John Curl's Blowtorch preamplifier part II

[Steve Eddy]

Originally Posted by Wavebourn

You won't believe. In person he is very nice.

I'm sure he is very nice when his neck is within arm's reach.

se

[jam]

I hope that you are being facetious..............

[Steve Eddy]

Originally Posted by jam

I hope that you are being facetious..............

No.

I'm being a smartass.

se

[anatech]

No, FM sidebands do not look like that. I do have some screen captures on another computer that I can access later on.

Looking at an FM modulated carrier will show a continuous series of sidebands because the frequency of the carrier is being swept up and down (= frequency modulation). The deviation (or how many KHz the carrier is offset) represents the amplitude of the modulating signal. How quickly the carrier is swept back and forth is dependent on the frequency of the modulating signal.

Now, that signal does represent what you might see with an amplitude modulated carrier. This is when you will see sum and difference products between the modulating signal and a carrier. That shouldn't be surprising to anyone as it is this process that creates harmonics of fundamental frequencies. Non-linearities in the transfer function will mix the two frequencies together giving us those new sum and difference frequencies. That is also called heterodyning, the way we change frequencies from the original carrier frequency (say 107.1 MHz) to another intermediate frequency that remains constant (say, 10.7 MHz). We apply the local oscillator frequency that is a constant 10.7 MHz difference from the frequency we want to receive. We just tune the filter to the difference frequency between the local oscillator and the frequency of interest. The constant frequency of the IF allows us to amplify and control the passband much more accurately. The FM modulation carries through this process ( it appears in both the sum and difference because one of the original frequencies is FM modulated). In fact, if we FM modulate the local oscillator and mix it with a carrier that is not modulated, the result is indistinguishable from the first case.

Anyway, that is roughly how both AM and FM RF stages work. In the AM case, the local oscillator (LO) is kept normally about 455 KHz above the frequency we want to receive. I'm pretty tired right now (it's about 1:20 AM here), so I apologize if I'm lacking continuity with this post. The intent is to try and explain what an FM modulated carrier looks like, and why the diagram in post 971 looks more like AM modulation (with many frequencies). I think some of the amplitudes are incorrect though, like 15 and 15.9 KHz as an example. How exactly did this diagram come to be?

-Chris

[john curl]

Yes they do. Do I have to show a picture? I have the same basic FM sidebands generated in my lab at the moment. It is easy to do this with the right test equipment.

[janneman]

Originally Posted by Steve Eddy

PERFECT!

You can give John the honor of setting me ablaze.

se

I'll propose Mark to auction off that right. Should pay fully for BAF2010

jd

[scott wurcer]

Just so young folks reading here can learn something from this exercise. When faced with a complex spectra vary each frequency a little and note the amount and direction that the spur of interest moves. This will give you the order and sign of each component. For instance F1 – 3*F2 will move positive one Hertz per Hertz with F1 and negative 3 Hertz per Hertz with F2. This will prove invaluable if 3 frequencies are inter-modulating.

If the spur does not move it is not related to that input frequency. If it never moves it is contamination.

Without the phase information it might be difficult to separate AM and FM.

Chris is correct that plot can be generated with nothing but AM, in fact an unbalanced passive mixer will create spurs at all those frequencies (and more).

[scott wurcer]

Originally Posted by janneman

Like so:

Over to you,

jd

Essentially you on on the right track. I just remembered one of my "bible" references the old ITT "Reference Data for Radio Engineers". Here is their plot. They also have a nice vector description of how AM and PM are not seperable without the phase information. There are no non-harmonically related frequencies here.

Anyone can repeat this, a single self biased npn mixer will produce all those spurs (at different amplitudes of course).

[simon7000]

Jan

RE: John, if you plot the following components:

2F2-4F1;
2F2-5F1;
2F2-6F1;
2F2-7F1;
etc.

where do these components fall on the graph?

Looking at 2F2 - 9F1 does seem to line up at the first previously unlabeled spike, but F2-5F1 is almost 10 db lower than the unlabeled spike. I would expect 9F1 from a square wave to be 5.1 db lower and 2F2 from a "sine" wave (ca 1970) to be at least 60 db lower. So for the spike to be about 9 db higher is interesting.

The time delay for a 741 type op amp is around .5 microseconds. This varies quite a bit and also does not correlate with the spike.

I do not like the term PIM for distortion introduced by time delay in a feedback system. The term is more often used for Passive Intermodulation Distortion. See http://www.boonton.com/products/pim2...et_BTN_WEB.pdf for how the RF folks measure this. If you see an uncanny resemblance to what some people think is happening with audio cables and connectors, you will understand why I think we should use PIM the same way as the RF folks.

ES

[Steve Eddy]

Originally Posted by janneman

I'll propose Mark to auction off that right. Should pay fully for BAF2010

Well, if I'd have known that, I'd have worked much harder this past year at being an even bigger ******* so that you could have netted enough to fly everyone to Hawaii for BAF2010.

se