Low power low pass filter?

NOYB · Post by **NOYB** » Sat Oct 24, 2015 11:25 am

I've just checked my Tugicom 1w board on the spectrum analyser and surprised to find it has awful harmonic performance. First harmonic only about -20dBc, and 3rd, 4th and 5th clearly visible. Before you ask - no I'm not over-driving the spectrum analyser (it's connected to 70 dB of attenuation and measuring -40dBm on the fundamental). This is nowhere near their claimed spec of -60dBc.

I know it's only 1w, but I'd like it to be cleaner, so I need a LPF. Any recommendations for something cheap yet effective? I've seen a few around the €30 mark, but they are spec'd for hundreds of watts - I only need it to handle 1w. I suppose I could make a pausey stub, but I'm not sure the notch would be stable enough over mid and long term. I don't mind spending a bit, but can I get something for less than €30 without having to make my own inducters?

teckniqs · Post by **teckniqs** » Sat Oct 24, 2015 12:11 pm

Couldn't you just try adding another basic filter after the output? ...Something like another 4 turn coil with 27 or 33pf down to ground, maybe even another 2 filter coils. Surprising though as these things shouldn't need any additional filtering....

thewisepranker · Post by **thewisepranker** » Sat Oct 24, 2015 2:17 pm

NOYB wrote: Before you ask - no I'm not over-driving the spectrum analys er(it's connected to 70 dB of attenuation and measuring -40dBm on the fundamental).

Are you using an external attenuator or the step attenuator in your analyser? If it is the latter then you might be overdriving the analyser as most can only handle +25 dBm, which is roughly 0.3 Watts. Some can handle +30 dBm, which is 1 Watt, but not all of them.

NOYB · Post by **NOYB** » Sat Oct 24, 2015 3:36 pm

Nope Wisepranker - external 100w 30dB attenuator then a couple of pads. You are right to question me - I know not many people may know the liklihood of over-driving the front-end of an analyser - but I've been there, done that and got the ticket!

Thanks Techniks - I was hoping to avoid that route - I don't have any wire, never done it, no experience of that - lazy I know!

Any more suggestions welcomed!

pjeva · Post by **pjeva** » Sat Oct 24, 2015 9:33 pm

Hold on, you have spectrum analyser but no wire?!
30€ is enough money to buy 1kW LPF. For one watt you need about a meter of enameled wire 0.8mm thick for 4 coils and 8 27pF ceramic capacitors. Total cost maybe 2€...

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Analyser · Post by **Analyser** » Sat Oct 24, 2015 11:07 pm

A simple low pass file can be made with 27pF capacitors down to ground either side of a 5t coil wound round a 1/4" former (trimming tool etc) or just use a 6mm drill bit. If you want more harmonic attenuation then just cascade identical sections (really two should be enough though).

From the results you're getting though, something sounds a bit fishy; the 2nd harmonic being only 20dB down would be as if there were no filtering whatsoever on the Tugi board and I thought they had some. Can you post a couple of pics of the board and your spectrum analyser screen?

NOYB · Post by **NOYB** » Mon Oct 26, 2015 3:34 pm

You are right pjeva and analyser. Although my systems knowledge is good, I have little experience down at component level. I've just got a nagging thought that for someone like me it's going to cost a lot more (because I don't have such components kicking around, and I expect I'll have to buy more than I need, pay postage etc). I'd still rather have a little kit that I know will work - but I'll give it a go anyway.

Just a thought - could the four coils actually be made out of one bit of wire with gaps between each - then I can scrape off the insulation and solder the capacitors down to ground? Or is that a silly idea? Or is veroboard suitable? (or too much stray capacitance?). I can hear the chortles from here...

s2000 · Post by **s2000** » Mon Oct 26, 2015 3:59 pm

Dont use veroboard or one piece of wire, filter needs a proper ground plane. Just buy some copper clad board and cut copper 'islands' for junctions if you dont want to etch...

http://www.gareth.net.nz/nrgworkshop/rf ... filter.htm

Analyser · Post by **Analyser** » Mon Oct 26, 2015 4:38 pm

You can actually use one piece of wire but its more fiddly. I wouldn't use veroboard though, just get some plain copper clad and use that. Lead lengths of the capacitors should be kept to a minimum (<2-3mm if poss.). You can also use tinned copper wire instead of enamelled copper wire to avoid the need for scraping the insulation off.

shorty · Post by **shorty** » Mon Oct 26, 2015 8:58 pm

You can use copper wire stripped from 1.5mm twin & earth cable used in house lighting circuits to make the filter coils, diy stores sell it by the metre for very little maybe 50p a metre, four capacitors 20p, you've got less than a quids worth of parts to make the filter posted by s2000.

NOYB · Post by **NOYB** » Fri Oct 30, 2015 4:06 pm

I've got loads of T&E - so I've run out of excuses now! Thanks lads for all the help.

Low power low pass filter?

Low power low pass filter?

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