[time-nuts] Broken Ovenaire OSC 85-50

[Mike Monett]

  > 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.

  Operating down  the  side of the resonance curve is  a  good  way to
  convert AM noise into phase 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.

  > The capacitively tapped circuit increases the current in the load.

  For a low impedance load. But we don't know what the load is.

  > 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.

  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.

  >> 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.

  Linear systems  do  not  care   where  the  filters  are  located. A
  zero-crossing detector   (limiter)   is   linear   through  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.

  However, none of this helps with the flicker noise generated  at the
  input of the limiter.

  This probably  contains most of the noise power, so  it  is doubtful
  that any arrangement of low-pass filters will do much good.

  Can you  post  a spice analysis of your approach to show  us  how it
  works?

  And don't forget the reference on the SPICE analysis of crystal osc.

  Regards,

  Mike