On Wed, 10 Dec 2003 23:28:06 GMT, wrote:

Myself I'd use an opamp to create a saw generator and then I can
control the swings.

Thanks, nospam. I can't imagine why I didn't think of this before. Got
just the circuit lying about for a 741 sawtooth too!
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

In article , Winfield Hill
writes
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Thats the way its done to pull crystals a long way.
The thought/real experiment to assist is to assume that the acoustic
resonator is resistive (zero phase) at series resonance.
The maintaining circuit can then be replaced by an equivalent resistor.
The circuit with resistor should oscillate at approx the resonator
frequency.The inductor across the varicap is selected to almost tune
out/parallel resonate with the varicap.
The series inductor is phase retard to ensure the maintaining circuit
tis zero phase . Adjustment of the varicap then moves the circuit above
and below the series resonance of the resonator.




--
ddwyer

In article , Winfield Hill
writes
Phil Hobbs wrote...

Tom Bruhns wrote:

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an
LC oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from
a decent varactor.

Sounds good. How about a specific example?

Thanks,
- Win

whill_at_picovolt-dot-com

Thats the way its done to pull crystals a long way.
The thought/real experiment to assist is to assume that the acoustic
resonator is resistive (zero phase) at series resonance.
The maintaining circuit can then be replaced by an equivalent resistor.
The circuit with resistor should oscillate at approx the resonator
frequency.The inductor across the varicap is selected to almost tune
out/parallel resonate with the varicap.
The series inductor is phase retard to ensure the maintaining circuit
tis zero phase . Adjustment of the varicap then moves the circuit above
and below the series resonance of the resonator.




--
ddwyer

W3JDR wrote:
Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
changed.

If you're not already using a "hyper-abrupt" type of varactor, you should
look into one. They offer a wider capacitance range.

What type of varactor are you using, and what's the frequency of the
resonator? What's the application...linear frequency modulation like FM or
data keying like FSK???

Joe
W3JDR


"Paul Burridge" wrote in message
...

Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

p.
--

"I expect history will be kind to me, since I intend to write it."
-

Winston Churchill



If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same
min. frequency. Im not sure that the high end won't be greater than
before, even though the capacitance ratio is the same, since the
fixed inductance is lower.

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.

W3JDR wrote:
Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
changed.

If you're not already using a "hyper-abrupt" type of varactor, you should
look into one. They offer a wider capacitance range.

What type of varactor are you using, and what's the frequency of the
resonator? What's the application...linear frequency modulation like FM or
data keying like FSK???

Joe
W3JDR


"Paul Burridge" wrote in message
...

Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

p.
--

"I expect history will be kind to me, since I intend to write it."
-

Winston Churchill



If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same
min. frequency. Im not sure that the high end won't be greater than
before, even though the capacitance ratio is the same, since the
fixed inductance is lower.

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.

If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same
min. frequency. Im not sure that the high end won't be greater than
before, even though the capacitance ratio is the same, since the
fixed inductance is lower.

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.


Kenneth,

F=1/((2*PI)*SQR(L*C))

If you double C, you have to halve L to maintain the same frequency. If you
do this, you only changed the LC ratio, not the delta tuning range.
The only way to get more delta F is to get more delta C. If you use the
switching technique, you'll have a discontinuous tuning curve (Vtune vs
Freq) which makes it hard to implement a closed loop tuning system. It can
be done, but the control loop gets complicated.

A previous poster suggested what I'd called a "synthetic reactance", which
is a series-parallel LC combination. This technique can produce very large
effective-capacitance changes with a modest varactor range, however it also
comes with the susceptibility of mode-jumping in the output frequency.

Joe
W3JDR

If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same
min. frequency. Im not sure that the high end won't be greater than
before, even though the capacitance ratio is the same, since the
fixed inductance is lower.

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.


Kenneth,

F=1/((2*PI)*SQR(L*C))

If you double C, you have to halve L to maintain the same frequency. If you
do this, you only changed the LC ratio, not the delta tuning range.
The only way to get more delta F is to get more delta C. If you use the
switching technique, you'll have a discontinuous tuning curve (Vtune vs
Freq) which makes it hard to implement a closed loop tuning system. It can
be done, but the control loop gets complicated.

A previous poster suggested what I'd called a "synthetic reactance", which
is a series-parallel LC combination. This technique can produce very large
effective-capacitance changes with a modest varactor range, however it also
comes with the susceptibility of mode-jumping in the output frequency.

Joe
W3JDR

On Sat, 13 Dec 2003 23:27:24 GMT, "W3JDR" wrote:

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.


Kenneth,

F=1/((2*PI)*SQR(L*C))

If you double C, you have to halve L to maintain the same frequency. If you
do this, you only changed the LC ratio, not the delta tuning range.
The only way to get more delta F is to get more delta C.

Suppose you mean greater Cmax/Cmin, larger delta C was already
achieved above, but as you say "it doesn't work"
The tuning range can be calculated as follows:
(Cmax/Cmin)^2 = Fmax/Fmin

If you use the
switching technique, you'll have a discontinuous tuning curve (Vtune vs
Freq) which makes it hard to implement a closed loop tuning system. It can
be done, but the control loop gets complicated.


Joe
W3JDR


You could always divide the tuning into two ranges and it shouldn't be
too difficult to adjust the trimmer capacitors

73
LA8AK
--
remove ,xnd to reply (Spam precaution!)

On Sat, 13 Dec 2003 23:27:24 GMT, "W3JDR" wrote:

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.


Kenneth,

F=1/((2*PI)*SQR(L*C))

If you double C, you have to halve L to maintain the same frequency. If you
do this, you only changed the LC ratio, not the delta tuning range.
The only way to get more delta F is to get more delta C.

Suppose you mean greater Cmax/Cmin, larger delta C was already
achieved above, but as you say "it doesn't work"
The tuning range can be calculated as follows:
(Cmax/Cmin)^2 = Fmax/Fmin

If you use the
switching technique, you'll have a discontinuous tuning curve (Vtune vs
Freq) which makes it hard to implement a closed loop tuning system. It can
be done, but the control loop gets complicated.


Joe
W3JDR


You could always divide the tuning into two ranges and it shouldn't be
too difficult to adjust the trimmer capacitors

73
LA8AK
--
remove ,xnd to reply (Spam precaution!)

A previous poster suggested what I'd called a "synthetic reactance", which
is a series-parallel LC combination. This technique can produce very large
effective-capacitance changes with a modest varactor range, however it
also
comes with the susceptibility of mode-jumping in the output frequency.

Joe
W3JDR
===================================

But perhaps the most important effect of this method of creating a
multi-band or wideband tuned circuit is big deterioration in effective
operating Q.


Once upon a time it was a popular PA tuned-tank, minimum-dip, arrangement.
At the higher frequencies the coils got hot. Too high a circulating current
in the tank. Poor efficiency. They didn't catch on!


Varactor diodes used in receiver and local oscillator circuits have a
relatively poor Q to begin with. Perhaps lower than coil Q. Series tuned
circuits in parallel with shunt tuned circuits only magnify adverse varactor
effects.
----
Reg, G4FGQ