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Old 6th Sep 2017, 12:25 pm   #1
PsychMan
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Default Mains RFI Filtering

I've noticed that some of my mains sets have pretty horrendous hum when tuned to my SSTRAN transmitter. I'm aware this topic has been covered to a great extent. My SSTRAN is located in my loft with a mains earth and I believe RF is being coupled into the electrical wiring.

Looking at potential solutions, I was puzzled why my DAC90, a "transformerless" set seemed to not be affected by this, I'd expect a transformer set to reject this kind of noise more effectively.. The set does however have a 0.01uf across the line. I've tried this with my transformer sets, and a capacitor across the mains input makes a dramatic reduction to this noise.

Wishing to make a more effective solution to this, I'm looking to build an RFI filter circuit into an extension board I use for my radios. Ive seen the following circuit:

http://www.zen22142.zen.co.uk/Circui...ainsfilter.htm

I lashed one up using a ferrite ring I nicked out of an old server power supply. In testing however, I've found this does no better than just using an X1 capacitor across the mains, does anyone have any suggestions on how it could be improved? Could it be the "ferrites" ive took out the PSU aren't any good? They are blue, not the traditional grey colour. I've wound them as a pair as suggested, would I be better off using 2 and have one for each side of the mains? I have several that came out of the PSU in varying sizes. Some were stacked, would this be useful?
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Old 6th Sep 2017, 2:51 pm   #2
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Default Re: Mains RFI Filtering

Can I suggest that it is better to prevent the RF getting onto the mains wiring, if possible. I built my own pantry transmitter using a circuit very similar to the SSTRAN. I found that hum performance varied dramatically between different houses and their mains wiring. In the current house, with PME, by far the best setup is to remove the shorting links on the chokes in the power input lines, to connect the transmitter 0v to mains earth and keep the tx antenna away from mains cables.
As for the ferrite rings, they sound fine. You could try winding L and N onto separate cores with a cap on the load side.
HTH

Ken
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Old 7th Sep 2017, 8:49 am   #3
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Default Re: Mains RFI Filtering

Thanks for your reply. Yes, perhaps you are right there... I was hoping to keep the TX as it is though, as it took a lot of trial and error to get a usable signal throughout the house. My house has TN-S, perhaps I'll have a play with the jumpers on the SSTRAN and see what can be achieved. At some point I plan to install a decent earth outside for radio purposes, and this might help also.

I imagine an RFI filter could be a worthwhile venture regardless, given how many devices cause noise to power lines. I'll experiment with some more ferrite rings and see what I can do
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Old 8th Sep 2017, 9:32 am   #4
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Default Re: Mains RFI Filtering

I did a few experiments last night, adding in the power supply chokes and audio source chokes, no discernible difference was heard. I assumed RF was getting into the mains through my use of mains earth for the antenna, but even when removing this completely, the hum was still present.

I decided to rule out any household sources of either the noise, or something that might be rectifying it, so went around methodically switching everything off, and still was not able to stop it. The hum is definitely coming from the TX. My next move will be to try running it off batteries and see if its hum free when completely isolated. If it does I might need to add the RFI filter to the TX power connection, which is actually much simpler and should fix hum throughout the house.

I think I'll try what you suggested, one ferrite ring each for live and neutral with as many turns as is practical, and also before and after the rings a resistor and class X 0.01uf capacitor across the supply.

Its such a good transmitter with excellent sound, but presently unusable on any mains sets downstairs because of this
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Old 8th Sep 2017, 10:11 am   #5
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Default Re: Mains RFI Filtering

Are you running the SSTRAN from a power supply? If so just try (at least temporarily) running it off batteries.

I've found modulation hum to be a problem too with 'pantry transmitters'. Sometimes it's much worse than other times. Complex causes explained elsewhere, so won't repeat it here, but it can be caused by very odd combinations of circumstances. For example, I've found when the electric blanket and cooker are both on it's bad! Switched cooker off and not so bad. Switched blanket off and it's gone!

Try this; sounds weird, but unplug ALL devices (and switch off all wired-in appliances) in your house. Switch off lighting circuits (or remove fuses) if you have any low energy bulbs, dimmer switches, low-voltage lighting SMPSUs. Try the SSTRAN and hum-prone radios again and assess problem. (A close neighbour's equipment could also be responsible.)

In this case it's not interference from lights, SMPSUs, etc, but various semiconductors in them connected to the mains that causes the problem.

I've also found that X-class caps across the mains in the radios, and also across the rectifier valves can help greatly. Also 0.01uF caps across the diodes in the SSTRAN PSU and X-class cap across its mains input.
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Old 8th Sep 2017, 10:53 am   #6
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Default Re: Mains RFI Filtering

Hi Iain, thanks for chipping in on this.

I am running the SSTRAN from an AC power supply (same spec as the one they ship with in the USA, at least in theory). So the rectification occurs in the SSTRAN itself (I believe the diodes have bypass capacitors on them already). The supply I'm using is just a small transformer inside an enlarged mains plug.

I will rig up a few batteries and try that next to see if there is any difference.

I was fairly sure I had isolated everything methodically last night, but as I type this at work, one thing I didn't isolate was my boiler

When on batteries, I will also switch off every breaker in my consumer, except my outbuilding supply, and run an extension lead from that to my test radio (Pye Fenman I radiogram).

Thanks again,
Adam
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Old 8th Sep 2017, 11:15 am   #7
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Default Re: Mains RFI Filtering

These can be very difficult ! The SSTRAN does have diodes bypassed by caps and I did find these essential. It sounds like you have tried most things except the battery power and moving the SSTRAN antenna as it could be direct pickup on your mains cabling in the loft, then re-radiation. If that is so and moving the SSTRAN is difficult then filters on the receivers maybe the only option. I guess that opening all the breakers would not stop the re-radiation but should (?) stop the 50Hz modulation of it.

rgds

ken
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Old 8th Sep 2017, 12:10 pm   #8
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Default Re: Mains RFI Filtering

Yes, it is proving quite tricky in my house! I tried the TX in a few places but had appalling coverage everywhere except the loft, which has got it just right, in terms of range.

Quite a contrast from the comments you see on American sites of "strong signal to the end of the block", I would be lucky to get it in the next room when it was downstairs! Probably much simpler in the USA with larger houses of timer frame construction and plasterboard walls. From the loft I get good signal to the end of my garden where it fades considerably, and a few steps out the front door.

I will persevere
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Old 8th Sep 2017, 12:35 pm   #9
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Default Re: Mains RFI Filtering

Of course it could be that your " good range" from the loft is because of mains re-radiation. I guess your rooms could have foil backed plasterboard, so accounting for the poor results horizontally, but something is certainly amiss with such poor range.
Some questions which might point to the cause - Is the antenna clear of other things which could mess things up ? Does the antenna trimming give a nice peak ? Did it hum when not in the loft (though with short range ) ?

Ken
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Old 8th Sep 2017, 12:49 pm   #10
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Default Re: Mains RFI Filtering

This 'mains hum' issue is a problem with direct-conversion receivers too: the issue is RF from the oscillator passing through the rectifier diodes and then remodulating the carrier with the hum.

Search for "DC receiver common mode hum": Doug DeMaw, W1FB has written some notes on how to solve this.
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Old 8th Sep 2017, 2:01 pm   #11
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Default Re: Mains RFI Filtering

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Originally Posted by Ambientnoise View Post
Of course it could be that your " good range" from the loft is because of mains re-radiation. I guess your rooms could have foil backed plasterboard, so accounting for the poor results horizontally, but something is certainly amiss with such poor range.
Some questions which might point to the cause - Is the antenna clear of other things which could mess things up ? Does the antenna trimming give a nice peak ? Did it hum when not in the loft (though with short range ) ?
Ken
Hi Ken, There was no noticeable hum before it went into the loft. I live in a 50s ex council house which has seen little refurbishment so no foiled backed plasterboard, though metal door frames in every room attached to metal lintels inside the wall...

Could well be re-radiation I guess, the peak I get at 1593khz is around 3.5v, at lower frequencies I've been able to get closer to 7v which seems to be normal.

I do have a light fitting in my loft, and while not right next to it, the wiring isn't too far away, perhaps that is the source of rf, I'll try disconnecting the other end of that cable. Its very tricky finding somewhere in my home to run the antenna vertically where it isn't near to electrical cabling (or cat6 for that matter!)
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Old 8th Sep 2017, 2:02 pm   #12
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Default Re: Mains RFI Filtering

Quote:
Originally Posted by G6Tanuki View Post
This 'mains hum' issue is a problem with direct-conversion receivers too: the issue is RF from the oscillator passing through the rectifier diodes and then remodulating the carrier with the hum.

Search for "DC receiver common mode hum": Doug DeMaw, W1FB has written some notes on how to solve this.
Thanks, I will take a read of that
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Old 8th Sep 2017, 2:29 pm   #13
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Default Re: Mains RFI Filtering

The Doug DeMaw 'diodes and chokes to filter the supply against hum" is shown in the attachment below.
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Old 8th Sep 2017, 2:40 pm   #14
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Default Re: Mains RFI Filtering

Cheers for that. I've seen a few examples with capacitors going to earth, which while it does create some earth leakage, might do a better job providing my RCD is ok with it.

I'm going to see what If I can narrow the source of this down a bit better, if its through the ac adaptor I'll knock up a filter and see if that improves it
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Old 8th Sep 2017, 2:44 pm   #15
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Default Re: Mains RFI Filtering

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Originally Posted by G6Tanuki View Post
The Doug DeMaw 'diodes and chokes to filter the supply against hum" is shown in the attachment below.
Ahah, yes. Also, when the diodes used in this application are 1N4xxx types, regardless of whether bypassed by caps, they generate copious RF!

Ultra-fast diodes to replace them are a good fix.
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Old 8th Sep 2017, 2:50 pm   #16
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Default Re: Mains RFI Filtering

The particular issue isn't the turnoff-time of the diodes; it's that there is RF passing through them (unwantedly) so they act as a classic diode-ring modulator and modulate the RF with the hum (and its harmonics) which is then radiated from the connecting-leads and power-supply mains-lead.

I'd have thought that ultra-fast diodes would be *better* as modulators so cause the problem to worsen?

Important thing is to keep the RF leakage away from the diodes. A common-mode choke in the lead between the PSU and the device-being-fed, along with rigorous bypassing of the diodes [bear in mind we're talking RF here, so low self-inductance of the bypass capacitors probably means a lot more than the actual capacitance used] is in order.
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Old 8th Sep 2017, 3:34 pm   #17
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Default Re: Mains RFI Filtering

If it only hums in the loft, that should be a major pointer to finding what is different there (although these things are all interactive). Another thought-in the loft, are you picking power off the lighting circuit ? If so, might be worth trying an extension from the main power circuit below.
Most interested in your battery test ! That should tell if it is conducted rf back onto the mains.
Incidentally, on my home build, fast diodes made no difference, bypass caps were essential anyway.

Rgds

Ken
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Old 8th Sep 2017, 5:12 pm   #18
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Default Re: Mains RFI Filtering

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Originally Posted by G6Tanuki View Post
The particular issue isn't the turnoff-time of the diodes; it's that there is RF passing through them (unwantedly) so they act as a classic diode-ring modulator and modulate the RF with the hum (and its harmonics) which is then radiated from the connecting-leads and power-supply mains-lead.

I'd have thought that ultra-fast diodes would be *better* as modulators so cause the problem to worsen?
Hmm, I agree with your first paragraph except that research shows ( widely discussed by David (Radio Wrangler on this forum )that these 1N4xxx diodes are intrinsically generative of RF, regardless of whether it is present anywhere else downstream of them. Hopefully , David will see this an respond. I can't remember the characteristics responsible...
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Old 9th Sep 2017, 9:24 pm   #19
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Default Re: Mains RFI Filtering

I've tried several things today, none of which have provided any results (least not the ones I wanted!)

1) run the TX on batteries - rigged up two pp3 batteries to give me 18vdc, no change in the hum, even removing my mains earth connection. So power supplies not an issue

2) disconnected the lofts fluorescent light fitting, which is a spur off an upstairs socket ring, so I removed the wiring from the fused switch box, no change. It's worth adding the hum is a little worse with it on, but it's rarely on.

3) moved the antenna to the other side of the loft draped horizontally this time but away from the floor, no change.

4) took the TX into the garden with batteries, rigged it up around 20-25 feet away from the room where the radiogram is in, retuned antenna. Admittedly my earth wire is shorter than standard as I had trimmed it for use into a mains earth connection. Results were very poor, no Hum, but hardly any signal strength at all.

5) walked into the house holding the TX and an audio source, when in the same room or the kitchen next door the signal is clear with no hum.


From the above I've reached the following conclusions / assumptions:

Hum is bad downstairs as most of the signal is coming from re-radiation down there, Upstairs nearer the TX it is better where the signal is strong.

My TX has p**s poor strength based on the experiences of others, and perhaps there is a fault with a component or my assembly somehow, I might look into this further.

Perhaps my house wiring is such that this TX will
Never perform well for me as others have experienced

The only options left are another TX, or heavily filter the mains I use for radios and live with it. Or modify the sstrans loading coil for better signal, while detuning it enough that I don't radiate much beyond my property...
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Old 9th Sep 2017, 10:03 pm   #20
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Default Re: Mains RFI Filtering

It is fairly easy to see if your transmitter has its carrier modulated with hum, just by looking at its scope probe or running it off batteries, but its probably not that. And also it probably won't make a big difference if you add RFI filters on to the mains input connections of the radios either as the same hum corrupted signal will come in via their antennas anyway, but there could be a small improvement, so its worth a try.

Most likely though it is the sets you have receiving most of the signal from re-radiation from the house wiring into them via their mains cables or from nearby mains wiring to their antenna. All of this is exacerbated with a weak transmission signal too.

I have managed to eliminate this issue completely for my own pantry TX systems. This is due to the fact I use a loop TX antenna and the fact the radios I have mainly are ferrite rod types and in the near field the magnetic signal strength with the rod on the right axis is very good and re-radiated mains corrupted signals get ignored as they are relatively too low level.

For one valve radio that has a wire antenna, I got rid of the problem buy having a plastic small antenna box for it with a tuned ferrite rod, the output to the radio's antenna a twisted pair of wires from a tap about 1/3 of the tuned winding. And placing the box in the right position, peaking the signal from the pantry TX, and nulling out the signal with hum.
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