RF Technology Eclipse Series Operation And Maintenance Manual page 25

5 CIRCUIT DESCRIPTION
current) if the oscillation level should increase, and similarly the gate bias will reduce if
the –ve peaks of the oscillation should reduce.
The Modulation bias is also adjusted by the modulation input (2PORT_MOD). This
signal is amplified by a VCA. The BALANCE DAC output is converted to a current by
U707 and Q700, and that then is used to set the gain of the VCA. The output of the
VCA is then attenuated by R725/R729, and this bias is then applied to varactor D701.
Note that the modulation bias is in anti-phase to the PLL bias.
Each VCO has its own gyrator feed circuit (Q707 and Q702). This is done to remove
any possible noise on the voltage rails from effectively modulating either VCO.
The drain current of each VCO can be switched off or on by MOSFETs Q701 and
Q703. Q701 is switched on when MOD_VCO_EN is high. MOD_VCO_EN is the user
programmable digital output from DAC U601 (see Sheet 6). Q703 is switched on when
the CPU output CHAN_VCO_EN (see Sheet 2) is high.
The output of each VCO is "sniffed", by a high impedance attenuator. In the
modulation VCO, R737 in series with the 50 ohm input impedance of U702 forms this
attenuator. In the Channel VCO, R745, R736, and the input impedance of U706 form
one such attenuator, and R741 in series with the input impedance of U703 forms the
other.
U702 amplifies the modulation VCO signal, which is then amplified again by U701,
then filtered to reduce harmonics, before arriving at the LO input of MX700 at a level of
around +5 - +7dBm.
The output of U702 is also attenuated by R733, and then re-amplified before becoming
MOD_VCO_OUT. MOD_VCO_OUT is then passed to the Modulation PLL (U602,
see Sheet 6). U703's primary role is to ensure that noise that becomes coupled by the
PLL chip, back onto its VCO input, does not couple back into the path to the mixer. If
this isn't done, then the mixer's LO input contains many harmonics of the reference
oscillator.
U703 performs both an amplification role, and an isolation role similar to U703's. Its
output (CHAN_VCO__OUT) is fed back to the Channel PLL (U604, see Sheet 6).
U706 and U701 are both designed to be in "gain compression", i.e. their outputs are
saturating, so that significant changes in their input levels will not significantly change
their output levels. U706, though, starts its gain compression at about 10dB lower than
U701. In this way, variations in output level caused by variations in the parameters of
the JFETS Q704, and Q705, will not significantly modify the levels of signals arriving
at the mixer MX700.
The output of U706 is attenuated by the network R714, R742, and R724, and then it is
filtered to reduce the VCO harmonics as much as possible. This filtered VCO output is
then brought to the RF pin of the mixer at a level of about –20 to –22dBm.
The output of the mixer is then run through a low pass filter to remove frequencies other
than (Fmod - Fvco). U700 then amplifies this signal to a level of about –6 to –8dBm.
RF Technology T50 Page 25