User's Manual

TXRX Systems Inc. Manual 7-9408-1.2 07/25/05 Page 17
61-38-05 UserMan page 17 of 38
Single channel bandpass filters are generally 4-
pole crystal filters using piezo-electric resonators.
A secondary but very beneficial effect of filtering is
instantaneous input and output frequency spec-
trum limiting which helps to prevent amplification of
unwanted channels.
While filtering can reduce or eliminate spurious
output signal, this is a tertiary function in the VHF
signal booster because this booster family uses
Class-A linear amplifiers that generate much lower
harmonic content than the typical Class C or D
amplifier used in typical landmobile transmitters.
The input and output filter assemblies used in the
model 61-38-05 signal booster systems are com-
posed of helical preselectors, 2” square bandpass
cavities, and 2” square notch cavities. These filters
all have a carefully shaped response curves that
define the pass windows for the booster.
The helical preselectors are composed of four cas-
caded helical cavities. The cavities are intercon-
nected with critical length cables to synthesize a
shaped response. The bandpass filters pass one
narrow band of frequencies (the passband) and
attenuate all others with increasing attenuation
above and below the pass frequencies. The inser-
tion loss setting determines the filters selectivity
and maximum power handling capability. Insertion
loss is set at the factory. The notch filters are used
to notch out a very narrow range of frequencies
and improve the skirt selectivity of associated
bandpass filters.
The filters used in the booster are factory pretuned
and do not require any adjustment. The filters are
easy to misalign. Being passive devices using sil-
ver plated contacts means they requires no mainte-
nance and will stay tuned indefinitely unless they
are physically damaged or tampered with. If it is
suspected that a filter is out of alignment, we sug-
gest returning it to the factory for re-alignment.
However, if the necessary test equipment is avail-
able then the tuning procedure outlined later in this
manual may be used to put it back on frequency.
DC Regulator (3-5969)
The DC regulator receives 24.7 VDC from the
power supply assembly through it's input 'TNC'
connector. Two regulator assemblies are used, one
for each signal branch in the bi-directional system.
each of the regulators provides two different output
voltages, +15 VDC and +21.7 VDC. A minimal volt-
age differential of 3 volts is required between the
input and the output of the 3-5969 regulator in
order to maintain proper operation. The regulator
assembly can provide up to 5 amps of total current.
The regulator circuit uses two conventional IC reg-
ulator chips. An LM338K is used to produce the
fixed 15 VDC, and an LM340K is used for the vari-
able output which is factory adjusted to +21.7 VDC.
Test jacks (red & black) are available on the regula-
tor chassis for measuring the input, fixed-output,
and the variable-output voltages. The regulator has
an access hole on the side of it's case for adjusting
the variable-output voltage. A thin blade screw-
driver is used to engage a trim-pot type variable
resistor R2 which is then rotated until the desired
output voltage is obtained. Adjustment of the regu-
lator is only required after making repairs to the
regulator circuitry.
OLC Assembly (3-6280)
The OLC assembly 3-6280 is used in both
branches of the system is divided into three
shielded compartments; one housing the RF to DC
converter, the second a DC control circuit, and the
third containing the PIN diode attenuator circuit. A
test point is provided for measuring the voltage that
is applied to the PIN diode attenuator. A second
test point allows measurement of the voltage sup-
plied by the converter to the DC control circuit (see
Figure 8
). Regulated 21.7 VDC is supplied to the
"TNC" female connector to power the assembly.
The RF to DC converter receives RF from the sig-
nal sampler and produces a negative polarity DC
output voltage that is proportional to the RF signal.
A Schottky Barrier diode is used as the detector.
Because this detector circuit has a very high input
impedance, the magnitude of the voltage that it
produces will vary if the length of the coaxial cable
which connects it to the signal sampler changes.
Therefore, it is important not
to change this length.
The voltage produced by the RF to DC converter is
directly proportional to the output signal strength of
the final amplifier. The voltage is supplied to the DC
control circuit at the non-inverting terminal of op-
amp IC2. A variable reference voltage is applied to
the inverting terminal of the same op-amp. Variable
resistor VR2 is used to set the magnitude of this
reference voltage and controls the level at which
gain reduction will start to occur. As the signal
strength increases the output voltage of the con-
verter, which is of negative polarity, becomes