[R-390] Synthesizer Phase Noise

[John Vendely]

Hi Bob,

You are confusing phase noise effects in receiver local oscillators with 
that of thermal noise in RF amplifiers and mixers.  Front end thermal noise 
can be overcome by added antenna gain, but phase noise certainly does not 
become a "non-issue" with increased antenna gain.   In fact, large, high 
gain antennas usually make the problem worse.  Large antennas often deliver 
huge off-channel signals to the receiver front end, where reciprocal mixing 
with L.O. phase noise sidebands translates this noise into the receiver's 
IF.  Weaker signals can be buried in the translated phase noise.  This 
effect is completely independent of the receiver's front end noise figure. 
Of course, if the antenna is positioned so the offending strong signal is 
off-axis, the effect can be reduced, but this is often not possible.  Often, 
the whopper interferer and the weak desired signal are both in the antenna's 
main lobe.  Consider the case of a station on the U.S. east-coast with its 
high gain directional antenna facing Europe, attempting to receive weak 
signals in proximity to an HF broadcast band.  The bigger than antenna, the 
worse the phase noise effects.

For such applications, receiver designers often trade off noise figure for 
improved front end dynamic range, but L.O. phase noise cannot be 
compromised.

73,

John K9WT

----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <r-390 at mailman.qth.net>
Cc: "Tisha Hayes" <tisha.hayes at gmail.com>
Sent: Sunday, August 16, 2009 4:54 PM
Subject: Re: [R-390] Synthesizer Phase Noise


> Tisha and others have pointed out the noise levels inherent with the
> external synthesizer.
>
> However, we have to remember that these were used with antenna systems
> that we could only dream and drool over.
>
> One particular receiver site of infamy, isn't very far from me.  I do
> remember when it was still up and running.  When you drove up the road
> to get to the Guarded entrance, you drove for at least a quarter mile
> along a massive curtain array.  That was only ONE of the VERY large
> number of antennas that were visible from the road.  The entrance road
> to the actual road, even had a firing range situated between the curtain
> systems and the Guarded entrance.
>
> Considering that this was a Rifle range, you "may" begin to get a feel.
> Just as you came to the guard, you could observe three or four RLPs,
> (Rotating Log Periodics), that were about 70 foot up in the air, and
> from the rear elements, 3MHZ was certainly within reason.
>
> When you use an antenna system(s) like this, noise becomes a non-issue.
> The gain of these systems had to be unreal.  But, that is what they were
> there for.  Simply to listen, record, store, and pass on to those places
> with initials.
>
> BTW, those curtain arrays were strung on poles the tops of which were
> WELL above the top of ANY power pole in the vicinity.  They were
> obviously Class 1 poles!
>
> Bob - N0DGN
>
> Tisha Hayes wrote:
>> I had my doubts about the capabilities of the synthesizer just by looking 
>> at
>> the block diagram and "imagineering" how they would do it. It is not
>> surprising to hear that phase noise was a problem.
>>
>> Someone could create a similarly functioning external synthesizer for the
>> R-390A using a variety of techniques with digital IC's by why bother?
>>
>> Tisha
>>
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>