A17 MODIMIX. If A17 is suspected, remove the A4 assembly. This
removes all modulator current and provides an unrestricted path for RF. If
full unleveled RF power is achieved, refer to A4 Troubleshooting. If power
is, still bad, disconnect W25 and check the RF output directly out of the
mixer (open loop power should measure approximately -12
dBm). Before
replacing this assembly, ensure that A16 Cavity Oscillator is functioning
properly.
RF SECTION, CIRCUIT DESCRIPTION
A1
6
Cavity Oscillator
RF OUTPUT Connector
Do not attempt to test any microcircuits, at a bias feed-
through or the RF connectors, with an ohmmeter. Resistance
measurements are rarely useful, and will often destroy a
working microcircuit. Measure DC voltages at the bias feed-
throughs with a high-impedance DC voltmeter only with bias
or control connections intact.
The RF Section includes the high-frequency microcircuits, with their bias
boards, that produce the actual RF output power. These components include
A12, A14 through A17, A19, and
DCI. All other plug-in assemblies function
essentially to control these RF components. The connections between
microcircuits and other assemblies are provided on the Overall Block Diagram.
Refer to the Overall Block Diagram circuit description for a more general,
functional description.
The A16 Cavity Oscillator provides a fixed 3.8
GHz RF output at approximately
+9 dBm which is mixed down by the swept YO output, yielding the heterodyned
low-frequency output. This source is extremely stable in both frequency and
amplitude. The
+20V and -lOV lines provide power for the A16 assembly. Two
large, separately-replaceable capacitors help filter these supplies to reduce
residual FM noise.
On Standard or Option 002 instruments, the RF output is directed to a female
type N "RF OUTPUT connector at the front panel. On plug-ins with Option
004 (with or without Option
002), the output is directed to the rear panel RF
OUTPUT connector.
The following troubleshooting procedure traces power levels through the RF
path. RF measurements should be made with a high-frequency spectrum
analyzer or an RF power meter. A type-N (female)-to-SMA adapter, along with a
short, flexible RF cable terminated at both ends with
SMA male connectors, will
make troubleshooting easier.
@
A16 CAVITY OSCILLATOR. Check power supplies. Check RF output
for approximately
+9
dBm at 3.8 GHz.
RF PATH TROUBLESHOOTING
A1
4
Amplifier
NOTE
NOTE
The A14 Amplifier provides approximately 40 dB of gain from 0.01 to 2.4 GHz.
The amplifier gain drops sharply at higher frequencies, providing a lowpass
nature which rejects the unwanted mixing products.
MAXIMUM RF UNLEVELED POWER
Assembly circuit descriptions are discussed in signal flow
order.
Many RF path failure symptoms are closely related to
A4
ALC
failures. Refer to A4 Troubleshooting for additional information.
@
Refer to this symptom under A4 Troubleshooting.
Opening RF connections within the
ALC loop will cause the loop to be
unleveled, producing abnormally high power levels (up to +20
dBm) and
harmonic distortion. The ALC loop includes all connections between the A17
ModIMix and DCl Directional Detector. (Figure 8-25, within the A4
Troubleshooting section, provides a graphic definition of the loop.) If necessary,
the modulators may be externally biased using the Open Loop Procedure
described in the A4 Troubleshooting Section. If possible, avoid opening the
ALC loop to make RF measurements. In any case, it is a good idea to begin
troubleshooting just outside the ALC loop.
A1
2
YIG Oscillator
The A14Al Amplifier Bias assembly provides the various bias currents for the
A14 amplifier. It is matched and attached to the microcircuit at the factory, has
no adjustments or replaceable parts, and cannot be replaced separately as an
assembly. The
+20V and L RFON lines provide the power. When the RF is
"off," the bias is removed, shutting off the amplifier completely.
@
DCl DIRECTIONAL DETECTOR. Select a CW frequency anywhere in
the band. Verify maximum unleveled RF output power. Check INT DET
0
output to be equal to or more negative than -0.2 Vdc. (It may be necessary
to perform INT DET
0 BIAS adjustment. Refer to Section V, Adjustments.)
For more information refer to A4 Troubleshooting.
The RF Path consists of the microcircuits and their bias boards that produce the
actual front-panel RF output. These microcircuits are sealed, cannot be
repaired, and are costly to replace. Ensure that associated control circuits
(i.e.
the other printed circuit boards) are working correctly before replacing any
microcircuit components. When certain of a failure in the RF components,
isolate the problem to a single microcircuit assembly.
The A12 YIG (Yttrium-Iron-Garnet) Oscillator (YO) is the solid-state tunable
microwave source. Its output frequency ranges from 3.81 to 6.2
GHz, with
approximately
+
12 to +14 dBm of output power. The oscillator's resonant tank
circuit is basically a small YIG sphere with a resonant frequency which depends
on the surrounding magnetic field strength. The magnetic field is established by
an opposing pair of electromagnetic "main coils." Changing the current through
the coils changes the magnetic field strength, and hence the frequency of
oscillation. The sphere is lightly coupled to a bi-polar transistor, providing the
gain necessary to sustain oscillation. A FET amplifier provides the final output
power gain.
A17
MODIMIX.
Check modulator bias line MOD 0. It should be slightly
negative. If it is approximately
+4 Vdc while A4TP6 is approximately +7.5
Vdc, the modulator diode is probably open. If MOD 0 is at 0.0 Vdc, but
A4TP6 is at 4-7.5 Vdc, troubleshoot the A4 PIN Mod 0 Driver and
connections to the modulator.
A1
5
DC Return
Three RF assemblies have bias boards attached directly to the microcircuit
packages:
The A1 5 DC Return is simply a shunt RF choke. The shunt inductor allows DC
currents to flow to ground while passing on the RF power with less than 0.6 dB
of insertion loss.
Failure Symptoms
The A14 Amplifier is directly attached to its bias board. The A14A1
Amplifier Bias assembly cannot be repaired, is not separately replaceable,
and is supplied with the A14 microcircuit.
The information below should be used to help systematically troubleshoot to the
individual RF assembly. Based on the failure symptom, the components most
likely to have failed are listed, with the most probable failure cited first. Hints
for ensuring that the RF Path is actually responsible for the failure are also
given. For troubleshooting information related to a specific assembly, refer to
Microcircuit Verification By Assembly
below.
DC1 Directional Detector
HARMONIC DISTORTION
The A12A1 YO Bias assembly supplies the biasing for the oscillator and YO
amplifier. This board is matched to the YO, and cannot be separately replaced.
There is a single adjustment (R4) on the assembly, optimizing the FET gate bias
for minimum harmonics in higher frequency, multiband RF plug-ins. R4 does
not need to be adjusted in the
83522A. The bias assembly provides Zener
protection against high-voltage transients that appear across the main coils. It
also supplies current for a resistive heater that helps maintain the oscillator at a
constant temperature.
A12 YIG OSCILLATOR. If harmonics are unnacceptable, check the
spectral purity of the YO output. If harmonics are less than 14 dB below the
fundamental, replace A12.
The
DCl Directional Detector serves two purposes: 1) detects the RF power
amplitude; and 2) samples a portion of the RF energy for use in the marker
circuits. The insertion loss for the entire package is less than 3.5 dB.
The
A12A1 YO Bias assembly includes two factory select resistors matched
to the A12 YIG Oscillator. The bias board is part of the A12 assembly and
cannot be separately replaced. If a bias board component
(e.g. protection
diode or variable resistor) has been externally damaged, it is acceptable
(and economical) to replace that individual component. However, a bias
board failure often indicates a failure inside the microcircuit and may
require that the entire assembly be replaced.
A simple resistive directional bridge samples a portion of the RF energy to a
diode detector. The RF is rectified and filtered, providing a voltage, proportional
to the peak RF amplitude, which is used for leveling. A single resistor
(AlORl)
biases the detector diode through feed-through El. Feed-through E2 carries the
detected signal, but also carries a second bias current from the A4 assembly for a
second, temperature compensating diode. An internal resistor helps protect the
static sensitive diodes.
@
A14 AMPLIFIER. Check the power level into A17 ModIMix, and trace
the problem back through the RF path if it is too low. Measuring power or
spectral content directly out of A17 or A14 will open the ALC loop, causing
maximum unleveled power and high harmonic distortion even without a
failure. Refer to A4 Troubleshooting and perform the Open Loop
Procedure. This procedure externally biases the modulator to level RF
power while the
ALC loop is open.
NOTE
All references to test points, pin connections, etc., can be
located on the RF Schematic, Figure
8-66.
The dynamic response of the YO (i.e. how fast frequency changes for a fast
change in coil current) is somewhat limited, due to the inductive and magnetic
delays of the electromagnet coils and poles. Delay compensation circuits help
during a sweep, but frequency modulation is limited to low modulation
frequencies. To allow high-frequency modulation, a smaller, faster, air-core FM
coil is added to the YO. Its magnetic field adds to the main coil's field, yet
frequency changes are far quicker.
NO RF POWER
A simple resistive tap samples a portion of the RF frequency for use in the
marker generation circuits. The SAMPLED RF output is aproximately 25 dB
below the front panel output power, and ranges from 0.01 to 2.4
GHz.
SPURIOUS DISTORTION
Many microcircuits are extremely sensistive to static electric
discharges (more so when the microcircuits are removed from
their bias boards or control circuits).
A12 YIG OSCILLATOR. Check power supplies and bias levels. OSC
BIAS (TP "ON") should be at -10 Vdc, with some shaping.
TP
"G"
should
be approximately -2 Vdc. Check TP
"M" for the waveform entitled
EMITTENCOIL, Figure 8-48, within the A6 Service Sheet. This waveform
represents the current across the main coil. Check the RF output directly at
the YO for approximately +14
dBm at several frequencies.
A17
MODIMIX.
Select a CW frequency anywhere in the band and check
RF output for spurs 3.8
GHz removed from the carrier. The mixer may be
leaking the swept LO frequency (3.81
-
6.2 GHz). However, the A14
Amplifier should filter these out.
A1
9
Step Attenuator (Option
002
Only)
Before handling a microcircuit, discharge your own body by
touching the instrument chassis or microcircuit package.
Avoid touching the center conductors of the RF connectors
and bias feed-throughs at all times.
On RF plug-ins equipped with Option 002, the A19 Step Attenuator provides up
to 70 dB of attenuation in 10 dB steps. Combined with the range of the ALC
loop, this yields a leveled power range of
+
13 to -72 dBm. The Step Attenuator
consists of three fixed attenuators, with 10, 20, and 40 dB of attenuation each.
Latching relays close contacts which either insert these attenuators into the RF
path or bypass them. The control and drive
circuitry for the attenuator is located
on the
A2
Front Panel Interface assembly. The insertion loss, with
0
dB
attenuation selected, is approximately 0.5 dB.
The A17
ModulatorIMixer mixes a fixed 3.8 GHz signal with the swept 3.81 to
6.2 GHZ YO output, producing the 0.01 to 2.4
GHz RF output. The swept YO
output acts as the Local Oscillator signal for the mixer. The internal PIN diode
modulator attenuates the fixed 3.8
GHz input, providing both amplitude
leveling and pulse modulation. The mixer has a high conversion loss, and
produces approximately -20
dBm of mixed output with
+9
dBm of 3.8 GHz
input and no modulator attenuation.
POWER DROP-OUTS
A12
YIG
OSCILLATOR.
If power is present and leveled across part of
either band, but drops out entirely for the rest of the band, suspect A12.
Check OSC BIAS for approximately
-
10 Vdc. Check the RF power directly
out of the YO. If it appears to be faulty refer to Section V, Adjustments, and
perform the A7 Oscillator Bias adjustments.
A14 AMPLIFIER. Check power supplies. Check the power directly out of
A14. This will open the ALC loop. Expect to measure approximately
i-20
dBm
unleveled RF output with high harmonic distortion. If this is
undesirable, refer to A4 troubleshooting and follow the Open Loop
Procedure to externally level the RF while opening the ALC loop.
Microcircuits should be stored and transported in static-
protective packaging. Never package microcircuits with
styrofoam, cellophane (unless treated for static), or adhesive
tape.
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