Apply 1V rms 70% 1kHz modulation 150kHz-1GHz carrier to input terminals, then to output terminals and measure the audio signal (demodulated) on the output.
These efforts will be wasted. Unmodulated carrier will be detected and shifts device(s) idle voltage/current, and it will give some shift in THD. So what?
These efforts will be wasted. Unmodulated carrier will be detected and shifts device(s) idle voltage/current, and it will give some shift in THD. So what?
dimitri said:
No they won't be wasted and demodulation performance is not an entirely reliable indicator/measure of RF immunity (eg fully complementary circuits)
Go back an read the previous discussion on input stage gain and signal handling capability Vs VAS signal handling capability and compensation methods.
Some amplifiers will overload and produce gross THD with very little injected RF carrier and some will do similar with significantly higher amounts of RF carrier.
Rfi
Hi Dimitri,
Looking for AM demodulation was also my first choice (although I've simmed it in a slightly different way), but don't you think 1V rms HF is a bit unrealistic? With such high amplitude almost every amp will get berserk, unless the amp is provided with a low pass filter at the input.
However, to make fair comparisons between different topologies, we should disable such filter.
Cheers,
Edmond.
PS: If no AM demodulation takes place, doesn't necessarily mean that no other kind of distortions may rise under influence of HF signals.
dimitri said:
Hi Dimitri,
Looking for AM demodulation was also my first choice (although I've simmed it in a slightly different way), but don't you think 1V rms HF is a bit unrealistic? With such high amplitude almost every amp will get berserk, unless the amp is provided with a low pass filter at the input.
However, to make fair comparisons between different topologies, we should disable such filter.
Cheers,
Edmond.
PS: If no AM demodulation takes place, doesn't necessarily mean that no other kind of distortions may rise under influence of HF signals.
Re: Re: Re: AM demodulation.
Hello Bob.
Thinking about this a bit more, I think that your amp is saved from gross VAS overload at RF by the input stage (100R+150pF) RC.
Suppose a 100mV RF carrier of 2MHz is injected into your amplifiers input. At 2MHz the loop gain is unity so the differential error signal ( E=Vin/(1+AB) ) = 50mV. At 2MHz your input stage has ~unity gain, so the VAS only sees ~50mV differential input. At higher frequencies the input stage gain heads well below unity.
I think that's about right, but it is nearly midnight now
Cheers,
Glen
Bob Cordell said:
Hello Bob.
Thinking about this a bit more, I think that your amp is saved from gross VAS overload at RF by the input stage (100R+150pF) RC.
Suppose a 100mV RF carrier of 2MHz is injected into your amplifiers input. At 2MHz the loop gain is unity so the differential error signal ( E=Vin/(1+AB) ) = 50mV. At 2MHz your input stage has ~unity gain, so the VAS only sees ~50mV differential input. At higher frequencies the input stage gain heads well below unity.
I think that's about right, but it is nearly midnight now
Cheers,
Glen
Re: Re: Re: Re: AM demodulation.
Hi Terry,
These are very good (and interesting) points. It is a particularly good point that a no-NFB approach forces you to do right by the input stage. The inverse of this is that the use of negative feedback can allow some less skilled designers (especially in the past) to use an un-degenerated input stage with all of its consequent badness.
Cheers,
Bob
Terry Demol said:
Hi Terry,
These are very good (and interesting) points. It is a particularly good point that a no-NFB approach forces you to do right by the input stage. The inverse of this is that the use of negative feedback can allow some less skilled designers (especially in the past) to use an un-degenerated input stage with all of its consequent badness.
Cheers,
Bob
homemodder said:
Hi Alex,
Some on the NFB thread have described it as phase lead compensation in the past, but I have always thought of it as just another way of doing dominat-pole integrating feedback compensation. In essence, we are still building a Miller integrator with it. We are just feeding back the capacitor current one stage earlier. Doing that encloses the input stage in the local Miller compensation feedback loop, with significant input stage linearization benefits.
Cheers,
Bob
Re: Re: Re: Re: AM demodulation.
Hi Glen,
That is a good point. I should remind others here that the purpose of the 100R + 150 pF RC across the cascode collectors of the input stage is to frequency compensate the loop that is formed locally by the Miller compensation capacitor feeding back to the input stage. I think that loop was designed to have a unity gain frequency in the range of 10-20 MHz.
Cheers,
Bob
G.Kleinschmidt said:
Hello Bob.
Thinking about this a bit more, I think that your amp is saved from gross VAS overload at RF by the input stage (100R+150pF) RC.
Suppose a 100mV RF carrier of 2MHz is injected into your amplifiers input. At 2MHz the loop gain is unity so the differential error signal ( E=Vin/(1+AB) ) = 50mV. At 2MHz your input stage has ~unity gain, so the VAS only sees ~50mV differential input. At higher frequencies the input stage gain heads well below unity.
I think that's about right, but it is nearly midnight now
Cheers,
Glen
Hi Glen,
That is a good point. I should remind others here that the purpose of the 100R + 150 pF RC across the cascode collectors of the input stage is to frequency compensate the loop that is formed locally by the Miller compensation capacitor feeding back to the input stage. I think that loop was designed to have a unity gain frequency in the range of 10-20 MHz.
Cheers,
Bob
While I think designing the circuitry for EMI resistance is important my experience points to layout and stability as being the dominant issues in emi/rfi resistance. Since your dealing with RF it can get into the circuit after the input stage if the "window" (the area inside the wire loop at the input of the VAS) is large enough. Even an output follower is a candidate. And as higher frequency energy becomes more common at 850/900/1800/1900 MHz (cell phones) then even a small window is a good antenna. And that window could be a slot in a ground plane.
I have used both fast rise pulses and RFI generators (walkie-talkies are great for this) to find local instabilities and fixing those will usually eliminate excess RFI sensitivity.
I have measured over a volt of RFI at 80 MHz in a customer's house near a TV transmitter. TV signals have a characteristic buzz from the vertical refresh that sounds like bad PS hum. This will be gone next year when TV becomes digital in the US. It will be a hash instead. I told the customer to move, fixing the problem on a moving coil input was almost impossible, and I didn't think it would be healthy to live in such a high RF field.
I have used both fast rise pulses and RFI generators (walkie-talkies are great for this) to find local instabilities and fixing those will usually eliminate excess RFI sensitivity.
I have measured over a volt of RFI at 80 MHz in a customer's house near a TV transmitter. TV signals have a characteristic buzz from the vertical refresh that sounds like bad PS hum. This will be gone next year when TV becomes digital in the US. It will be a hash instead. I told the customer to move, fixing the problem on a moving coil input was almost impossible, and I didn't think it would be healthy to live in such a high RF field.
1audio said:
A good article in EDN about layout and the RF environment. A number of interesting comments.
<<http://www.edn.com/article/CA6515348.html>>
john curl said:
A transformer!?
My God! You should have told him to take up residence in Sutro TUNNEL instead.
Dear Glen,
I'm appreciating your enthusiasm, but should admit that there are some gaps in your education. Please read some of these books http://www.hottconsultants.com/book.html to be able to talk about EMC issues.
I ordered the Toshiba JFETS 2SK170 and 2SJ74 from MCM. I received a partial shipment (the rest are due this Friday) and they appear to have a JP suffix. Earlier posts have suggested there are different quality parts and given the low price at MCM I assume these are not the high quality parts. I have three questions:
1. Is the JP in fact the suffix or a country abbreviation, and
2. If the JP is the suffix, what does that mean?
3. If the JP is not the suffix, how to ID the suffix?
Thanks.
Lindsay
1. Is the JP in fact the suffix or a country abbreviation, and
2. If the JP is the suffix, what does that mean?
3. If the JP is not the suffix, how to ID the suffix?
Thanks.
Lindsay
RFI
Hi Dimitri,
I don't think Glen is completely wrong by just looking at the harmonic distortion. Suppose that under influence of a strong HF signal, both tops of that signal (i.e pos. and neg.) are equally compressed. In that case, no AM demodulation will detected, as the whole thing is symmetrical (i.e. no rectifying). However, the tops of an superimposed AF signal will also compressed (maybe even more), which is easily detected by a HD analysis.
Nevertheless, I will start tomorrow with sims of AM demodulation effects, by subjecting several front-ends to a 0.3V 10MHz carrier, 70% modulated with a 10kHz sine. Measuring of the demodulation products will be done by means of an FFT.
BTW and OT, what's the correct spelling: "an FFT" (130k hits on Google) or "a FTT" (55.7k hits) ?
Cheers,
Edmond.
dimitri said:
Hi Dimitri,
I don't think Glen is completely wrong by just looking at the harmonic distortion. Suppose that under influence of a strong HF signal, both tops of that signal (i.e pos. and neg.) are equally compressed. In that case, no AM demodulation will detected, as the whole thing is symmetrical (i.e. no rectifying). However, the tops of an superimposed AF signal will also compressed (maybe even more), which is easily detected by a HD analysis.
Nevertheless, I will start tomorrow with sims of AM demodulation effects, by subjecting several front-ends to a 0.3V 10MHz carrier, 70% modulated with a 10kHz sine. Measuring of the demodulation products will be done by means of an FFT.
BTW and OT, what's the correct spelling: "an FFT" (130k hits on Google) or "a FTT" (55.7k hits) ?
Cheers,
Edmond.
Re: RFI
Thanks Edmond. And yes, that is EXACTLY why I proposed the alternative unmodulated carrier test (see my previous reference to symmetrical circuits).
Some symmetrical amplifier stages with wide bandwidth will readily behave as a limiting amplifier when over driven with an AM modulated RF carrier and contribute very little demodulation.
As a consequence a measure of the amplifiers demodulation performance will not necessarily indicate how badly an injected RF carrier will effect the amplifiers ability to linearly amplify audio signals.
Cheers,
Glen
Oh, well if you say so.
Edmond Stuart said:
Thanks Edmond. And yes, that is EXACTLY why I proposed the alternative unmodulated carrier test (see my previous reference to symmetrical circuits).
Some symmetrical amplifier stages with wide bandwidth will readily behave as a limiting amplifier when over driven with an AM modulated RF carrier and contribute very little demodulation.
As a consequence a measure of the amplifiers demodulation performance will not necessarily indicate how badly an injected RF carrier will effect the amplifiers ability to linearly amplify audio signals.
Cheers,
Glen
dimitri said:
Oh, well if you say so.
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