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Old 25th January 2011, 02:39 PM   #21
SY is offline SY  United States
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First things first: define what you mean by "measuring the effects of RF."

Is this for a product design that will be used in many different environments? If so, there are lots of standards out there which give a guide to quantifying RF immunity.

Is this just for your own use? Then try to be realistic about the RF challenges you face, and apply sound engineering prophylaxis.

Is this troubleshooting or characterizing an existing piece of kit? If so, what are the issues? Random noise from time to time? Continuous degradation? Buzz? AM detection?

With a target, it's obvious where to aim things, even with those very misunderstood conventional test tools.
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Old 25th January 2011, 02:49 PM   #22
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I have been working in audio electronics for a long time 50+ years. My full time job is an RF engineer (38 years) doing cell phone and two way radio design work. I have seen effects caused by RF devices affect the sound in subtle ways that are not easilly measured by typical steady state measurements in the audio band. I have also seen and chased "ghosts" where it looks like RF was affecting the audio equipment, but in reality the RF source was whacking the test equipment.

The worst offender is the cell phone. Here in the US there are several different cellular standards each with its own characteristics that affect things differently. All cell phones operate with 0.6 watts of RF power or less. Two way radios can also affect consumer electrical equipment.

Background information:

The most popular world wide cell phone standard is GSM. GSM is used in several frequency bands worldwide from 380 MHz to 2.4 GHZ. 900 MHz and 1.8 GHz are the most common in the US and used by older T-mobile and AT&T phones. GSM transmits a constant envelope carrier, meaning that the RF carrier is held at a constant amplitude during transmission. The information is carried by phase modulating the carrier. GSM uses Time Division Multiplexing (TDMA) which means that the transmitter is pulsed on and off at a base rate of about 8 Hz with multiple timeslots available, making the apparent pulse rate faster. In effect the carrier is 100% AM modulated with narrow pulses. This makes a GSM phone a strong interference generator whose effects are easilly heard. Many consumer devices like computer speakers, TV sets, and audio equipment will buzz in the presence of a GSM cell phone. It's effects on audio equipment are easilly measured in the audio band.

iDEN is an older standard used by Nextel (Sprint) in the US and Telus in Canada. It is also used in South America and Japan. It transmits at 800 MHz and 900 MHz. It uses TDMA with an 11 or 22 Hz rate AND the RF carrier is amplitude modulated. iDEN will get into anything remotely succeptible to RF and its effects are easilly heard and measured.

CDMA evolved from military secure communications. (Sprint and Verizon in the US) The system uses Code Division Multiplexing to spread a fairly low data rate across a wide frequency range. The transmitter does not pulse, it operates continuously during transmission. The information is modulated onto the RF carrier using a form of Amplitude Modulation, meaning that the RF carrier power is changing with the applied data. The data is combined with the spreading code to create a data rate in the 1.5 MHz range. The key here is that the RF frequency AND the modulation are both far above the audio range.

3G and 4G standards also use Amplitude Modulation with a higher data rate in the 1 to 10 MHz range. Again the RF frequency AND the modulation are both far above the audio range.

Hand held two way radios like carried by police and ham radio operators use Frequency Modulation but the RF power output can be as much as 10 watts. The RF frequency can range from 30 MHz to 1.3 GHz.

Automobile based two way radio equipment can cover 1.8 MHz to 2.4 GHz and the RF power level can be as high as 100 watts. Some ham radio equipment can produce over 1 killowatt of RF power. Both AM and FM modulation modes are used.

When RF radiation impacts consumer electrical equipment some of the RF energy will be picked up on the external wiring and even the internal circuitry. This RF energy can be rectified by ANY nonlinear device that it reaches. The rectified RF will create a DC voltage across this nonlinear element. The most common nonlinear element is a semiconductor junction. Ordinarily the rectified RF will be in the microvolt range and present no obvious effect other than a small bias shift. If the RF field is constantly changing in strength (ANY amplitude modulated or pulsed signal) there will be a constantly changing DC bias shift. Frequency Modulation does NOT vary the RF field. If the rate of change is in the audio range the effect can be plainly audible. This is why you hear a GSM or iDEN cell phone in your cheap unshielded computer speakers. This effect can be heard and measured, although since scope probes and test leads are good antennas the mere connection of a scope probe can turn a clean circuit into a succeptible one.

The area where difficulty lies is when the rate of change in the RF signal strength varies at a rate above (or far above) the audio range. This is the case with CDMA, 3G, and 4G cell phones. There will be some change in the operating point of internal circuitry but the rate of change can be in the 1 to 10 MHz range. You will not hear this directly, nor will most audio test equipment. In some cases its effects can be plainly heard and measured as an increase in distortion. In more subtle cases the effects can cause mild distortion effects or compressing of dynamic range that can not be easilly measured without disturbing the effect.

So, if you have a piece of equipment that can "hear" a GSM cell phone, you can bet it is also affected by CDMA and 3G or 4G phones. The effect may not be as obvious but you can probably find a spot where the phone does something to the sound that doesn't quite sound right, but doesn't cause blatant distortion. You can't always measure this with a sine wave and distortion meter.

I have seen one case where a 1KW ham radio transmitter blew the tweeters in a neighbors speakers without the stereo even turned on (SS pioneer receiver in the 1970's).
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Old 25th January 2011, 03:03 PM   #23
jkeny is offline jkeny  Ireland
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Originally Posted by DF96 View Post
..... The fact that something is in principle measurable does not guarantee that it will be measured!
Agreed, I presented scope shot measurements of RF attenuators & their reduction of RF signal reflections by twice the amount that the signal is reduced. i.e a 10dB attenuator reduces reflections by 20dB approx. Two other people confirmed this with their own tests & measurements. Now a third, Pano, has also confirmed this double attenuation. SY could not measure this doubling effect for whatever reason.
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Old 25th January 2011, 03:08 PM   #24
SY is offline SY  United States
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Quote:
So, if you have a piece of equipment that can "hear" a GSM cell phone, you can bet it is also affected by CDMA and 3G or 4G phones. The effect may not be as obvious but you can probably find a spot where the phone does something to the sound that doesn't quite sound right, but doesn't cause blatant distortion. You can't always measure this with a sine wave and distortion meter.
Why would one even try? This is a job for spectrum analysis.
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Old 25th January 2011, 04:02 PM   #25
jkeny is offline jkeny  Ireland
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Pano, did find one anomalie - the RF attenuators worked as predicted (double attenuation of reflections) on a properly terminated 75ohm line but when used on a 1M terminated line, it showed equal signal & reflections attenuation (no doubling). Any ideas what might be the reason for this?
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Old 25th January 2011, 04:14 PM   #26
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Nope- not that this has anything to do with the topic- RF. With a 1M load, the attenuation isn't correct, however, when I scoped it, the attenuation was spot-on. So besides being ad hoc (where did 1M come from?), your speculation is unsupported by fact.

Care to get back to the topic?
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Old 25th January 2011, 04:24 PM   #27
jkeny is offline jkeny  Ireland
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Nope- not that this has anything to do with the topic- RF. With a 1M load, the attenuation isn't correct, however, when I scoped it, the attenuation was spot-on. So besides being ad hoc (where did 1M come from?), your speculation is unsupported by fact.

Care to get back to the topic?
It is an example of measuring signals in the RF domain & the practical difficulties that might be encountered. How do we know how RF might effect audio if we can't even measure it accurately, consistently? I believe that it is educational to learn how to do this & this example throws up an anomalie that the RF experts may well be able to shed some light on for the benefit of all. I believe that it is very much on topic!

It's by studying & explaining anomalies that we can learn! If you don't want to discuss your test conditions that's fine but Pano also experienced this as stated. The 1M was his test condition, not mine (probably a scope input impedance?).

Last edited by jkeny; 25th January 2011 at 04:29 PM.
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Old 25th January 2011, 04:41 PM   #28
DF96 is offline DF96  England
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A cable itself (Bybeed or not) is unlikely to convert RF to audio, so if the audio effects of a cable are due to RF attenuation then this will only be seen via RF measurements. (Cable terminations can convert RF to audio via the rusty bolt effect.)

If the effect of stray RF is to shift device bias points, then this might or might not show up in distortion tests. The problem is that distortion tests usually assume that the effect is constant (being caused by the circuit), so may smear the result and just give an average. It is conceivable that rapidly varying distortion could sound worse than constant distortion with the same average level.
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Old 25th January 2011, 04:52 PM   #29
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Why would one even try? This is a job for spectrum analysis.
I got this fancy 3G cell phone from AT&T (Samsung Galaxy) It makes calls in the 3G mode unless the coverage is bad or the network is busy, then it reverts to GSM. I can tell the difference if I am sitting here in front of the computer by the sound coming from the computer speakers. At home the Sony TV will make the GSM buzz if the phone is in GSM mode, but the audio will subtly distort as I walk around the TV if the phone is using 3G.


I routinely test my electronic creations with a cell phone. It is expected that some equipment will misbehave when approached with an iDEN or GSM phone. I look for bad stuff. I was a design engineer on many of the early iDEN phones (1997) and we had a lot of fun in the local Circuit City store before people knew what a Nextel phone was. I found that leaving the phone on the passenger floorboard of a 1984 Dodge Daytona can cause unexpected car stalling when the phone starts transmitting. Its best to find this stuff early in the design cycle.
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Old 25th January 2011, 05:01 PM   #30
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I realise this is a bit OT, but if a line is properly terminated there are no reverse reflections to be attenuated. I don't know why audio people would want to measure things like this. If it does what it should, then you have merely confirmed that the textbooks are correct. If it appears not to, then you have merely confirmed that you are measuring the wrong thing. This assumes you live in a rational universe where RF transmission lines propagate signals according to well-known principles.

Please can we kill off this ghost? Then talk about the actual topic.
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