[R-390] Re: Spurious Resonances (was images)

[Tony Casorso]

Hi Bob,

Maybe the "spurious response" subject is confusing. I should have titled it 
differently. I made it spurious because I shouldn't have titled the original 
thread "images". These are produced by non-linearity, not by mixing at the 
receiver mixer. Also, when I took the pictures at first, there was an extra 
peak near 1.5MHz so I had "spurious resonance" on my mind. The problem was 
the offset control on my sweeper.

As I said, the pictures just show the response of the preselector. With a 25 
Ohms source I am getting about 15 db attenuation at 1.5MHz relative to the 
peak at 3MHz. With a 2K source it is more like 25 to 30db (it's hard to be 
sure how fat that line is at 1.5MHz).

Maybe the title should be "spurious thread" ;) .

Tony


----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: "Tony Casorso" <canthony15 at msn.com>
Cc: <r-390 at mailman.qth.net>
Sent: Saturday, October 27, 2007 9:04 AM
Subject: Re: [R-390] Re: Spurious Resonances


> Hi
>
> What are you picking off the output with?
>
> Picking off at the plate would be a *much* better idea if you are  trying 
> to measure the input circuit. You will have far less impact  from the 
> probe that way. If you want to suppress the effect of the  output tank, 
> swamp it with a low probe impedance.
>
> Bob
>
>
> On Oct 27, 2007, at 10:35 AM, Tony Casorso wrote:
>
>> How lucky can I get? I posted a set of images this morning that had  a 
>> "spurious response" in it. It turns out it was cause by the  settings on 
>> my sweeper (DC Offset was cranked over to one side for  some reason). The 
>> mailer wus unable to post it for some reason so I  get to try again with 
>> good pictures.
>>
>> Here are links to couple of pictures. I am sweeping the preselector  in 
>> my R-390a. The left edge of the sweep is 1.5MHz. The right edge  is 
>> 4.5MHz. The center is approximately 3MHz. The receiver is tuned  to 
>> 3.00MHz and the power is off. I am measuring the response at the  grid of 
>> the RF stage.
>>
>> The broader response picture is with the 25 Ohm (gen with 50 Ohm 
>> rterminator) sweep gen directly connected to the balanced input. The 
>> sharper response is with a 2.2K resistor in series with one side of  the 
>> balanced input.
>>
>> It looks like I was right that low source impedance broadens the 
>> frontend response.
>>
>> http://i25.photobucket.com/albums/c90/tonysradios/IMG_0825.jpg
>>
>> http://i25.photobucket.com/albums/c90/tonysradios/IMG_0826.jpg
>>
>> Tony
>>
>> ----- Original Message ----- From: "Tony Casorso" <canthony15 at msn.com>
>> To: <r-390 at mailman.qth.net>
>> Sent: Saturday, October 27, 2007 8:09 AM
>> Subject: [R-390] Re: Spurious Resonances
>>
>>
>> I should havethought about it for a few more minutes.
>>
>> Maybe the sweeper has a strong second harmonic that is visible near  1.5.
>>
>> Tony
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>