[time-nuts] Square to sine wave symmetrical conversion (part 2)

Bob Camp kb8tq at n1k.org
Sun Jul 26 09:44:52 EDT 2015


Hi

Ummm …. errr…. 

The oscillator loop has a limiter in it or it’s not going to work very well.
You can *easily* get <-160 dbc/Hz with an oscillator that has harmonics in the 
-10 to -20 range. With some tweaking you can get into the 170’s. 

It’s not the limiting by it’s self, it’s *how* the limiting is achieved. 

In most cases the “oscillator output” you are looking at is not the signal from 
the oscillator stage it’s self. You are looking at a signal that has been through 
several (like tuned) buffers before you see it. The same narrow band tuned stages
can be used to “clean up” harmonics on any signal. The old style rack mount 
OCXO’s did a *lot* of this. 

Bob

> On Jul 25, 2015, at 11:47 PM, jerry shirᴀr <radio.n9xr at gmail.com> wrote:
> 
> "Not necessarily.  Many oscillator circuits do not deliver a good sine wave
> to begin with"
> 
> This is very true. However if it is worse than -30dB harmonic sinewave back
> stream then the oscillator is probably extremely high in phase noise
> anyway. Since the threshold is off center, the phase noise of the 20% duty
> cycle squarewave will have additional amounts from the AM noise on the
> signal adding in from that threshold offset. The reason we want 50% is to
> get the cleanest signal possible with what we have to work with.  :)
> 
> Jerry N9XR.
> On Jul 25, 2015 3:17 PM, "Alexander Pummer" <alexpcs at ieee.org> wrote:
> 
>> it is relative easy to make a perfect 50% square wave from almost any
>> input wave form
>> 
>> U1 could be any-- fast enough  for the desired frequency--comparator or a
>> transistor pair similar to Charles Wenzel's  circuit, R1 C1 is a long time
>> integrator,[ RxC >> 1/f of the incoming frequency] the voltage at "A" is
>> proportional with the duty cycle, U 2 is some high gain low noise, low
>> input offset voltage high input impedance amplifier, the duty-cycle is set
>> by R4/R5,  fine tuning with R6, C2v removes the noise Vr is well stabilized
>> reference voltage,
>> To set up the circuit the output should be connected -- with DC decoupling
>> -- to a spectrum analyzer's input for watching the second harmonic [a
>> perfect 50% duty cycle square wave lacks of even harmonics ..] of the input
>> frequency, which has to be adjusted to minimum with R6, using the same
>> stile resistors for ±0,1%, R4 and R5, with value 100 times of R6 a very
>> good temperature stability could be achieved. for better short time
>> stability R2 's top could bealso connected to Vr,
>> If the drive capability of U1 is not enough for the load non-inverting
>> buffers could be inserted to U1's output, of course that circuit wil
>> contribute some phase noise/ jitter too.
>> 73
>> KJ6UHN
>> Alex
>> 
>> 
>> On 7/25/2015 1:34 AM, timeok at timeok.it wrote:
>> 
>>> 
>>> bypassing the inverter you will improve phase noise. Yuo will probably
>>>> need a sine buffer at 10MHz to drive 50 ohms.
>>>> 
>>> This separator con be the solution:
>>> http://www.timeok.it/files/hp5065AoptH10v200.pdf
>>> high input impedance, output 50Ohm, high power handling and low additive
>>> phase noise.
>>> 
>>> But before measure using an oscilloscope the output of the GPS OCXO
>>> directly on the output pin to verify if there is a sine-wave or square
>>> 
>>> Luciano
>>> timeok
>>> Message sent via Atmail Open - http://atmail.org/
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>> 
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