[TenTec] RE: [Orion] Orion subreceiver

[Carl Moreschi]

All I am talking about is the image response from the 14 khz IF.  If you
have a IF desired signal at 45 mhz and you mix it with 45.014, you will get
the desired signal to be 14 khz.  But if another very strong signal is at
45.028, and you mix with 45.014, you will also get this undesired signal at
14 khz.  It is the job of the 45 mhz roofing filter to remove this 45.028
image response.

Carl Moreschi N4PY
Franklinton, North Carolina
n4py at earthlink.net
----- Original Message -----
From: "Bill Tippett" <btippett at alum.mit.edu>
To: <tentec at contesting.com>
Sent: Wednesday, January 05, 2005 4:14 PM
Subject: Re: [TenTec] RE: [Orion] Orion subreceiver


> 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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