100khz LORAN receiver mains hum

[dr pepper]

Continuing with my loarn experiments I built a 100khz receiver to pick up loran pulses.
I get a lot of 100hz hum, if I power the circuit from my bench psu I hear loran with a load of 100hz messing up th esignal, if I pull the - or + power connection the circuit slowly dies with the supply cap discharging, still hum, but if I pull both supply conns the hums goes away, and recption decreases but is still there.
So i assume the rx is using the mains wiring as the ant, capacitively coupling through transformers, and 100khz is a decade multiple of 100 hz, hence the hum, other low freqs like 60khz work ok.
So how do I keep rf away from the power rails?

 

[JimB]

The facetious answer is:

" It hums because it does not know the words"


On a more sensible note, what sort of a receiver is it?
TRF, Direct Conversion, Superhet ?

What are you using for an antenna?
Using the mains sounds like a recipe for disaster.

JimB

 

[dr pepper]

Thanks for marking the centenary of that one.

Trf, using first a Ta7642 (Zn414), and now a wideband op amp, I wanted to avoid having an IF as its going to be a freq reference, and dont want many variable delays.
The ant is a 24" square 1.5mH wire loop, 52 turns, resonated with a variable cap, 10-500pf.
I tried a loop stick but the s/n was poor.

I pulled a differential choke out of an smps, and connected inline with the Dc supply, cant get the signal anymore, but I can get radio 4 on 198 khz, I guess my ant is a pile if junk, I did verify it resonates at 100khz, and it picks up my sig gen Ok, maybe its the pulsed nature of loran, others have used multi turn loops though.
The Ne602 thet say works at Dc and has 25dB of gain, I was wondering abotu using a pair of fets to buffer the ant, and feed the '602 with a differential signal, with a local osc of 0hz.

 

[JimB]

Thanks for marking the centenary of that one.

You're welcome.
Come to think of it, the ZN414 cannot be far off its half century, I first saw them around 1972/3.
Where does time go when you are enjoying things?

As for your hum problem, my first guess is that the large loop makes a nice pick-up coil for all the 50hz magnetic fields around your workbench and the high input impedance of the ZN414 just lets it all come whizzing* through.

JimB


* a technical term used when I am in a in a frivolous mood.

 

[rjenkinsgb]

Do you have an earth on the receiver? If its floating, that may cause a problem.

 

[dr pepper]

I thought that too jim, but if I battery power the thing the hum almost goes away.
Thats kind of a problem as I want to connect this to other mains powered equipment.

Rj, nope no earth I thought that would only make things worser.
I'll try connecting the - rail to mains earth, also try it to the ground side of the loop (lowest petential).
The wiring in my house is from the 50's, so how good it'll be I dont know.

 

[dr pepper]

I have been researching commercial stuff on the net.
Seems that magnetic or B field ants are not common, e field whips seems to be what boats & planes use.
I spose a tuned loop could ring or otherwise distort the phase of the incoming signal, and picoseconds will make a difference to a accurate fix.
My pcket Vlf receiver picks up loran outside, kinda, but not inside, so I need a better ant system, and some way of filtering.

 

[dr pepper]

Before I give up on the mag loop I'm going to try shielding it and using it with a differential amp, so neither side of it is grounded.

 

[atferrari]

Hola Doctor, could you say your location and that of the target station?

 

[dr pepper]

I'm in lancashire and the station is anthorn cumbria.

 

[unclejed613]

you may have a ground loop problem... in other words you have two grounds that connect to a ground at different locations, and you have a leakage current setting up a voltage drop that gets amplified by your receiver. there are a couple of ways to solve this, first run your power supply from an isolation transformer (or use an open-frame isolated linear power supply), and have all of the grounds in the equipment in-line all the way back to the antenna shield, and ground the antenna shield to a ground stake (a good one, copper at least 5 feet long and 1/2" diameter (bigger is better))... or a "star ground" (not as good unless everything is inside the same box), again having the DC supply isolated, and the center of the "star" grounded to the main water pipe that comes into the house.

 

[dr pepper]

I have anisolation transformer feeding the whole bench.
Looks like it might be a ground loop problem your right, the noise appears when I connect the 'scope ground lead.
A quick unearth (now reconnected) of the 'scope made it worse.
The o/p of the rx is a digital pulse train, so if I isolate the power with an electronic version of an austin transformer I could isolate the o/p with a opto.
In the Uk I'm failry sure you cant have a ground stake connected to the electrical system, and therefore water pipes, you can only do that if you have a TT power supply, mines a TNC, in other words I have an earth line from the sub, TT does not.

Using 2 op amps as a differential amp with no ground connection to the ant it works a lot better, but it does require a dual supply.

 

[k7elp60]

I am in the, and I thought all Loran-C was shut down.

 

[JimB]

I thought all Loran-C was shut down.

And then people realised how vulnerable GPS is to jamming.

JimB

 

[dr pepper]

I dont know how long it'll be on the air, its still on trial, the pandemic cost will probably shut it down.
Have a play here you can pick it up on a sdr receiver in the netherlands 100khz.

http://websdr.ewi.utwente.nl:8901/

 

[unclejed613]

LORAN has been revived as a GPS backup, but a lot of LORAN stations were decommissioned and stripped before somebody realized GPS needed a backup, so the number of LORAN chains back on the air are smaller than the original system...... this is why we used to upgrade stuff redundantly over long periods of time instead of just jumping into the new tech whole hog without thinking.... LORAN C went into operation in 1962 and LORAN A was decommissioned in the 1980s... it took time back then because the user's equipment was expensive and took a long time for everybody to get new stuff... but there's also the problem that nobody foresaw with GPS, and that's (relatively inexpensive) SDR hardware that can spoof GPS... spoofing any radio based system is so inexpensive that it's no longer the domain of state agencies with unlimited cash... anybody with a budget of a few thousand (dollars, euros, pounds, you name it) can buy hardware (and use free software) to generate just about any radio signal imaginable, the only problem then being whether you are spoofing a weak low power signal (GPS signals come in from earth orbit from satellites using very limited transmit power) or a huge earth based transmitter with kilowatts or megawatts of available power (land based LORAN stations)... it's obvious that land based stations are almost impossible to spoof, but satellite based stations are relatively easy... so even though there was a big mistake in charging in and decommissioning older technology without thinking through the ramifications, it seems LORAN C is going to be around for a while longer (and a few Chinese LORAN A stations were still reported to b operating at least up until the mid 1990s... whether they are still operating is anybody's guess)...

a quick listen of KiwiSDRs in the area on the LORAN A channels of 1750, 1800, 1850. and 1900 khz reveals no LORAN A pulse trains, so even the Chinese chains are now off the air... KIWI SDRs can be found by map locations at http://rx.linkfanel.net/

the radio plays and displays IN the BROWSER so you don;t need any SDR equipment or antenna... demodulators available are AM, CW, SSB, FM, and some even have digital (SAM and DRM)... there are also decoders for FAX and some of the amateur digital modes...

 

[dr pepper]

Eloran from Anthorn I beleive is still on trial, hence my comments.
You missed one thing, an antenna big enough to spoof gps will go in the boot of a car, one to spoof loran would be either be massive, or require megavolts to be able to transmit any real power.
Plus loran has authentication codes, I notice every so often theres a bit of a skip, so I wonder if there is some randomness to detect spoofing.

 

[rjenkinsgb]

Back to the interference problem - how about using transformer coupling to the antenna, a few turns over another tuned coil?
That could eliminate any mains hum type pickup; or centre tap the antenna loop and ground that, feeding your balanced input stage from the ends?

You could also try wrapping the antenna coil in aluminium foil & ground that - that's a technique I've seen used with metal detector search coils to prevent noise pickup.
You just leave a small gap at one point to avoid a "shorted turn" effect.

 

[dr pepper]

Yes centre tap ground is an idea, good suggestion, that only requires one op amp instead of 3.
Not sure I could make a balun the rx input impedance is in the order of megohms.
I had thought of making the ant shielded, thats not so hard to do.