inductor tempco

Interesting, it's sort of DC bootstrapped, so despite looking like a 
follower, it'll actually have high loop gain!

Be careful it doesn't also oscillate, I suppose (cap on base and 
emitter!).

The offset voltage needs to be smaller though.  Or the oscillator side 
needs to be biased up slightly.  Otherwise it's too close to the Vcb limit 
anyway.  Well, using a schottky diode would help I think, so it limits to 
more like 0.5-0.6Vpk.

Running the oscillator at much higher Vce would also save fT, but that's 
not a problem with an already blazing fast transistor.  The flatter Ccb 
might be handy I guess.  Or heck, use a slower/fatter one, and modulate 
Vee for frequency control? :-p

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com


"piglet"  wrote in message news:n4m3j4$p77$1@dont-email.me...

On 14/12/2015 04:40, John Larkin wrote:
>
> In the circuit that I posted, amplitude is limited by conduction of
> the transistor c-b junction. That probably has costs.
>

Adding a one-transistor ALC loop may fix that. As in:

<https://www.dropbox.com/s/ltmihwc0gnaaxdd/ALC_JLcolpitts.jpg>

piglet 

"Phil Hobbs"  wrote in message news:566F1DFC.6010607@electrooptical.net...
>Like this (a tutorial by a friend of mine from grad school, Mark 
>Rodwell):
> <http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece218b/notes/Oscillators2.pdf>
>
>As he says on the first page,
>"...a well designed LC oscillator like the Colpitts must be current
>limited: that is, we allow the active device to go into cutoff for part
>of the period of oscillation. This reduces the effective gm of the
>device, averaged over the period, and thus gives the needed gain
>compression. It also benefits phase noise as we will later see. The
>simulated drain current for a common collector Colpitts oscillator below
>illustrates that the device can be in cutoff for much of the period."


Conversely, if it were a voltage mode amplifier (transresistance or pure 
voltage gain), voltage clamping would be preferable -- and the resonant 
tank would have to be series resonant, not parallel.  (The output would be 
current through the tank, rather than voltage across it.)

But transistors are gm devices first and foremost, so the parallel 
resonant and gm limited operation is preferable.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

"Bill Sloman"  wrote in message 
news:fef3fd86-3b54-4d45-a3c0-5d0e773ce748@googlegroups.com...
>> I also wonder how a PCB ground plane effects L and Q. I guess I'll
>> order a kit and try it.
>
>The stray field from a cylindrical coil mounted parallel to a ground 
>plane would induce some current in the ground plane, and >lower the 
>inductance and the Q, but if you think about the currents induced in the 
>ground plane, it can't be much.


It'll be less than 5% change.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

On Tuesday, 15 December 2015 04:39:23 UTC+11, John Larkin  wrote:
> On Mon, 14 Dec 2015 11:53:59 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
> >On 12/14/2015 11:22 AM, John Larkin wrote:
> >> On Mon, 14 Dec 2015 09:59:01 -0500, Phil Hobbs
> >> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> >> 
> >>> On 12/14/2015 09:51 AM, makolber@yahoo.com wrote:
> >>>> OP
> >>>>
> >>>> I don't understand your orignal question.
> >>>>
> >>>> Is the coil for the PLL VCO?
> >>>>
> >>>> Most modern PLL ICs have on chip VCOs.
> >>>>
> >>>> Mark
> >>>>
> >>>>
> >>>
> >>> John is usually doing something weird like instant-on VCOs that are
> >>> phase locked eventually.  His stuff is pretty much all time-domain.  You
> >>> could instant-on that Colpitts by putting a Schottky across the top
> >>> capacitor and swinging the supply from +5 to -5.
> >>>
> >>> Cheers
> >>>
> >>> Phil Hobbs
> >> 
> >> Yes, it is an instant-start oscillator that is eventually phase locked
> >> to an OCXO. So any slow loop, like an ALC, or any inherent slow
> >> amplitude limiting mechanism isn't good. I want steady-state as soon
> >> as possible. It's a messy crossover between time domain and RF-type
> >> issues.
> >> 
> >> The inductor is a big part of the problem. Nasty things, inductors.
> >> 
> >> 
> >
> >Just dropping the gain some and moving the bias so that it clips at the
> >emitter would go a long way.  A third cap, from the top of the tank to
> >the base, lets you optimize the loaded Q.
> >
> >Cheers
> >
> >Phil Hobbs
> 
> My Colpitts will be amplitude limited by clipping, either the c-b
> junction of the BFT25 or by the startup circuit. Either way, the gain
> (emitter current) determines how hard it clips, and that is easily
> tuned. Amplitude stabilizes in a couple of cycles. I have no idea how
> clipping (as opposed to some delicate AGC mechanism) affects things
> like phase noise.

It's a class C oscillator and the make-up current (small compared with the standing current at Q around 100) is a Dirac spike, with all the harmonics of the fundamental up to limit imposed by the width of the spike (or the bandwidth of the transistor).
 
> Spice is not useful in simulating this,

Why not? You have to plug in real values for the inductor (but Spice now includes a bunch of Wirth inductors with real values for the parallel and series resistances and capacitances). It doesn't have Spice models for fast transistors, but they are on offer on manufacturer's web-sites.

> and I don't have the budget or the learning time to use fancy RF tools.

Not to mention lacking the enthusiasm, either.

> It's solder-and-fiddle time,
> I guess. I really need to do that anyhow, for tempcos and such.
> 
> The actual design needs to leave lots of knobs to turn, to allow
> tweaking.

Or be done properly in the first place, to minimise the post-assembly tweaking.

-- 
Bill Sloman, Sydney

On Tuesday, 15 December 2015 06:04:22 UTC+11, Tim Wescott  wrote:
> On Sun, 13 Dec 2015 23:10:24 -0800, Bill Sloman wrote:
> 
> > On Monday, 14 December 2015 12:26:17 UTC+11, John Larkin  wrote:
> >> These look great, very high Q. I'm thinking about a 50 or maybe 100 MHz
> >> Colpitts oscillator PLL.
> >> 
> >> http://www.coilcraft.com/1515sq.cfm
> >> 
> >> I wonder if the thermal expansion tempco of the FR4 board will stretch
> >> the coil and change its native tempco. FR4 is variously cited as being
> >> around +5 to +17 ppm/K, which isn't bad. That might even reduce the
> >> tempco of the inductor.
> >> 
> >> I also wonder how a PCB ground plane effects L and Q. I guess I'll
> >> order a kit and try it.
> > 
> > Why a Colpitts? If you wanted a clean and reasonably fast tuneable sine
> > wave oscillator you could use a pair of Analog Devices fast multipliers
> > - the AD834, AD835 or ADL5391 are all fast enough for a 100MHz
> > oscillator.
> > 
> > One multiplier would provide the adjustable in-phase feedback to keep
> > the amplitude where you wanted it, and the other would provide
> > adjustable (positive or negative) quadrature input to allow you to pull
> > the frequency up or down. At 100MHz, half a metre of coax could provide
> > the quadrature component to be fed into the second multiplier. A coax
> > delay line isn't a broad-band solution, but sufficiently broad-band for
> > something you might otherwise fine-tune with a varicap.
> > 
> > It's a more complicated solution than you'd come up with on your own,
> > but it would be easier to explain to customers than the traditional
> > approach.
> 
> Resonant time bases (like an LC tank) provide better phase noise and 
> stability characteristics than non-resonant solutions (even, I'm pretty 
> sure, your 1/4-wave line).

The 1/4 wave line wasn't part of the tank. It's just one way of getting a quadrature signal that you can feed back to tweak the frequency. It's the kind of thing you can build into the printed circuit layout if you have enough space.
 
> Not knowing what JL is planning I can't say which approach is better, but 
> if he needs good stability and phase noise characteristics, than building 
> something around a good LC oscillator is the way to go.

He eventually got around to revealing the "instant start-up" aspect (which isn't impossible - as has been pointed out - but isn't something I've thought about in any detail).

-- 
Bill Sloman, Sydney

On Tuesday, 15 December 2015 03:24:13 UTC+11, John Larkin  wrote:
> On Mon, 14 Dec 2015 18:46:06 +1100, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
> 
> >On 14/12/2015 18:10, Bill Sloman wrote:
> >> On Monday, 14 December 2015 12:26:17 UTC+11, John Larkin  wrote:
> >>> These look great, very high Q. I'm thinking about a 50 or maybe 100
> >>> MHz Colpitts oscillator PLL.
> >>>
> >>> http://www.coilcraft.com/1515sq.cfm
> >>>
> >>> I wonder if the thermal expansion tempco of the FR4 board will stretch
> >>> the coil and change its native tempco. FR4 is variously cited as being
> >>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the
> >>> tempco of the inductor.
> >>>
> >>> I also wonder how a PCB ground plane effects L and Q. I guess I'll
> >>> order a kit and try it.
> >>
> >> Why a Colpitts? If you wanted a clean and reasonably fast tuneable sine wave oscillator you could use a pair of Analog Devices fast multipliers - the AD834, AD835 or ADL5391 are all fast enough for a 100MHz oscillator.
> >>
> >> One multiplier would provide the adjustable in-phase feedback to keep the amplitude where you wanted it, and the other would provide adjustable (positive or negative) quadrature input to allow you to pull the frequency up or down. At 100MHz, half a metre of coax could provide the quadrature component to be fed into the second multiplier. A coax delay line isn't a broad-band solution, but sufficiently broad-band for something you might otherwise fine-tune with a varicap.
> >>
> >> It's a more complicated solution than you'd come up with on your own, but it would be easier to explain to customers than the traditional approach.
> >>
> >
> >It might be quite noisy though. Multipliers tend to be noisy because the 
> >"LO port" is in small-signal operation all the time, and therefore the 
> >devices contribute noise. If you replace the multipliers in your scheme 
> >with hard-driven mixers then it would be less noisy as the switching 
> >devices in the mixer core don't contribute to the current noise when 
> >they are fully on or fully off. The harmonic content in the mixer output 
> >current should not matter as the tank will filter it out, and even if it 
> >didn't, filtering out 3rd and higher harmonics is not very difficult 
> >unless the tuning range approaches an octave.
> >
> >Chris
> 
> A BFT25 costs us 36 cents.

And thousands of dollars worth of tweaking.

In low volume production, what you pay for the components over the commercial life of the device is dwarfed by what you spend on designing, debugging and documenting what you build.

A more expensive, but more predictable device, in a more designable circuit, can often come out cheaper over-all, particularly if you can get it right first time.

-- 
Bill Sloman, Sydney

On 15/12/15 07:14, Tim Williams wrote:
> Cliff: your clock appears to be about a half day behind everyone...
>
> Tim

Thanks. For some reason this Linux virtual machine doesn't synchronize 
via VMWare, even though it's configured to. I'll have another look at it.

On Mon, 14 Dec 2015 11:46:06 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

>On Monday, December 14, 2015 at 8:22:18 AM UTC-8, John Larkin wrote:
>
>> Yes, it is an instant-start oscillator that is eventually phase locked
>> to an OCXO. ...
>> The inductor is a big part of the problem. Nasty things, inductors.
>
>If you waste some current, it can also be the solution: instead of 
>an instant-start  by applied voltage spike, you can 'idle' the tank
>by switching a high regulated current through the coil.
>When you unclamp the coil and the loop amplification takes
>over, it's already got stored energy in I-squared-L, so it
>doesn't need to accelerate up to full amplitude.

Sure, the idle initial condition is current through the inductor,
applied from the impedance that best damps the tank. I need to start
and stop the oscillator ASAP. We've done that lots of times, with LCs
and with coaxial ceramic resonators.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Mon, 14 Dec 2015 15:04:03 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 12/14/2015 02:04 PM, Tim Wescott wrote:
>> On Sun, 13 Dec 2015 23:10:24 -0800, Bill Sloman wrote:
>> 
>>> On Monday, 14 December 2015 12:26:17 UTC+11, John Larkin  wrote:
>>>> These look great, very high Q. I'm thinking about a 50 or maybe 100 MHz
>>>> Colpitts oscillator PLL.
>>>>
>>>> http://www.coilcraft.com/1515sq.cfm
>>>>
>>>> I wonder if the thermal expansion tempco of the FR4 board will stretch
>>>> the coil and change its native tempco. FR4 is variously cited as being
>>>> around +5 to +17 ppm/K, which isn't bad. That might even reduce the
>>>> tempco of the inductor.
>>>>
>>>> I also wonder how a PCB ground plane effects L and Q. I guess I'll
>>>> order a kit and try it.
>>>
>>> Why a Colpitts? If you wanted a clean and reasonably fast tuneable sine
>>> wave oscillator you could use a pair of Analog Devices fast multipliers
>>> - the AD834, AD835 or ADL5391 are all fast enough for a 100MHz
>>> oscillator.
>>>
>>> One multiplier would provide the adjustable in-phase feedback to keep
>>> the amplitude where you wanted it, and the other would provide
>>> adjustable (positive or negative) quadrature input to allow you to pull
>>> the frequency up or down. At 100MHz, half a metre of coax could provide
>>> the quadrature component to be fed into the second multiplier. A coax
>>> delay line isn't a broad-band solution, but sufficiently broad-band for
>>> something you might otherwise fine-tune with a varicap.
>>>
>>> It's a more complicated solution than you'd come up with on your own,
>>> but it would be easier to explain to customers than the traditional
>>> approach.
>> 
>> Resonant time bases (like an LC tank) provide better phase noise and 
>> stability characteristics than non-resonant solutions (even, I'm pretty 
>> sure, your 1/4-wave line).
>
>A 1/4 wave line is a resonant solution.
>
>> 
>> Not knowing what JL is planning I can't say which approach is better, but 
>> if he needs good stability and phase noise characteristics, than building 
>> something around a good LC oscillator is the way to go.
>> 
>
>Cheers
>
>Phil Hobbs

I've done similar instant-start oscillators around 600 MHz, using 1/4
wave coaxial ceramic resonators. They start out as pulse-propagating
"digital" delay-line oscillators and, as the corners round off,
gradually transition to sine waves. That can be mitigated some by
starting them softly, launching a slower initial edge.

CCRs are great for acuracy and tempco and Q, but they start around 500
MHz, are sort of expensive, and tend to have Zo values around 10 ohms,
so need a lot of initial-condition current, which implies low
oscillation amplitude. 500 MHz is too high for our FPGAs, so we'd need
a divider too.

Real coaxial cables are a mess. They are big and ugly to install on a
board. Teflon has a phase change right around room temp, so frequency
tempco is awful. Polyethylene melts when you solder the braid.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Mon, 14 Dec 2015 14:13:39 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Mon, 14 Dec 2015 15:04:03 -0500, Phil Hobbs wrote:
>
>> On 12/14/2015 02:04 PM, Tim Wescott wrote:
>>> On Sun, 13 Dec 2015 23:10:24 -0800, Bill Sloman wrote:
>>> 
>>>> On Monday, 14 December 2015 12:26:17 UTC+11, John Larkin  wrote:
>>>>> These look great, very high Q. I'm thinking about a 50 or maybe 100
>>>>> MHz Colpitts oscillator PLL.
>>>>>
>>>>> http://www.coilcraft.com/1515sq.cfm
>>>>>
>>>>> I wonder if the thermal expansion tempco of the FR4 board will
>>>>> stretch the coil and change its native tempco. FR4 is variously cited
>>>>> as being around +5 to +17 ppm/K, which isn't bad. That might even
>>>>> reduce the tempco of the inductor.
>>>>>
>>>>> I also wonder how a PCB ground plane effects L and Q. I guess I'll
>>>>> order a kit and try it.
>>>>
>>>> Why a Colpitts? If you wanted a clean and reasonably fast tuneable
>>>> sine wave oscillator you could use a pair of Analog Devices fast
>>>> multipliers - the AD834, AD835 or ADL5391 are all fast enough for a
>>>> 100MHz oscillator.
>>>>
>>>> One multiplier would provide the adjustable in-phase feedback to keep
>>>> the amplitude where you wanted it, and the other would provide
>>>> adjustable (positive or negative) quadrature input to allow you to
>>>> pull the frequency up or down. At 100MHz, half a metre of coax could
>>>> provide the quadrature component to be fed into the second multiplier.
>>>> A coax delay line isn't a broad-band solution, but sufficiently
>>>> broad-band for something you might otherwise fine-tune with a varicap.
>>>>
>>>> It's a more complicated solution than you'd come up with on your own,
>>>> but it would be easier to explain to customers than the traditional
>>>> approach.
>>> 
>>> Resonant time bases (like an LC tank) provide better phase noise and
>>> stability characteristics than non-resonant solutions (even, I'm pretty
>>> sure, your 1/4-wave line).
>> 
>> A 1/4 wave line is a resonant solution.
>
>I don't think it is the way that Bill is proposing to use it.
>
>When properly terminated for the job a 1/4-wave line is resonant, but 
>Bill's talking about using it as nothing more than a 90 degree delay 
>line, not as a resonant element.

One classic gated oscillator is an ECL AND gate and a length of coax
looping the output back to one input. It's pretty awful in practice.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com