©2009 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 8/10/11
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standard.
The channel frequency is trimmed pre-
cisely on frequency with a small variable ca-
pacitor, which is accessible through a hole in
the top of the TCXO shield can. The proper
tool is a plastic wand with a small ceramic or
metal bit in the end.
To perform this adjustment, it is first nec-
essary to verify that the discriminator is prop-
erly adjusted. Do this by connecting a dc
voltmeter to TP4. Connect a signal generator
set for 10.700 MHz to TP5 (left side of coil L9),
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 antenna 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 trimmer capacitor for 3.3Vdc (to
match the voltage obtained with the 10.700
MHz signal).
Setting Channel Frequency.
The channel frequency is determined by
frequency synthesizer circuits, which use a mi-
crocontroller which is programmed at the fac-
tory. If you need to change frequency,
contact the factory to get another micro pro-
grammed for the new frequency settings.
ALIGNMENT.
A complete alignment is needed whenever
the frequency is changed by more than about
1 MHz. Alignment ensures that the frequency
synthesizer is optimized at the center of the
vco range and that all stages are tuned to
resonance.
Equipment needed for alignment is a sen-
sitive dc voltmeter, a stable and accurate sig-
nal generator for the channel frequency, and a
regulated 13.6Vdc power supply with a 0-200
mA meter internally or externally connected in
the supply line.
The slug tuned coil should be adjusted
with the proper .062" square tuning tool to
avoid cracking the powdered iron slugs. Vari-
able capacitors should be adjusted with a plas-
tic tool having a small metal bit. (See
website.)
MID MAX MIN
Capacitor Adjust ment
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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.
a.
Set the SQUELCH pot fully counter-
clockwise and the VOLUME pot just a little
clockwise.
b.
Connect speaker and +13.6 Vdc. You
should hear white noise.
c.
Connect voltmeter to TP1 (insert
probe tip in pad on board). Adjust vco coil L1
for +2Vdc. (Although the vco will operate
over a wide range of tuning voltages from
about 1V to 5V, operation is optimum if the
vco is adjusted to 2V.)
d.
Connect voltmeter to TP2 (insert
probe tip into pad). Adjust doubler variable
capacitor C21 for a peak. Note: the peak will
be small.
e.
Adjust buffer variable capacitor C24
for a peak. This will be just a slight peaking
(increase) of the voltage.
f.
Adjust C32 and C33 for a peak. Then,
re-peak C21, C24, C27, and C33 to ensure they
are all at maximum.
g.
Connect stable signal generator to
TP-2. Set generator to exactly 10.7000 MHz.
Use a frequency counter or synthesized signal
generator so the frequency is accurate. Set
level just high enough for full quieting. At 20
µV, you should notice some quieting, but you
need something near full quieting for the test
(about 200µV).
h.
Connect dc voltmeter to Discrimina-
tor pad E4. Adjust discriminator transformer
T3 for +2.0Vdc.
Be careful not to turn the slug tight
against either the top or bottom because the
winding of the transformer can be broken.
i.
Connect signal generator to J1 using a
coax cable with RCA plug. Adjust signal gen-
erator to exact channel frequency, and turn
output level up fairly high (about 1mV).
j. If necessary, adjust trimmer in TCXO to
net the crystal to channel frequency, indicated
by +2.0Vdc at Discriminator output terminal
pad. The frequency normally will be very
close; so you may not need to adjust it at all
except to compensate for aging.
Note:
There are two methods of adjusting
the mixer and front end. One is to use a volt-
meter with test point TP-3. The voltage at this
point is proportional to the amount of noise
detected in the squelch circuit; so it gives an
indication of the quieting of the receiver. With
SQUELCH control fully ccw, the dc voltage at
TP-3 varies from -0.5 Vdc with no signal (full
noise) to +0.9 Vdc with full quieting signal.
The other method is to use a regular profes-
sional SINAD meter and a tone modulated sig-
nal.
In either case, a weak to moderate signal
is required to observe any change in noise. If
the signal is too strong, there will be no
change in the reading as tuning progresses; so
keep the signal generator turned down as re-
ceiver sensitivity increases during tuning.
If you use TP-3 with a voltmeter, the signal
can be modulated or unmodulated. If you use
a SINAD meter, the standard method is a 1000
Hz tone with 3 kHz deviation.
k.
Connect dc voltmeter to TP-3. Set
signal generator for relatively weak signal, one
which shows some change in the dc voltage
indication at TP3. Alternately peak RF ampli-
fier and mixer variable capacitors C29, C30,
and C31 until no further improvement can be
made. When properly tuned, sensitivity
should be about 0.2µV for 12 dB SINAD.
T1 and T2 adjust the crystal filter for
minimum distortion. These are adjusted at
the factory and should not be disturbed. If
you must adjust them, inject a signal exactly
on channel frequency with a 1000Hz tone and
5kHz deviation. Adjust alternately for mini-
mum distortion.
THEORY OF OPERATION.
The R306 is a frequency synthesized uhf
fm Receiver. Refer to the schematic diagram
for the following discussion.
Low noise dual-gate mos fet’s are used for
the RF amplifier and mixer stages. The output
of mixer Q6 passes through an 8-pole crystal
filter to get exceptional adjacent channel se-
lectivity.
U4 provides IF amplification, a 2
nd
mixer to
convert to 455 kHz, a discriminator, noise am-
plifier, and squelch. Ceramic filter FL5 pro-
vides additional selectivity at 455 kHz. The
noise amplifier is an op amp active filter
peaked at 10 kHz. It detects noise at frequen-
cies normally far above the voice band. Its
output at pin 11 is rectified and combined
with a dc voltage from the SQUELCH control
to turn a squelch transistor on and off inside
the ic, which grounds the audio path when
only noise is present. Inverter Q7 provides a
dc output for use as a COS signal to repeater
controllers.
The injection for the first mixer is gener-
ated by voltage controlled oscillator (vco) Q2.
The injection frequency is 10.700 MHz below
the receive channel frequency, and the vco
operates at one half the injection frequency.
The output of the vco is doubled by Q3 and
buffered by Q4 to minimize effects of loading
and voltage variations of following stages
from modulating the carrier frequency. The
buffer output is applied through a triple tuned
circuit (L4, L5, and L6) to gate 2 of mixer Q6.
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 C2 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 temperature compensated
crystal oscillator (tcxo). 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 varactor
diode D1, which varies its capacitance to tune
the tank circuit formed by L1/C16/C17. C13
