Block diagram of the IFL system
Most of the local FM broadcast stations have their transmitters on this site, not to mention the presence of 4 television stations and many many paging, remote base, and other land-mobile transmitters. Because of this, the RF noise level on the site is quite high and can have a tendency to mask weak signals.
In the early 1980's, when the Utah Amateur Radio Club (UARC) installed a 2 meter repeater on site (now on 146.02/62 - and you can read the exciting story of the repeater's installation here) it was decided to locate the receiver some distance to the north, down the ridge from the main transmit site. This horizontal and vertical separation greatly reduced receiver noise floor and even today this repeater is one of the best around from a sensitivity standpoint. It is not unreasonable to work it handie-talkie w/rubber duck from over 60 miles away.
On establishment of the remote site, power and signals need to be
transferred
between the two. Since it was decreed to be a receive-only site, it was
decreed that NO transmitters (of any kind!) were to be permitted.
Fortunately
the two sites are close enough (approximately 500 feet) that wire may
used
for an interconnection. When a connection this long is made between two
sites, especially on a mountaintop (i.e. a lightning magnet!) special
precautions
need to be made:
Putting all of this information together, we have a repeater that:
For power, low voltage AC is available and this may be upconverted to standard 120VAC to operate equipment, or the equipment may be designed to operate directly from that voltage. For audio, video, and control signals, the solution is not quite as simple. If it were just audio, one could carefully use standard audio transformers to move audio along twisted-pair lines. Control might even be encoded as an audio signal, too, but what about video? Remember that the medium one uses for transmitting video must have a bandwidth extending from under a few hertz to about over 4 MHz and be well-behaved in terms of amplitude, phase and delay!
Since over-the-air transmission of video (on any band) is not an
option,
we must string cable and transmit on that. Owing to previously-stated
conditions,
it would have to be balanced line, so twinaxial cable is being used.
Having
such an RF conduit, it made sense to make all other signals RF as
well...
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As mentioned above all signals must be balanced. Because we need to transmit video, they will need to be RF. These baluns below perform the necessary conversion from a 50 ohm unbalanced (coax) line to 100 ohm (balanced) line. In addition to performing the balun function several important design features have been added to protect the equipment connected to it from lightning:
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Great care has been taken on these baluns to optimize their
performance.
For example, the tap-point of the Faraday shields have been carefully
selected
to maximize common-mode rejection at higher frequencies. There is also
a small trimmer-potentiometer at one end of the Faraday shield to null
out imbalance in the Faraday shield itself caused by the proximity of
the
unbalanced secondary to further enhancing the longitudinal
isolation
of the system.
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The IFL (InterFacility Link) video modulator (above)
frequency-modulates
the received video onto an RF carrier. This is one of two that are
planned
and it operates on a center frequency of 40 MHz.
This modulator has a very sharp input video lowpass filter to keep
wideband noise modulated on the 40 MHz carrier from making it
take
up too much bandwidth on the video IFL system. This lowpass filter has
a cutoff frequency of 5.5 MHz and it has been delay-equalized to
prevent
distortion of the video signal. The signal is then pre-emphasized in a
conventional manner to boost the high-frequency components of the video
before modulation. It is then amplified to several volts peak-to-peak
and
applied to an NE564 to frequency-modulate the carrier.
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Because the NE564 uses an R/C oscillator (and for other reasons...) it is not particularly frequency-stable with temperature. To keep it on frequency a PLL is employed (the diagonally-mounted board.) This modulator has been tested from a cold-start at 10 degrees F to an operating temperature of over 150 degrees F and found to be very stable.
On the receiving end of the IFL there is the matching receiver. This
also uses an NE564, but as a demodulator.
About 80 db of gain is used prior to the demodulator to bring it up
to limiting: Two cascaded MAR-6 MMICs are used in the first stages,
followed
by an MC1350.
When being constructed, it was noted that the demodulator output of
the NE564 was of very high impedance. It was not enough to just hang an
emitter follower on it but a complementary pair, along with a
compensating
capacitor, was required to get it to work well. This output is followed
by a lowpass filter identical to that in the modulator to get rid of
noise
and carrier bleed-through. The signal is amplified, run through a
de-emphasis
network to attenuate and compensate for the pre-emphasis at the
transmit
end and then amplified to proper video levels.
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When tested, this demodulator worked reasonably well over a very wide temperature range, but since it is free-running, it is confined to within its lock range. As a system, the following attributes were measured:
It should be noted that the audio information is not carried
as a subcarrier on the video carrier. To carry a subcarrier on the
video
would reduce the signal-to-noise ratio because of intrinsic
non-linearities
as well as increasing the power-bandwidth of the carrier. It would also
make the audio signal more vulnerable to wideband noise in the video
passband.
For this reason, the audio carriers are entities unto themselves. The
audio carrier modulators/demodulators are still being constructed.
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The IFL system is designed to be able to carry 2 FM video carriers
and
a multitude of audio and control carriers. The primary video carrier is
centered on 40 MHz and has a peak deviation of approximately 4 MHz
while
the secondary video carrier is centered on 19 MHz with a peak deviation
of 3 MHz. Between 1 and 8 MHz are the audio and control carriers: These
are typically WFM (75 KHz deviation) and are arranged such that the
harmonics
of the lower frequency carriers do not fall near the center frequencies
of the higher-frequency carriers.
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There are two IFL Filter assemblies: The one at the Receive site, and the one at the Transmit site.
The IFL filter assembly at the receive site (not shown) is
primarily
responsible for limiting the bandwidth of the signals being modulated
there.
They confine the video signal to their respective bandwidths, and
low-pass
filter the 1-8 MHz audio/control carrier system. The signals coming
from
the various modules are combined via ferrite hybrid combiners, passed
through
a main highpass/lowpass filter assembly (at approximately 55 MHz and 1
MHz, respectively) and sent to the balun.
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Since Farnsworth Peak is the home of a TV channel 5, there is also a notch filter tuned to this frequency. Additionally, the lowpass filters are of the elliptical type, with the notches chosen to correspond with the 2 meter band: This is just a precaution in case any IFL energy were to escape from the twinax system.
The IFL filter assembly at the transmit site (shown above)
is
more complicated in that its filters are much sharper (see the spectrum
analyzer displays.) Their purpose is to provide the noise-bandwidth
limiting
of the video carriers prior to FM limiting. The main lowpass filter on
this assembly is also of the elliptical type, with its notches landing
in the middle of the TV channel 5 passband providing over 60 db of
attenuation.
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Common to both filter assemblies is two audio/control carrier
input/output
ports and a test port. The two audio/control ports accommodate
the
presence of both receive and transmission of
subcarriers
from each site. The ferrite combiners provide a degree of isolation
above
and beyond those in the filters of the associated
modulators/demodulators.
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There are three different types of IFL audio modules: The Demodulator, the Communications Quality audio modulator, and the Broadcast Quality audio modulator.
The Demodulators are based on the Signetics NE605. This IC is a complete FM receiver-on-a-chip. It has an input mixer, IF stages, and demodulator and RSSI (Received Signal Strength Indicator) outputs. All one needs to do as supply a local oscillator tank (a crystal will do nicely,) a few IF filters (280 KHz ceramic filters in this case) and a quadrature coil. These demodulators operate with 10.7 MHz IFs using wideband FM. Their 20db SINAD sensitivity is better than 20 microvolts and their ultimate signal/noise ratio is well over 65 db. Their mediocre sensitivity is a result of the inputs being designed more for simplicity than sensitivity since the minimal signal levels will be in the hundreds-of-microvolts range, anyway.
The Communications Quality modulators are based on the 74HC4046. These are synthesized modulators with the '4046 serving both as the Frequency/Phase detector and the modulated VCO. They are programmable in 25 KHz steps and useable from a few 10's of KHz to 8 MHz. They provide only about 40 db Signal/Noise ratio (at a deviation of 75 KHz) owing to the noisy nature of the '4046. This is more than adequate for communications-quality audio and telemetry.
The Broadcast Quality modulators are similar to the Communications
Quality
ones in that they also use the 74HC4046 Phase/Frequency detector.
Instead
of the on-board VCO, they use an external discrete-component L/C
oscillator.
These are also programmable in 25 KHz steps and have been tested and
found
to perform as well as commercially-available subcarrier modulators.
Any questions? Email to find out more.
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