Soundcard_SDR_Ground_(Earth)_Loops

This page is partially compiled from some of my other pages. I have tried to include everything related to soundcard SDR ground loops.

SPIKE AT THE CENTRE. 

The exact centre of a soundcard SDR display sometimes puzzles newcomers. It must be understood that the exact centre represents zero frequency at the soundcard. In respect to the SDR tuning this is the local oscillator frequency which mixes with RF signals both above and below it. There are two mixers producing I and Q signals that have a 90° phase difference. Each side of the centre represents an increase in "audio" frequency. Thus two signals, equally spaced each side of the local oscillator will produce the same "audio" frequency but because of the phase difference between the IQ signals the SDR program is able to separate RF frequencies both above and below the local oscillator.
Note I use "audio" to describe the signals sent to the soundcard. These signals are actually the IF signals for the SDR and may be any frequency up to half the sampling rate of the soundcard. They are not intelligible sounds and some of the frequencies may be well above the range of hearing. These signals are processed by the SDR program and the demodulated "audio" IF signal goes to the speakers.


The two top pictures show a cheap USB sound device.

Some soundcards will produce a single very sharp spike in the centre. This is not the local oscillator, because it is at zero frequency at the soundcard, nothing can be received here. What is actually seen is due to the analogue to digital converter (ADC) in the soundcard. This may be ignored, some SDR software reduces the visual effect, some, often more expensive soundcards only have a small spike. But no soundcard can receive anything here, when tuning the centre is best avoided. 


HUMP OR MOUND, LOTS OF SIGNALS AT THE CENTRE.

The central peak is a combination of the ADC spike and the fundamental of the powerline frequency.

The next two pictures are mid-priced Creative XiFi


The fundamental frequency of the powerline noise (50 or 60Hz) will be greater than it's harmonics and produces two signals that look like the ADC spike.


Here is what it looks like with Rocky, only the centre and right of the display shown.
Sometimes a bad power supply can cause this. Try a battery to eliminate the possibility.

SDRs like the Softrock use the audio card to convert the received signals from analogue to digital. Because the SDR utilises the maximum sensitivity of the audio card it is susceptible to minute currents  that may flow due to ground loops, bad screening or just bad grounding.

WHAT IS A GROUND LOOP?

Well, it is a loop formed by multiple grounds to the SDR.

A common example is one formed by connecting a power supply.
A PSU often has it's negative terminal connected to the powerline (mains power) ground.
A computer case and the shield of all cables connected to it is often also connected to this powerline ground. 
So a loop is formed if you connect the power supply negative to the SDR and also connect the audio shield from the SDR to the computer.
The SDR is then grounded by the PSU ground AND the computer ground.

One way of removing this loop could be to remove the powerline ground at the PSU or computer. BUT THIS IS OFTEN DANGEROUS.

A better way may be to use a double insulated power supply that does not need grounding. 

Or use a battery. 

OR just connect the positive lead of the power supply. This way the negative of the PSU is connected to the SDR by way of the powerline ground, through the computer ground and then by way of cables connecting the SDR to the computer.
This seems a big circuit BUT there is no loop.
Rather than rely on the shields of the audio cable(s) a separate ground could be made from the SDR to a point on the computer near the soundcard. Multiple ground connections between the SDR and computer are unlikely to be a problem especially if they are similar length and run together.

Switch-mode power supplies can be a problem. These may often be used with laptops. Sometimes their filtering is poor and noise is picked up, maybe looking like a ground loop problem. Another supply, filtering or a really good ground may be the answer. This situation will show up bad screening, bad audio cables. Unplug the SDR, see if anything is picked up on the cables. With an Ensemble check the isolation transformer is connected correctly. https://sites.google.com/site/g4zfqradio/softrock_ensemble_isolation_transformers


An SDR should be constructed with ground loops in mind. Tony KB9YIG designs his Softrock range in such a way as to make that easy. [BUT see VHF Ensemble, at bottom] However the constructor MUST bear this in mind when connecting it to the outside world. Some models have a position for a link to connect the antenna ground to Softrock ground. DO NOT FIT THIS. Connections to antenna, power supply, maybe other devices and computer will all be grounded. A loop is formed if by any means two ground connections are made between the SDR and computer.
It is not recommended to  use a single "long wire" antenna but if you do you will need to ground the antenna socket (return). As the antenna is not otherwise grounded this is acceptable but only for this configuration. 

When mounting the SDR in a metal box the best way would be to mount the antenna socket on the box. This would ground the box to the antenna ground, yes, the antenna co-ax SHOULD be connected to a good ground! The SDR's ground connections should all be insulated from this box. If the SDR box is instead  connected to SDR ground then should this box touch another grounded object a loop is formed.

I have connected many Softrocks in various ways. Sometimes I have connected carelessly and seen enormous ground loop effects on the SDR program display. In all cases I have found the best results are obtained by making the screen of the audio connections the only connection between the SDR and the computer.

One way of reducing the effect of ground loops is to connect everything to a good ground with thick wire and well terminated connections. This will help to equalise the ground potentials. In one case I did this so well that I could not see the effects of a loop I purposely connected.

Some use transformers as a way of avoiding ground loops. These may actually pickup noise if placed near a source of radiated energy. I have seen pickup from transformers, monitors and low-energy lamps. In addition transformers may introduce phase shift or other distortions. I have known a situation where a DRM signal was decoded with a direct connection but could not be decoded when the SDR was connected through transformers. In my opinion good results may always be obtained without transformers.

CURING GROUND LOOP PROBLEMS.

The basic technique is to remove ground connections until the loop is found.
Sometimes low quality cables can pickup interference.
In cases of received interference find out where to look. Remove the antenna, try battery power, even disconnect everything.

How far someone goes to cure this effect is a matter of opinion. Some go to extremes to remove it completely. But a few lines around the centre may be considered acceptable, just avoid this area when tuning. Programs like PSDR-IQ automatically avoid this area for receive.




VHF Ensemble.
Tony has not included antenna ground isolation in this model.
Depending on your configuration a ground loop may be formed.
Look here http://sites.google.com/site/g4zfqradio/vhf_ensemble Isolating the antenna socket one way of removing this loop.



June 2012


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