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The Myth of IF Repeaters
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Feel free to print this page for
handy reference |
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When
adequate fresnel zone clearance cannot be attained,
repeaters are generally used for Studio-Transmitter Links
or Inter-city Relays. Such repeaters can either be of the
IF feethrough type or down-to-baseband or regenerative
type. Unfortunately, there is an industry-wide
misconception that the IF repeater feedthrough type offers
much better end-to-end audio and RF performance. These
misconceptions are a carryover from the days of very
non-linear modulating and demodulating circuitry.
To understand the performance
difference between IF repeaters and the regenerative
repeaters, one must understand the factors that determine
signal-to-noise ratio, sensitivity, distortion,
selectivity and stereo separation for a multi-hop STL or
Inter-city relay.
The saturation signal-to-noise
ratio is determined by the phase noise of the first local
oscillators of the transmitter and the receiver. The
sensitivity of the radio is based on the noise figure of
the receiver. For a relatively high front end gain
receiver, the noise figure is determined by the first
preamplifier. As both repeater schemes use the first local
oscillators and preamps, sensitivity and signal-to-noise
ratios, for most intents and purposes, is the same.
Distortion is determined by non-linearities in the
modulator and demodulators, group delay of the IF filters,
and the filtering of Bessel sidebands by the IF filters.
Selectivity is determined by the shape of the IF filters.
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Today's technology, with
linear varactors and digital pulse counting demodulators,
result in distortion being determined by the IF filter.
Consequently, IF repeaters offer no distortion benefits.
In addition, the absence of subsequent conversion stages
and additional filtering results in compromises in
selectivity and leaves an IF repeater prone to
oscillations or extraneous signals propagating through the
system. Stereo separation is determined by the phase and
gain relations across the complete stereo band. With an IF
feed-through approach there are no means of correcting for
phase and gain relations of the composite stereo. In a
regenerative system, stereo separation and crosstalk can
be improved by adjustments.
Furthermore, an IF
feed-through repeater, compared to a regenerative type,
offers inferior performance at high and low signal levels
severely limiting the RF dynamic range of the receiver. IF
repeaters also result in lower third-order intercepts,
lower adjacent-channel rejection and higher FM thresholds.
From an implementation point, an IF feethrough repeater,
by virtue of the requirements for a fixed IF frequency,
can also limit the frequency agility of both the
transmitter and receiver.
The above analysis has been
general. Critical analysis, and Moseley experience with up
to three-hop STL/ICR systems, clearly indicates that the
more expensive IF feedthrough repeaters result in an
inferior STL/ICR system.
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