[time-nuts] 10MHz Square to Sine Wave Conversion

[Bob Camp]

Hi

But your 3 pole will not be as good as my 5 pole. My 5 pole will not be as good as the next poster’s
13 pole. My 5 added traps will not do as much as the next poster’s 13 traps.

What *will* happen as all of these parts are added:

1) It becomes a real mess to properly lay out and align
2) Even with good equipment, you will need ever more accurate parts to implement it
3) The sensitivity of the result to minor parts variation will keep going up. (I get -180 dbc here and “only” -120 dbc 1% away).
4) The odds of anybody actually building one go down probably as the square of the number of parts involved. 

The simple filter topology posted earlier by Charles is indeed quite adequate. You can get -60 dbc harmonics without
going very crazy on the design. Part values can either be calculated from formulas that have existed for > 80 years
or you can play with simulation. 

Bob
 
> On Jul 17, 2015, at 4:07 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> I was thinking along these lines.
> Cooking up a 3-pole filter in the form of a Pi-filter should be a good start, and then add traps for third and possibly fifth overtones that will not get much damping initially can be done if you need it pretty clean.
> 
> Cheers,
> Magnus
> 
> On 07/17/2015 04:07 AM, Graham / KE9H wrote:
>> All you need is a 10 MHz low pass filter.
>> 
>> How far down do you need the harmonics/spurious to be?
>> 
>> If 40 dB suppression of the 2nd and 3rd harmonics is adequate,
>> (you can't see the distortion with the eye on an oscilloscope)
>> you can make your own for about $2 in parts, not including a PC board or
>> housing.
>> 
>> Feel free to copy the low pass filter (L1, C9, C10) from here:
>> http://openhpsdr.org/wiki/index.php?title=EXCALIBUR
>> 
>> Or for about $35, you could get the same performance from an inline BNC
>> filter from Minicircuits.
>> 
>> http://www.minicircuits.com/pdfs/BLP-10.7+.pdf
>> 
>> If you need more harmonic suppression, buy two and put them in series.
>> 
>> --- Graham
>> 
>> ==
>> 
>> 
>> 
>> --- Graham
>> 
>> ==
>> 
>> 
>> 
>> On Thu, Jul 16, 2015 at 12:49 PM, skipp Isaham via time-nuts <
>> time-nuts at febo.com> wrote:
>> 
>>> re: 10MHz Square to Sine Wave Conversion
>>> 
>>> The GPSDO I recently acquired outputs a 10 MHz square wave. I'd like
>>> to convert it to a sine wave and I am looking for suggestions and info re
>>> any reasonable pre-made circuits and/or boards. No sense reinventing the
>>> wheel if I can avoid it.
>>> 
>>> Otherwise I will start from scratch and make a new wheel....
>>> 
>>> Thank you in advance for your replies.
>>> 
>>> Regards,
>>> 
>>> skipp
>>> 
>>> skipp025 at yahoo dot com
>>> 
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