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Re: Transystems Modifications in simple terms.



In simple terms,

You need to REMOVE THE STUB no matter what. This stub attenuates the band of
interest (2400). In it's original application it attenuates the image
(2400).

If using a 123 MHz IF:
1. You have twice the noise being converted to 123 because the stock filter
does not offer any image attenuation.
2. You can place a stub at the original location and cut it to notch the
image at 2155 without affecting 2400.
3. you can replace the filter with any ISM band filter like (2450 +/- 50
MHz) to get rid of the image and get the opmimum performance.

If using a 145 MHz IF:
1. You might be ok with the stock filter since the image has SOME
attenuation caused by the filter skirt.
2. You can place a stub at the original location and cut it to notch the
image at 2111 without affecting 2400. This will give you lower noise.
3. you can replace the filter with any ISM band filter like (2450 +/- 50
MHz) to get rid of the image

Now this makes sense to me. It did not make sense how leaving the 2400 stub
helps the amateur radio configuration.

Pieter Ibelings
N4IP

----- Original Message -----
From: "Estes Wayne-W10191" <W10191@motorola.com>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, May 08, 2001 3:28 PM
Subject: Re: [amsat-bb] Transystems Modifications


> Ward WC0Y wrote:
>
> The 122 to 222 MHz IF output band is below the LO frequency.  Its image
input is in the 2400 to 2500 MHz band where we hams want it to operate.  The
designers could not allow the 2400 to 2500 MHz span to add noise and
interference, so they added the tuned stub after the LNA to filter it out.
That stub is what removes the image noise and keeps a good noise figure when
using a 123 MHz IF.
> So, one image noise span is suppressed for each IF frequency case; for the
123 MHz IF it is the stub, and for the 145 MHz IF it is the RF filter
roll-off.
>
> Wayne replies:
>
> With a 2278 MHz L.O. and no filtering, both 2401 and 2155 MHz will mix
down to the 123 MHz IF.  We only want to "hear" the 2401 MHz signal.  To
accomplish that, the mixer input needs either a 2155 MHz notch filter or a
2401 MHz bandpass filter.
>
> I don't understand Ward's statement that the 2400 MHz stub (notch) will
reduce the undesired signal unless he is referring to the MMDS application
where 2400 MHz is the undesired signal.  For use with AO40, the undesired
signal is 2155 MHz with a 2278 MHz LO and 123 MHz IF.  Or 2111 MHz with a
2256 MHz LO and 145 MHz IF.
>
> I understand how Pieter Ibeling's 2400 MHz ceramic bandpass filter would
greatly reduce any noise mixing down from 2155 MHz.  Howie DeFelice reported
that the 2155 MHz signal does indeed get through the mixer and stock filters
(even stronger than the desired 2400 MHz signal because of the 2400 MHz
stub).  It appears to me that the Murata ceramic filter would offer a
significant improvement in the downconverter's performance under every
circumstance that has been discussed (stub cut or not cut, 123 or 145 Mhz
IF).  So would a longer stub cut to 2111 or 2156 MHz.
>
> I think that anybody who uses the AIDC3733 with a helix or 60cm dish
should add the Murata 2400 MHz ceramic filter.  Maybe it's not necessary
with higher-gain antennas.  It may also be necessary if there are any
obstructions such as trees.
>
> Wayne Estes W9AE
> Mundelein, IL, USA
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