You need to keep stray inductance and capacitance between components and connections to an absolute minimum, as the tiniest amount of undesirable coupling between components can easily ruin that 60dB you're looking for.As for the component choice, metal film resistors should do fine. Despite the helical cut of metal film resistors, the inductance is still quite small and shouldn't be too much of an issue at 500MHz and below. The best would be SMD resistors but these can be difficult to use without a PCB.An example using metal film resistors:http://roland.cordesses.free.fr/rfcoupler.htmlNote also the use of 3 resistors in series, instead of a single higher value resistor. This reduces overall parasitic capacitance and also moves the coupled port physically away from the main RF path. Especially important if you're going for 60dB coupling! It might actually be easier to build a 40dB coupler and throw additional attenuators in-line as required. The greater the attenuation you want from your coupler, the more difficult it will be to build and have a flat response.

author: E Kafeman
author_rank: Regular Contributor
timestamp: May 15, 2020, 02:23:06 am
content: Result is much as expected, wired resistors high above ground creates coupling traces/antennas which connection improves with frequency. A kind of auto-transformer.Keep resulting RF current loop for each resistor as small as possible and as close as possible above ground is a must.Problem is not related to type of through-hole resistor even if it is very recommended to avoid them.If afraid for small SMD resistors, SMD 0804 are huge resistors. Glue them directly at a PCB while keeping PCB groundlayer at opposite side and use as short and lossless connections as possible => avoid pig tails when connecting ground at BNC ground to PCB-ground. Solder wick do I sometimes use as short groundwire with low losses.Cut PCB to real well fitting size and any ground wire can be designed with minimal length, 1-2 mm. Center pins at BNC connectors should be soldered diretly at PCB.Either use double side PCB and manually cut away all copper except a T-shaped 1 mm wide trace or use CU-tape on backside of a single side PCB to create needed traces.High impedance input to SA will increase measured noise level but it is probably not an important factor in this case.This shows an example of how to keep any wires as short as possible with low loop-sizes:

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 15, 2020, 02:41:22 am
content: There was some very useful info in this recent thread (8 pages but worth the time):https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 15, 2020, 03:00:43 am
content: Quote from: mio83 on May 06, 2020, 08:25:18 pmThe bigger SMD capacitors and resistors like 0805 are very easy to solder.