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Re: Mode-S Antennas



The PLL is nice for two reasons:

It can lock to a good reference at an arbitrary freq.  I have several 10MHz 
and 10.240MHz TXCO reference oscs that I've picked up at our company's 
surplus store.

The other reason is that the PLL circuit can be physically MUCH smaller than 
an osc/multiplier circuit.  I've got samples of those parts, and I can lay 
out a board that is about twice the size of a postage stamp... including the 
pic.  The PIC is really simple-- an 18F83/84 can be programmed from parallel 
port on a PC, and only has to wake up and send a stream of bits to the PLL 
and then go to sleep.
Very trivial.

PLL Pitfalls:

Phase Noise.  Obviously if Drakes work, then the PLL's can be designed to 
work fine with SSB.  For our use a slow lock time, like a second can help 
greatly with phase noise.  It can be done.  I've already designed the LO on 
Nat'l Semis website-- they have an online PLL design tool, and phase noise 
works out to be good in this config.

Those damn small parts:  The pc board is going to be fun.  The pitch on 
these parts are SSOP, or .05" in between leads!  Photographic etching is 
necessary.. maybe even professionally.

As far as IF's go:  the 120 MHz IF the Drakes use are really good too.
At 24 MHz away from the uplink, if the transmitter is still noisy there, 
it'd be easy to filter.  And a second conversion to 50 MHz is trivial as 70 
MHz clock oscs are available.  I don't think a 50 MHz
IF will work directly from 2400 MHz unless you use image reject mixers.  I 
could be wrong about that, though.. I'd have to check minicircuits specs on 
the different mixers they offer.

One other comment:  If there's plenty of gain before the mixer, a diplexer 
isn't necessary-- use a 3 or 6 db pad after the mixer.  I've done it before, 
and it works wonders.  Same pad is important if you try to drive a 
diode-ring mixer with a TTL clock osc like I've implied before.  The mixer 
is happy with those pads terminating the LO and IF ports.

Fred W0FMS

>From: "Dave Metz" <Dave.Metz@mut1.muscanet.com>
>Reply-To: davemetz@muscanet.com
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>CC: amsat-bb@amsat.org
>Subject: Re: [amsat-bb] Mode-S Antennas
>Date: Tue, 13 Feb 2001 11:00:25 -0600
>
> > From:          "Frederick M. Spinner" <fspinner@hotmail.com>
> > To:            amsat-bb@AMSAT.Org
> > Subject:       Re: [amsat-bb] Mode-S Antennas
> > Date:          Tue, 13 Feb 2001 15:15:58 -0000
>
> > The hard part is the oscillator.  I'm kicking around the idea of using
> > a Maxim MAX2751 VCO ($3.50 in one-seys, $1 in Qty) and some sort of PLL
> > circuit-- the simple one in the Drake-- a fixed divider and a XOR phase
> > comparator-- or more complex like a National LMX2326 and a PIC to 
>program it
> > (same parts count, and minimal difference in price-- and more 
>flexible)...
>
>A quick look through the Mini-Circuits catalog shows a lot of nice
>parts.  A HP MGA MMIC as the front end, a MiniCircuits DBM as a
>passive mixer followed by a diplexer and maybe a ERA MMIC
>post amp.  Simple!
>
>Then we get to  the hard part,  the oscillator!
>Converters are easy, its the LO that is difficult.  There are some
>advantages to using a conventional crystal LO as opposed to a
>PLL.   For one thing, phase noise will be lower.  Also no PIC is
>required.  Keep in mind that there are many of us that are not
>programers and most likely never will me.  Ordering a xtal from
>ICM is trivial, getting a PIC programed is not (at least for me).
>
>The idea of placing the LO (or at least the low frequency portion
>of it) in the shack and piping the signal up to the converter is a good
>one.  I considered it long ago and made that idea part of my plans.
>
>In my shack, 2M will be used for the IF for the 1.2Ghz uplink transmitter.
>This precludes using 2M as the 2.4Ghz receive IF.  My plan is to use
>a UHF IF, say below 70cm, no point in cluttering up that band either.
>Maybe 400Mhz, then using a second converter to move that down to
>28Mhz.  This would allow me to use my "good" HF receiver as my
>main sat receiver.
>
> >
> > If I/we can do that, the filters, amps and mixers are commercially
> > available.  But, no it won't be a $25-$50 downconverter-- but it could 
>be
> > smaller and higher performance than the Drake.  Right at the back
> > of a Helix feed would be nice-- or in a dipole reflector feed for the
> > Conifers...  Maybe send a TXCO high-stability freq reference (nice thing
> > about PLLs) from the shack up one cable, and the signal down another.. 
>lots
> > of cool ideas here....
> >
> > If I only had more time to work on things like this....:-)
>
>You and me both!!!
>
>I have only gotten as far as some developmental work on taming
>the conventional Butler oscillator circuit.  I have learned a few things.
>LIke the inductor you always see strapped across the crystal should
>not be there.  And that ICM can provide crystals that will oscillate on
>frequency!  I've solved some drift problems as well and have just
>started my next LO project.  One that will evolve into my 1.2Ghz
>up converter.
> >
> > Fred W0FMS
>
>
>--73-- David WA0AUQ

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