There have been other TV-derived references in the past (e.g. "Low Cost TV-derived Frequency Reference",
Electronics Australia Oct-Nov 1993).
However the performance has often been poor due to signal path effects, modulation of the reference by TV video,
and poor locking. This present design uses a high performance sync separator, and a micro controller with special phase lock firmware,
to provide exceptional performance. The remaining issues with a reference of this type centre around knowing when the TV source
is "on reference" and when it is not, and this is also addressed in the present design.
Some of the features of the unit are:
Micro Section
Follow the Reference Schematic
as you read this description. (The other version is similar - just cut-down somewhat).
Power for the Precision Reference unit can come from two sources - a simple plug-pack connected to the points labelled "+10V" and "-0V", and a
backup battery, B1. The supplies are isolated from each other by diodes D3 and D4.
These diodes also prevent reverse supply damage.
When the plug-pack output (nominally 10-12V DC) is higher than the battery voltage, D3 is reverse biassed,
D4 is forward biassed, and the unit operates from the AC supply.
When the AC supply fails, D3 conducts, D4 is reverse biassed, and the unit continues to keep time from the battery.
If power failures are frequent, a 6-cell 500mAH or better NiCd or NiMH battery pack is recommended.
The backup isn't necessary if the clock feature is unused, so omit B1 and D3.
Regulator U1 provides 5V with sufficient current to operate the micro controller, sync separator and
buffer/modulator device. The total load is only about 20 mA. It is convenient to operate the Exciter
from a small SLA (gel-cell) battery, and the power amp from an AC supply. If the power fails, at least
10mW will leak through the power amp. Battery operation is intended only to keep the clock accurate,
as the phase-lock TV reference source will be lost.
The RS232 transmit signal from the micro to the PC requires a negative voltage supply, and to keep supply current
to a minimum, this voltage is derived from the mostly idle PC transmit data line, which
is normally negative. TR1 converts to RS232 levels and transmits the serial data. Note how its collector
supply is derived from the RS232 receive line at J2 pin 3, via D1 and R28. C8 maintains the -10V supply even when
data is being received from the PC. TR2 is the RS232 receiver, with D2 catching the reverse voltage to
prevent reverse bias damage. The RS232 connection is purely for time setting and phase monitoring.
Full phase-locked operation continues with the PC off or disconnected.
J1 is the programming header, a simple 2x5 pin connector. The micro can be programmed in the unit.
The operating frequency and ID message, stored in EEPROM, are changed in the same way.
X1 is the reference crystal, which operates in a variable crystal controlled oscillator configuration (VCXO).
The oscillator is the main reference oscillator in the micro, and so is used for all timing. The components
C2 and C3 set the precise crystal oscillating frequency, which can also be trimmed over a range of about 30 Hz by
the variable capacitance diode D2. In the prototype a 47V Zener diode is used as the varicap. The voltage
on the varicap is provided by the micro via a pulse-width modulated output, operating at about 100 Hz,
but with 8 Hz fine adjustment timing components. The PWM output is averaged by R36 and C8 to remove most of the
PWM and leave a clean DC ERROR signal. Since the bandwidth of the varicap frequency shift is very low, the
remaining PWM ripple is not modulated onto the carrier (4 Hz sidebands about 60dB down, and 100 Hz sidebands
unmeasureable).
In the Reference Phase Comparator version, the micro operates directly from the 5MHz or 10MHz
local reference, and U3/1 is used instead as a buffer for the incoming reference to drive the micro. There is
no phase lock, and the ERROR signal is simply used with a phase indicating meter. The firmware remains the same. Telemetry to the PC
provides an indication of the phase drift, and the PC also calculates the frequency offset of the local reference
from the TV sync source. This is a good way to check whether the TV station is "on reference" or not.
A unit of this type can be fitted inside a local reference unit and provides continuous calibration updates
while the unit is in use.
Firmware
The micro controller has three program loops - main, 2 kHz divider, and 50 Hz TV sync. The reference oscillator
operates a 16 bit divider, used as the phase
counter. When the defined count is reached (every 500µsec), the timer interrupts, the 1 kHz output is toggled,
the pulse width modulator stepped, and a divide by 2000 firmware counter stepped, allowing seconds to be counted. The PWM is oversampled and generates 8 bit resolution in only 10ms, rather than the expected 128ms.
These are all tasks requiring high timing precision, either to ensure accuracy, or prevent carrier phase noise.
Once every second the 2000 software counter overflows, and a flag is set. In the main program, when this flag
is seen, the 1Hz pulse is generated, UTC time is incremented, and the serial output updated. These are all tasks which
are not precision time dependent, but still occur within a few microseconds of the precision event which
triggers the activity (the setting of the flag). Otherwise the main program does very little and the micro remains asleep untill needed.
The third task happens when the 50 Hz TV frame sync reference arrives. The content of the phase counter is
sampled (in hardware, so no precision is lost), and the interrupt generated. In the interrupt, time precision
is unnecessary; the phase error calculation is made and the new error value provided to the pulse width modulator.
Time is kept in packed BCD format (Binary Coded Decimal, where the lower four bits
represent the units 0 - 9, and the upper four bits represent the tens), which makes counting more tricky, but simplifies display. Every time the
UTC time is incremented, the BCD values need to be checked to ensure they follow the BCD format,
which means checking that the low nibble is <10 and the high nibble <6. The hour is checked to see
that it remains <24, and is set to zero when 23 is exceeded.
The two local reference versions do not have ID or keying, and the Comparator version has
no phase lock. It is used only for monitoring a local OCXO reference and comparing it to the TV reference.
Both these versions retain the accurate clock, which is available only via the serial message.
TRANSMIT (DATA)
The serial communications data transmitted to the monitoring PC is of the form:
HH:MM:SS PPPP<CR><LF>
where PPPP represents the phase of the local counter sampled by the external reference, the
50 Hz TV frame pulses. The value of PPPP can be from 0000 to one less than the divisor
required to divide the clock reference to 2 kHz, and is expressed in 16 bit HEX. For
example, operating on 3600 kHz, the value is (3600 / 2) - 1 = 1799 or 0707HEX.
The Precision Reference also transmits responses to some of the time
setting commands (those in red in the table below). These
three commands should not be used while transmitting, as they disturb the carrier
phase as well as the UTC timing. (This is why the PC monitoring software
is not equipped for time setting).
RECEIVE (COMMANDS)
There are no front panel controls for the clock section of the Precision Reference.
After all, the clock is Rubidium Standard accurate, and battery backed,
so what would need adjusting? All adjustment is achieved using the serial
link to a PC. There are eight commands:
Hnn Set UTC hours to nn (BCD) hours
Mnn Set UTC minutes to nn
Snn Set UTC hours to nn
+ Add one to UTC seconds
- Subtract one from UTC seconds
> Increment local time offset by one hour
< Decrement local time offset by one hour
R Retard clock by 10 ms
|
The commands should be used in the order shown while setting the clock. The commands shown
in red
cause the clock to stop while they are entered, so must not be used during time-keeping.
They also disturb the PWM part of the frequency control, causing frequency disturbances.
Lower case letters are accepted and used as upper case. Invalid commands are ignored. The clock can
be set with a simple ASCII terminal program, such as Windows TERMINAL, or even HYPERTERM.
By comparing the Precision Reference seconds ticks with a reference such as a GPS receiver
with 1pps output, the clock can be set to within 10ms of real UTC time, and when calibrated, may stay there
for months on end (dependent only on the TV network). If the calibration of the VCXO is good, it
will be quite adequate even without the TV reference, but the seconds ticks may need to be pulled
back in line before the next transmission session.
The Precision Reference is used as a means of monitoring the
performance of your high stability local reference (such as a 5MHz or 10MHz OCXO) compared with another reference. This version can be used with or without phase lock. Using phase lock, it operates with the
Phase-locked Local Reference design.
Used without phase lock, this version is used with the Phase Comparator for References hardware version. The precision OCXO it monitors runs unaffected, but the phase and
frequency are monitored, and the phase feedback output indicates phase via a meter and via the same PC software.
The PC software supports all versions and all applications. It operates by displaying the oscillator
phase graphically over time. It also calculates the frequency offset by accumulating the phase
error across the screen. The phase display easily shows drift and short-term variations, while
the frequency display indicates long term frequency and averages out short-term errors.
PC Software monitoring a locked reference
(Click on image to view full size)
The above image is shown with negative colours for improved clarity (the actual background
is black). The top half of the screen constitutes the main graphical display.
The red line in the main display is phase, measured from one minute after
oscillator switch-on, with a duration of 10 hours. Above it, the black line with the brown "lump"
at the left end is the frequency measurement. The error at the beginning is caused by the
phase drift of the oscillator as the unit warms up. The frequency error is never greater than -1 part
in 108. If there had been any disturbance of the TV sync refernce during the operating
period, it would show clearly as a change in phase followed by a large change in frequency offset.
The lower display is the "fast phase" display, used mostly during calibration.
Below that is a circular phase meter and a text display of the operating phase and frequency parameters
and program settings. The current frequency offset (largely irrelevant with a locked oscillator)
is shown at the top right above the main display.
The PC software is used for initial calibration, and in order to monitor long term phase performance.
Long term performance monitoring is also a measure of the reliability of the TV network reference.
The PC software is available as both executable and source code (if you order the source code, you get both). The PC software is a DOS executable program, which will operate
from Windows™ 3.1, 95, 98 or 2000 (because it is a graphics program, on older computers it may
require full-screen rather than "DOS Box" operation). Computer requirements are minimal - a 486
processor and a serial port. Given the simplicity of the communications protocol, you could
easily write your own phase and time display or data logging program. To recompile the source code you
will need Microsoft Quick Basic 4.2 Compiler or similar. The source code may also run under "QBASIC"
which was supplied with DOS 6, but it will not compile to a stand-alone executable.
The source code and executable programs
for micro and PC are available from the author at a very modest cost. See the Micro Page
for details.
You can edit and compile the source code for yourself by getting
a copy of the AVR Assembler free from Atmel.
You will also need a copy of the ISP, or "In System Programming" software, free
from the same source.
You will also need to use the ISP software to modify the EEPROM map file supplied,
in order to set the carrier frequency, the required serial data rate, and the ID message.
If the unit is not used as a transmitter, the message is not necessary, but the other values
will still need to be set. Instructions are supplied with the firmware.
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