[Boatanchors] Collins receiver distortion

[Dave Brown]

Hi Gary- This is something I have been meaning to investigate for some 
time.
  Any idea how much contribution there is from the output stage 
itself?
Interesting stuff- you should ask on the collins reflector as well.
73
 Dave, ZL3FJ

----- Original Message ----- 
From: "Gary Schafer" <garyschafer at comcast.net>
To: <Boatanchors at mailman.qth.net>
Sent: Tuesday, May 10, 2005 1:12 PM
Subject: [Boatanchors] Collins receiver distortion


>I have always noticed audio distortion in the Collins S line 
>receivers.
> I have acquired an HP 3586C "wave analyzer" also known as a 
> selective voltmeter. Made some measurements on the audio of my 
> Collins 75S3. With a carrier tuned in (crystal calibrator) to 
> produce 1000 hz audio tone I measured the 2nd and 3rd harmonic 
> levels in the audio.
>
> With the agc on and full rf gain the second harmonic was down 30 db. 
> The 3rd harmonic was down about 40 db.
>
> With the rf gain backed down to just below the point where the agc 
> is active the distortion is much better. The 2nd harmonic is down 40 
> db and the third is also down 40 db!
>
> It is noticeable when listening to any of the S line receivers that 
> as soon as the rf gain is raised so that the agc is working the 
> audio distortion goes up. This can be heard on SSB signals also.
> I have in the past played around a bit with the bypassing in the 
> audio stages to try and clean it up but have been unsuccessful.
> The problem has something to do with the agc. Maybe it clips the rf 
> at the detector and some of the artifacts get through to the audio?
>
> Now that I have identified it as second harmonic energy it may help 
> in resolving. Haven't had time to dig in further.
> Anyone else had any success in improving the Collins audio?
>
> As a comparison I also measured my Kenwood ts430 at 1000 hz. 2nd 
> harmonic was down 57 db and 3rd was down 53 db! Much cleaner than 
> the collins.
>
> Just to see if the audio circuit was rolling things off in the ts430 
> I also used a 500 hz tone and got 54 db down for the second harmonic 
> and 47 db down for the third.
>
> It did not make any difference in distortion on the ts430 whether or 
> not the agc was functioning.
>
> 73
> Gary  K4FMX
>
>
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