Ouch! Relay stepped attenuator? Or solid state components? On Mon, Nov 21, 2016 at 11:00 PM Bob Camp <[email protected]> wrote:
> Hi > > There are packages you can put on a fairly standard HP signal generator > that will > do the mismatch stuff without spending all the money Sperient wants for > one of > their machines. One suggestion: If you *do* go with the HP solution, > running the > attenuator up and down to simulate fast fades (think urban canyon) will > fry the generator > in about 30 days … I have empirical data …. > > Bob > > > On Nov 21, 2016, at 8:53 PM, Scott Stobbe <[email protected]> > wrote: > > > > I haven't used one personally but a spirent gps simulator would let you > do > > a try it and see. > > > > It will be interesting to see if out of the growing sdr community an open > > source gps simulator emerges. > > > > On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) < > > [email protected]> wrote: > > > >> People state it is desirable to have a GPS antenna well clear of > >> obstructions, which I believe is to stop reflections. But there is > another > >> source of reflections which I suspect could be just as problematic. > >> > >> Whilst the input impedance of the antenna input terminal on a GPS > receiver > >> is probably marked 50 Ohms, I'd be somewhat surprised if it was very > close > >> to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- > 2 > >> Ohms, but this varies, not only between different makes/models of > cables, > >> but even on the same real of cable.The output of the pre-amp is most > >> unlikely to have a 50 Ohm source impedance. In fact, the output > impedance > >> might be close to 0 Ohms, as it may be driven by a voltage source, > without > >> any 50 Ohm resistor. > >> > >> Anything not immediately absorbed by the GPS receiver is going to be > >> reflected back up the coax, and could be reflected multiple times. > >> > >> I just looked on my HP 8720D VNA, and see I can reduce the output power > to > >> -70 dBm, which would should not do any damage. It will be interesting to > >> see just what the input impedance of the GPS receiver is. I'm tied up > with > >> doing my accounts over the next few days, but later I will look. > >> > >> If reflections on the antenna/cable/receiver are a problem, then > >> attenuators can improve the match, but of course they reduce the signal > >> level too. A more intelligent, but more difficult solution, is to build > a > >> matching network. For that one would need a VNA to measure the > impedance in > >> the first place. > >> > >> Dr. David Kirkby Ph.D CEng MIET > >> Kirkby Microwave Ltd > >> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 > 6DT, > >> UK. > >> Registered in England and Wales, company number 08914892. > >> http://www.kirkbymicrowave.co.uk/ > >> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please) > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
