[time-nuts] GPS Modulation and 10 MHz Delay Lock (Jim Lux)
jimlux at earthlink.net
Wed Oct 3 00:26:33 EDT 2012
On 10/2/12 8:40 PM, johncroos at aol.com wrote:
> Hi Jim -
> Thanks for the update on the modern GPS receivers. I was aware that the
> modern ones
> do not have a classical analog tracking loop, much less a bunch of them.
> However it is a useful concept for purposes of explanation that you do
> not need the 1 pps to
> lock up the 10 MHz VCXO - which was my main point.
> The Tbolt block diagram in the manual Figure 5-10 shows the 10 MHz VCXO
> output going to
> the receiver and also to the output. The 1 pps comes from the "cpu and
> support" circuit.
yes.. the Tbolt is an example of a receiver that shifts the frequency of
the reference so that it can generate an "on the mark" 1pps (and
generate an accurate 10 MHz as well...)
> - On jamming - maybe so, but the effect of the receiving correlator is
> to spread the energy of a CW interferer
> and concentrate the energy of the signal with the matching PN modulation
> is it not????
Yes, if the receiver is linear (e.g. say you do a sliding code
correlator and slide until you get the peak, with the correlator using a
But since most (inexpensive) receivers have hard limiters/1 bit
quantizers in front of the correlator, what goes into the correlator is
a square wave at the jammer frequency, and you can slide your PN code
all day and not get a peak.
There's some analysis out there that tells you how many bits you need
for a given Jammer/Signal ratio, but in general, if the jammer is 20 dB
over the signal, you need 3-4 bits.
Back in the day, when bits were very expensive, that's why narrow band
excisers were popular.. basically you'd run something like a PLL to
recover the tone jammer, and subtract it out.
More information about the time-nuts