[Drake] Drake Xtal Eliminator - Spurs anyone?

[Bob Camp]

Hi

The amazing thing is just how far down you can hear spurs. One of the 
things about the good old R4 series is that it has a fairly small 
number of spurs due to the crystal based conversion process. It took a 
number of years for us to get back to anything close the R4 performance 
with synthesized radios.

If you want to go nuts about it the "correct" way to check the 
synthesizer it to use a direct conversion receiver. You drive one port 
of the mixer in the receiver with a clean voltage controlled crystal 
oscillator. The other port is driven by the synthesizer. You lock one 
to the other with a narrow bandwidth PLL. The audio out of the mixer = 
spurs on the synthesizer.

Since you can pretty easily hear stuff that's 120 db down weird setups 
like this are often need to see what's going on.

Of course you could hook the synthesizer to a less capable receiver and 
it wouldn't be as much of a problem :)

	Take Care!

		Bob Camp
		KB8TQ



On Aug 10, 2004, at 7:04 PM, Tom Hammond wrote:

> Hi Bob:
>
>> If you are getting extra beat notes to either side of the signal 
>> that's generally an indication of phase modulation of your VCO.  I 
>> would check the parts in the filter between the phase detector and 
>> the VCO to be sure they are all the right value and working properly. 
>> Since the spurs are phase modulation changing the size of the 
>> coupling cap may not help things much.
>>
>> Since you square up the reference oscillator as soon as it goes into 
>> the PLL chip it's harmonics will be running around everywhere. They 
>> almost immediately get on the power and ground lines to the digital 
>> section of the PLL. Makes listening to WWV a bit tough.
>
> MANY thanks!  Will try to check those avenues of solution out soon.  
> I've been planning on putting it on a spectrum analyzer anyway. That 
> might be the excuse I need to actually DO it!
>
> Much appreciated.
>
> 73,
>
> Tom Hammond    N0SS
>