[R-390] Image problem solved - Not.

Fri Oct 26 06:28:40 EDT 2007

A 2nd order product (2*F1) drops 2 dB for each 1dB drop of the input signal and a 3rd order
product(2*F1-F2  and 2*F2-F1) drops 3 dB for each 1 dB drop of the input signal. A step attenuator
in series with the antenna will help answer the question as to whether the problem is in the
receiver or external to it. While listening to the undesired signal click in say 6 dB of
attenuation. If the undesired signal drops by ~12 dB it's a 2nd order product. If it drops by ~18 dB
it's a third order product. If the undesired signal drops by 6 dB it isn't a 'phantom' signal being
generated by the receiver, however, it could still result from poor image rejection. Running the
known frequency and oscillators numbers will show whether or not it could be an image.  It's a good
idea to click in an a few dB before running the test and leave it in to help 'flatten' the match
between antenna and receiver.

Jay W1VD

----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: "Tony Casorso" <canthony15 at msn.com>
Cc: "R-390 List" <r-390 at mailman.qth.net>
Sent: Thursday, October 25, 2007 10:36 PM
Subject: Re: [R-390] Image problem solved - Not.

Hi

Second order distortion should follow a normal power law. More or
less, you should get a larger drop in the "spur" than in the signal
generating the spur. I'd have to dig out my old course notes to be
sure, but a 2:1 ratio in db sounds right for second order.

Bob

On Oct 25, 2007, at 10:03 PM, Tony Casorso wrote:

> Hi Mark,
>
> Thanks for asking around for me. I did check the AGC (-11 volts to
> 1st RF amp grid on a strong local station). The thing is, the
> actual signal at the frequency I am tuned to is not that strong. I
> am tuned to 3.3MHz where there is actually no signal being
> transmitted. The signal that I am hearing is being transmitted at
> 1.65MHz. Normally, AGC does not even operate in this situation
> because there is no signal at 3.3MHz where I am tuned. So, once I
> hear this harmonic, it's already too late for AGC to prevent it.  I
> don't think I am explaining this very well.
>
> Suppose the 1.65MHz signal was just below the threshold of causing
> this problem? I would hear nothing at 3.3MHz and there would be no
> AGC at all. Now turn up the 1.65MHz signal a little bit and I have
> the problem. How could AGC have prevented it? I'm not sure thats
> much better.
>
> Anyway, I still say that the front end preselctor is way too broad
> in this situation. Talking to Bob gave me an idea that is probably
> all wet. If the impedance of my antenna system at 1.65MHz were
> really low, say a few ohms, would this kill the Q of the
> preselector at that frequency? Putting a 220 Ohm resistor in series
> with one side of the balanced input reduces the 1.65MHz signal at
> te input to the RF amplifier enough that I no longer hear anything
> at 3.3MHz. Increasing it to 2200 Ohms reduces the 1.65MHz signal at
> the RF amp grid when tuned to 3.3MHz by 30db while only reducing
> the signal when tuned to 1.65MHz by 8db. This says to me that the Q
> is much better.
>
> I wasn't going to keep this up here on the list but people keep
> asking me questions... :).
>
> Tony
>
>
> ----- Original Message ----- From: "Mark Huss"
> <mhuss1 at bellatlantic.net>
> To: "Tony Casorso" <canthony15 at msn.com>
> Cc: <r-390 at mailman.qth.net>
> Sent: Thursday, October 25, 2007 7:40 PM
> Subject: Re: [R-390] Image problem solved - Not.
>
>
>> For what it is worth, Tony, I asked around about your problem with
>> people who have had extensive experience with troubleshooting
>> R-390A's. All four said the same thing. AGC on the First RF Stage
>> is driving it too hard, and into distortion. I just gave the
>> symptoms, not that you had worked on the AGC Transformer. That may
>> not be it, but it is the first place they would start.
>>
>> Tony Casorso wrote:
>>> Sorry everybody,
>>>
>>> I lied. I didn't mean to but I did. Tonight I tried to repeat the
>>> experiment and it doesn't work.
>>> Lifting the ground from one side of the balanced input does
>>> absolutely nothing to help the situation.
>>>
>>> I'll bet that I had a bad connection to one side of the balanced
>>> connector since I had to jerry rig it to connect my BNC from the
>>> antenna to the balanced input without ground. I hear no harmonic
>>> at all when only one pin of the balanced input is connected to
>>> the antenna. I also hear only a faint hint of a harmonic when I
>>> use the unbalanced input.
>>>
>>> So, I still have the problem (if it's really a problem). It's not
>>> an image problem really. I'm sorry I called it that. It has
>>> nothing to do with the IF.
>>>
>>> I'm going to go away now. I'll make sure everything is repeatable
>>> before I bring it up again.
>>>
>>> I feel like the Rockies must feel after the disaster in Fenway
>>> last night. Well, maybe not quite that bad.
>>>
>>> Tony
>>>
>>>
>>>
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>>
>>
>> -- 
>> --------------------------------------------------------------------- 
>> -
>> Some people are like a Slinky .. not really good for anything,
>> but you still can’t help but smile when you shove them down the
>> stairs.
>> --------------------------------------------------------------------- 
>> -
>
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