Maintenance Manual
10
Local Oscillator Buffer
The injection frequency (354.7 ~ 414.7
㎒
) from the synthesizer circuit is applied to the base of LO buffer
transistor Q105. The output signal level is +3dBm and is connected to the input of mixer circuit (Q201).
Mixer
The RF signal from the front-end and the injection frequency from the LO buffer is connected to the input
of mixer(Q201), The mixer generates a 1
st
IF (UHF : 45.3
㎒
, VHF : 21.4
㎒
) signal on the output of the
mixer circuit (Q201). The signal level of VCO local frequency is 2dBm
±
2dBm and at this time, the
conversion gain is approximately 0dBm. The Mixer (Q13) uses n-channel dual gate MOS-FET in order to
prevent spurious harmonics.
The signal on the output of the mixer is applied to the 1
st
IF band-pass filter. The Crystal filter (XF31)
selects 1
st
IF frequency out of frequencies generated from the mixer part. The Crystal filter matching
circuit consists of L73, C257, C223, C208, and R209.
The crystal filter output is amplified by bipolar IF amp transistor Q54. The output signal of IF amplifier
connected through coupling capacitor C260 to the input of IF IC.
IF Detector circuit
The IF IC (U301) consists of the 2
nd
Mixer, the 12.5 / 25KHz ceramic filter, the Discriminator, and the
noise squelch circuit.
The 1
st
IF (UHF : 45.3
㎒
, VHF : 21.4
㎒
) signal is applied to an input of second mixer circuit (internal
U301) through coupling capacitor C260. A second Local injection frequency of 44.845
㎒
(VHF : 20.945
㎒
) is internally applied to an other input of the mixer. This result in a second IF of 455
㎑
on the output
of the mixer. This signal pass through the 2nd IF filter (455
㎑
) to the internal IF amplifier. The
microprocessor selects the ceramic filter for the suitable channel spacing (12.5
㎑
or 25
㎑
) out of 2
ceramic filters. The W_CON signal is high, 25
㎑
ceramic filter is selected and N_CON signal is high,
12.5
㎑
ceramic filter is. Accordingly, the output of IF IC (pin#3) selects only the 12.5
㎑
signal
through F104 (25
㎑
ceramic filter) and F103 (12.5
㎑
ceramic filter).
When the channel spacing 12.5
㎑
and 25
㎑
are selected, Q55 and Q56 make the audio sound and the
noise squelch in the same level.
The squelch is adjusted in 10 steps by supplying the noise voltage of
U301 (pin#14) to P2.6 A/D port of MPU (U51).
