FM transmitter

[Kevin Aylward]

Dbowey wrote:

Fred posted:
Are you saying it is a tuned circuit because C4 is a variable
capacitor? I think it would be simpler to say it is a tunable
colppits because one of the C's is variable, otherwise people
(me) might think "tuned circuit" as in "pure LC circuit". This
circuit is so bad in the way it is drawn, the LC circuit gives
the impression it's a pure tuned circuit. The circuit is
difficult enoght without this added confusion. The person who
drew this circuit probably didn't quite understand it.


I think the circuit is drawn quite well, and that you need to study a
bit more before you critique other's work.

The L and C form a "tank circuit" which is tunable by varying the
value of C4. I could also tune it by varying the inductance by
compressing or stretching the coil. I can also tune it by varying
the applied voltage to the circuit. Whether it is "pure" or all
screwed up, is all in your mind.

Whether the type of circuit is a Colpits or something else, is not
particularly relevant to how the circuit tunes.

However, regardless of all the previous posts, this is not a Colpits.

I disagree. It is. The circuit clearly functions by action of capacitor
reactance at the be and ce terminals, with inductive reactance at the bc
terminals. This is *not* debatable. Its how it works. End of story. Go
and do the math as I outlined.

It looks like a form of Pierce oscillator.

This is probably a bit of symantics. The name itself is not important,
its how the impedances act at the oscilation frequency.

I have subsequently done a little search, and this paper here certainly
supports my view, not that I require such support.

http://www.oscilent.com/esupport/TechSupport/ReviewPapers/IntroQuartz/vigtypes.htm

Note what terminals are the base, emitter and collector, and where the
capacitors and xtal are located.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
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"That which is mostly observed, is that which replicates the most"
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"quotes with no meaning, are meaningless" - Kevin Aylward.

 

[fred]

I think the circuit is drawn quite well, and that you need to study a bit more
before you critique other's work.

yes, you are right, for some reason I thought the circuit was
not drawn correctly, but it is, this is my mistake, I admit it.

The L and C form a "tank circuit" which is tunable by varying the value of C4.
I could also tune it by varying the inductance by compressing or stretching
the coil. I can also tune it by varying the applied voltage to the circuit.
Whether it is "pure" or all screwed up, is all in your mind.

we have this

T-base, +Vcc
|
|
|
|-----|-----|
| | |
| | Cbe
L C4 |-------T-emitter
| | |
| | C5
| | |
|-----|-----|
|
|
|
T-collector


which is the same as
|
|
|--|--
| Cbe
X1 |------
| C5
|--|--|
|
|

This *is* colppits becaese X1 is inductive even with C4, but
changing C4 will increase/decrease the indutiveness.
Remember, if X1 is inductive, we have colppits, otherwise
we have hartley.
Obviously you can change the frequency of oscilation by changing
any one of these term, X1 (C4 or L), or Cbe (by changing base
voltage)

 

[Watson A.Name - \"Watt Su]

"fred" <fred222@btinternet.com> wrote in message
news:2685a85f.0402170901.3162ab3@posting.google.com...

I said it was a Colpits, which it is. It is also a tuned circuit.
However, the tuning is *not* at *resonance*. Just off resonance
there is
a fast rate of change of phase with respect to frequency, which
results
in better frequency stability. The tuned circuit is tuned so that it
has
a net +jx impedance. It is not a pure real impedance.

Are you saying it is a tuned circuit because C4 is a variable
capacitor? I think it would be simpler to say it is a tunable
colppits because one of the C's is variable, otherwise people
(me) might think "tuned circuit" as in "pure LC circuit". This
circuit is so bad in the way it is drawn, the LC circuit gives
the impression it's a pure tuned circuit. The circuit is
difficult enoght without this added confusion. The person who
drew this circuit probably didn't quite understand it.

To add even more confusion.. In the circuit that I use, I add a 5 to 8
pF capacitor between the emitter to ground, across the emitter resistor.
This helps swamp out any changes in the transistor's capacitance and
makes the frequwncy more stable.

I use a coil of 5 turns of 22 AWG (6.4 mm) enameled wire close wound on
a 3/16" (4.8 mm) air core, which is about 120 nanohenrys. With a 22 pF
tank capacitor and the remaining capacitance of the circuit, about 5 or
6 pF, the circuit oscillates at about 90 MHz, the low end of the FM
band.

Here's an even more Colpitts circuit, using two caps to split the tank
like a regular Colpitts osc.
http://www.zen22142.zen.co.uk/Analysis/colfr.htm