©2005 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 5/17/05
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OPTIONS.
Repeater Use.
E5 provides a COS (carrier oper-
ated switch) output which may be
connected to a COR module to turn a
transmitter on and off. The output
level is about 8V unsquelched and 0V
squelched. There is a resistor in se-
ries with the output to limit current.
Therefore, the voltage that appears at
the COR board will depend on the
load resistance at the input of that
board. For best results, be sure that
the input resistance of the COR board
is at least 47K. If the input resistance
is too low, no damage to the receiver
will occur; but the squelch circuit hys-
teresis will be affected.
If your repeater controller uses
discriminator audio, rather than the
speaker output, filtered discriminator
audio is available at E4. The level is
about 2V p-p.
Note that discriminator
audio is not de-emphasized or
squelched.
If you need audio which is
squelched, take it from Repeater Au-
dio terminal E1.
If your controller uses low level
audio and has a high input imped-
ance (20K or higher), squelched audio
can be obtained from E1 independent
of the VOLUME control.
Discriminator Meter.
If you wish to use a discriminator
meter and you are handy in designing
with op-amps, you can run a sample
of the dc voltage at
DISCRIMINATOR
output terminal E4 to one input of an
op-amp and tie the other input to a
voltage divider pot set to provide a ref-
erence voltage of about +3.3Vdc.
S-Meter.
There is no s-meter function, as
such, available in i-f amplifier ic's
made for professional receivers; how-
ever, a signal strength indication is
available at test point TP-4. This volt-
age is a function of the noise level de-
tected in the squelch circuit. It also
varies with SQUELCH control setting.
With the SQUELCH set to where the
squelch just closes, the dc voltage at
TP-4 is about -0.5V with no signal and
+1V with full quieting signal. You can
tap off this test point with a high-
impedance circuit, such as an op-
amp, to drive a meter or a computer-
ized repeater controller.
Subaudible Tone Decoder.
To use our TD-5 Subaudible Tone
Decoder or a similar module, connect
its audio input to
DISCRIMINATOR
terminal E4. If you want to use it to
mute the audio (instead of inhibiting a
repeater transmitter as is normally
done), connect the mute output of the
TD-5 to E1 on the receiver.
ADJUSTMENTS.
Frequency Netting.
All crystals age a little over a long
period of time; so it is customary to
tweak any receiver back onto the pre-
cise channel frequency once a year
during routine maintenance. Because
modern solid state equipment doesn’t
require much routine maintenance,
many receivers don’t get their oscilla-
tors tweaked as a matter of routine
any more, but they should.
The adjustment should be done
using an accurate service monitor or
frequency counter. Of course, make
sure the test equipment is exactly on
frequency first by checking it against
WWV or another frequency standard.
The channel frequency is trimmed
precisely on frequency with a small
variable capacitor, which is accessible
through a hole in the top of the shield
can on the TCXO. The proper tool is a
plastic wand with a small metal bit in
the end.
To perform this adjustment, it is
first necessary to verify that the dis-
criminator is properly adjusted. Do
this by connecting a dc voltmeter to
E4. Connect a signal generator set for
10.700 MHz to TP3, and set the level
for a relatively strong signal so there
is very little white noise. Adjust dis-
criminator coil T2 for 3.3Vdc. Then,
reconnect the signal generator to an-
tenna connector J1, and set it for the
precise channel frequency. You can
also use a strong signal on the air if
you are sure it is right on frequency.
Adjust the TCXO capacitor for 3.3Vdc
(to match the voltage obtained with
the 10.700 MHz signal).
Setting Channel Frequency.
The channel frequency is deter-
mined by frequency synthesizer cir-
cuits, which use a dip switch in
conjunction with programming in a
microcontroller to set the channel.
The microcontroller reads the dip
switch information and does mathe-
matics, applying serial data to the
synthesizer ic whenever power is ap-
plied. Following is a discussion of
how to set the dip switch to the de-
sired channel frequency.
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NOTE:
If the frequency is
changed more than about 1 MHz, a
complete alignment of the receiver
should be performed, as described in
later text. Optimum operation only oc-
curs if the synthesizer is adjusted to
match the frequency switch setting and
all the tuned amplifier circuits are
peaked for the desired frequency.
To determine what channel fre-
quency to use, the microcontroller
adds the frequency information from
the dip switch to a “base” frequency
stored in eprom used for microcontrol-
ler programming. Each model of the
R302 Receiver has a particular base
frequency. For example, the R302-2
has a base frequency of 144.000 MHz,
as shown in Table 1.
Dip switch settings are binary,
which means each switch section has
a different weighting, twice as great as
the next lower section. Sections have
weights such as 5 kHz, 10 kHz, etc.,
all the way up to 2.56 MHz
. (See Ta-
ble 2 or the schematic diagram for
switch values.)
The system sounds cumbersome,
but it really is fairly simple, and you
don’t need to do this frequently. A
piece of paper or a small calculator is
handy to aid in determining which
sections of the switch to turn on.
When done, you might want to record
the switch settings in table 3 for fu-
ture reference.
Begin by subtracting the base fre-
quency, e.g., 144.000, from the de-
sired frequency to determine the total
value of all the switch sections re-
quired to be turned on.
If the difference is more than
5.120MHz, turn on dip switch section
1, and subtract 5.120 from the start-
ing frequency. Otherwise, turn off
section 1.
Likewise, look at the remainder
frequency and see if you can subtract
2.560 from it. If so, turn on dip
switch section 2, and subtract 2.560
from to get a new remainder. Other-
wise, turn off section 2.
Do the same for each of the other
sections, from highest to lowest
weighting, in sequence. Each time
you consider the remainder, turn on
the switch section with the highest
weighting which will fit within the re-
mainder without exceeding it. Each
time it is found necessary to turn on a
switch section, subtract the value of
that section from the remainder to get
the new remainder.
As an example
, let us consider
how to set the Receiver for 146.94
MHz. The following discussion is bro-
ken down into steps so you can visu-
alize the process easier.
a. 146.940 - 144.000 base freq. =
2.940 MHz remainder. Turn on
switch #2, which represents the larg-
est increment to fit remainder.
b. 2.940 - 2.560 value of switch
#2 = 0.380 MHz. Turn on #5, which is
0.320 MHz, the largest increment to
fit the remainder.
