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Re: NOTES on AIDC 3733



I meant a 26.4 crystal might oscillate at 8.8.


----- Original Message -----
From: "Pieter Ibelings" <elpieter@hotmail.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Thursday, April 05, 2001 21:29
Subject: [amsat-bb] NOTES on AIDC 3733


> Here are some notes on the Transystems downconverter:
>
> The chip marked 510 is a Fujitsu MB510 prescaler. This chip can divide and
> incomming signal (up to 2.7 GHz) by 128,144,256, or 272 times. This
divided
> signal in the 8 or 16 MHz range (with a 2.3 GHz signal in) is compared to
> the VCO and locked by the HC4046. What does this mean? It means that you
> have a choice of four crystals to achive the desired LO frequency. Here is
a
> list:
>
> For a 2256 MHz Local Oscillator:
>
> 128x  17.625 MHz
> 144x  15.66667 MHz
> 256x  8.8125 MHz
> 272x  8.294118 MHz
>
> The desired division ratio is selected by putting either ground or Vcc on
> pins 3 and 6. More information on the MB510 can be found here:
> http://www.fujitsumicro.com/pdf/mb510.pdf?sec=prescalers
>
> Look around in the junkbox and see if any of the crystals that you have
> might work better with a different divide ratio. Some of the components in
> the 8 MHz oscillator might need to be changed when using the other divide
> ratios. The loop components might also need tweaking if going to the 16
MHz
> ones. Keep in mind that most crystals higher that 20 MHz or so might be
> overtone crystals. A crystal marked 28.4 MHz might oscillate close to 8.8
> MHz. I have not tried the CB crystals sold at Radio Shark.
>
> Another interesting point is that the phase noise can be improved by
several
> dB's by changing the multiplication factor. The calculated degradation of
a
> PLL of this type is 20 x log(multiplication ratio). This value is
> theoretical and you actually get more degradation. This means that the
> degradation from the phase noise of the reference will be:
>
> x128    = 42.1 dB
> x144    = 43.2 dB
> x256    = 48.2 dB
> x272    = 48.7 dB
>
> From these numbers it is possible to improve the phase noise by 6 dB by
> going to a 17.625 MHz crystal. Improvements will be less due to slight
> degradation of the Q of the crystal going from 8.8 to 17 MHz.
>
> Another very important note is that the bandwidth of the bandpass filters
> have passband at both the desired frequency (LO+IF) and the image (LO-IF).
> So when using a 2256 LO and receiving a signal at 2401, you are also
> receiving at 2546 MHz. This undesired signal or noise power passes through
> the bandpass and ends up as noise at 145 MHz.  This will create a 3dB
error
> in the noise measurements. LO leakage can also degrade these measurements.
>
> The chip marked R24 is the VCO transistor. The wavy trace on the lower
left
> pin is the 2278 MHz resonator inductor . It is bypassed to ground and has
a
> 220 ohm bias resistor. The 1PZ chip is the varactor . The varactor is fed
by
> a 10K resistor marked 103. The round test pad to the right of the resistor
> is used to test the lock voltage. Idealy you want the voltage on this pad
to
> be the same before and after the conversion of the LO frequency. I have
not
> measured mine but it needs to be in the middle of the voltage rails so
that
> the PLL has range to compensate for the temperature and other drifts. On a
> locked PLL the voltage can be adjusted by changing any of the resonator
> components (varicap, series cap, etched inductor).
>
> The signal out of the VCO goes to an Agilent (HP) AT-41511 transistor. The
> datasheet is here:
> http://literature.agilent.com/litweb/pdf/5965-8929E.pdf
>
> Most of the resistors around it are for biasing. The three resistors after
> it are a small attenuator pad going into the mixer. This pad can be
removed
> to make a stronger signal source. The mixer diodes are inside the chip
> marked SH2. There are two diodes inside that package.The 2.4 GHz signal is
> feed on the upper left of the mixer and the IF comes out below the etched
> "U". This "U" looks like a LO notch or something.
>
> Receiver stages:
> The small coax at the input of the second filter is a notch filter. It can
> be adjusted to notch the image (around 2546) by cutting it. How much? You
> need test equipment. I would just remove the thing.  The part marked 414
is
> an Agilent AT-414xx transistor for additional gain. The LNA board works on
> 12 volts and requires no negative supply. It can be used for a much better
> downconverter.
>
>
> Things I don't like or would do different:
>
> 1. I would remove the input filter. This would buy me better noise figure.
> Overload should not be a problem if aiming the antenna  away from powerful
> transmitters.The second filter would be modified to filter the image
> frequency and the LO. For weak signal work it is very important to filter
> the LO at the receiving frequency (2401). The LO specially on these types
of
> converters has a lot of undesirable noise at 2401. This degrades your
noise
> performance.
>
> 2. The LO is a very poor performer. This design is just not suitable for
SSB
> work! I would totally get rid of the LO and replace it by a 2256 design
> based on a 94 MHz multiplied crystal in an oven.
>
> The mixer should be ok. I plan on taking the converter to work this
weekend
> to see if I can come up with reproducible modifications to improve this
> converter. I personally think that it can be modified to really perform
well
> for mode S reception. I will keep everyone updated.
>
> Pieter N4IP
>
>
>
>
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