# LC oscillator

Discussion in 'Electronic Design' started by Johnson F., Oct 8, 2004.

1. ### Johnson F.Guest

Hi,

consider this CMOS gate "LC" oscillator circuit :

http://www.discovercircuits.com/PDF-FILES/parallelosc.PDF

I want to use L1=1uH and C1=1500pF. The circuit must oscillate close to
4Mhz. It will be used as part of a simple inductance meter, the frequency
being measured to find an unknown the inductance value (the inductance to be
measured, that can range from 1uH to 100mH, is placed in series with L1).

*But* should C2 also be increased to 1500p, that is, be the same value as
C1?

Thanks for any hints!
John

2. ### John PopelishGuest

Your changes lower the impedance of the resonator enough that the CMOS
inverter may not be a good match. Take a look at a somewhat similar
oscillator based on a comparator (lower output impedance and lots more
gain). It was designed as part of an inductance/capacitance meter.
http://my.integritynet.com.au/purdic/lc-meter-project.htm#circuit-osc

3. ### Andrew HolmeGuest

L1, C1 and C2 form a capacitively tapped tuned circuit:

+------+ Equal capacitors in series
| | C = C1/2 = C2/2
--- |
C1 --- ) Tuned circuit f = 1/(2.pi.sqrt(LC))
| L )
--- ) f = 0.23/sqrt(L.C1)
C2 --- |
| | This is the equation in the PDF
+------+

4. ### Johnson F.Guest

Mmmh! So I would have to use something like 3000pF for both C1 and C2 to get
the desired 1500pF!!!
As John says in the other reply, the CMOS gate may have a hard time to
oscillate.
I forgot to mention I want to use the 74HC4060 with its built-in oscillator
gates.

(sorry for my english...)
John

5. ### Tim ShoppaGuest

Not disputing your points, but C1 = C2 is not a necessity. C2
in combination with the 10K resistor and the output impedance of the
CMOS gate form a RC low-pass filter. Making C2 bigger moves the
cutoff frequency down, and this can take away the loop gain and
take the circuit out of oscillation.

In the PDF schematic, RC is 4.7uS (f = 212000 Hz) which is fine for a 125
kHz oscillator. Making C be 1500 pF without changing the 10K will
make RC be 15uS (f = 66kHz) which is way too low for an oscillator at
4MHz. The "right" thing to do is use a much smaller C. Making the 10K
resistor smaller will help to an extent but quickly you'll be at the
limit of the output impedance of the CMOS gate.

I suspect that with 10K and 1500pF, if the thing oscillates, it will
be a RC relaxation oscillator at around 66kHz.

Tim.

6. ### Tam/WB2TTGuest

L - C 1 - C2 forms a pi network. The C1/C2 ratio determines the "gain" from
output to input. I would keep C1 and C2 the same value, or you run the risk
of either not enough gain, or burning out the CMOS input. As has been
pointed out, 1 uH is way smaller than what is typically used at 4 MHz. It
will probably oscillate if you make L= 10 uH, C1 = C2 = 300 PF, and change
the 10K to 1 K.

I really don't think this is the circuit you want to use for an inductance
meter, and vote for plan-B. Search the net for inductance meter or
inductance measurement. I would look for some kind of RL circuit. For one
thing, frequency would vary linearly with L. Whatever you use, you can't
measure 1 uH to 100 mH all in one range. Also, the 4XXX stuff is way too
slow to run at 4 MHz, your error might be 50 % of the calculated frequency.

Tam

7. ### Johnson F.Guest

Obviously, I have to reconsider the oscillator.
John

8. ### BanGuest

Why don't you copy or buy this inexpensive instrument? Works very well. I
have one standing here and I'm happy with it.

9. ### Johnson F.Guest

Just for the fun of it!
I was more or less copying from this one.

http://www.electroniquepratique.com/article.asp?mag=EP&num=271&article=15

Measures from ~100nH to ~1H using 74HC gate and a BF245A JFET for the
oscillator followed by a 74HC4040 to divide the frequency by 32 then fed
into a computer port. Maximum frequency is ~4Mhz at the oscillator and
~125Khz at the computer port.
I was trying to simplify the oscillator using only a 74HC4060. Simplifying
and modifying circuit is part of the fun I got from a project like this and,
so far, it was a successfull method with most other projects I've done. I
have realized that some circuits from magazines and other sources *can* be
simplified without drawbacks (or very little) and I also learn a great deal
during the process.

Regards!
John