N9DG wrote:
> I think that you'll find
> that these "ghost" signals are at +/-12 or 14 kHz away
> (depending on radio model) as you move away from the strong
> signal that is causing them. What is happening is that they
> are images that fall inside the the first IF roofing filters
> bandwidth.
N4PY wrote:
> The image signals should be at twice the third IF away or 24 to 28 kHz away
> and only on one side of the signal and not both. The current roofing
> filter width is fine for this except it probably has an ultimate rejection
> of about 60 to 80 db. Very strong signals can still push thorough this.
I missed something here. If you mean 3rd order IMD products,
they are formed when two interfering signals (f1 and f2) mix as
2f1-f2 and 2f2-f1. Assume you are tuned to 14020. An IMD product
can be formed by 2 different combinations of interfering signals at
the following frequencies:
14021 & 14022 (2*14021 - 14022 = 14020 IMD product).
14018 & 14019 (2*14019 - 14018 = 14020 IMD product).
The rule is that IMD can most easily be formed whenever two
interfering signals (from either side) fall within the 1st IF
roofing filter. For a +/- 6 kHz roofing filter (i.e. 12 kHz
total BW), this means 2 signals spaced 3 kHz apart at 14023
and 14026 will create an IMD product at 14020 per the above
example. These same signals fall well outside a +/- 500 Hz filter
(e.g. Orion's 1000 Hz filter) and will not cause IMD products.
Note that some IMD can also be formed if only ONE of the
interfering signals is within the roofing filter passband, this
is why you see gradual degradation as interfering spacings are
decreased (i.e. it's not a brick wall response).
In general, the closer strong signals are spaced, the worse
the IMD performance in any receiver, but such close spacings are
the real world in most contests and large DX pileups (e.g "listening
1-2 kHz up").
I think Duane had it right above but perhaps Carl is making some
other point I don't understand. The problem with ANY up-conversion
receiver (i.e. general coverage) is that its 1st IF MUST be in the
VHF range, and it is extremely difficult (and expensive) to make
narrow filters at that frequency versus 9 MHz which Orion/Omni use.
For example a 0.01% BW filter at 45 MHz is 4.5 kHz but a similar
0.01% BW filter at 9 MHz is 900 Hz. This is why it is much more
difficult to make narrower filters at VHF 1st IF's. Yaesu's FT9000
3 kHz roofing filter is the lowest I have seen for any VHF IF,
and it will be extremely difficult for Icom or Yaesu to push this
much lower in bandwidth. The real problem is their choice of up-
conversion architecture, which is necessitated by general coverage.
73, Bill W4ZV
P.S. As I said before, with Inrad's new filter, Ten-Tec will likely
occupy the top two categories in any IMD ranking of current rigs;
it will be interesting to see which (Omni or Orion) comes out on top.
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