RSP RFI

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JMG
Posts: 14
Joined: Thu Mar 26, 2015 9:58 pm

RSP RFI

Postby JMG » Tue May 24, 2016 9:15 pm

Numerous articles and YouTube videos have been published on how to deal with RFI issues. Most proposals end up proposing creative shielding designs or by asking for a metallic casing. With limited reliable results.

RF interference manifests itself as a higher noise floor and as spurious signals. To analyze these issues, the RSP has been taken to an EMC test range.

The measured results highly depend on the test frequency and the setup of the test configuration. All tests were performed at a frequency of 149MHz. The length of the cables connected to the antenna port was 80cm. Different results may be obtained for other frequencies and setups, e.g. in SW bands.

How do unwanted signals enter the receiver?

The most common way how spurious signals may appear is by loading the receiver with strong signals from the antenna. Less efficient antennas, filtering or even a resistive attenuator may help. But this is a topic for a different discussion.

The cables connected to the antenna and the USB port form a dipole antenna. The signal picked up between the ground terminals of these cables spreads over the board and finds its way to the receiver. This received signal has been measured to be >40dB lower compared to a real antenna placed at the same position.

A good and proven practice to keep this antenna signal from entering the receiver is to add a choke (ferrite) to the antenna, to the USB cable or to both. The choke can be placed quite elegantly within the casing over the cable connecting the antenna port and the board.
The chokes do not keep the USB signals from entering or leaving the board, however. These signals are bound within the shielding of the cable.

A really efficient solution (to be considered for future designs) is to short the ground terminals of the antenna and the USB connector with as low an impedance as possible. Both the connectors should be close together and not at opposite sides of the casing. Connecting the ground of the two ports results in a signal attenuation of >15dB.

The board itself may also receive signals. The parasitic antennas on the board are quite inefficient. The received power is >50dB below that of a real antenna. Hence, this effect is of less importance.

Due to the limited shielding effect of coax cables, the antenna cable itself may pick up RF signals. These signals will not be suppressed by the chokes placed over the cable. The signal received via this path has been measured to be >30dB lower compared to a real antenna. These tests have been run with a well-terminated RG58U cable of length 80cm. This effect may appear for strong close-by signals entering the antenna cable. The shielding effect of coax cables has not been measured yet at lower frequencies, such as those of noise emitted by switching regulators. The effect could be worse there.

In summary, the most important impact will be from signals picked up by the dipole antenna formed by the cables connected to the antenna and the USB input port of the receiver. These signals have to be blocked from entering the receiver. Signals penetrating the coax shield may also result in RFI. The effect of signals entering directly into the board is of minor influence. For the given test, setup no effect on the noise floor has been observed.
Last edited by JMG on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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JMG
Posts: 14
Joined: Thu Mar 26, 2015 9:58 pm

Re: RSP RFI

Postby JMG » Sat Mar 18, 2017 11:12 am

A common mode rejection filter has been added to the USB data lines. This will give some additional improvement in noise suppression. Addding the filter to a RSP2 is not easy at all. The traces on the board are VERY small.
In conjunction with a very short connection between the ground ports of the USB- and the antenna connector all the ferrites may be omitted.

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CaptainNemo
Posts: 106
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: RSP RFI

Postby CaptainNemo » Mon Mar 20, 2017 10:32 am

Thanks JMG for sharing with us this summary.
There is a lot of post on this issue, I find your summary/report is well structured an full of interesting insights.
I look forward to hear more hints from you.

------------------

What about the PC connected?
I ever used a laptop, since I started this hobby.
There are particular hints when using a laptop vs a desktop or a deskside?

bye
gio

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DanubeBCL
Posts: 116
Joined: Sat Jul 02, 2016 12:39 pm

Re: RSP RFI

Postby DanubeBCL » Mon Mar 20, 2017 5:31 pm

The major problem is not RFI created in our own house or created by the construction of the receiver itself (shielding, computer noise a.s.o). We BCLs all normally have the technical skill to overcome such noise with shielding, choke filters, common mode chokes, grounding, galvanic isolation, whatever. Most of us have listening and technical experience for more than 50 years and know how to deal with electromagnetic noise.

The major problem is noise coming from badly filtered devices in the neighbourhood (like switched PS, lawn mowing robots and all the rest) and devices specially designed to distribute high level broadband noise (like homeplug powerline PLC/PLT modems). We cannot do anything against such noise which comes in through our antennas for these reasons:
- The neighbours are stubborn and do not co-operate.
- The authorities do not take any action.
- The noise levels are legally allowed thanks to intensive lobbyist work of the manufacturers.
- Courts often judge to the detriment of the complainant with the argumentation that noisefree old style radio reception is no longer necessary because there are other sources of information available.
I once had a talk with a lawyer who said: "It is merely impossible to convince a judge that listening to a weak radio station 10000 miles away transmitting most of the time in a language you do not understand is vital for your personal lifestyle."

The "noise" created in our own devices is the least problem.

73, Heinrich

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