[Premium-Rx] real basic question on stability etc.

[Carcia, Francis A HS]

Gary,
 Cool article in QEX I read it last night.  fc

-----Original Message-----
From: Gary Geissinger [mailto:ggeissinger at digitalglobe.com] 
Sent: Thursday, September 01, 2005 1:58 PM
To: premium-rx at ml.skirrow.org
Subject: RE: [Premium-Rx] real basic question on stability etc.

Guys,

A few ideas with regard to frequency stability / calibration.

1.  Look in the July/August 2005 issue of QEX magazine(issue no. 231).
There is an article I wrote on performing the AFC operation using DSP
software control.  While transmitters and receivers with synthesizers
are much better these days, that is not necessarily so; especially when
listening to low power signals in the "tropical bands".  This approach
works on SSB signals very well.

2.  Look in the September 2005 issue of CQ magazine.  There is an
article I wrote on aligning receivers (premium too) with either no
instruments or minimial instruments and obtaining a highly accurate
calibration.  This method has been used on Watkins-Johnson receivers
with excellent results.

3.  These days I have a setup that pretty well solved my stability
issues.  I have a Rockwell Jupiter GPS receiver ($80 from Fair Radio)
with a 10 KHz reference output.  This output is horrible...drift, phase
noise, jump corrections of frequency that occur at a 1 Hz rate.
Fortunately the errors I've observed are zero mean.  I built a phase
locked "cleaning filter" for it. The filter tracks the 10 KHz and
generates a 10 MHz reference using an ovenized variable frequency
crystal oscillator that is Stratum 3E in quality.  I set the PLL loop
filter to a 10 second time constant with a damping coefficient of 1.25.
I divide the 10 MHz to 1 MHz and 5 MHz.  On the 1 MHz and 10 MHz I then
low pass filter them and generate 50 Ohm, 0 dBm, sinewaves that I put
into my Watkins-Johnson receivers reference inputs.  It takes 5 minutes
for the reference to settle to 1 part in 10^7, and 3 days to fully meet
spec due to the oven system.  I've compared it to top end ovenized
counters and standards...it is good to at least 1 part in 10^9th short
term whenever I have watched it.  Long term, it is probably much better;
i.e., I've never seen it slip a cycle.  Given the long time constant and
over damping, the phase noise is not an issue.  Total cost:

$80 for the GPS board from Fair Radio
$30 for Bud box, small parts
$1.65 for the MC145170 PLL chip from Digikey
Free for the sample of the Conner-Winfield ABOV5S3E VCOCXO

Using a WJ8615 receiver feeding a WJ9477 tuner I've never seen a tuning
error as much as 1 Hz across the HF / VHF bands.  I have trouble
measuring better than that.

BTW, you can use a cheaper oscillator and get okay (but not as good)
results.  The $25 dollar VTCXO from International Crystal isn't bad.

73's,

Gary WA0SPM  





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-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of Kevin
Sent: Thursday, September 01, 2005 10:49 AM
To: premium-rx at ml.skirrow.org
Subject: Re: [Premium-Rx] real basic question on stability etc.


Hi:

I think the satellite based handsets are on 1500 Mhz via INMARSAT
(Standard M, etc).

Cheers.

Kevin

Ben Dover wrote at 11:45:53 am on Thu, Sep 1 2005:
premium-rx at ml.skirrow.org

Hello John.


>> AFC would definitely have been needed to track the frequency shift 
>> due
to the doppler effect due to a low earth orbit satellite - 
noise/interference would not have been a problem. <<


Yes, AFC definitely IS an asset in LEO bird reception because of Doppler
shift, but based on practical experience here with the NOAA weather
satellites I'd have to say that interference IS a definite problem. It
came as a BIG 
surprise to me that digital satellite phones share the 137 MHz band with
the NOAA birds, not to mention that at times two satellites share the
same frequency and are mutually visible, thier frequencies seperated
only by the different Doppler shifts and shift directions of each bird.

73's,

Tom, W9LBB


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