[Elecraft] XG3 Quesiton

[Wayne Burdick]

A crystal oscillator generally produces something approaching a  
sinewave, with varying degrees of harmonic distortion depending on how  
much output power the designer was trying to get out of it.

The XG3's signal is quite stable as it is derived from a crystal  
oscillator. But its output will be very close to a square wave, and if  
the RX/TX stages that follow don't have a lot of filtering of their  
own, you'd be better off filtering the XG3's signal with a simple 5- 
element low-pass filter. The ARRL Handbook has examples.

A crystal oscillator will have one more advantage over an XG3,  
usually: lower phase noise. The XG3's phase noise is a relatively  
constant -105 dBc/Hz at all offsets, whereas a crystal oscillator  
might be down to -140 dBc/Hz at 1 kHz and -150 to -160 at 10 kHz.

The practical implication of this is that the XG3 should not be used  
as the local oscillator for a high-power station (100 W+). That -105  
dBc/Hz figure for phase noise would translate to wideband noise on key- 
down if you have hams as neighbors. The phase noise is less of a  
concern at QRP levels.

73,
Wayne
N6KR


On Oct 25, 2012, at 10:17 AM, Phil Hystad wrote:

> Follow on question...
>
> What type of signal (wave?) does a crystal, as used in old rigs like  
> the Eico 720, produce?  I am thinking that the oscillator of  
> something like in the 720, would clean up the output of the XG3 a  
> little bit.  I was actually thinking of using the XG3 as a crystal  
> replacement type of VFO in my Eico 720 but I never even got around  
> to think too much about it (probably because I was having too much  
> fun putting together Elecraft kits and operating them).
>
> PEH's iPad
> K7PEH
>
> On Oct 25, 2012, at 10:00 AM, Wayne Burdick <n6kr at elecraft.com> wrote:
>
>> Lee Buller wrote:
>>
>>> Does the XG3 have the stability and output power to be used as a VFO
>>> for older
>>> (1950-60s) CW and Phone rigs like the T-50, DX-40, etc.?
>>
>> Yes, with appropriate level conversion, and possible filtering (the
>> output of the XG3 is closer to a squarewave than a sinewave, so a  
>> low-
>> pass filter may be needed).
>>
>>
>>>
>>> Are there software programs that have been written that allows "VFO"
>>> operation?
>>
>> You can use XG3 Utility for this, or write your own app pretty  
>> easily.
>> The full command set is documented in the manual.
>>
>> Wayne
>>
>>
>>>
>>> Lee - K0WA
>>>
>>>
>>>
>>> In our day and age it seems that Common Sense is in short supply.
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>>> Sense divine?
>>>
>>> Common Sense is the image of the Creator expressing revealed truth
>>> in my mind.
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>>>
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