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Radiated immunity testing for product that has an antenna and receiver?

treez

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Hi,
Say you've got a product to go for radiated immunity testing, and it is mains powered, and comprises a metal box containing a 60W offline SMPS, and a tx/rx module. The metal box feeds 12 metres of coaxial cable to a straight antenna of length some 10's of cm.
The metal box also feeds a 12v isolated cable (screened) to an open metal box which contains a 10W battery charger.
How do you get it to pass radiated immunity at 10V/m?
After all, the product has an antenna, which is designed to pick up radiation and feed it to its receiver....so its bound to fail?
 

frankrose

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How do you get it to pass radiated immunity at 10V/m?
It depends on a bit more factors I think, for example the excitation frequency, and everywhere will it get maximum 10V/m? And to answer this at least you should share how it failed on a test (after leveling how much was the lowest radiated power where it has passed already, which frequencies/bands are affected).
After all, the product has an antenna, which is designed to pick up radiation and feed it to its receiver....so its bound to fail?
10V/m is quite high for a receiver if it is in-band excitation. If the receiver operates at 2.4GHz and excitation comes 900MHz I think there is no issue.... or maybe. You should share more about the receiver, its input and simulate the effect of 10V/m in-band radiation.
 

KlausST

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Hi,

radiated noise test focusses on the frequencies that are "not meant for operation".
The test for an FM sender for X MHz excludes the X MHz (plus bandwidth) from the test.

You will se the X MHz on the frequency plot, but it is allowed to exceed the usual "radiated noise limit" there.

Else no Bluetooth, no cellular phone, no WiFi device ... could pass the test.

Klaus
 

frankrose

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You are right probably, when a receiver is used in the device under radiated immunity test with high as 10V/m excitation it is always out of the band test, I don't know. I meant test for a blocker/interferences/channel selectivity, but those should be applied next to lower radiation level I guess.
 

treez

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If you have an antenna matched up for 240MHz, and the system fails radiated immunity at 60MHz, then is there some offtheshelf filters that people normally buy for this? The 60MHz makes the receiver circuitry not "see" the antenna and it gives an "antenna missing" fault. How do we get round this? We also have many metres of coaxial cable going to the antenna, and wonder if we have to account for this with some kind of matching circuit, so we dont fail radiated immunity?
 

G4BCH

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If you place a band pass filter centred at your radio's operating frequency then this will help with both immunity and spurious output tests. Your antenna should be matched to 50ohms so if the transmitter and reciever are both matched then there should be no problem.
You don't say what frequency the transmitter and receiver are aperating at or which specifiaction you are testing against, I'm guessing that from the 10V/m that it is CE. All the tests have detailed procedures that the test house will folow. If you have not already got a copy of the specification you are working to towards meeting then I would recommend that you do, it will tell you over what frequency range you need not test for receiver immunity and spurious emissions and how to do it. The Transmitter will have its own separate approval specifiacation to meet.
Some of these documents may seem expensive but compared to product development costs and the cost of a re-test it is well worth it.
 

treez

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Thanks, what is happening is that we have a transceiver, which regularly transmits at 240MHz to the antenna. (and it transmits during the radiated immunity test, because its part of the normal operation) However, when we take it for radiated immunity testing, then at 60MHz (60MHz from Radiated immunity antenna), the transmitter does not work properly at its 240MHz transmit frequency, and this must be due to the 60MHz being transmitted by the radiated immunity test antenna, being picked up by our equipments antenna. What happens is that the hardware detects too low current in the transmitter, and so puts out an "antenna missing" fault, and thence we fail the radiated immunity test.
How can we stop this from happening? ie, how can we stop the 60MHz from screwing up the transmitter such that it somehow cant transmit at its frequency of 240MHz?
This incoming 60MHz is somehow screwing up the transmitters ability to transmit at its 240MHz. -The transmitter has a current monitor and this is recording low current when the 60MHz is incumbent. This causes software to put out "antenna missing" fault....and thence we fail the radiated immunity test.

Its difficult to explain this because there are two antennas here, one is the radiated immunity test antenna (which is sweeping from 30MHz to 1GHz) , and the other is the antenna on the equipment that's undergoing radiated immunity testing.
 

G4BCH

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It sounds like the monitoring / protection circuits are being afected by the 60MHz RF. Without knowing eexcatly what you have I'm making some guesses here.
Do you have a VSWR monitor on the transmitter? It soounds like you do from the 'antenna missing' fault. If so then you need a filter in the antenna path to reduce the out of band signal. Another possibility is to only monitor the VSWR reflected output after the transmitter has been activated, there should be a large enough forward to reverse power ratio to stop reported failure. I'm assuming here that the problem is caused by RF coming from the antenna and not directly radaited onto the PCB.
If it is signal level at the PCB then you need better screening and filtering.

Just re-read your reply and it looks more like the DC circuitry being affected. Again making educted guesses, you need to reduce the RF coupling into your current monitoting circuit. If it is a fairly standard circuit with a low value resistance in series with the supply and a high gain differential amplifier across the resistor, try adding some resitance (as high as you can get away with and still have the circuit function) in series with the inputs to the op amp close to the op amp and place some decoupling at the op amp inputs. You may need to take care with the decoupling in case it causes instability in your circuit.

I have had success in tracking down the sensitive parts of circuits by making a small dipole (a few cm per side), or a loop if magnetic couplinng is needed and using as high power signal generator as I could find to probe different parts of the circuit. Once you have found the sensitive parts then you can start to investigate how to reduce the sensitivity to theh RF field. You may need an amplifer to get enough signal 10 to 20dBm from a signal genrator is not always enough it may take up to a watt.
Curing emission and susceptiblily problems just come down to filtering and screening, 10V/m at HF & VHF is not too bad 100V/m or more in the GHz can be a right pain, but again it comes down to good filtering and screening.
 

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