From: "Michael A. Terrell"
Subject: Re: Receiving multiple ~100kHz time code broadcasts
Date: Tue, 03 Sep 2002 14:49:56 -0400
Organization: Some day, some time
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Paul Keinanen wrote:
> On Mon, 02 Sep 2002 10:33:47 +1200, Jim Granville
> > First place to focus on, is a high Q ferrite antenna : Q's of 60-180
> >give bandwidths of 1KHz-500Hz, plus ferrite gives virtual area gains.
> > A Q of 6000, is a bandwidth of 10Hz, and you will see some of the Atmel
> >appnotes use XTALs for the IF filters.
> > Antenna Q is 'signal for free', so should be a key focus.
> > It has no quantise errors, and no dynamic range issues, and
> >no beat frequency effects. ( but might need Tempco / auto-tune )
> While a high antenna Q is a very good thing for the dynamic range,
> there are some practical problems if multiple stations on different
> frequencies are needed.
> The required capacitance changes are quite large (if you want to keep
> a reasonable L/C range and impedance level), so you would need quite a
> few capacitance diodes intended for broadcast AM to tune a large
> frequency range. In practice, you may have to switch in fixed
> capacitors to tune to other frequencies.
> Even for single frequency operations, the auto-tune range must be
> quite wide if you want to avoid manual frequency setting during
> manufacturing (which is a cost issue if large series are produced).
You can use a cheap analog mux to select the cap. I've used this
with AGC circuits to select the time constant. You can also use a
varactor to tune the antenna to the desired frequency. The Atmel T4227
data sheet shows the different modulation Methods used, and uses a
crystal on the selected frequency as a series resonator for better
selectivity. The output can be configure active high, or active low,
and it has a power on pin to shut it down, if needed. The sheet also
explains the ferrite antenna design. It will take a lot of digital to
replace a simple analog design. As far as a wire antenna, Microdyne
used to use a Fluke VLF receiver to pick up WWVB on 60 KHz, and the long
wire antenna was over 200 feet to get a usable signal. Have fun with
your DSP ideas.
An old fart since August 5th, 2002!
Michael A. Terrell
Check out my web site and tell me what you think.
I'm up to 160 pages, and still working on it.