Service manual

Page 26
Transmitter: Transmitter, TxBPF and 100W PA boards
Refer to the block diagram on page xxx and schematics starting on page xxx.
The transmitter in the Sienna is completely separate from the receiver. They do not share local
oscillators or bandpass filters as is the case in most transceivers. This allows the transmitter and
receiver to be operated at the same time, i.e. in full duplex mode, a useful mode for satellite op-
erations in particular, where you need to be able to listen to your downlinked signal while trans-
mitting. It’s also useful if you should want to transmit audio code practice on AM, or re-
transmit receiver audio on a different frequency.
Microphone and Line Processing
Balanced microphones can be used with Sienna. The balanced lines are fed into U6, a precision
preamp with programmable gain. The gain is set by a digital pot in U14. For most low output
microphones, the default setting provides very high gain. High output mics can also be used, in
which case the gain can be reduced by changing the resistance via a menu option. The preamp
output and the unbalanced line input from the phono connector on the rear panel are mixed by
U9 (pins 12,13,14), a precision, low distortion, rail-to-rail op-amp, to provide three outputs—
one for the balanced modulator, one for the PC’s line input, and one for FM and VOX detec-
tion. The VOX/FM path includes a pre-emphasis circuit, which is simply a high pass filter. FM
receivers employ “de-emphasis”, or low pass filtering, which cancels out the effect of the pre-
emphasis. U9 (pins 5,6,7) mix the input from the receiver board (RxLine) with the external Line
In signal. This output becomes the “Line” signal.
The output of the balanced preamp goes to jumper JP12, allowing it to be tested without the
load of the subsequent amplifiers, and allowing for a future audio equalizer to be inserted.
Line/Mic selection
U19 is a low distortion CMOS switch that allows either the Line or Mic outputs to drive the
balanced modulator. Note that for digital modes, there are two ways for the signal to get into the
transmitter—via the unbalanced Line In connector on the transmitter board’s phono connector,
or via “RxLine”. When the internal PC is used, its sound card output appears on this line.
Balanced Modulator and CW keying
The selected input is fed into the balanced modulator (U7). Pot RV1 is used in a calibration step
to null the carrier in SSB modes. In AM, the balance is removed by turning on Q22, allowing
the carrier to feed through. The local oscillator, TXBFO, runs at about 10.7MHz. Thus the first
IF is 10.7MHz +/- audio frequencies.
In CW and FM modes, the LO is switched to Q21. The keying circuit (Q19, Q23) waveshapes
the keying signal via low pass filter consisting of C9 and R20 and applies it to the drain of Q21.
The waveshaped CW signal then appears at the source of Q21. Diodes D12 and D13 and ca-
pacitor C130 keep the RF from bleeding through to the subsequent stages when the transistor is