Usually GPS receivers have CPU clock frequencies that are not an
integer multiple of the chipping rate: that's why there is the
sawtooth correction in the timing units. The LO frequency for the
receiver front-end is not a problem due to the fractional-N PLL. The
Motorola M12 seems to use a 40MHz clock, uBlox receivers use 48MHz
clock...

On Tue, Oct 27, 2015 at 3:06 AM, Brooke Clarke brooke@pacific.net wrote:

Hello Brooke,

It has been a while since we emailed each other - hope you are doing well.

The 10.949297 MHz oscillator in GPS receivers was (I think) chosen by
Rockwell in the early days when they wanted a frequency that work for both
L1 and L2 receivers.  If you multiply the 10.949 times 128 you get 1401.51
MHz. for the local oscillator in the receiver.  That will give you a first
IF of 173.91 MHz.  If you add 1401.51 MHz and 173.91 MHz you get 1575.42
for the L1 channel.  If you subtract 173.91 MHz from 1401.51 MHz you get
1227.6 MHz for the L2 channel.

So the receivers you have were most likely designed to work as either L1 or
L2 or maybe both.  Using the 10.949 MHz oscillator frequency made it easy
for the receiver manufacturers to produce receivers for the military that
would be for L1 and L2 channels.

Hope this helps.
Best regards,
Doug, K4CLE

On Mon, Oct 26, 2015 at 9:06 PM, Brooke Clarke brooke@pacific.net wrote:

Brooke
An interesting question and like you I have seen the odd frequencies.
For the 10.230 MHz since they come out exactly at the carrier frequency
that would tend to speak to a direct conversion receiver.

At least on the much older receivers that had IF stages at either 75 Mhz or
35 Mhz and therefore some other frequency would be needed. (There could
also be a very wide range of IF frequencies besides the two I mention.) I
will guess the 10.9 Mhz is exactly that when multiplied up either comes up
high or low by the IF frequency offset they wanted.
Regards
Paul
WB8TSL

On Mon, Oct 26, 2015 at 10:06 PM, Brooke Clarke brooke@pacific.net wrote:

Hi Doug:

Thanks very much.  I've added a diagram to my GPS frequencies paragraph:
http://www.prc68.com/I/DAGR.shtml#GPSs
scroll down a little.

Hi Paul:

The 10.23 is a key number for a transmitter.  For example the Stanford Telecom 5001 test transmitter which uses a high
quality 10.23 xtal oscillator:
http://www.prc68.com/I/5001A.html
I think the Northern Telecom GPS Satellite Simulator STR2760 synthesizes the needed 10.23.
http://www.prc68.com/I/NTgpsSTR2760.shtml

The first generation Trimble Trimpack (PSN-10) GPS receivers were all built on their key patent.  It's a direct
conversion receiver using a hard limiter with I&Q signals.
http://www.prc68.com/I/Trimpack.shtml#Pat
The prior art GPS receivers used heterodyne receivers which were much more expensive.

Mail_Attachment --
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Baker Douglas wrote:

On 10/26/15 7:06 PM, Brooke Clarke wrote:

you might choose a frequency that when used as a direct sampler, causes
the GPS signal to alias to a convenient offset frequency (then all
Doppler shifts wind up being positive, for instance).

try 38.65 MHz for instance, that makes both L1 and L2 alias down to
around 9.2 MHz, which is slightly less than 1/4 of the sample rate, and
nicely accommodates the bandwidth of the signal