LC oscillator

[Johnson F.]

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

 

[John Popelish]

"Johnson F." wrote:

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

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
--
John Popelish

 

[Johnson F.]

"Andrew Holme" <ajholme@hotmail.com> a écrit dans le message de news:
966757eb.0410080226.1ed71d27@posting.google.com...

"Johnson F." <johnf-nospam@losalanos.com> wrote
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?

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
+------+

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

 

[Ban]

Johnson F. wrote:

Thanks for your advices!
Obviously, I have to reconsider the oscillator.
John

Why don't you copy or buy this inexpensive instrument? Works very well. I
have one standing here and I'm happy with it.
http://www.aade.com/lcm2binst/LC2Binst.htm
--
ciao Ban
Bordighera, Italy