[R-390] Re: Spurious Resonances
Tony Casorso
canthony15 at msn.com
Sat Oct 27 13:10:24 EDT 2007
Hi Bob,
OK. I get this. But I was not trying to show any distortion in the pictures.
All I was trying to show is that the Q of the preselector degrades when the
source impedance is low.
Thats it.
If it's no surprise then so be it. The consequence of this is that, with the
receiver tuned to 3 MHz, a 1.5Mhz signal at the antenna terminals is going
to be 10 to 15db stronger at the grid of the RF stage if the antenna
impedance is low . This stronger signal is more likely to cause overload and
harmonic generation in the RF stage. The downstream filtering in the plate
of the RF stage and the plate of the mixer stage will kill the 1.5MHz signal
so the AGC does not respond to it. So the RF stage has a strong signal at
it's grid (1.5MHz) and no AGC to compensate for it. So it distorts and
produces harmonics in the plate circuit which is tuned to 3MHz. The
harmonics are amplified and fed through the mixers and onward right through
the receiver.
I don't think the loading of a 10Meg probe is going to kill the Q that much
at 3MHz (my "sharp" picture is made with the probe connected after all). The
capacitance will shift the resonant peak slightly at 3MHz. Even if I did
reduce the Q with my scope probe by some amount, the low impedance source
reduces it by a much greater amount. If I had a 100x probe I might see a
40db attenuation at 1.5MHz relative to the peak. But I would still see the
15 db attenuation with the low impedance source.
I found an antenna impedance calculation program on the web for inverted Ls
where you can specify the length of the horizontal part and the vertical
part as well as the length and gauge of the ground lead and the gauge of the
antenna wire. I tried it for my antenna dimensions using zero for the length
of the vertical segment. Not that I believe it, but it came out 18 ohms at
1.65MHz.
I wonder if I could sweep it with the antenna connected?? Probably too much
BCB stuff to see what is going on down there.
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 10:36 AM
Subject: Re: [R-390] Re: Spurious Resonances
> Hi
>
> Any filter is going to respond to it's termination impedances. Both the
> input and output impedance are significant.
>
> The grid of the RF stage looks like > 100 meg ohms resistive at RF. It's
> got a bit of capacitance, but not a whole lot.
>
> If you look at the preselector like a single tuned circuit, it's going to
> have Q. The Q is going to be determined by:
>
> 1) The resistive portion of the source resistance driving the
> preselector, relative to the reactances.
> 2) The Q of the coils themselves
> 3) The resistive portion of the load resistance, relative to the
> reactances.
>
> The "relative to the reactances" part gets a bit crazy because you
> normally have part of the resistances in series form and the other part
> of the resistances in parallel form. There is a great thing called the "Q
> formula" that lets you convert them all back and forth. It was a staple
> in EE texts up through about 1954.
>
> Vacuum tubes have really low Gm compared to just about anything else. To
> get a stage to have gain, you need very high impedances. The normal way
> to make stuff work was to run impedances way higher than we do today. You
> can easily fake yourself out going from solid state to tube. Loading is a
> much bigger deal when checking tube circuits.
>
> A measurement you can make on a solid state amp with very little
> interaction, is much more difficult to make on a tube circuit. An easy
> way to see this is using a DVM on an R390. The numbers don't come out
> right. The input impedance of the DVM is not as high as that of a VTVM.
> The DVM is fine on solid state, it's not up to the task with tubes.
>
> By going to the plate for your measurement, the load resistance is no
> longer interacting with the preselector at all. The source resistance is
> still there, but the antenna has source resistance as well. Once you have
> the load impedance out of the way you can be reasonably sure that what
> you are measuring is "real".
>
> Since you are after second order distortion, every db you knock down the
> input counts as two db at the output. If you were talking about third
> order (intermod) distortion, each db of attenuation would count three db.
> A 30 db selectivity in the preselector will take second order distortion
> down 60 db and third order down 90 db. At low levels, a tube amp should
> have second order distortion down > 30 db with no preselection at all.
>
> If the response you see is at exactly twice the input frequency, it's
> second order distortion. The conversion scheme in a 390 does not produce
> other spurs that follow a 2:1 ratio like that. You can write a book about
> all the possible spurs that can occur (and people have). The math is
> pretty simple. It's been mentioned before in this thread. Take the two
> sine waves, add them or subtract them. Take that and raise it to an
> integer power. Powers of 0,1,2,3,4, and 5 are commonly used. If you have
> a mixer you have three signals rather than just two. The same "raise it
> to a power" is used. You can include am, fm, and pm in the signal
> equations. AM, and FM modulation transfer differently through each spur
> generating process. AM is transfered by all of them to the extent that
> the amplitude of the output will follow he amplitude of the input through
> the db/db ratio for that response.
>
> So much fun .....
>
> Bob
>
>
>
>
>
> On Oct 27, 2007, at 11:39 AM, Tony Casorso wrote:
>
>> Hi Bob,
>>
>> I'm not sure what you are getting at. I used a 10x scope probe connected
>> to the Grid of the RF stage. I want to measure the response of the
>> preselector. I do not want to swamp anything. The two pictures show a
>> sharp response from the preselector in one case, and a not so sharp
>> response in the other case. The only difference is the impedance of the
>> signal source connected to the antenna.
>>
>> 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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