©1998 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 12/11/02
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quency counter or synthesized signal
generator. Set level just high enough
for full quieting. At 1 uV, you should
notice some quieting, but you need
something near full quieting for the
test (about 20µV).
g. Connect dc voltmeter to TP-6
(top lead of R31 on right side of
board). Adjust discriminator trans-
former T2 for +3.3Vdc. Note that the
transformer is fairly close from the
factory and usually only requires less
than ¼ turn in either direction.
0
Be careful not to turn the slug
tight against either the top or bottom
because the winding of the transformer
can be broken. The tuning response is
an S-curve; so if you turn the slug sev-
eral turns, you may think you are going
in the proper direction even though you
are tuning further away from center
frequency.
+3.3Vdc
Figure 1. Discriminator tuning
h. Connect signal generator to J1
using a coax cable with RCA plug.
Adjust signal generator to exact chan-
nel frequency, and turn output level
up fairly high (about 1000µV). Adjust
frequency trimmer in TCXO to net the
crystal to channel frequency, indi-
cated by +3.3Vdc at test point TP-6.
Note:
There are two methods of
adjusting the mixer and front end.
One is to use an fet voltmeter with test
point TP-5, which is the top lead of
R26. The voltage at this point is pro-
portional to the amount of noise de-
tected 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-5 varies
from -0.5 Vdc with no signal (full
noise) to +0.8 Vdc with full quieting
signal.
The other method is to use a regu-
lar professional SINAD meter and a
tone modulated signal.
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
receiver sensitivity increases during
tuning.
If you use TP-5 with a voltmeter,
the signal can be modulated or un-
modulated. If you use a SINAD meter,
the standard method is a 1000 Hz
tone with 3 kHz deviation.
i. Connect fet dc voltmeter to
TP5. Set signal generator for relatively
weak signal, one which shows some
change in the dc voltage indication at
TP5. Alternately peak RF amplifier
and mixer coils L4-L8 until no further
improvement can be made.
When properly tuned, sensitivity
should be about 0.15 to 0.2µV for 12
dB SINAD.
0
Mixer output transformer T1
normally should not be adjusted. It is
usually set exactly where it should be
right from the factory. The purpose of
the adjustment is to provide proper
loading for the crystal filter, and IF
misadjusted, ripple in the filter re-
sponse will result in a little distortion
of the detected audio. If it becomes
necessary to adjust T1, tune the sig-
nal generator accurately on frequency
with about 4.5kHz fm deviation using
a 1000 Hz tone. In order of prefer-
ence, use either a SINAD meter, an
oscilloscope, or just your ears, and
fine tune T1 for minimum distortion of
the detected audio.
THEORY OF OPERATION.
The R301 is a frequency synthe-
sized vhf 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 Q5
passes through an 8-pole crystal filter
to get exceptional adjacent channel
selectivity.
U4 provides IF amplification, a 2
nd
mixer to convert to 455 kHz, a dis-
criminator, noise amplifier, and
squelch. Ceramic filter FL5 provides
additional selectivity at 455 kHz. The
noise amplifier is an op amp active fil-
ter peaked at 10 kHz. It detects noise
at frequencies normally far above the
voice band. Its output at pin 11 is
rectified and combined with a dc volt-
age 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. In-
verter Q6 provides a dc output for use
as a COS signal to repeater control-
lers.
The injection frequency for the first
mixer is generated by vco (voltage con-
trolled oscillator) Q1. The injection
frequency is 10.700 MHz below the
receive channel frequency. The out-
put of the vco is buffered by Q2 to
minimize effects of loading and voltage
variations of following stages from
modulating the carrier frequency. The
buffer output is applied through a
double tuned circuit to gate 2 of mixer
Q5.
The frequency of the vco stage is
controlled by phase locked loop syn-
thesizer U2. A sample of the vco out-
put is applied through the buffer stage
and R1/C3 to a prescaler in U2. The
prescaler and other dividers in the
synthesizer divide the sample down to
5kHz.
A reference frequency of 10.240
MHz is generated by a TCXO (tem-
perature compensated crystal oscilla-
tor). The reference is divided down to
5 kHz.
The two 5kHz signals are com-
pared to determine 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 capacitance 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 refer-
ence frequency from the tuning volt-
age to avoid whine.
In order for the synthesizer to lock,
the vco must be tuned to allow it to
generate the proper frequency within
the range of voltages the phase detec-
tor in the synthesizer can generate,
roughly 1Vdc to 8Vdc.
Serial data to indicate the desired
channel frequency and other opera-
tional characteristics of the synthe-
sizer are applied to synthesizer U2 by
microcontroller U1. Everything the
synthesizer needs to know about the
band, division schemes, reference fre-
quency, and oscillator options is gen-
erated by the controller. Information
about the base frequency of the band
the Receiver is to operate on and the
channel within that band is calculated
in the controller based on information
programmed in the eprom on the con-
troller and on channel settings done
on dip switch S1 and jumper E6-E7.
Whenever the microcontroller boots at
power up, the microcontroller sends
several bytes of serial data to the syn-
thesizer, using the data, clock, and
/enable lines running between the two
ic’s.