©2008 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 6/3/10
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the power supply to limit at 700 mA.
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Note:
Meter indications used as refer-
ences are typical but may vary widely due to
many factors not related to performance, such
as type of meter and circuit tolerances. Typi-
cal test point indications are for the 144 MHz
band unit and may differ for other bands.
c.
Set dip switch for desired frequency.
d.
Connect voltmeter to TP1 (top lead of
R6). Adjust vco coil L1 for +2Vdc. (Although
the vco will operate over a wide range of tun-
ing voltages from about 1V to 5V, operation is
optimum if the vco is adjusted to 2V.)
e.
Connect voltmeter to TP2. Adjust
buffer coils L3 and L4 alternately for a peak,
typically about +0.3V.
f.
Alternately, adjust driver coil L5, PA
input coil L8, and PA output capacitor C44 for
maximum output. Note that coil tuning may
be very broad. There may be small interac-
tions between tuning controls, so repeat until
no more interactions are noticed.
g.
At full drive, the total current drawn
by the exciter should be 400-600 mA, and the
RF output should be about 2 to 4W. Maxi-
mum output obtainable varies with many fac-
tors, so don’t worry if it is a little lower or
higher than expected.
Note that full output may not be possible
when operating on a power supply less than
13.6 Vdc. Power output falls rapidly as oper-
ating voltage is reduced. For example, on a
sample 144MHz unit, output level of 3.3W at
13.6Vdc was reduced to 2W output at 10Vdc.
This does not necessarily mean that the
unit cannot be used on lower B+ voltage,
however, since it is hard to distinguish even a
2:1 reduction in power on the air. And some-
times, you may wish to deliberately restrain
the output level to be conservative. Reducing
the power supply voltage is a good way to do
it. Just don’t operate below 10Vdc because
the voltage regulators would fall out of regula-
tion with too low an input. And do not reduce
voltage by putting resistance in series with the
supply; you want a well regulated/filtered
power source.
After tuning the exciter into a known good
50 ohm dummy load, it should not be retuned
when later connected to the antenna or
power amplifier. Of course, the antenna or pa
should present a good 50 ohm load to the ex-
citer.
h.
Perform the carrier frequency and
audio level adjustments given on page 2 to
complete the alignment of the exciter.
THEORY OF OPERATION.
The T301 is a frequency synthesized vhf
fm exciter. The carrier frequency is generated
by voltage controlled oscillator Q1. The out-
put is buffered by Q2 to minimize effects of
loading and voltage variations of following
stages from modulating the carrier frequency.
The resultant signal is amplified in successive
stages to provide 2 to 4 Watts output into a
50
Ω
load.
The frequency of the vco stage is con-
trolled by phase locked loop synthesizer U2. A
sample of the vco output is applied through
the buffer stage and R1 to a prescaler in U2.
The prescaler and other dividers in the synthe-
sizer divide the sample down to 5kHz.
A reference frequency of 10.240 MHz is
generated by a TCXO (temperature compen-
sated crystal oscillator). The reference is di-
vided down to 5 kHz.
The two 5kHz signals are compared to de-
termine what error exists between them. The
result is a slowly varying dc tuning voltage
used to phase lock the vco precisely onto the
desired channel frequency.
The tuning voltage is applied to carrier
tune varactor diode D1, which varies its ca-
pacitance to tune the tank circuit formed by
L1/C20/C21. C16 limits the tuning range of
D1. The tuning voltage is applied to D1
through a third order low pass loop filter,
which removes the 5kHz reference frequency
from the tuning voltage to avoid whine.
Modulation is applied to the loop filter at
R19. Audio or data signals are amplified by
U5a, limited by D4/D5, and applied to R19
through low pass filter U5b. The first op amp,
U5a, provides pre-emphasis so that higher au-
dio frequencies deviate wider than lower fre-
quencies. The second op amp, U5b, provides
a 12dB/octave rolloff for any audio or data
modulation products over 3000 Hz to prevent
splatter interference to other nearby chan-
nels. A direct modulation input is provided
through E5 and R37 for use with a subaudible
tone (CTCSS) encoder.
A lock detector in the synthesizer ic pro-
vides an indication of when the synthesizer is
properly locked on frequency. In order for it
to lock, the vco must be tuned to allow it to
generate the proper frequency within the
range of voltages the phase detector in the
synthesizer can generate, roughly 1Vdc to
5Vdc. If the vco does not generate the proper
frequency to allow the synthesizer to lock, the
lock detector output turns off U5c, which pro-
vides operating bias to the pre-driver ampli-
fier, thus preventing the exciter from putting
out signals which are off frequency. This fea-
ture ensures that the signal will reach the an-
tenna only after the carrier locks on
frequency.
Serial data to indicate the desired channel
frequency and other operational characteris-
tics of the synthesizer are applied to synthe-
sizer U2 by microcontroller U1. Everything the
synthesizer needs to know about the band, di-
vision schemes, reference frequency, and os-
cillator options is generated by the controller.
Information about the base frequency of the
band the T301 is to operate on and the chan-
nel within that band is calculated in the con-
troller based on information programmed in
the eprom on the controller and on channel
settings done on dip switch S1 and jumper E6-
E7. When the microcontroller boots at power
up, it sends several bytes of serial data to the
synthesizer, using the data, clock, and /enable
lines running between the two ic’s.
+13.6Vdc power for the exciter is applied
at E1. This B+ input is keyed on and off to
control when the exciter transmits a signal.
There is a jumper trace under the board run-
ning to E4, which allows power to be applied
constantly to the synthesizer circuits if de-
sired. This is convenient for applications
where the exciter will be keyed on and off
regularly. Because the microcontroller must
boot before it can send data to the synthe-
sizer, there is a short delay in generating the
carrier when power is first applied to the syn-
thesizer circuits.
RF amplifier stages are powered directly
by the +13.6Vdc. However, all the lower level
stages are powered through voltage regula-
tors for stability and to eliminate noise. U4 is
an 8Vdc regulator to power the vco, buffer,
and phase detector in the synthesizer. Addi-
tional filtering for the vco and buffer stages is
provided by capacitance amplifier Q3, which
uses the characteristics of an emitter follower
to provide a very stiff supply, eliminating any
noise on the power supply line.
Resistive voltage dividers provide lower
voltages which are regulated because they are
based on the regulated 8Vdc from U4. U5d
provides a stiff +5Vdc supply for the frequency
synthesizer and microcontroller.
TROUBLESHOOTING.
General.
Checking dc voltages and signal tracing
with an RF voltmeter probe and oscilloscope
will work well in troubleshooting the T301. A
dc voltage chart and a list of typical audio lev-
els are given to act as a guide to trou-
bleshooting. Although voltages may vary
widely from set to set and under various op-
erating and measurement conditions, the indi-
cations may be helpful when used in a logical
troubleshooting procedure.
The exciter draws about 30 mA of current
when just the synthesizer and audio circuits
are operating. When the exciter is generating
an RF output, it draws a total of about 450-
600 mA.
RF Amplifier Circuits.
You can use an RF probe with a dc voltme-
ter or scope to check the relative RF levels at
the input and output of each stage. The out-
put level should always be higher than the in-
put level of a given stage. Also, check the dc
operating and bias voltages for each stage.
The pre-driver stage gets its bias only when
the lock detector in the synthesizer is locked;
so if that bias is missing, check the synthesizer
and vco to see why it isn’t locked.
Synthesizer Circuits.
Following is a checklist of things to look
