SL1461SA
Lock range
The easiest way to centralise the VCO is to input an RF
carrier which is being modulated by a low frequency
squarewave. The tuning coil(s) should be adjusted until the
AFC voltage toggles between 0.2V and VCC-0.7V. The smaller
the FM deviation of the squarewave used, the more accurate
the setting will be.
Lock range should be symmetric about the centre of the
S-curve. When the oscillator is sitting in the centre of the
S-curve, the two video outputs will be at the same DC voltage.
RF oscillator design
The standard application circuit for the SL1461SA is
showninFig.3ThelayoutoftheVCOtankshouldfollownormal
good RF techniques - ie as compact as possible. This will
minimise parasitics, thus giving improved VCO linearity and
stability. The PCB layout used for testing purpose is shown in
Fig. 10.
A pre-emphasised video input containing black to white
transitions can also be used for this setting, since the DC
content in a pre-emphased video is much less than that in non
pre–emphasised video. This is important as any dc content in
theinputwaveformwillintroduceanoffsetintheAFCtransition
point.
The setting can be confirmed by measuring the DC
voltage on the two video outputs, the voltages should be the
same when the oscillator is centred around the incoming
frequency. This DC measurement must be carried out with an
unmodulated carrier of the required frequency. Modulation
must not be present, since by definition, the dc voltages would
be changing, thus making accurate measurement difficult.
Setting up of oscillator
The VCO should be set up so that the desired input RF
frequency is at the centre of the lock range. This will coincide
with the centre of the S-curve and the point at which the AFC
toggles when set to zero deadband.
10
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