Elecraft XV Transverter Owner's Manual - Manual (Page 28)

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Circuit Details
Separate schematic diagrams are provided in Appendix A for each of the
transverter RF PCBs. The location of the circuits on the schematic
diagrams is similar to the block diagram except for the control logic. The
Controller, display LEDs and associated drivers are on the front panel
PCB shown on a separate schematic diagram. The same front panel PCB
is used on all of the transverters.
Receive Circuits
The first active device in the receiver is Q3, a low noise PHEMT that
provides an exceptionally low noise figure. The current through Q3 is set
for optimum performance by potentiometer R13.
U1 further amplifies the incoming signal which is then passed on to mixer
Z1 through relay K2 and an RF band-pass filter.
The local oscillator signal is generated by crystal oscillator, Q1. In the
XV50, XV144 and XV222, the local oscillator signal is amplified by Q2,
and in the XV432 by U9. The local oscillator signal is 28 MHz below the
RF signal frequency on all bands.
In the XV50 the second harmonic of the local oscillator at 44 MHz is very
close to the 50 MHz pass band, so a low-pass filter is used between
amplifier Q2 and mixer Z1 to attenuate the 44 MHz signal.
The intermediate frequency (I.F.) output of mixer Z1 passes through
another band-pass filter that includes a diplexer formed by C57, L17 and
R25. C57 and L17 are series resonant at 28 MHz, so the I.F. signal passes
through to the band-pass filter section. C57 and L17 present a high
impedance to frequencies removed from the 28 MHz I.F. These signals
are terminated by R25.
LEDs D10 and D11 are used as limiters to protect the mixer during
transmit as described below. Inductor L9 resonates with the capacitance of
the LEDs at 28 MHz to avoid attenuating the I.F. signal.
The 28 MHz I.F. signal is routed around the transmit signal attenuator to
the I.F. port selector circuits by relays K6 and K7. Relays K8 and K9
disconnect the I.F. RX and TX/RX ports whenever the transverter is not in
use as described below. This permits several transverters to be connected
to the same external rig without loading the signal lines.
If separate transmit and receive connections are used for the 28 MHz I.F.,
the receive line is connected to J2. A jumper is placed across JP1 pins 1
and 2 and across JP2 pins 2 and 3. JP1 routes the receiver signal through
relay K9 to K6. JP2 routes the transmit signal through the attenuator.
If the external rig uses a single connection for both transmit and receive,
jumpers are placed across JP1 pins 1 and 2 and across JP2 pins 1 and 2. In
receive mode, relay K7 routes the I.F. signal to the RXout/IF2 output at
J3. In transmit mode relay K7 routes the 28 MHz I.F. signal coming in at
J3 to the attenuator.
When the external rig uses a common I.F. connection for both transmit
and receive, a cable connected to J2 will carry the transmit and receive I.F.
signals to another transverter.
Transmit Circuits
In transmit mode, the 28 MHz I.F. signal at J3 is routed through the I.F.
port selector circuits to the I.F. level control. The I.F. level control either
amplifies or attenuates the I.F. signal to the proper level for the mixer. The
I.F. level control can handle signal levels of from -20 dBm to +39 dBm
from the external rig. For low-level I.F. input levels amplifier Q6 may be
switched into the circuit by jumpers JP5 and JP6. For higher level I.F.
input levels, jumpers JP3 and JP4 permit adding or bypassing a fixed 30
dB attenuator as needed. Potentiometer R22 permits continuous
adjustment of the attenuation.
Mixer Z1 uses the local oscillator signal to produce an RF output at the
desired transmit frequency. Unwanted mixer products are attenuated by
the RF band-pass filter.
Relay K2 routes the transmit RF signal to RF driver U6, which amplifies
the signal to drive RF power module U7. A different RF power module is
required for each band. The RF power modules for the 144 MHz and 220
MHz bands require a bias adjustment to set the amplifier current at the
proper level. The bias is adjusted by R39, which is set for the proper total
current drain on the 12 volt line. The current drain is measured by
connecting a DMM to TP3 and TP4
RF from power module U7 passes through a low-pass filter and relay K1
to the antenna connector. In the XV432, an additional relay, K10, grounds
the input to the receive RF amplifier during transmit.