[time-nuts] Broken Ovenaire OSC 85-50

[Bruce Griffiths]

Mike Monett wrote:
>   > Mike Monett wrote:
>
>   >> A couple of things. First, trying to measure the currents  in the
>   >> circuit with  a ferrite toroid won't do you much good.  You don't
>   >> know what the currents should be, and the secondary of the toroid
>   >> transformer requires  a termination resistor.  The  value changes
>   >> with the turns ratio.
>
>   >> Just from  looking  at  the  circuit,  the  RF  currents  will be
>   >> extremely low.  This  requires  a large number  of  turns  on the
>   >> secondary, which  will  probably resonate at or  below  the 10MHz
>   >> operating frequency due to stray capacitance from  the connection
>   >> to the scope. So it is unlikely you will get any  useful progress
>   >> in this direction.
>
>   > Uncalibrated speculation isnt helpful.
>
>   > Estimates of  the actual current would be more  helpful  than mere
>   > hand waving.
>
>   All the  discussion up till now has been handwaving. And  you forgot
>   the termination  resistor  that   is   required  on  the transformer
>   secondary. I  provide  means to get the true  voltages  and currents
>   later.
>
>   > Tektronix current   probes   don't   seem   to   suffer  from such
>   > limitations.
>
>   > If the  current  is  very  low then a  low  noise  preamp  is also
>   > necessary.
>
>   Obviously. But the currents are likely to be in the microamp region.
>
>   Not only  is  that  very hard to measure,  especially  at  10MHz, it
>   doesn't do any good if you don't know what they are supposed to be.
>
>   But why  bother  trying  to  measure the  current.  If  you  have an
>   accurate spice  model, you know the voltages in  the  circuit. These
>   can be  measured  much  easier and more  accurately  than  trying to
>   measure microamps in a 10MHz crystal tank.
>
>   >> However, from  the  values  on your  schematic,  the  output tank
>   >> circuit resonates  at  9.602MHz with a Q of 9.6. So  the  tank is
>   >> already well  below   resonance,   which   attenuates  the output
>   >> voltage.
>
>   >> Any stray  capacitance  you  add to the  circuit  will  bring the
>   >> resonant frequency lower, further aggravating the loss in signal.
>
>   >> The output tank is tapped with the 75pF and 91pF in  series. This
>   >> further attenuates the signal.
>
>   >> I'd change the circuit to a single capacitor across the tank with
>   >> a small trim capacitor to tune it to resonance.
>
>   > This is usually a bad idea.
>
>   > Unless the  circuit  components have  been  altered,  the designer
>   > intended that the collector load be capacitive.
>
>   > Using a  resonant  circuit  tuned   to  resonance  at  the crystal
>   > frequency as a load inevitably degrades the amplifier  phase shift
>   > tempco and the phase noise.
>
>   Without putting the circuit in spice, it seems he is  operating near
>   the -3dB point. The slope of the phase vs frequency is pretty linear
>   from the +/- 3dB points through resonance.
>
>   So it  doesn't  matter much where the tank is  tuned.  It  will give
>   about the same phase noise anywhere.
>
>   
If and only if the tank operating point is within the region where the
phase slope is approximately linear.

>   Operating down  the  side of the resonance curve is  a  good  way to
>   convert AM noise into phase noise.
>
>   
A good oscillator has very low AM noise.
>   > A detuned  tank avoids the dc voltage drop and  the  flicker phase
>   > noise associated with just using a collector resistor as a load.
>
>   I think  he really meant to tune the tank to resonance.  The problem
>   may simply be incorrect values shown for the tank components.
>
>   
Perhaps the design is so old that the designer was unaware of the phase
noise implications of using a tuned tank.
>   > The capacitively tapped circuit increases the current in the load.
>
>   For a low impedance load. But we don't know what the load is.
>   
The load for the oscillator buffer is well defined,
The output stage load lies somewhere between 0 and 330 ohms.
>   > A common  base amplifier could be used with some advantage  in the
>   > output buffer but there are better circuits.
>
>   >> To get  the  signal  into 50 ohms  for  distribution,  I'd  add a
>   >> limiter if  you  can  tolerate a square wave  output,  or  a good
>   >> emitter follower  if you need a sine wave. Take  the  output from
>   >> the collector of the 2N2369 to get the maximum signal amplitude.
>
>   > Emitter followers are not usually a good idea as they are somewhat
>   > intolerant of short circuits (accidents do happen)  and capacitive
>   > loading.
>
>   Short circuit protection is easy to add.
>   
Yes but instability due to capacitive loading can only be cured by
careful design.
Using a series resistor in series with the output  can work but reduces
the signal level.
>   Capacitive loading  means  the tank would be  operating  further off
>   resonance, and the basic circuit diagram shows this is unlikely.
>
>   > There are single transistor circuits with better reverse isolation
>   > than an emitter follower.
>
>   Right now  there  is nothing on the output except the  tank.  So any
>   added isolation would be an improvement.
>   
Its better to do the analysis and design an appropriate stage than just
chuck in an emitter follower.
>   >> Your original  post mentions an output amplitude of 20mV.  If the
>   >> normal amplitude  is around 2V, this represents a  loss  of 40dB.
>   >> This is  a huge loss in signal. The circuit  obviously  worked at
>   >> one time, so there may well be some other hidden problem.
>
>   >> It is possible the crystal is damaged, but this seems unlikely. A
>   >> crystal oscillator probably won't even start if the  signal level
>   >> is down 40dB.
>
>   >> You can check the oscillator and crystal in SPICE.  Normally, the
>   >> high Q of the crystal will make the analysis very slow.  It could
>   >> take many  hours  for  the simulation  to  begin  oscillating and
>   >> stabilize at the final amplitude. The transient analysis requires
>   >> a very  fine  time step for accuracy, and you  could  run  out of
>   >> memory before the simulation was complete.
>
>   > Not so  (although some Spice variants may still  suffer  from this
>   > problem) this may once have been true with a slow PC.
>
>   Depends on the time step. To get any accuracy, you need a  fine time
>   step. This is slow on any computer, and it eats a lot of memory.
>
>   > It depends  on the actual oscillator circuit  some  circuits start
>   > faster if  one  sets up a suitable initial  condition  such  as an
>   > initial current in the inductor in the crystal  equivalent circuit
>   > but you  have  to  get the  current  right.  With  some oscillator
>   > circuits doing this can slow the simulated oscillator startup.
>
>   >> I have  developed  a  much  faster  way  of  analyzing  a crystal
>   >> oscillator in  SPICE.  Instead of requiring tens  or  hundreds of
>   >> thousands of simulated cycles, this method gives accurate results
>   >> in only  a  few dozen cycles. For  more  information,  please see
>   >> "SPICE Analysis of Crystal Oscillators"
>
>   > This isn't new its been around for decades.
>
>   Please, Bruce,  show me one reference that uses my approach.  Do not
>   confuse previous  attempts that inject a starting  impulse  into the
>   tank to get the oscillation going.
>
>   

>   My method initializes the tank to the exact point in the cycle where
>   the current through the crystal motional inductance is at maximum.
>
>   You can  calculate this current exactly, and set  the  oscillator to
>   whatever crystal dissipation you desire.
>
>   When the  transient analysis starts, the tank  proceeds  through the
>   cycle as  if it had been running forever. It does not  need hundreds
>   or thousands  of  cycles  to get  the  amplitude  stabilized.  It is
>   already stabilized,  and  the only thing you have to do  is  let the
>   electronics catch up.
>
>   This does not occur with previous methods of injecting a  pulse into
>   the tank. This still take many cycles to get the  oscillator running
>   and to stabilize the amplitude.
>
>   The next  trick  is  to  measure   the  amplitude  of  the  peaks to
>   parts-per-million accuracy  so  you  can  see  if  the  amplitude is
>   increasing or decreasing.
>
>   This relies on the peak search capability in Microcap SPICE. LTspice
>   and PSPICE  do  not  have the capability to  do  this,  and Microcap
>   didn't have  it in previous releases. So I am  pretty  confident you
>   have never seen this approach before.
>
>   Please provide references to support your claim.
>
>   >> http://pstca.com/spice/xtal/clapp.htm
>
>   >> You can estimate the value of the crystal ESR by finding the Q of
>   >> your crystal and working backwards.
>
>   >> Thanks,
>
>   >> Mike
>   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   from your next post:
>
>   > Blindly adding a wide bandwidth limiter will degrade the phase noise
>   > if the input signal slew rate is too low.
>
>   > In such  cases ts better to use a cascade of limiters  each  with an
>   > output filter and a well defined gain at the zero crossing.
>
>   > The output  filter and the gain (at zero crossing) of each  stage is
>   > selected to minimse the jitter at the output.
>
>   > Bruce
>
>   Bruce, you have mentioned this many times. I have a hard time seeing
>   how this could work.
>   
Its almost trivial, you actually need to maximise the zero crossing
slope to noise ratio for each stage of the limiter.
Using a wide bandwidth limiter reduces the output noise to slope ratio
below that of a limiter with the optimum bandwidth.
A few Spice simulations or hand calculations with a fixed gain limiter
will quickly show that there is an optimum filter cutoff frequency that
produces the lowest output jitter.
The slope gain required to ensure that the jitter is well below that of
any subsequent logic is readily calculated.
If the signal slew rate is high enough the limiter design is relatively
non critical.
Its also necessary to optimise the gain distribution within the limiter.
Using a single high gain limiter is far from the optimum approach for
low slew rate signals.
For a given signal and slope gain there is also an optimum number of
limiter stages.
However even at 1Hz there isn't usually a great deal of improvement
beyond 6 limiter stages.
At low frequencies the performance of a single stage limiter can be
1,000,000 times worse than a 6 stage limiter.
At higher frequencies 1-3 stages may suffice, depending on the signal
slew rate and the jitter characteristics of the following circuitry.

>   Linear systems  do  not  care   where  the  filters  are  located. A
>   zero-crossing detector   (limiter)   is   linear   through  the zero
>   crossing.
>
>   
A limiter isn't a linear system except near the zero crossing.
>   So all you really need is one limiter with sufficient gain,  and one
>   filter on  the output to cut the high frequency  noise  generated at
>   the input stage of the limiter.
>
>   
It has been obvious (for example it has long been known and shown in
practice that the performance using such a system is inferior to a
system using a set of cascaded limiters with each successive  stage
having a greater gain and bandwidth than the previous stage however
despite several attempts the optimum gain and bandwidth distribution
remained unknown) for decades before the Collins paper that this isn't
true in general. If one only needs a modest slope gain then a low gain
single stage limiter with an appropriate output filter may suffice.
>   However, none of this helps with the flicker noise generated  at the
>   input of the limiter.
>
>   
What mechanism do you have in mind?
If you are naive enough to use a limiter stage without (series) feedback
to linearise its performance near zero crossing then flicker noise may
be an issue.
If the limiter stage has too high a dc and low frequency gain then
flicker noise can also be a problem.
With sufficient feedback and not too high a dc gain the limiter flicker
phase noise can easily be made much lower than that of a crystal oscillator.
>   This probably  contains most of the noise power, so  it  is doubtful
>   that any arrangement of low-pass filters will do much good.
>
>   
Only if you limit yourself to using a limiter without signal frequency
feedback in the active region.
A limiter stage generates very little noise when its output is actually
at one of the limits.
>   Can you  post  a spice analysis of your approach to show  us  how it
>   works?
>
>   
Oliver Collins did the fundamental analysis in the mid 90's:
"The Design of Low Jitter Hard Limiters" IEEE transactions on
Communications, Vol 44 No 5, May 1996 pp 601-608 (no it isn't free, you
either have to pay to download it, or find a library that gets this Journal)
His analysis (possibly with suitable extensions to take into account the
fact that device noise tends to increase at high frequencies) also
applies to RF limiters.

Its almost trivial to extend Collins' analysis to the case where the
noise of each limiter stage isn't the same as the other limiter stages.
A Spice analysis isn't particularly helpful.
Whilst one can use it to estimate the output noise and output slope for
each limiter stage, simulating the resultant jitter is usually a little
problematic.

>   And don't forget the reference on the SPICE analysis of crystal osc.
>   
Spice is merely a tool for numeral solution of a set of nonlinear
differential equations, the approach you use (and those that others use)
has been obvious for some 400 years.
>   Regards,
>
>   Mike
>
>   

Bruce