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Old 4th Aug 2021, 5:27 pm   #1
Skywave
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Question FM oscillator circuit: questions

Attached is a circuit, found on the www, of a simple frequency-modulated RF oscillator which uses a varicap diode. (I have added only the cct. diag.; I have the supporting text stored, but have added only the cct. diag. for brevity).

Now I don't have a great deal of experience of designing circuits using varicaps (although I do understand their fundamental principles) and although this circuit looks interesting, I'm a bit puzzled about the selection of the frequency-determining passive components.
The coil is stated as being 0.15 µH and the author claims that this cct. is tunable over the range 75 to 150 MHz. By my calculations, for 75 MHz, the effective capacity needs to be 30 pF and at 150 MHz, 7.5 pF. However, looking at the circuit, I can see that 30 pF is obtainable, but not 7.5 pF. (The C ratio is 4:1; the f ratio is 2:1, so at least that bit is consistent). And how do the two series-connected 82 pF capacitors effect the selected resonant frequency?
(Incidentally, the 1N757 is a 9.1 volt Zener diode; a 2N918 will be a suitable replacement for the 2N3563)

Am I missing something here or is my analysis in error?

Al.
Aug. 4th.
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Old 5th Aug 2021, 2:46 pm   #2
regenfreak
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Default Re: FM oscillator circuit: questions

It looks like a series LC tank if you ignore the two 82pF caps. I wish some electrical engineer can draw an equivalent circuit for this as I find it confusing too. it looks a bit like a Colpitts oscillator with the two 82pF but not quite. I came across puzzling circuits like this before.

From the datasheet of the varactor MV107 min = 19.8pF and max = 24.2PF, ignoring the effect of the 82pF capacitors,the varactor is in series with the 5pF, so the maximum series capacitance is 4.138PF. The overall Cmax = 34.1pF .

83pF looks like a voltage divider for the output.

Correct me if I am wrong, the frequency modulation is achieved by the amplitude fluctuations of Ib changing Miller capacitance of the transistor and hence frequency modulation. Therefore you have to somehow include this shunt input capacitance when you apply the LC resonance equation.
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Old 5th Aug 2021, 2:55 pm   #3
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Default Re: FM oscillator circuit: questions

Clapp.

Lawrence.
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Old 5th Aug 2021, 2:59 pm   #4
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Default Re: FM oscillator circuit: questions

Thanks

this is the resonance equation:

https://www.electrical4u.com/clapp-oscillator/
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Old 5th Aug 2021, 3:02 pm   #5
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Default Re: FM oscillator circuit: questions

the AC signal input is actually varying the voltage applied to the varicap which in turn is modulating the oscillator frequency.
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Old 5th Aug 2021, 3:06 pm   #6
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Default Re: FM oscillator circuit: questions

Quote:
the AC signal input is actually varying the voltage applied to the varicap which in turn is modulating the oscillator frequency.
I see.


1/C1 + 1/C2 + 1/C3 = 1/C
C= 18.587pf

L = 150nH f = 95.3MHz bingo
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Old 5th Aug 2021, 3:12 pm   #7
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Default Re: FM oscillator circuit: questions

https://www.electroschematics.com/fr...ed-oscillator/

Lawrence.
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Old 5th Aug 2021, 6:35 pm   #8
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Default Re: FM oscillator circuit: questions

Instead of using varactor frequency moderation, this is the Miller effect moderator with Colpitts:

https://armymunitions.tpub.com/Mm032...dulator-16.htm
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Old 5th Aug 2021, 9:21 pm   #9
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Default Re: FM oscillator circuit: questions

The tunable range is set by adjusting the 0.15uH coil (aided by the trimmer c)

The varactor is only required to produce the FM, so perhaps +/- 100kHz max.



The circuit oscillates when the series chain which includes Lt parallel resonates with the two 82pf caps in series, i.e we need an inductor which resonates with about 40pF.

As Lt is configured in its own series tuned circuit, this must be held above resonance to yield an inductive reactance.

The effective inductance of this series circuit needs to be about 62nH at 100MHz, i.e much less than the actual inductor value, due to the circuit operating not far from series resonance. At much higher frequencies the effective inductance is asymptotic to the value of Lt, i.e 150nH.
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Old 5th Aug 2021, 10:33 pm   #10
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Question Re: FM oscillator circuit: questions

Quote:
Originally Posted by regenfreak View Post
From the datasheet of the varactor MV107 min = 19.8pF and max = 24.2PF, ignoring the effect of the 82pF capacitors,the varactor is in series with the 5pF, so the maximum series capacitance is 4.138PF. The overall Cmax = 34.1pF .
The varactor is an MV2017: I assume your above "MV107" is a typo.

For this type of varicap diode, I would expect its Cmin. to be a lot less than the 19.8 pF you have stated. I searched the www for the datasheet for the MV2107, but found nothing. Where did you get your data from for this varicap, please?

Quote: "the varactor is in series with the 5pF, so the maximum series capacitance is 4.138PF"
"The 5 pF"? The capacitance in series with the coil comprises the trimmer (10 pF max.) and a 22 pF in parallel with it. So the maximum series capacitance is not 4.138 pF.

Al.
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Old 5th Aug 2021, 10:40 pm   #11
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Arrow Re: FM oscillator circuit: questions

Thank you all for your replying posts. Having read those replies, it now seems that this cct. is not as straightforward in terms what determines its fundamental freq. of operation as I first anticipated.

I'll have to build this simple circuit and test it. When I've done that, I'll send in my results.

Al.
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Old 5th Aug 2021, 11:43 pm   #12
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Default Re: FM oscillator circuit: questions

Quote:
For this type of varicap diode, I would expect its Cmin. to be a lot less than the 19.8 pF you have stated. I searched the www for the datasheet for the MV2107, but found nothing. Where did you get your data from for this varicap, please?

Quote: "the varactor is in series with the 5pF, so the maximum series capacitance is 4.138PF"
"The 5 pF"? The capacitance in series with the coil comprises the trimmer (10 pF max.) and a 22 pF in parallel with it. So the maximum series capacitance is not 4.138 pF.


It is from
http://pdf.datasheetcatalog.com/data...101-MV2115.pdf


I didn't make it clear. Assume CTmax = 24.2pF,

4.1pF is the overall capacitance for 5pF and 24.2pF in series.

Referring to the Clapp resonance equation on this page:

https://www.electrical4u.com/clapp-oscillator/

I misread the number as I did it in a rush, I put 20pF instead of 22pF

C3=4.1+22+10=36.1pF

C1=C2=82pF

Aplying Clapp with 1/C= 1/C1 +1/C2+1/C3

C=19.197pF and assume L = 150nH F = 93.8MHz. Right or wrong. With a homebrew FM transmitter, you take any arbitrary frequency not coincides with any station.
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Old 5th Aug 2021, 11:59 pm   #13
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Default Re: FM oscillator circuit: questions

Just note that the Clapp equation in above web page is wrong, it should not be 1/C inside the square root but C instead which I corrected in my calculation above. See equations 11.13 and 11.14:

http://educypedia.karadimov.info/lib...2Lecture24.pdf

The equivalent circuit is in figure 11.5 in which L is in series with C1, C2 and C3 (which are also in series)

Last edited by regenfreak; 6th Aug 2021 at 12:12 am.
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Old 6th Aug 2021, 12:46 am   #14
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Default Re: FM oscillator circuit: questions

In datasheet, the tuning ratio is 2.9 which is the CT at 2v divided CT at 30V. The Ct min and max are only applicable for "VR=4V"
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Old 6th Aug 2021, 3:52 am   #15
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Default Re: FM oscillator circuit: questions

The varactor diode capacitance versus bias voltage law doesn't give you an accurately linear frequency/voltage characteristic. It becomes better as the varactor is used to get lower and lower amounts of frequency swing.

In ye olden days when reactance valves were (almost) the only show in town, one method of getting better linearity was to design a basic modulated oscillator for very little frequency deviation, on a much lower frequency, and then employ a chain of frequency multipliers.

The multipliers not only multiplied the carrier frequency, they also multiplied the deviation. So the deviation required on the base oscillator was so low that its characteristic curve started to look more like a straight line. You might think that the frequency multiplier chain would also increase things so that the non-linearity would be scaled up too, back to where it was. But the nature of the curvature helps and the error in the end actually improves.

This trick was used by Edwin Armstrong, the inventor of FM, and is usually referred to as 'The Armstrong Method' of FM generation. (Humourous people often use the same term to refer to rotating an antenna by hand rather than with a geared motor.)

Another variety, once common in 2 metre amateur radio transmitters is to start with a crystal oscillator on a sub-multiple of the wanted transmit channel, and to use a chain of frequency multipliers to bring it up to the final frequency. This is ripe for using the Armstrong Method, and crystal oscillators don't pull easily (which is good in this case) but with switched crystals for different channels, the crystals won't necessarily give the same deviation for the same audio drive. You could have a bank of deviation cal pots to go with the crystal bank, but the switching and setting up is tedious. Instead, it was common to put a phase modulator circuit after the crystal oscillator, and to shape the audio applied to it with an integrator filter (Phase is the integral of frequency because frequency is the rate-of-change of phase). Phase modulators are also a bit low in the amount of modulation they can give, so the Armstrong Method helps them out and gives you the wanted deviation.

The Armstrong Method was commonly used to get broadcast FM transmitters down to hifi levels of distortion.

David
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Old 6th Aug 2021, 7:38 am   #16
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Default Re: FM oscillator circuit: questions

Of topic-ish. Ye is actually pronounced the. Old English used to have a symbol called the thong, that was pronounced th, it looked a bit like the Greek lower case rho. Icelandic still has the thong in its everyday language.

So why ye? Well when Caxton was putting together the symbols for his printing press in the mid 1400's he decided that it would complicate things somewhat to have something that hung over from early language, so he based his alphabet on the usual 26 letter alphabet.

But he still needed to have something that took the sonic value of the thong. So he picked y, and since there was no word in the English language ye, y in that context became the thong.

So ye is pronounced the. And always was since Caxton

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Old 6th Aug 2021, 12:53 pm   #17
regenfreak
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Default Re: FM oscillator circuit: questions

Quote:
The varactor diode capacitance versus bias voltage law doesn't give you an accurately linear frequency/voltage characteristic. It becomes better as the varactor is used to get lower and lower amounts of frequency swing.
For homebrew FM transmitter project, the band tuning range is not critical.

I built a solid state FM stereo kit a few months ago. It uses three VHF B910 varactor diodes tuning for the FET amp, mixer and oscillator LC tanks. The attached graph is the capacitance versus reversed voltage. It looks fairly linear.

Varactor tuning diodes are often used in series with a padder capacitor or two varactor diodes back-to-back.

Quote:
n ye olden days when reactance valves were (almost) the only show in town, one method of getting better linearity was to design a basic modulated oscillator for very little frequency deviation, on a much lower frequency, and then employ a chain of frequency multipliers.
The multi-stage valve amplifiers have both tuned LC input and output operating in class C with non-sinusoidal waves rich in harmonics.
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Old 6th Aug 2021, 7:35 pm   #18
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Default Re: FM oscillator circuit: questions

In#17, we have "The attached graph is the capacitance versus reversed voltage. It looks fairly linear".
It may look linear on a log - lin scale, but that does not make it linear on a lin - lin scale surely?
Les.
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Old 6th Aug 2021, 9:41 pm   #19
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Default Re: FM oscillator circuit: questions

And then you have the capacitance to frequency characteristic set by the choice of topology and values of the oscillator circuit F = 1/(2*pi* root(L*C)) which is not linear in C
VCOs are pretty non-linear most usually for any but trivial frequency shift.

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Old 6th Aug 2021, 10:20 pm   #20
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Default Re: FM oscillator circuit: questions

Quote:
Originally Posted by Craig Sawyers View Post
Of topic-ish. Ye is actually pronounced the. Old English used to have a symbol called the thong, that was pronounced th, it looked a bit like the Greek lower case rho. Icelandic still has the thong in its everyday language.

So why ye? Well when Caxton was putting together the symbols for his printing press in the mid 1400's he decided that it would complicate things somewhat to have something that hung over from early language, so he based his alphabet on the usual 26 letter alphabet.

But he still needed to have something that took the sonic value of the thong. So he picked y, and since there was no word in the English language ye, y in that context became the thong.

So ye is pronounced the. And always was since Caxton

I'll get my coat...

Craig
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