[time-nuts] Broken Ovenaire OSC 85-50
xde-l2g3 at myamail.com
Fri Jul 3 23:52:33 UTC 2009
> 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
> Tektronix current probes don't seem to suffer from such
> If the current is very low then a low noise preamp is also
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
>> 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
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.
>> You can estimate the value of the crystal ESR by finding the Q of
>> your crystal and working backwards.
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, 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
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
And don't forget the reference on the SPICE analysis of crystal osc.
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