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Re: choice of IF frequencies



If you have a high Q filter (helical or interdigital) on the output the
third harmonic of the IF at 1305-1314 MHz (for 435-438 MHz input) can be
attenuated sufficiently. A 2 pole 20 MHz wide filter  (Q=63) will provide at
least 20 dB of attenuation 40 MHz from the center frequency. The third
harmonic is already 40 dB down so this will be enough.

My original message was answering a question why Down East Microwave
transmit converters didn't support a 435 MHz IF. These products (of which I
own several) use printed microstrip filters that are constrained to a Q of
about 10. At 1270 MHz the bandwidth of these filters is over 100 MHz so they
will do nothing to reject a spur 40 MHz away. Because they are typically 6
pole filters they will have 70 dB or more attenuation at the image frequency
which is 288 MHz away for a 2 meter IF.

73,

John
KD6OZH

----- Original Message -----
From: "Robert Snyder" <kd1vv@mediaone.net>
To: "laura halliday" <marsgal42@hotmail.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Friday, 08 December 2000 21:51 UTC
Subject: Re: [amsat-bb] choice of IF frequencies


> I think the problem isn't receiving (AO-40 doesn't have an L-band
> downlink) but transmitting.  You have to meet FCC spurious emissions
> requirements.  Also, it's not the image frequency (that's way away for a
> 435 MHz IF) it's the 3x1 mixer spur.  But some manufacturers (e.g.
> Parabolic AB) are advertising a 435/1268 MHz up converter, so how are
> they dealing with the problem?
>
> 73
> Bob KD1VV
>
>
> laura halliday wrote:
> >
> > John Stephensen, KD6OZH <kd6ozh@gte.net> wrote:
> >
> > >Yes, it can be done and most of the 1296 MHz transverters
> > >in the 1960's and 1970's used a 28 MHz IF. However, they
> > >all used fairly large narrowband interdigital filters built
> > >up from sheet metal and tubing. This was required to get a
> > >Q high enough to reject LO and image frequencies...
> >
> > There is of course nothing preventing you from building
> > an image-reject mixer, in which case you can use any IF
> > you want. Even baseband. All you need is enough image
> > rejection to keep the image from killing the noise
> > figure. If you can't be bothered, you can also just
> > eat the noise figure hit. Based on what I've heard from
> > AO-40 so far, this won't be much of a problem...
> >
> > Laura Halliday VE7LDH     "Que les nuages soient notre
> > Grid: CN89mg               pied a terre..."
> >                                    - Hospital/Shafte
> >
____________________________________________________________________________
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> >
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