g >noise. That got me scared a bit that I could be jamming something somewh ere so I >immediately turned off my supply. With such low power and at 100 MHz I don't >expect my interference would travel very far. But at least now I understand the >importance of shielding.
I will probably invest in one of these little fellows:
On a sunny day (Sun, 28 Jul 2013 03:04:45 -0700 (PDT)) it happened "M. Hamed" wrote in :
I do not want to critisise other posters, but really, the 100 MHz was 5 to 7 turns 8 mm diameter coil spaced over 1 cm with one of those air dielectricum caps as tuning cap. Open up any old FM radio, and look, or better yet, this is an ebay FM transmitter board tuned to 107.2 I use
The "Q' story as quoted is a bit different:
Bandwidth: B = f0 / Q
From:
- the aerial or amplifier feeding the tuned circuit for example - will contain present resistance.
**For a series resonant circuit, the higher the inductance and the lower the capacitance, the narrower the filter bandwidth (meaning the reactance of the inductance, L, and the capacitance, C, at resonant frequency will be relatively high compared with the series source/load resistances). **For a parallel resonant circuit the opposite applies; small inductances reduce the damping of external circuitry.For exmaple, this is LTspice my run for the 25 MHz DVB-S transmitter exciter filter at 25 MHz with a 220 pF capacitor and 184 nH coil:
And this with a 22 pF capacitor and 1.84 uH coil (factor 10 change, same frequency)
Now look at the bandwidth of the first one, compared to the second, At 50 MHz (2 x f0) attenuation is better than 33 dB.
And then the second one: at 50 MHz (2 x f0) attenuation is about -14 dB, not very usable as filter.
That is why I wrote in a previous post that bigger C is better, for a given circuit load (1 k Ohm here), in a parallel setup.
Bigger C is also better as there is less influence from parasictic capacitances, like input C of transistors, PCB wiring, test leads, what not. And less turns, made with nice silvered wire, on a good coil former is more stable than some ferrite with unkown properties, on top of that sensitive to magnetic infuences.
Look at the silver wire:
Remove the core, and it should give you 100MHz with that big variable capacitance. Big bigger diameter helps.
That is very nice, really.
On a sunny day (Sun, 28 Jul 2013 03:27:32 -0700 (PDT)) it happened "M. Hamed" wrote in :
If you are on-frequency and have a clean power suply, then your radio should become very quiet, as it sees an 'unmodulated' (= stable in frequency) carrier.
No.
With high Q inductors, L/C dominates the resonant impedance.
R is equivalent *series*, not parallel resistance, BTW, ie. R=wL/Q
If you've found the previous thread, see my second posted Spice simulation to that thread (the current-fed, constant Q, one). That demonstrates dependence of Zr on L/C.
--
"Design is the reverse of analysis"
(R.D. Middlebrook)
For example, this is LTspice my run for the 25 MHz DVB-S transmitter exciter filter at 25 MHz with a 220 pF capacitor and 184 nH coil:
And this with a 22 pF capacitor and 1.84 uH coil (factor 10 change, same frequency)
Now look at the bandwidth of the first one, compared to the second, At 50 MHz (2 x f0) attenuation is better than 33 dB.
And then the second one: at 50 MHz (2 x f0) attenuation is about -14 dB, not very usable as filter.
That is why I wrote in a previous post that bigger C is better, for a given circuit load (1 k Ohm here), in a parallel setup.
P.S. Actually this is really simple to understand if you look at his configuration as a combined high and low pass (as that is what it is, 'resonance' is just a phase zero frequency where the complex part cancels).
in --- R ----- out | gives better attenuation at higher frequencies for bigger values of C. === | ///
and:
in --- R ----- out | gives better attenuation at lower frequencies for smaller values of L. L usually smaller values of L have lower resistance due to less turns, | causing a dead short versus a slight output at DC. ///
:-)
That's misleading. The Q of each circuit, as drawn, is not the same; the difference in bandwidth is due to Q, not L/C ratio.
The effective Q of the 220pF circuit is 38. The effective Q of the 22pF circuit is 3.8.
That's a 10:1 bandwidth ratio, right there. Nothing to do with L/C ratio.
Assuming the same wire gage, a 1.84uH inductor will have sqrt(1000) times the resistance of a 184nH coil. That's about thirty times. You used the same resistance in both cases.
Your simulation doesn't prove your case, regarding "bigger C", which is wrong, anyway.
Now go figure how to simulate L/C ratios properly...
--
"Design is the reverse of analysis"
(R.D. Middlebrook)
On a sunny day (Sun, 28 Jul 2013 09:13:58 -0700) it happened Fred Abse wrote in :
I think you misunderstand something. You can actually only talk about Q ( > 1 ) if you get some voltage increase, that would be the case, as you mentioned, if you were driving with a current source.
That current source could be a collector of a BJT (pretty flat Ic versus Vce).
In this case, you may notice from the simulation that the output voltage is always LOWER than the input voltage, so from your POV Q is < 1 perhaps.
In the most ideal case, in 'resonance' the impedance of the LC is infinite (if it has a huge Q by itself) and the circuit is just a resistor, and, given the infinite impedance of the LTspice voltmeter, Uin = Uout.
That reduces the circuit to a low pass and high pass outside f0, and in the case of low - and high passes, L and C, DO make all the difference,
Look here at the situation without L:
That was a joke right ;-) although it is true, at DC the R will give a residual voltage... more for a multi turn L.
It think you pay way to much attention to R of L in this, logically as you still think infinite current drive.
If you really want to see Q put up voltage swing, do this
C L zero output impedance amp --|>--- R ---||-----L ----| gnd | out
Are you arguing with LT spice?
Well, I gave a practical example, but you seem still fixed on your current source. The above circuit shows how to drive from a low impedance voltage source.
In the oscillator example this is all about low impedance drive however, the LC is loaded with a fixed resistor, and there is no 'infinite source' driving it. So maybe you should do some self inspection...
But if that is your religion... ;-)
Hooray. You are actually doing very well. Good enough to make jealous of all the fun you are having. Keep going.
?-)
Late at night, by candle light, "M. Hamed" penned this immortal opus:
Congratulations, you've just discovered the buffer amplifier, to isolate the oscillator from the load. From there you can take the signal and do something with it without disturbing the frequency.
And now you know to take parasitics into account. Good work!
At that power it's probably detectable at some tens of metres at most.
- YD.
-- Remove HAT if replying by mail.
Late at night, by candle light, "M. Hamed" penned this immortal opus:
So recalculate for some more reasonable L, like 60 nH. You probably already know, but when you have two of f, L or C you can calculate the third by
f = 1/(SQRT(2 * pi * L * C)) L = 1/(4 * pi^2 * f^2 * C) C = 1/(4 * pi^2 * f^2 * L)
For winding the coil, do a search on "inductor calculator", there's a lot of them on-line.
- YD.
-- Remove HAT if replying by mail.
Very nice pictures. May I ask you what material you are using that is surrounding the coil?
uld become very quiet,
It turned out connecting my cheap frequency counter is the source of this n oise. It probably totally messes up the biasing. When I don't have that con nected, I get exactly the effect that you mentioned. It's pretty cool when the frequency on my radio matches the frequency readout on my scope!
I have to admit I am getting confused by the L/C ratio. I read it in another book that for parallel resonance the L/C ratio should be small. A bit counter-intuitive to me.
Thanks! It's been a lot of fun and a lot of learn> Congratulations, you've just discovered the buffer amplifier, to
I am seeing a strange effect though. When I am not probing at the FET the frequency is slightly higher (someone at work happened to have a wireless sniffer). So the load on the FET affects the tank frequency. My only guess here would be Miller effect?
#6
--- The fact that every cap in my oscillator circuit seems to be able to affect the frequency bothered me. The fact that I couldn't analyze the circuit bothered me more. I ended up taking a shot at trying to figure out how f is related to all the C's.
After a few agonizing hours with the math and seven pages of symbols, I actually figured it out and came up with a formula.
2pi.f = sqrt ( (c3*c4 + c2*c3 + c2c4) / ( L * (c1*c3*c4 + c2*c3*c4 + c1*c2*c3 + c1*c2*c4)) )I put down the formula in an Excel spreadsheet that would computer the frequency and it almost always matched simulation especially if the cap ratios are reasonable. C1, C2 are the tank caps and C3,C4 are the tap caps.
Further simplification can be made if C3,C4 >> C1,C2 then the frequency could be simply:
2pi.f = sqrt ( 1 / (L * (C1+C2)) )This is the first time I analyze such a circuit and probably the most math I have ever done outside of college. It's also the first oscillator that I could mathematically figure out. proud of myself!
On a sunny day (Wed, 31 Jul 2013 09:50:32 -0700 (PDT)) it happened "M. Hamed" wrote in :
? air? ferrite core < > 0 || < >|| plastic former || < >||0 0 || < >|| || < >||0 silvered wire 0 || || || ||0 0 || || =============|=
-------------------
--------------------- PCB
If you do not have it yet, get the ARRL Handbook (a slightly old printing will do). It will save you countless hours of guesswork.
Link:
-- Tauno Voipio, OH2UG
On a sunny day (Wed, 31 Jul 2013 09:52:32 -0700 (PDT)) it happened "M. Hamed" wrote in :
Yep!
Now if you put some music from a sound card via a capacitor of say 100 nF, and a series resistor of say 100 k, to the base or collector of that oscillator, you get an FM transmitter, especially if you have some collector resistance. Because if Ic varies then Vce varies, and the capacitance of the collector base junction varies, and that slightly changes the frequency. Then you have your first radio transmitter,
You can change frequency too by connecting a reversed biased diode across the coil, and modulate the bias with the sound card (or mp3 player or whatever) audio: + | [ ] 100 k ~10 pF |--------------||---> to coil hot side
---- | / \ [ ] 100k
--- | | === 100n /// | | 1Vpp audio from soundcard or player
The diode should be a small signal diode, or simply use a transistor with base and emitter connected to ground and the collector at the 100 k (NPN if + supply), so the collector acts as a revere biased diode. Variations are endless, almost _anything_ will FM modulate your circuit. Not all very linear, but that can be fixed too...
Q: is there a #7 etc in the works?
I guess this deserves an update. It's been almost a year and a half. Since then I gave up radio and RF because it was too difficult and too time consu ming and a lot of material had to be digested.
However since then I got my amateur radio license, learned Morse code, and added a few more tools to my toolbox. Then I stopped all radio related acti vities and got busy with other things. But after a year and a half I couldn 't resist coming back to radio and electronics. The idle soldering iron, th e dusty scope, and untouched part boxes didn't give me a good feeling. I re alized I was approaching this the wrong way and I was making it too difficu lt for myself.
So, I will have less ambitious goals, and I will be starting a new Log :)
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