Page 12
RF Technology R50
the possibility of replacing MX401 with a mixer that requires a higher level of
LO drive (but an improved IP3 figure).
5.6 Audio Pr ocessing Section (Sheet 6)
The discriminator output from the RF section is connected, in this audio section, to a
peak detector formed by U604 and C623. This peak detector output is passed
to an A/D input of the Microcontroller to be used as a simple deviation meter.
This raw detected audio is then attenuated by R642 and R655 before being filtered.
This attenuation is required to ensure that the audio signal will fit into the
dynamic range limitation of the CTCSS chip, U602.
The raw, unfiltered Discriminator Audio signal is attenuated by R658 and R657
before being connected to the rear panel connector P3.
U605B, R642, R643, R644, C626, C627 and C628 form a 5
th
order ellyptical low pass
filter with 3dB cut-off at 4.2kHz.
U602 is a CTCSS tone encoder and decoder. U602 can receive and transmit NRZ
data, and as such can be used, with suitable firmware, to receive or detect DCS
codes.
The CPU accesses U602 via the serial bus using MOSI, SCLK, and the low active
Select signal CTCSS_SEL. When U602 detects new CTCSS tones, or detects
the absence of a tone, or receives bytes of NRZ data, it signals the
Microcontroller by asserting the low active signal SUBTONE_IRQ.
The 4MHz crystal, X401, is used as the reference for decoding CTCSS tones, or
setting the bit rate for NRZ data. U602 has a built-in high pass filter, between
pins 10 and 11, which can eliminate any subtone, or low frequency NRZ data.
Whilst the Microcontroller can enable or disable this filter, in the R50, it is
always enabled. The subtone output from Pin17 is amplified by U605C. The
output from U605C is ac-coupled to the rear panel connector P3, through
capacitor C604.
The audio paths in U602 are all dc-biased to Vcc/2. The audio paths before and after
U602 are zero-biased.
U609B is set up as an inverting amplifier with a nominal gain of unity at 1kHz. If the
signal FLAT, is asserted low, analogue switch U607A is enabled. This sets the
gain at unity across the audio band. If, on the other hand, the digital signal
DE_EMPHA is asserted low, analogue switch U607B is enabled, and the gain
becomes proportional to frequency. Thus de-emphasis is applied to compensate
for any pre-emphasis applied at the exciter.
The audio is connected to the Line outputs and the DIR_AUD output, when the digital
signal SQ is asserted low.
The audio is connected to the speaker amplifier when the digital signal MON_SQ is
asserted low.
U609C is the amplifier for the 600 ohm line output. U609D is the amplifier for the
direct audio output (DIR_AUD). The output level of the 600 ohm output is
adjustable via VR601 on the front panel.
U604 is the power amplifier for internal and external speaker. The volume can be
adjusted by VR602 on the front panel.
RL602 may apply, or remove, 12VDC to the 600 ohm line output. Whether this
occurs or not, and whether the voltage is applied when the squelch opens or
