[time-nuts] To use or not to use transmission line splitters for GPS receivers

Bob Camp lists at rtty.us
Wed Oct 10 07:09:43 EDT 2012


…. and if we have to go to something more exotic than simple two pole filters the group delay (and it's variation) has got to go up.

At least some of the HP splitters have RF filters in them. The same is true of GPS receivers. A receiver or splitter in the attic will have many of the same group delay issues as an antenna. I know, who would put one in the attic. Just how warm does that rack get as the air-conditioning cycles and the vents clog up? 


On Oct 10, 2012, at 4:11 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Hi!
> I forgot to mention, but the peak group delay of a pole pair is d_peak = 2*Q/w0 = Q / (pi * f0)
> Hence, the group delay increases linearly with increasing Q values. Shift the Q, and your delay vary, shift the center-frequency, and you dip off the peak.
> Cheers,
> Magnus
> On 10/09/2012 10:55 PM, Magnus Danielson wrote:
>> 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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