On 10/09/2012 09:27 PM, John Ackermann N8UR wrote:
Here's a link to a USNO paper that measured the tempco of three GPS
amplifiers: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA490830
They found that amplifier filtering was the prime cause of tempco, and
the narrowest bandpass amplifier they looked at had a group delay range
of 4 nanoseconds over the range of -15 to +45 degrees C.
This is a good paper. I've read it before. It presents three strategies
for GPS amplifiers:
1) Wide-band amplifier, represented by the AOA Wideband amplifier
2) Narrow-band amplifier with peaks, represented by the AOA narrow band
amplifier
3) Narrow-band amplifier with no peaks, represented by the KW microwave
phase-stable narrow band amplifier.
The wide-band amplifier has around 4 ns group delay, and it is fairly
flat and stable. Since there isn't much delay to start with, it doesn't
change a whole lot either. Since the amplifier isn't very flat, it also
has some variations in group delay. It's fairly natural. The downside is
that it has no suppression of interference, so we should do some damping.
The second case tries to achieve just that, but in order to create steep
slopes around the pass-band, they have used two resonances, one on each
side of the pass-band. You see the peaking effect on the gain curve of
figure 1, but oh... they show up clearly in the group delay measurement
of figure 2 too. This is expected from the theory, as these two
pole-pairs has fairly high Q, their group delay will show this property
in the direct vicinity of their respective resonances, just as their
contribution to gain will do. So, nice steep slopes and good
suppression, but lots of group delay, and by that higher sensitivity to
environmental effects, i.e. temperature.
The third example shows wider but much flatter amplitude response, and
essentially flat group delay. This is what you expect from maximum flat
group delay filters such as Bessel/Thompson. No wonders those are
specified as measuring filters for digital transmission. Lesser delay,
and lesser sensitivity. The downside is that the cost of steep slopes
comes from a higher number of needed poles/zeros.
Just as I expect from traditional signal theory.
Again, you get what you pay for.
Now you know why I want a network analyzer reaching this area at home.
Cheers,
Magnus
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