need help with fm circuit study

The circuit in question is

http://www.geocities.com/myelectronicsrevision/fmcct1.jpg

(you may need to copy and paste this link in a new browser
if you don't get anything)

The left portion is just an audio amplifier which is easy. The right
portion, circled in blue, is an fm transimitter. Is this a Hartley
oscillator. I do understand the modulation bit, which is performed
by the variable capacitor/diode D, but if we forget the fm
modulation for a moment, how does the oscillator work, are there
any books I can find an explanation about such a configuration?

 

Hi,

The reason, I guess, that you haven't yet received a reply to your
posting, is that the explanation of how that particular oscillator
works (in the time domain) is notoriously long-winded. In my relative
youth, we used to have oral theory exams where you had to stand up and
explain circuits to the instructor without any aids (not even a piece
of chalk). The erstwhile multivibrator (of which this circuit is a
variant) was a favourite of their's and so I don't intend to try here.

Call me chicken if you like but, unlike Marty in "Back to the
Future", I don't care. The problem is that you can get a rough-and-
ready "This is how it works" but the finer detail does get somewhat
involved. Sufficient to say, look-up multivibrator and push-pull
circuits, put them both in a poke (take the pig out first though),
shake and there you are.

As for books, try an older copy (cheaper and probably better for
you) of the "ARRL Radio Amateur's Handbook" which is updated every
year. It isn't filled with FM circuits but should be one of the first
books on any RF start-up's bookshelf. In the general electronic sense,
go for Horowitz & Hill's "Art of Electronics", not cheap but worth it.
Not only that you get to praise, or complain to one, of the writers,
Winfield Hill, who is a denizen of sci.electronics.design. We are all
waiting for the 3rd edition but may not live that long

Finally, no it isn't a Hartley. All oscillators that have a tapped
tuned circuit are not by Mr. Hartley. Notice that C8 decouples the
centre-tap to ground which cannot then take part in the feedback
process. The trimmer capacitor TR2 incidently is there to tune out the
leakage inductance of the antenna coupling coil. The antennas on these
things are usually short with respect to a quarter wavelength and
present a capacitive reactance, so TR2 helps resonate that coil with
the antenna.


Cheers - Joe

 

The reason, I guess, that you haven't yet received a reply to your

Normally oscillators are simply amplifiers with an added feedback such
that A*beta = -1. Is it possible to idetify the input and output in this
case, and the feedbach path? Is it a differential amplifier (long tailed
pair?) ?

The hartley oscillator usually is two coils and in the middle there is
earth, which is why I was asking. C8 for RF is short circuit, i.e. we
have earth between the coils, which, anyway, seems a bit redundant
as the center tap is also connected to the battery, which is also earth
for ac, but I am sure there is some reason for it...

 

I can see though that this is not a Hartley due to the symmetry of the
circuit. The split coils should connect output to input, but here I cannot
see that this is happening.

 

[Joe McElvenney?]

You didn't finish reading the instructions. Throw in an astable
multivibrator coincident with two mirror-imaged Hartley oscillators.

Does this help?

Good Luck,
Rich

 

You didn't finish reading the instructions. Throw in an astable

which instructions are that ? no I haven't got a clue about mirror
Hartleys, though I wouldn't mind if you wanted to expand, it does
sound interesting. (or if you can say from which book you found
this information)

 

This is a Hartley oscillator.

The cross-coupled capacitors makes it look like a multivibrator, and the
push-pull topology tends to obscure the classic Hartley circuit at first
glance. You can recognize the Hartley if you erase one of the transistors
and move the AC ground from the emitter to the collector of the remaining
transistor. Then you can move the coil so that the bottom end is grounded,
the tap is at the emitter, and the top end goes through the coupling
capacitor to the base.

I recommend the ARRL handbook, especially if you can find a vintage one from
the 60's to augment the current ones.

Neil

 

The circuit looked familiar, and as something more than a multivibrato
with a tuned circuit.

Checking the RSGB "VHF-UHF Manual" from 1972, it sure looks like
the oscillator in there referred to as the "Kaliatron" oscillator.
It doesn't really say much about it, though, even though there is
a sample circuit, tube-based, and an FET-based "GDO".

I'm sure I've seen a similar circuit, either a variation or just
under a different name. There was a 1964 issue of QST that had
an article about phasing SSB ringt on 144MHz, and I have a vague
feeling that they suggested an oscillator like this (albeit crystal
controlled), because of stability. But I'm not certain of that and
can't easily dig out the magazine. It was a less than common oscillator
circuit.

Michael

 

Heh - I just made it up. What I meant was two Hartley
oscillators, with the schematics drawn such that they're
mirror images of each other, and you put one on top of
the other so that they share the coil. Coincidentally,
when you do that, the circuit comes out looking almost
exactly like an ordinary astable multivibrator, where
the coil(s) is(are) the collector load(s).

HTH!
Rich

 

What ?!"£$%$£~~## How can you say that tapped tuned circuits are
not by Hartley, this is absolute ignorance, open this winfield book you
are recommending and read about Hartley & Colppitz, this is exactly
what they are, i.e. tapped tuned circuits.

 

Phil posted:

<< What ?!"£$%$£~~## How can you say that tapped tuned circuits are
not by Hartley, this is absolute ignorance, open this winfield book you
are recommending and read about Hartley & Colppitz, this is exactly
what they are, i.e. tapped tuned circuits.
Don't be such a !!?!"£$%$£~~##. Not ALL circuits that have a tapped coil are
Hartley oscillators.

Go read the book again.

Don

 

Or this:
http://www.radioelectronicschool.com/reading/reading27.pdf

 

Don't be such a !!?!"£$%$£~~##. Not ALL circuits that have a tapped coil
are

I am not anything mister. The poster said no circuit with tapped coil is by
Hartley. Hartley is a tapped coil isn't it!

 

phil posted (that dbowey posted):

I am not anything mister. The poster said no circuit with tapped coil is by
Hartley. Hartley is a tapped coil isn't it! >>

It has become clear to me that American english is not your native language, so
I understand your occasional comprehension problem.
If you will go back and read the poster's comments more carefully, you will see
that he did not say what it is you have attributed to him.

It would help if you would post a quote of other poster's comments rather than
post a paraphrase.

Just as a guess, is THIS the comment with which you disagree? "...Finally, no
it isn't a Hartley. All oscillators that have a tapped tuned circuit are not by
Mr. Hartley...."

As an aside, it is interesting to note that in your language and in english
(American and U.K., I believe) "!!?!"£$%$£~~##" has the same meaning, so I hope
there is no confusion over it.

Don