October 13, 2015
CWTD Episode #75
Adventures in GPS Data Display, Oscillator Timing and
With guest designers
Dave AD7JT and Mike WA8BXN
This is Part 3 in our
series of CWTD episodes chronicling the design and build of a GPS-Disciplined
Oscillator. In Part 1, we dealt with the VCXO. In Part 2, we
talked about Oven Controlled Crystal Oscillators and started going
through the Oven Controller electronics. But, as typical in so
many of our various projects on the bench, we've
found that the more we dig into a topic, the more we find that we need
to better understand!
So this month we snug up a number of loops that
we loosely tied in the previous two episodes concerning GPS receiver
basics, timing and "oscillator disciplining" implications, oven control
and temperature display.
We also "tear down" the popular NEO-6/7/8 GPS
modules available on eBay these days for less than the cost of a
sandwich and cup of coffee. No longer will it be intimidating to
experiment with these postage stamp-sized boards and antennas. Taking
the mystery out of them and showing how we can use these little gems in
the shack is what it's all about.
We also touch on some select topics in a popular
"GPS Compendium" document from u-blox, a popular manufacturer of little
GPS receiver modules. We are all guaranteed to learn something new
in this part of the discussion.
Further, we report on some fabulous progress
that the design team has been making on the GPS Display Terminal that we
excitedly announced last time. In fact, we presented this really cool
little instrument at the Mid-Atlantic States VHF Conference in
Philadelphia last weekend ... and the audience interest was simply
palpable! How exciting it was to learn that the same platform for the
popular SNA Kit can also run our firmware for a hand-held GPS terminal
displaying latitude, longitude, maidenhead grids, accurate time,
satellite signal strength, etc ... *and* serve as an accurate RF signal
*** *NEWS FLASH* *** We've now received the "CWTD Oven
Control Kits" from our good friend Hans Summer of QRP Labs. As discussed
last time, Hans created a derivative of his larger OCXO/Synth Kit
containing only the circuits we need to control our VCXO boards. The
CWTD Oven Control Kit is now posted and available for ordering at the
bottom of last month's "Oven"
episode whiteboard ...
BTW, the VCXO Kits are now SOLD OUT, so we hope that
everyone who wanted one got it already. If not, PCBs are still
available, but you'll need to collect the parts yourself.
So sit back and listen in to our discussions ... and toss
us some questions along the way too, based on your own experiences on
73, George N2APB and Joe N2CX
Window during show ...
"Mike WA8BXN": same data format as RS-232, just different voltages
<20:52:38> "Mike WA8BXN":
<20:58:53> "Mike WA8BXN": Fortunately you don't have to know the
details of how it works to make use of the resource!
<21:01:47> "George N2APB": Ain't that the truth for so many things
<21:05:52> "Joe N2CX": Ain't no tubes to replace these days!
<21:12:19> "Mike WA8BXN": The "wandering" gives you a speed and
heading when standing still.
<21:12:44> "Dave_W4VU": US Civilian aviation - with ground based
WAAS system is 3 meters 95% of the time - can bring an aircraft down
to 200 feet from the runway
<21:12:57> "Mike WA8BXN": DOP is not in meters!
<21:13:18> "Joe N2CX": A couple of meters is close enough to aim a
<21:18:32> "George N2APB": Thank you Dr. Strangelove!
<21:27:14> "Mike WA8BXN": 99.9% of everything can be done with the
touch screen, there is an onscreen keypad as well
<21:29:34> "Mike WA8BXN": More depth on some of this on the Yahoo
group for CWTD
<21:31:53> "Mike WA8BXN":
1) Our approach thus far with the OCXO Project
can get a kit of the "Oven Control" parts that we discussed in the
last episode (see Schematic 1 / 2 below, and
pictured below), as a kit of parts that you can start adding
to your VCXO. This is a special kit
for us from Hans Summers of QRP Labs, and contains the oven
control component (that we will use), as well as a 10 MHz
crystal oscillator components and enclosure pcb set that are
included for our eventual use downstream. In order to
save us cost, it does not contain the Si5351
synthesizer chip that his standard kit has ... Hence this is
the special "CWTD OCXO Kit" from QRP Labs ... thanks Hans!
[You can see the manual for assembling this at
http://www.qrp-labs.com/ocxokit.html ... But remember to
save the pcb enclosure and 10 MHz crystal oscillator for
[Note: To complete the GPS-DO
project, we are designing a "motherboard" to to slip inside
the enclosure, holding the VCXO, Oven Control board,
temperature display circuits, power supply and the little
NEO-xx GPS receiver boards ... Should be available by the
Full information on the
"special kit" that Hans is supplying for us is at ...
2) The CWTD "GPS
... $14.30, free shipping
... $21.25, free shipping
- 100% brand new and high
GPS Chip parameters:
- Receiver type 72-channel
u-blox M8 engine
- GPS/QZSS L1 C/A, GLONASS
L10F, BeiDou B1
- SBAS L1 C/A: WAAS, EGNOS,
- Galileo-ready E1B/C
- Nav. update rate1 Single
GNSS: up to 18 HZ
- Concurrent GNSS: up to 10
- Position accuracy2 2.0 m
- Acquisition2 Cold starts:
- Aided starts: 2 s
- Reacquisition: 1.5 s
- Sensitivity2 Tracking &
Nav: –167 dBm
- Cold starts: –148 dBm
- Hot starts: –156 dBm
- Assistance AssistNow GNSS
- AssistNow GNSS Offline (up
to 35 days)3
- AssistNow Autonomous (up
to 6 days)
- OMA SUPL & 3GPP compliant
- Oscillator TCXO
- Crystal (NEO-M8M)
- RTC crystal Built-In
- Noise figure On-chip LNA
(NEO-M8M). Extra LNA for
- lowest noise figure
- Anti jamming Active CW
detection and removal. Extra
- onboard SAW band pass
- Memory ROM (NEO-M8M/Q) or
- Supported antennas Active
- Odometer Travelled
- Data-logger For position,
velocity, and time (NEO-M8N)
- Operating temp. –40° C to
- Storage temp. –40° C to
85° C (NEO-M8N/Q)
–40° C to 105° C (NEO-M8M)
- RoHS compliant (lead-free)
- Qualification according to
- Manufactured and fully
tested in ISO/TS 16949 certified production sites
- Uses u-blox M8 chips
qualified according to AEC-Q100
- Supply voltage 1.65 V to
3.6 V (NEO-M8M)
- 2.7 V to 3.6 V (NEO-M8N/Q)
- Power consumption4 23 mA @
3.0 V (continuous)
- 5 mA @ 3.0 V Power Save
- (1 Hz, GPS mode only)
- Backup Supply 1.4 to 3.6 V
- NEO-M8N-0 u-blox M8
Concurrent GNSS LCC Module,
- TCXO, flash, SAW, LNA,
- Timepulse Configurable
0.25 Hz to 10 MHz
Connector & Cable for the GPS Boards
2X U.FL Mini PCI to RP-SMA Pigtail Antenna WiFi Cable
... $3.40, free shipping
3) GPS Compendium Topics ...
Select Topics ...
Satellite Navigation is a method employing a
Global Navigation Satellite System (GNSS) to accurately
determine position and time anywhere on Earth. Satellite
Navigation receivers are currently used by both private
individuals and businesses for positioning, locating,
navigating, surveying, and determining the exact time in an ever
growing list of personal, leisure and commercial
applications. Using a GNSS system, the following values can
accurately be determined anywhere on the globe (Figure 1): 1.
Exact position (longitude, latitude and altitude coordinates)
accurate to within 20m to approx. 1mm. 2. Exact time (Universal
Time Coordinated, UTC) accurate to within 60ns to approx. 5ns.
Speed and direction of travel (course) can be derived from these
values, which are obtained from satellites orbiting the Earth.
Speed of travel may also be determined directly by means of
Doppler shift measurements.
GPS (the full name of the system is:
NAVigation System with Timing And Ranging Global Positioning
System, NAVSTAR-GPS) is intended for both civilian and
military use. The civilian signal SPS (Standard Positioning
Service) can be used freely by the general public, while the
military signal PPS (Precise Positioning Service) is available
only to authorized government agencies. The first satellite was
placed in orbit on February 22, 1978, and it is planned to have
up to 32 operational satellites orbiting the Earth at an
altitude of 20,180 km on 6 different orbital planes. The orbits
are inclined at 55° to the equator, ensuring that at least 4
satellites are in radio communication with any point on the
planet. Each satellite orbits the Earth in approximately 12
hours and has four atomic clocks onboard.
2) Basic Principles of Satellite Navigation
3) At least 4 satellites are necessary ...
4) Coordinate Systems
5) Satellite Coverage
6) The GPS Signal
Satellite navigation signals are generated
using a process known as DSSS (Direct Sequence Spread
Spectrum) modulation18 . This is a procedure in which a
nominal or baseband (not to be confused with the baseband
chip in the receiver) frequency is deliberately spread out
over a wider bandwidth through superimposing a higher
frequency signal. The principle of spread-spectrum
modulation was first devised in the 1940s in the United
States, by screen actress Hedy Lamarr and pianist George
Anthell19 . This process allows for secure radio links even
in difficult environments.
GPS satellites are each equipped with four
extremely stable atomic clocks (possessing a stability of
greater than 20·10-12 ) 20. The nominal or baseband
frequency of 10.23MHz is produced from the resonant
frequency of one of these onboard clocks. In turn, the
carrier frequency, data pulse frequency and C/A
(coarse/acquisition) code are all derived from this
frequency (Figure 43). Since all the GPS satellites transmit
on 1575.42 MHz, a process known as a CDMA (Code Division
Multiple Access) Multiplex 21 is used. The C/A code plays
an important role in the multiplexing and modulation. It is
a constantly repeated sequence of 1023 bits known as a
pseudo random noise (PRN) code. This code is unique to each
satellite and serves as its identifying signature.
The C/A code is generated using a feedback
shift register22. The generator has a frequency of 1.023 MHz
and a period of 1023 chips23 , which corresponds to 1ms. The
C/A code is a Gold Code24 , which has advantageous
correlation properties. This has important implications
later on in the navigation process in the calculation of
7) GLONASS: the Russian System
8) The Three GNS Systems: GPS, GLONASS,
4) The Midnight
GPS Display Terminal ("GDT")
... A handheld terminal that
dynamically interprets and displays the live NMEA serial data feed
coming from any GPS receiver module!
The Midnight GPS
Display Terminal (GDT)
monitors the output of a GPS receiver and displays satellite signal
strength as well as location and time information. It can also control
the frequency and duty cycle of a timing signal generated by the GPS
receiver and synchronized with the precision atomic clocks in the GPS
satellites. The result is an inexpensive but very precise signal
generator tunable from 1 Hz to 10 MHz in increments of 1 Hz.
The GDT dynamically interprets and displays
the live NMEA serial data feed from any GPS receiver module with a
serial interface. Additional screens show raw NMEA messages and non-NMEA
proprietary messages. Messages can be recorded in files on an SD card
using the GDT's FAT16 file system which is compatible with virtually all
Windows and Linux systems. Recorded messages can be played back on the
GDT or transferred to a PC for further analysis.
Another screen controls the TimePulse output
of a u-blox GPS receiver. This feature provides an extremely accurate
time base in a signal generator format. The frequency range covered
ranges from below audio up into the HF range.
Another screen turns the GDT into an atomic
clock displaying local time and UTC time. The clock time is
synchronized with the clocks on board the GPS satellites.
The GDT software also runs on the Midnight
Scalar Network Analyzer (SNA) hardware
that can easily and quickly be installed in a MSNA thus converting it to
a GDT. The GDT can be converted back to an MSNA just as easily. The
GDT application will also run on other existing and future QVGA 16
platforms (e.g., the NAT).
GDT Firmware ... is ready-made to
load right onto your NAT and SNA instruments! Just
right-click and save to your local computer,
place onto the NAT/SNA SD Card and use the UPLD command to load it.
GDT Quick Reference Guide ...
Beta Version 1.0 'oh'
Inexpensive handheld unit with color graphic
display and touch panel displaying GPS receiver data & more!
Live display of GPS Receiver Data* ... Time,
Date, Lat/Log, Satellite Signal Strength, ...
Super Accurate Clock ... Displays
UTC and Local time
Generator ... Programmable
"timepulse" output (1pps, x-MHz)**
Data Stream ... Displays NMEA
sentences for all satellites being received
Setup Mode ... Specify
serial baud, timezone, LCD orientation, ...
SD Card ... For
software updating, GPS data collection, ...
System ... On
SD Card, file/data management
320x240 pixel color
graphic LCD ... On
SD Card, file/data management
Touch-Screen ... For
convenient field use or as display on bench
(optional) ... For
convenient data entry/edit
external connection of GPS receiver (not included); 3.3V
serial data (Rxd/TxD)
all GPS receivers have ability to generate output
frequencies. Supported receivers currently include u-Blox
1. Current UTC time and date synced to satellite
2. Fix: None (< 3 sat), 2-D fix (3 sat), or 3-D fix (> 3 sat).
3. Current location, speed and heading.
4. PDOP (Position Dilution Of Precision) … smaller is better.
1. Carrier to Noise ratio (Car/Noise) shown in dB.
2. Car/Noise not shown for values too low for receiver to track.
3. Only 12 of In View satellites are shown at a time; tap screen
[SPACE] to toggle between three groups of 12 satellites.
4. Total number of satellites In view is fixed by receiver
location and time.
5. Total number being tracked depends on Car/Noise ratios.
6. White satellite numbers (Sat) are US GPS satellites, pink
numbers are Russian GNOASS satellites.
1. Current TimePulse n frequency (1 Hz to 10 MHz).
2. Current TimePulse n duty cycle (in percent of period).
3. Four configurations (see Signal Generator mode)
Shows local and UTC time and date continually updated with data
Large display readable from a distance.
User can select of which time is displayed in large characters and
on permanent buttons
Time is synchronized with atomic clocks on board GPS satellites.
GDT will maintain time when receiver disconnects or loses lock
Main time display changes from black characters to red characters
when GDT takes over
Local Maidenhead coordinate shown too.
User enters local time zone offset from UTC in Setup mode (- for
West of Greenwich, + for East). Format: sHHMMSS.
SIGNAL GENERATOR MODE:
- Gives user control over frequency and
duty cycle of receiver TimePulse signal(s).
- Digital square wave outputs at 3.3 V
- Provides a precision reference
frequency synced with the atomic clocks on the GPS
Most u-blox receivers have one TimePulse output, receivers with
special timing capabilities have two TimePulse outputs.
These receivers have a 'T' suffix on their model numbers
- Each TimePulse has two settings, one
for when receiver does not have satellite lock and one for
when receiver has satellite lock.
- Each of the four TimePulse
combinations can be independently set.
- Virtual buttons select one of the four
- Once set, TimePulse parameters are
retained by receiver until power is cycled.
- GDT can command the receiver to save
TimePulse parameters so same parameters are used after each
- Frequency range from 1 Hz to 10 MHz in
1 Hz steps
- Duty cycle range from 0.00001% to
- Frequency and duty cycle of the
currently selected TimePulse combination is displayed on the
- Application: precision digital signal
generator for testing digital circuit performance.
- Application: tweaking system clock
frequency to improve precision.
- Application: precision testing of
Pulse Width Modulation (PWM) logic.
- Application: (with output shaping)
precision signal generator for general RF use and testing.
- Application: "discipline" a VCXO (
"GPS Disciplined Oscillator" or "GPSDO").
- Application: provide precision
reference timing for PLL control.
- Application: propagation delay
- Possible future application: user
controlled delays could be used to generate two TimePulse
outputs with a precise phase relationship.
GPS-T: TERMINAL DISPLAY:
GPS receivers broadcast one NMEA message set per second. Messages
are parsed to extract the data shown on GPS-Graphic and GPS-Clock
Messages are color coded as follows:
WHITE - Recognized
messages containing data we use.
2. GREEN - Recognized messages containing no data we use.
3. YELLOW - Recognized messages containing information
about satellites not in the first 12 in view.
4. LIGHT RED - Unrecognized NMEA messages including NMEA-sanctioned
Using the GPS Display Terminal with a GPS Receiver:
Miles, KE5FX site ... <www.ke5fx.com>
... A great assemblage of links having to do with high precision,
accuracy and stability techniques can be found on the. You can literally
spend days reading up on techniques, software, measurement results and
the history of timing and frequency control here. Just plan on spending
an afternoon to browse it.
2) “Resources for
Precision Timing, Stability, and Noise Analysis” ... <http://www.ke5fx.com/stability.htm>
... If you want in-depth knowledge this is THE place to go. Plus,
you can download plenty of extremely valuable application notes and
technical papers for future reference.
3) PLL reference
papers that are a good read ...
AN-885 Introduction to Single Chip Microwave PLL's (TI’s LMX1501A)
MM74HC4046 CMPS Phase Lock Loop ...
Experiment #4 - CMOS 4046 Phase-Locked Loop...
CMOS Phase-Locked-Loop Applications Using the CD54/74HC/HCT4046A and
CD4046B Phase-Locked Loop: A Versatile Building Block for Micropower
Digital and Analog Applications ...
ADF4110 PLL …
ADF4001 PLL …
And a simple Phase Detector chip + a simple VCO ...
Phase/Frequency detector ...
Supposedly "goes well" with ...
VCO with vvc control ...