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

[Tony Casorso]

Hi Jim,

I used a 10x scope probe which has about 10pf and 10Meg. The extra 
capacitance just shifts the resonant peak a little to the left. I can 
re-peak at 3MHz with the antenna control. At 30Mhz the probe would make a 
much bigger difference.

Tony
----- Original Message ----- 
From: "Jim M." <jmiller1706 at cfl.rr.com>
To: "'Tony Casorso'" <canthony15 at msn.com>; "'Bob Camp'" <ham at cq.nu>
Cc: <r-390 at mailman.qth.net>
Sent: Saturday, October 27, 2007 10:27 AM
Subject: RE: [R-390] Re: Spurious Resonances (was images)


> When you connect a test probe to a high impedance tube circuit, especially
> the grid, the probe may change the circuit parameters and cause strange
> behavior.  A "10X" probe may not be high impedance (in terms of tube type
> circuits).  Also, any signal picked up by the probe itself can show up in
> the measurement.  A VTVM or RF voltmeter having very high impedance probe
> inputs is preferable (10's-100's of megohms).  Not sure this is affecting
> your tests or not...just my 2 cents worth.
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
> On Behalf Of Tony Casorso
> Sent: Saturday, October 27, 2007 11:51 AM
> To: Bob Camp
> Cc: r-390 at mailman.qth.net
> Subject: Re: [R-390] Re: Spurious Resonances (was images)
>
> 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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>>
>
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