# VCO Design

Discussion in 'Electronic Design' started by redhat, Jun 23, 2005.

1. ### redhatGuest

In VCO design using negative resitance amplifier, i want to design the
resonator first,should it designed to resonate at the wanted center
frequency or this frequency will chage when connecting the negative
resistance amplifier,so, what should i do first? what is the steps of
design?

Regards

2. ### Andrew HolmeGuest

All circuits contain stray capacitance and inductance. The higher you
go in frequency, the more significant these strays become. Transistors
have junction capacitance. These strays and parasitic reactances can
be hard to predict, but they can be minimised, and they should be
small. It's common practice to include a small trimmer capacitor to
correct for them. The circuit you posted recently in s.e.b included a
trimmer. Be aware though: it's hard to make a stable, accurate
LC-controlled oscillator at frequencies much above 10 MHz.

An empirical approach is often taken to RF design. Why not make a
prototype? It could save you a lot of time and you will learn from it.
Don't try to work out every last detail before soldering two
components together.

3. ### redhatGuest

in designing vco with negative resistance analysis , the output
reactance of the resonator should be equal to the input reactance of
the reflection amplifier, so the resonant frequency will be the same as
the resonant frequency of the resonator, am i right? what about the
value of Q, i have no specific phase noise requirement, what value of Q
should i take?

4. ### Andrew HolmeGuest

The magnitude of the negative resistance produced by the amplifier
needs to exceed the loss resistance of the inductor.

Any reactance seen looking into the amplifier will appear in series
with the tuned circuit and will slightly alter its resonant frequency,
but this should not be a significant proportion of the total reactance.
Resonant frequency should primarily be governed by the tuned circuit
components.

Inductor Q is the ratio of its reactance to loss resistance. You want
this to be as high as possible. Plucking figures out of the air, for
an oscillator, with no specific application, I would say 20 is rubbish;
50 is OK; 100 is good.

5. ### Andrew HolmeGuest

[snip]

Colpitts and Clapp circuits can be analysed as negative resistance
oscillators, and the capacitive divider is shown as part of the
negative resistance amplifer, even though it is part of the tuned
circuit. If the dotted line - where tuned circuit meets "reflection
amplifier" - is drawn to the left of the capacitive divider, there
would be equal and opposite reactances either side of the line. But
the oscillator frequency is not the same as the resonant frequency of
the series LC components to the left alone, because the capacitive
divider contributes significant reactance.

6. ### MarkGuest

[snip]
I am glad to hear you say that. I have always had a problem with the
concept of the negative resistance connected to a tank circuit. My
problem is that the transistor by itself cannot create the negative
resistance, it needs the impedance transformation created by the tank
circuit to create the negative resistance. So it really is all about
where you "put the dotted line" as you say. The negative resistance
may be one concept for the thought process but there is no actual
physical way that you can disconnect the tank circuit from the
transistor, and connect a network analyzer, and measure a negative
resistance. The tank circuit must also be attached to create the
negative resistance because it creates the impedance transformation.
So the tank circuit and transistor TOGETHER create the negative
resistance, but this is just another way of saying that you connect an
amplifier to a tank circuit and the amplifier makes up for the losses
of the tank and for the output power so it oscillates. To me, the
amplifier concept is much more straight forward compared to the
negative resistance concept.

Mark

7. ### redhatGuest

i am going to use this resonator:
http://www.geocities.com/aezzat3/resonator.jpg
i have a coil that has Q of about 48 at the specified center frequency
950MHz, should i find another coil with higher Q to have a high
effective Q, why does the Q of the coil affects Qe that much? Qe=-92.3
for QL=100 , Qu=48

8. ### redhatGuest

i searched for another inductor but the maximum Q that i found for an
inductor at this frequency is about 70, may be i didn't search well
,but if i want to have higher Q what should i do? is there anyway to
increase the Q of the vco?

9. ### redhatGuest

i searched for another inductor but the maximum Q that i found for an
inductor at this frequency is about 70, may be i didn't search well
,but if i want to have higher Q is there anyway to increase it? what is
the meaning of negative Q