[time-nuts] ThunderBolt question

Sat Jun 6 13:03:23 UTC 2020

Hi

Do a little research and You will find that a 3 db noise figure is as good as it gets
with a receiver who’s front end is a perfect match to the transmission line ….

(Signal goes down 6 db when you match, noise drops by 3 db ….). 

Unless you are using some sort of ferrite isolator on the front end, a low noise
device is not going to be one that matches the line. 

Bob

> On Jun 6, 2020, at 8:51 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> Basically true, Tom, but with some caveats:
> 
> The GPS system was originally designed, as you say, to provide
> positions and velocity
> information to military users in the field.  Errors of a few meters were of
> little consequence.
> 
> But now, we have the time-nuts, surveyors, etc, who are decidedly pushing
> the envelope
> of GPS accuracy.  Some users want sub-nanosecond timing accuracy, surveyors
> want
> mm-level position accuracy, etc, and doing these things indeed requires
> extreme attention
> to detail.
> 
> As I understand it, multipath components that are delayed by about a few
> chips or more
> basically appear as a wee bit of added noise and have negligible
> consequence unless
> they are comparable to the direct signal in power.  However, MP components
> whose
> delay is less than about a chip time have the effect of tugging the
> apparent temporal
> location of the direct signal to some new value.  Note that this is in the
> range of practical
> round trip cable delays in at least some installations.  At Arecibo, for
> example, the one-way
> cable lengths from GPS antennas on the roof to their respective receivers
> was about
> 200 ft.  Round trip was thus 400 ft physical, hence up to about 600 ft
> depending on
> what the cables were using for the dielectric.   That's about 0.6 chip
> length for the C/A
> code GPS signal.
> 
> Another factor which will influence the error introduced will be the RF
> phase relationship
> between the direct and delayed signal component.
> 
> Gee, this stuff gets complicated ...
> 
> Dana
> 
> 
> On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes <tholmes at woh.rr.com> wrote:
> 
>> Dana...
>> The question that comes to mind is just how much effect a weak a long
>> delayed reflection will have on overall system performance since it will
>> only matter to SV’s with poor S/N. The modulation scene which allows all
>> the SV's to transmit on the same frequency has to be pretty robust in the
>> face of both widely varying signal strengths and multiple signals arriving
>> at different times. It’s a similar scheme to CDMA cell phones, which
>> operate in a much more difficult environment with regard to signal
>> strengths, multi-path, and number of on channel signals. And those work
>> amazingly well.
>> 
>> Further, I am led to believe that once you have enough SV’s in view to
>> get a good set of ‘readings’, ionospheric effects are the limiting factor
>> until you go to a multi-band receiver.
>> 
>> Yes, to wring the last ounce of performance out of GPS takes attention to
>> the details, but don’t lose sight of how it was designed to work for users
>> in less than optimum (military field operations) in the first place.
>> From Tom Holmes, N8ZM
>> 
>>> On Jun 6, 2020, at 7:14 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
>>> 
>>> It's one thing to maintain lock in a multipath environment, quite
>> another
>>> thing
>>> to get "full" accuracy of GPS measurements of PVT.
>>> 
>>> An interesting difference between my scenario of poorly matched
>> impedances
>>> and "ordinary" multipath is this:  In the poor matching scenario, all the
>>> received
>>> signals will be impaired identically, while in the ordinary multipath
>>> scenario,
>>> signals from different satellites will suffer different (and
>> time-varying)
>>> multipath
>>> impairments.  I'm not at all sure what effect this difference will have
>> on
>>> final
>>> outcome, but my gut feel is that the case where all signals are impaired
>>> identically
>>> could lead to worse effects.
>>> 
>>> Dana
>>> 
>>> 
>>>> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes <tholmes at woh.rr.com> wrote:
>>>> 
>>>> Dana...
>>>> 
>>>> I think that you are neglecting two important mitigating factors.
>>>> 
>>>> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those
>>>> reflections quite a lot from one end to the other.     It amounts to 2
>> - 3
>>>> dB in 25', depending on cable quality.
>>>> 
>>>> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order
>> of
>>>> 20% of the incident power, about  7 dB. I am rounding a lot here just to
>>>> keep the math easy...for me.
>>>> 
>>>> By the time a reflection has made the round trip from the receiver back
>> to
>>>> the antenna and them back to the receiver, which is how the delay would
>>>> have
>>>> to manifest itself, it will be down at least 15 dB from its original
>> self,
>>>> and probably more. Given the coding of GPS signals which allows several
>>>> satellites to share a common frequency band, that is not going to be
>> much
>>>> of
>>>> a problem. And if only one end of the path actually is 75 ohms, then
>> there
>>>> won't be a delayed signal.
>>>> 
>>>> Tom Holmes, N8ZM
>>>> 
>>>> -----Original Message-----
>>>> From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Dana
>>>> Whitlow
>>>> Sent: Friday, June 05, 2020 9:01 PM
>>>> To: Taka Kamiya <tkamiya9 at yahoo.com>; Discussion of precise time and
>>>> frequency measurement <time-nuts at lists.febo.com>
>>>> Subject: Re: [time-nuts] ThunderBolt question
>>>> 
>>>> I'd like to point out that mismatches at the ends of an antenna cable
>> *can*
>>>> cause trouble.  When both ends are mismatched, each bit of detail in the
>>>> signal
>>>> gets partially reflected back and forth, each time delayed by the round
>>>> trip propagation
>>>> delay in the cable, and so you have something like multipath going on.
>>>> Fortunately the
>>>> successive reflections get weaker with time, generally quite rapidly.
>>>> Since many
>>>> GPS users seem very concerned about multipath resulting from poor
>> antenna
>>>> placement,
>>>> I think this factor should be considered as well and not just get swept
>>>> under the rug.
>>>> 
>>>> The amplitude of the "multipath" resulting from cable mismatches
>> depends on
>>>> the product
>>>> of the voltage reflection coefficients at the two ends of the cable.  If
>>>> either end is perfectly
>>>> matched, then the quality of the match at the other end is not
>> significant
>>>> vis-a-vis apparent
>>>> multipath problems and only affects transmission loss.
>>>> 
>>>> But when there is a mismatch on both ends, then the length of the cable
>>>> comes into play
>>>> as well.  A longer cable means more delay between successive
>> reflections,
>>>> which is just
>>>> like multipath involving longer delays between the direct and the
>> reflected
>>>> signals.
>>>> 
>>>> Cheers,
>>>> 
>>>> Dana      (K8YUM)
>>>> 
>>>> On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts <
>>>> time-nuts at lists.febo.com> wrote:
>>>> 
>>>>> 50 ohm / 75 ohm question is really irrelevant in this kind of thing.
>>>>> Trmble itself says in manual, not to be concerned with this apparent
>>>>> mismatch.
>>>>> In my particular case, I have a home lab standard and existing
>> system.  I
>>>>> have an antenna and network of distribution amplifiers.  They are all
>> 50
>>>>> ohms and N connectors.  Some ports have BNC adapters attached.  I have
>>>>> pretty much standardized everything to SMA, N, or BNC.
>>>>> 
>>>>> I boxed a power supply, T-bolt, and buffer amp in a metal case.  I
>> bought
>>>>> a short cable (RG58) that goes from F to BNC.  On back of the case, I
>>>> have
>>>>> BNC to N adapter.  I also have a few adapters that goes from F to BNC
>> for
>>>>> the test bench.  It really doesn't matter what you use, as long as it
>>>> makes
>>>>> a solid connection.
>>>>> 
>>>>> Advantage of F connectors and RG6 are, cheap, abundant, and low loss
>> for
>>>>> the size.  Advantage of having house standard is, less adapters and
>> less
>>>>> headache.....
>>>>> 
>>>>> ---------------------------------------
>>>>> (Mr.) Taka Kamiya
>>>>> KB4EMF / ex JF2DKG
>>>>> 
>>>>> 
>>>>>   On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario <
>>>>> ka3zyx at comcast.net> wrote:
>>>>> 
>>>>> I have a Trimble ThunderBolt GPSDO that I just received. It has an F
>>>>> connector for the antenna input, and BNC connectors for the 1 pps and
>> 10
>>>>> MHz outputs. Is the receiver input impedance really 75 Ohms, or is it
>> 50
>>>>> Ohms and they just used the F connector to distinguish it from the
>>>>> others? What do people do, just use a 50 Ohm antenna?
>>>>> 
>>>>> Thanks
>>>>> 
>>>>> Robert DiRosario
>>>>> 
>>>>> KA3ZYX
>>>>> 
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