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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