[TenTec]General 2nd order questions

[Steve N4LQ]

Ok ...So what do you call these products? If you have the bandpass open to 
2khz and you hear 2 carriers about 500hz apart and they beat together, 
producing a 500hz tone. What is that? Is this occurring in the product 
detector or in the audio section? I've heard this on several rigs, some 
worse than others.
It can't be happening in the RF or IF stages because the 500hz tone is way 
out of the bandpass. It must be happening in the product detector. Is there 
a measurement for this?

Steve N4LQ
----- Original Message ----- 
From: "Bill Tippett" <btippett at alum.mit.edu>
To: <tentec at contesting.com>
Sent: Monday, January 17, 2005 4:51 PM
Subject: [TenTec]General 2nd order questions


> N4LQ wrote:
> >Bill etc.
> I can make little sense of the ARRL's test. For the VI+, they state the
> signals can be either S9 or S9+40db. Then they show a chart but don't 
> reveal
> which input level they used. They mention 3rd and 5th order IMD but not 
> 2nd
> Order IMD.
>
>         Yes, I noted they did not document their results well.
> I belive this test was done prior to KC1SX coming on board in 1998,
> and many of the previous test engineer's results were "sloppy" IMHO.
>
> >Then for the Orion, things are a little more clear. They still don't 
> >mention
> 2nd order however. Another thing I need help on: They say the input 
> signals
> are spaced 100hz apart then they state that these produce tones of 900hz 
> and
> 1100hz. The difference of which is 200hz. This looses me.
>
>         I don't believe 2nd order is a problem since these
> products fall well outside the IF.  Remember that the test signals
> are at 14 MHz, so sum and difference are not an issue (e.g. 14200.9
> and 14201.1 (200 Hz apart as they stated, not 100 Hz as you said).
> Sum and difference of those is ~0 and 28 MHz, well outside the IF.
> The problem is 3rd order products, which are 2f1-f2 or 2f2-f1 (e.g.
> 2 * 14200.9 - 14201.1 = 14200.7, OR 2 * 14201.1 - 14200.9 = 14201.3,
> both of which fall within an SSB passband).  5th order products are
> 3f1-2f2 or 3f2-2f1, and fall another 200 Hz each side of the 3rd order
> products but still within the passband.
>
> >The Orion seems to exhibit a huge difference in IMD between fast and slow
> agc. What does agc have to do with steady carriers used in this test? Why
> would agc make any difference at all since the agc voltage should be
> constant as well as the gain of the amplifiers the agc controls?
>
>         I once heard an explanation for this but have forgotten it.
> The net effect of Slow AGC may be to prevent the AGC from attempting
> to follow rapid transitions creating overshoot problems.  Perhaps
> someone who knows can explain.  Orion's primary AGC (it has two)
> is in the DSP so indeed it is digitizing the signal at the 36 kHz
> IF and could react (or over-react) to rapid changes in the input.  The
> lesson I take away is to use as slow an AGC as possible (good idea for
> very weak signals anyway as YT1NT has observed).
>
> >I think what Clark is hearing is simply the product of 2 carriers within 
> >the
> passband. Example: Tune rx to 7100khz. Two carriers present are at 7100.5
> and 7100.7. These produce a 500hz and 700hz tone in the speaker but also
> produce the difference of 200hz and the sum of 1200hz, both can be heard
> with the 1.8mhz filter engaged. So now we have 4 tones all of which
> constitute "2nd order products.
>
>         No.  Explained above.  3rd and 5th order are the concerns.
>
> >What are the requirement that make a product detector immune to these? Is 
> >it
> all based on linearity?
>
>         Yes, if all components were linear there would be no problems.
> This reminds me of Icom and W5YR trying to explain why DSP would
> eliminate the need for all crystal filters.  If a perfect world,
> that is true...but the world isn't quite perfect yet.
>
>                                                 73,  Bill  W4ZV
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