NMR Spectrometer (Versions 1 & 2 ) for 3H Labs
The circuitry for versions 1 & 2 ( usually Ver1 left hand side & Ver2 right hand side
of 3H lab) are basically the same however the following differences exist between
version 1 & 2:Version 1
Does not incorporate a phase shifter to vary the phase of the RF transmit
pulse.
The RF attenuator used to control the amplitude of the transmit pulse is not
incorporated on the main PCB but is a commercial self contained unit mounted on the
outside of the die cast case containing the main PCB.
The variable voltage supply for adjusting the attenuation is
mounted externally to the main PCB die cast case.
The transmit conditioning circuit components and the receiver conditioning
circuit components are incorporated onto the same PCB.
Input attenuation is not incorporated on the PCB but applied externally using
inline BNC attenuators.
Does not have low pass 4.7MHz filter on output.
Version 2
Phase shifter and associated control voltage circuitry to vary the phase of the
RF transmit pulse incorporated on transmit PCB.
The RF attenuator and associated control voltage circuitry used to control the
amplitude of the transmit pulse is incorporated on the transmit PCB.
The transmit conditioning circuit components and the receiver conditioning
circuit components are contained on separate PCBs and die cast cases.
Input attenuation incorporated on the transmit & receive conditioning PCBs.
Does have low pass 4.7MHz filter on output.
The following applies to both versions 1 & 2. (unless otherwise indicated)
Front panel BNCs
TRANSMIT IN (V1)
TRANS IN
(V2)
Transmit pulse RF input from Pulse Blaster
TRANSMIT MONITOR (V1)
TRANS MON
(V2)
Output enabling monitoring of transmit pulse.
The effect of the TRANSMIT POWER control
will be observed at this output but not the effect
of the PHASE control (V2 only)
TRANSMIT OUT (V1)
TRANS OUT
(V2)
Output to power amp input.
Q REF IN (V1)
Q IN
(V2)
Q input from Pulse Blaster
I REF IN (V1)
I IN
(V2)
I input from Pulse Blaster
RECEIVER IN (V1)
SIG IN
(V2)
Signal input from probe head
The above inputs and outputs are standard 50 ohm terminations thus if a ‘scope is
used with the TRANSMIT/TRANS MON output the ‘scope should be terminated in
50 ohms. If the standard input impedance of the ‘scope (1Mohm) is used the
monitored output will be twice that actually fed to the TRANSMIT/TRANS OUT bnc
Q OUT
Quadrature output
I OUT
In phase output
The Q OUT & I OUT are intended to drive loads of 1Mohm or greater i.e. normal
scope input the 0.1uF DC blocking capacitor on the output of the AD829s in version 1
and on the output of the 4.7 MHz filter on version 2 create a high pass filter with the
load resistance thus 1Mohm or greater is required.
Front Panel Controls
TRANSMIT POWER
Multiturn control used to set the transmit
pulse amplitude and hence the power of
of the transmit pulse. NB The dial is not
calibrated and the numbers are for user
reference only. Full power will occur
before the dial reads maximum.
PHASE (V2)
Multiturn control used to set the phase
of the transmit pulse from 0 to approx 200
degrees. NB The dial is not
calibrated and the numbers are for user
reference only. The control is non linear
particularly above a dial reading of approx 70.
+180 (V2)
In the down position this switch introduces an
additional 180 degrees of phase shift to the
PHASE control
RECEIVER GAIN
In the up position the receiver stage gain is
approx 50. In the down position (HIGH) the
receiver stage gain is approx 250
NB the higher gain setting increases the dead
time of the receiver by 10uS.
IQ GAIN
In the up position the IQ stage gain is approx
20. In the down HIGH position the IQ stage gain
is approx 100.
PREAMP GAIN(V2)
This multiturn control which is housed in a
separate die cast box can be used to alter the
probe head preamp gain.
RF power amp.
Approximately 50mV rms (140mV p-p / -13dbm) is required at the INPUT of the RF
POWER AMP in order to produce 100W in to 50ohms at the OUTPUT of the RF
POWER AMP.
SAFETY WARNING
BE AWARE that at this level of RF power, voltages of 70V rms ( 200V p-p) are
present at the output of the RF POWER AMP. THIS CAN RESULT IN
PERSONAL INJURY. The output of the RF POWER AMPLFIER must always
be connected via a suitable coaxial cable to a properly terminated (50 ohm) and
enclosed load.
Circuit description.
Transmit conditioning circuit version 1
RF input via the TRANSMIT IN bnc (CON3) is buffered by U2 the output of which is
applied to the TRANSMIT OUT bnc (CON2). U2 has a gain of +2 via R7 & R6 this
gain thus compensates for the divider action of the back termination provided by R5
before the transmit pulse is output to the RF POWER AMP.
Similarly U1 buffers the rf pulse before feeding the pulse to the TRANSMIT MON
bnc (CON1)
Receiver conditioning circuit version1
Cross diodes D1 & D2 protect the input of AMP1 from being overdriven. AMP1 is a
MAR 8ASM supplied by Mini Circuits and has a gain of approx 30 db(+30). Further
gain is supplied by U6 (+2) or U7 (+10) depending on the position of the RECEIVER
GAIN HIGH (G_SELECT) switch this switch inhibits either U6 or U7 by grounding
pin 8 of either U6 or U7. U8 buffers the selected output before feeding the signal to
the power splitter DIV1 .
The 3 db loss at the outputs of DIV1 is compensated for via U13 and U12 which then
drive the RF inputs of the mixers MIX1 & MIX2 .
The LO input of MIX1 is driven with the Q REF which is buffered via U10. Similarly
the I REF is buffered by U11before driving the LO input of MIX2.
U5 & U9 buffer the Q & I signal coming from the IF output of MIX1 & MIX2 .The
gain of these op amps is selected using the IQ GAIN switch (IQG_SELECT) this
switch energises an rf relay which shorts out resistors R20 & R21 in the feedback
loop of U5 & R27 & R28 in the feedback loop of U9 thus increasing the gain of the
op amps. When the IQ GAIN is in the up position the resistors are not shorted and the
gain is 20. When IQ GAIN is in the down HIGH position the resistors are shorted and
the gain is 100.
RF Power Amp
The RF power amp consists of a first stage 20W amplifier the output of which is used
to drive the second stage 100W amplifier. Approximately 5W is required at the input
of the second stage to drive the amplifier to full power output into 50 ohm. This
corresponds to 50mV rms (-13dbm) drive to the first stage input.
Switch mode power supplies provide the 28V for the 20W amp and 12V for the
140W amp.
Linear power supplies provide the +/-5V and +/-15V for the receiver and transmitter
signal conditioning circuits.