Comment from blog reader:
I think the real question here is valid for all testing. Do we test according a repeatable verifiable and implementable and above all safe and secure way or do we test an EUT as in real life conditions? f.i. all signals can be present at one time blue tooth, GSM, WiFi radio communication and emitting at various levels and intervals creating different harmonics in time and even combined harmonics.And this all within a proximity of less than 10cm of the EUT. How can you test this kind of situation?
Reply:
Dear reader, thank you for your interesting comments.
I believe the power levels of bluetooth and WiFi are limited by the FCC and equivalent authorities to a few hundred milliwatts, so I don’t see these as a significant threat. Also, the drive in medical equipment today is wireless connectivity, so modern equipment will be immune to signal levels from its integrated antenna. The power level of a cell phone is 2000 milliwatts, significantly higher than WLAN levels. However I see your point on multiple simultaneous threats. My thoughts on this are that though not ideal, testing one threat at a time seems to work. For instance, military aircraft are subjected to high test fields a single frequency at a time, and these aircraft tend not to fall from the sky. An argument I shall be making later on, is that the tests should check susceptibility to real cell phone signals, not susceptibility to simple amplitude modulated signals that in no way represent the real threat waveform.
Aug 12, 2012 Post
The basic formula for calculating the field strength at a distance d from the antenna is:
Field strength in volts per meter
E = sqrt (30*G*P) / d
Where:
G is the linear gain of the antenna at the frequency of interest
P (in watts) is the RF power entering the antenna connector
30 is the constant of proportionality (actually 120pi/4pi, let me know if you want to know why)
d is the distance from the antenna in meters
Rearranging so we can establish the power required at the antenna connector to create a specific field strength gives:
P = (d.E)^2 / 30.G
A reasonable assumption for say a frequency of 80MHz is an antenna gain of 5dBi (linear gain of 3.2), so for a field strength of 20 v/m (36v/m modulation peak) at 3m, the power required at the antenna connector is:
P = (3*36)^2 / 30*3.2
P = 11,664/96
P = 121.5 Watts
This is optimistic as it does not include overhead for systems losses, including cable / switch losses, coupling between the antenna and the room, an allowance for field uniformity, etc. A sensible overhead is 6dB, giving the power required at the amplifier output connector at approximately 500W.
If the test distance is reduced to 1m, the power required drops to:
P = (1*36)^2 / 30*3.2
P = 1,296/96
P = 13.5 Watts
Adding the system loss overhead of 6dB the power required from the amplifier is 54W.That is the required amplifier power is reduced nine-fold, raising the possibility that the existing amplifiers could be used to generate 20v/m (36v/m modulation peak) at 1m test distance
More of the same in the next blog
– Tom Mullineaux
