Analog Devices AN-412 Application Notes

AN-412

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Video-Standard Selection Circuit for the AD722 Using Low Cost Crystals

The AD722 has various options for receiving a subcarrier
reference frequency. This signal is input to FIN, Pin 3.
The choices are: a logic-level (TTL) clock signal at either

1 × FSC (Subcarrier Frequency = 3.579545 MHz for NTSC;

4.433619 MHz for PAL) or 4 × FSC; or just using a parallel
resonant crystal at 1 × FSC, whereby the on-chip oscil-

lator circuit will drive the crystal. All three options are
available for either NTSC or PAL operation.

If the lower cost, stand-alone crystal operation is
desired, there is only a single pin available to connect
a crystal. This does not directly avail itself to selecting
between the two different crystals required for either
NTSC or PAL operation in systems that offer both
video standards.

A low cost crystal selection circuit can be made that, in
addition to the two crystals, requires two low cost
diodes, two resistors and a logic inverter gate. The cir­cuit selection can be driven by the STND signal that
already drives Pin 1 to select between NTSC and PAL
operation for the AD722.

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NORWOOD, MASSACHUSETTS 02062-9106

APPLICATION NOTE

617/329-4700

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A schematic for such a circuit is shown in Figure 1. Each
crystal ties directly to FIN (Pin 3) with one terminal and
has the other terminal connected via a series diode to
ground. Each diode serves as a switch depending on
whether it is forward biased or has no bias.

Pin 1 (STND) of the AD722 is used to program the inter­nal operation for either NTSC (HIGH) or PAL (LOW). For
NTSC operation in this application the HIGH signal is
also used to drive R1 and the input of inverter U1. This
creates a LOW signal at the output of U1.

The HIGH (+5 V) signal applied to R1 forward biases CR1

with approximately 450 µA of current. This turns the

diode “on” (low impedance with a forward voltage of
approximately 0.6 V) and selects Y1 as the crystal to run
the oscillator on the AD722. The bias across the diode
does not affect the operation of the oscillator.

The LOW (0 V) output of the inverter U1 is applied to R2.
This creates a 0 V bias condition across CR2 because its
cathode is also at ground potential. This diode is now in

Y1

R1

10kΩ

NTSC

U1

HC04

Y2

PAL

R2

10kΩ

CR1
IN4148

Figure 1. Crystal Selection Circuit

CR2
IN4148

0-5pF

Optional

NOTES: Y1 = 3.579545MHz

STND

1

3

FIN

Y2 = 4.433620MHz

AD722

the “off” (high impedance) state, because it takes
approximately 600 mV of forward bias to turn a diode “on”
to any significant degree. The “off” condition of the
diode does, however, look like a capacitor of a few pF.

For PAL operation, the STND signal that drives Pin 1 is
set LOW (0 V). This programs the AD722 for PAL opera­tion, deselects the NTSC crystal (Y1), because CR1 has
no bias voltage across it and selects the PAL crystal (Y2)
by forward biasing CR2.

In order to ensure that the circuits described above
operate under the same conditions with either crystal
selected, it is important to use a logic signal from a
CMOS type logic family whose output swings fully from
ground to +5 V when operating on a +5 V supply. Other
TTL type logic families don’t swing this far and might
cause problems as a result of variations in the diode bias
voltages between the two different crystal selection
modes.

FREQUENCY TUNING

A parallel resonant crystal, which is the type required for
the AD722 oscillator, will work at its operating
frequency when it has a specified capacitance in parallel
with its terminals. For the AD722 evaluation board, it
was found that approximately 10 pF was required across
either the PAL or NTSC crystal for proper tuning. The
parallel capacitance specified for these crystals is 17 pF
for the NTSC crystal and 20 pF for the PAL crystal.

The parasitic capacitance of the PC board, packaging
and the internal circuitry of the AD722 appear to be con­tributing 7 pF–10 pF in shunt with the crystal. A direct
measurement of this was not made, but the value is
inferred from the measured results.

With the crystal selection circuit described above, the
unselected crystal and diode provide additional shunt
capacitance across the selected crystal. The evaluation
board tested actually required no additional capacitance
in order to run at the proper frequency for each video
standard. However, depending on the layout, some cir­cuits might require a small capacitor from FIN (Pin 3) to
ground to operate with the chrominance at the proper
frequency.

SUBCARRIER FREQUENCY MEASUREMENT

It has been found to be extremely difficult to measure
the oscillation frequency of the AD722 when operating
with a crystal. The only place where a CW oscillation is
present is at the FIN pin. However, probing with any type
of probe (even a low capacitance FET probe) at this node
will either kill the oscillation or change the frequency of
oscillation, so the unprobed oscillating frequency can­not be discerned. Neither the composite video nor
chroma signals have the subcarrier represented in a CW
fashion (the LUMA signal does not contain any of the
subcarrier). This makes it virtually impossible to accu­rately measure the subcarrier frequency of these signals
with any oscilloscope technique.

Two methods have been found that can be used to mea­sure the subcarrier oscillating frequency accurately. The
first method uses a spectrum analyzer like the HP3585A
that has an accurate frequency counter built in. By look­ing at either the COMP or CHROMA output of the AD722
a spectrum can be observed that displays the tone of the
subcarrier frequency as the largest lobe.

The CHROMA or COMP output of the AD722 should be
input into the spectrum analyzer either by means of a

scope probe into the 1 MΩ input port or a 75 Ω cable that
can be directly terminated by the 75 Ω input termination

selection of the HP3585A. Each of these signals has
present at least the color burst signal on almost every
line which will be the dominant tone in the frequency
band near its nominal frequency. Sidelobes will be
observed on either side of the central lobe spaced at
50 Hz (PAL) or 60 Hz (NTSC) intervals due to the vertical
scanning rate of the video signals. There will also be
sidelobes on either side at about 15.75 kHz intervals, but
these will not be observable with the span set to only a
few kHz.

The center frequency of the spectrum analyzer should
be set to the subcarrier frequency of the standard that is
to be observed. The span should be set to 1 kHz–3 kHz
and the resolution bandwidth (RBW) set to between
10 Hz to 100 Hz. A combination of wider frequency span
and narrower RBW will require a long time for sweeping
the entire range. Increasing the RBW will speed up the
sweep at the expense of widening the “humps” in the
subcarrier tone and the sideband tones.

Once the subcarrier is located, it can be moved to the
center of the display and the span can be narrowed to
cover only that range that is necessary to see it. The
RBW can then be narrowed to produce an acceptably
fast sweep with good resolution.

The marker can now be placed at the location of the
subcarrier tone and the frequency counter turned on.
The next scan across the location of the marker will
measure and display the subcarrier frequency to better
than 1 Hz resolution.

A second means for measuring the subcarrier frequency
of an AD722 operating from a crystal involves equip­ment even more specialized than a spectrum analyzer.
The technique requires a Tektronix VM700A video sys­tem measurement instrument.

The VM700A has a special measurement mode that
enables it to directly measure the frequency of one
subcarrier in a video waveform with respect to an inter­nally stored reference or a simultaneously supplied ref­erence. The instrument gives a reading of the relative
frequencies of the reference and test signals in units of

0.1 Hz. This is not a direct reading of the subcarrier fre­quency in MHz but a relative reading in Hz of the differ­ence in frequency between the two signals.

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