Frequency Agile Plug-On UHF Transmitter
Rio Rancho, NM
Input Limiter
The Digital Hybrid Wireless
™
transmitters employ a
digitally-controlled analog audio limiter just before the
analog-to-digital converter. The limiter has a range of
more than 30 dB for excellent overload protection. A
dual release envelope makes the limiter acoustically
transparent while maintaining low distortion. It can be
thought of as two limiters in series, connected as a fast
attack and release limiter followed by a slow attack and
release limiter. The limiter recovers quickly from brief
transients, so that its action is hidden from the listener,
but recovers slowly from sustained high levels, to both
keep audio distortion low and preserve short term dy-
namic changes.
Generally speaking, some limiting is desirable in normal
operation to improve the signal to noise ratio of the
system. The limiting action is not audible and does not
create distortion. A highly trained ear would hear only
the compression of the peaks in the audio signal, which
is desirable with most recorders and many sound rein-
forcement systems.
No Pre-Emphasis/De-Emphasis
The signal to noise ratio of the Digital Hybrid Wireless
™
system is high enough to preclude the need for conven-
tional pre-emphasis (HF boost) in the transmitter and
de-emphasis (HF roll off) in the receiver. Pre-emphasis
and de-emphasis in an FM radio system usually pro-
vides about a 10 dB improvement in the signal to noise
ratio of the system, but the high frequency boost in the
transmitter must be removed in a purely complementary
manner or else the frequency response of the original
audio signal will be altered.
Pre-emphasis can also cause distortion in the receiver.
As this signal is passed through the IF filters in the
receiver, distortion can be produced, most noticeably at
full modulation. De-emphasis cannot be applied until
the signal is converted into audio, so there is no way
around this problem short of eliminating pre-emphasis
altogether. Neither of these problems occur in the Digi-
tal Hybrid system.
Pilot Tone Squelch
The Digital Hybrid Wireless
™
system uses one of 256
different ultrasonic tones between 25 and 32 kHz, that
modulate the carrier to operate the receiver squelch.
The pilot tone frequency is chosen according to
which of the 256 channels has been selected by the
frequency switch setting. The basic benefit of the pilot
tone squelch system is that the receiver will remain
muted until it receives the pilot tone from the matching
transmitter, even if a strong RF signal is present on the
carrier frequency of the system.
Wide-Band Deviation
A ±75 kHz deviation improves the capture ratio, signal
to noise ratio and AM rejection of a wireless system
dramatically, compared to the more commonly used ±
15 kHz deviation.
Long Battery Life
Switching power supplies throughout the design allow
over 4.5 hours of operation using a single 9 Volt alka-
line battery. The battery contacts are spring loaded to
prevent “rattle” as the unit is handled.
Frequency Agility
The transmitter section uses a synthesized, frequency
selectable main oscillator. The frequency is extremely
stable over a wide temperature range and over time.
Two rotary switches, located on the side panel of the
unit, provide 256 frequencies in 100 kHz steps over a
25.5 MHz range. This alleviates carrier interference
problems in mobile or traveling applications.
