Download General notes on duplexers

General notes regarding Duplexers used by Tony Horsewill
The probes used by Tony have a single coil in which sits the sample and so the coil acts
as a source during the transmit phase and then as a receiver during the receive phase.
The powers involved are in the order of a 100 watts (high voltage typically 200Vp-p)
during the transmit phase and then micro watts (very low voltage) during the receive
phase. A RF power amplifier supplies the transmit pulse and a sensitive preamplifier
boosts the receive signal to the post amplifier stages of the spectrometer however the
output of the transmit amplifier and the input of the receive amplifier which are
diametrically opposed to each with regard to power handling are effectively connected to
the coil at the same time . The purpose of the duplexer is to passively direct the high
transmit energy to the probe coil whilst protecting the sensitive input of the receive
preamplifier during the transmit phase and then direct the low energy signal to the
preamplifier during the receive phase.
Duplexer circuit description.
The transmit pulse enters the duplexer via the N Type socket and the crossed diode pairs
Dn3, Dn4 & Dn1, Dn2. The number of diode pairs depends on the noise level of the
power amp when the transmit pulse has ended and the output of the power amp is
effectively zero. Because the diodes have the inherent forward voltage drop of typically
0.6V to 1V any noise at the output of the off state power amp will not pass through the
diodes and thus will not interfere with the receive signal. Slight distortion of the transmit
pulse occurs near zero for the same reason but this is insignificant compared to the high
voltage of the transmit pulse and the resonance effect of the sample coil. The 1N4148
diodes used are small, fast and because they are back to back allow negative & positive
signal excursions to pass thus the on diode always prevents the reverse voltage of the
off diode being exceeded.
Let us assume that the parallel resonant circuit composed of Ltr and Ct has been tuned
for maximum resistance and zero phase at the Larmor frequency and that the series
tuned circuit composed of Ltr and Cr has been tuned for minimum resistance and zero
phase at the Larmor frequency.
When the high voltage transmit pulse arrives at point X the diodes Dt1 & Dt2 are turned
on ( effectively shorted out) thus completing the parallel resonant circuit Ltr and Ct and
presenting a high impedance (typically several Kohms) to the transmit pulse and
therefore the vast majority of the energy is directed to the 50ohm impedance of the
probe. The crossed diodes Dr1 & Dr2 form an effective short to ground at the input to
the preamp for any voltage large enough to turn the diodes on. Thus the input to the
preamp is typically 1V maximum which will not damage the preamp and whilst the
preamp will saturate it will rapidly recover. The preamp has thus been protected from
the high energy transmit pulse.
Similarly during the receive phase when the signal is typically a few mV max the diodes
Dt1 & Dt2 are not turned on and the signal sees Ltr in series with Cr and since the
resistance of the series tunes circuit is low (typically 2ohms) the receive signal arrives at
the 50 ohm input of the preamplifier. Because the receive signal is well below the
forward voltage drop of all the diodes they are effectively open circuit as far as the
receive signal is concerned.
Tuning the Duplexer
The availability of suitable tuneable capacitors will determine the value of Ltr. If
compression capacitors typically 100pF or 250pF max are used they can be increased in
value by adding high voltage silver mica capacitors in parallel. Once a value for Ct (Ct
will equal Cr) has been chosen then Ltr can then be wound. Typically Ltr will be approx.
0.4uH with a Ct of 100pF for resonance at 26MHz. Ltr should be wound on something
like a 6mm cylinder of plastic and wrapped in PTFE tape do not use small ferrite cores as
these may be saturated by the transmit pulse. I made the coils empirically by winding a
few turns on the former and then measuring the inductance at the required frequency
using the VIM, having obtained the value for that particular number of turns the number
can they be adjusted to the required value. The usual formula for a resonant circuit is
used to obtain approximate values:F(r)=1/2∏√LC
With the components soldered on a suitable PCB and mounted in a die cast box the
following procedure can be used to tune the system.
Disconnect Cms from point X if connected.
Short links J1 & J2
Connect VIM via BNC and set to required frequency.
Adjust Ct for maximum magnitude at zero phase on the VIM.
Remove links J1 & J2
Short link J3
Adjust Cr for minimum magnitude at zero phase on VIM.
Remove link J3
Reconnect Cms if necessary.
The diodes need to be shorted out for tuning with the VIM as the signal output by the
VIM is only in the order of 10 mVs which will obviously not turn on the diodes.
Matching
As mentioned in Notes regarding Tony Horsewill NMR Probes. ( Typically14-36MHz) the
sample coil is series resonant tuned resulting in a resonant impedance of a few ohms at
zero phase. Because this low impedance is not equal to the characteristic impedance of
the 50ohm semi rigid co ax used in the probe an impedance transformation results at
the top of the probe and the impedance as measure by the VIM at the mini UHF
connector will indicate an inductance (typically 10-200ohms @ +30 to +80 degrees
depending on the frequency) this can be used to our advantage as by adding a capacitor
in parallel Cmp and then a capacitor in series Cms the probe can be match to 50 ohms
which is the condition for maximum power transfer in the transmit and receive phases.
Please be aware that the impedance transformation is a function of wavelength so
different co ax lengths and higher or lower frequencies will give different transformations
which could be inductive or capacitive.
However with the probe length used in Tony’s probes and in the 14 -36MHz range the
transformation results in an inductive characteristic.
Again an empirical approach has been employed in the matching process. Initially I
obtained the values via calculation and Smith Charts but once an approximate value for
Cmp & Cms were obtained the matching can be achieved by alternate adjustment of
Cmp & Cms and observing the effect via the VIM and after getting a feel for how
capacitor adjustment affects the impedance then extra capacitance can be added in
parallel or smaller trimmers can be used as required until a good match of 50ohms 0
degrees or as near as possible can be achieved.
In the 14 to 36MHz range a suitable starting value for Cmp & Cms would be 200pF. If
adjustment increases the value of the trimmer and the match moves closer to 50ohms @
0 degrees but cannot be reached because the trimmer is at maximum then place a fixed
value silver mica in parallel and readjust the trimmer. If the match approaches 50ohms
but the trimmer cannot be reduced any more then replace the trimmer with a smaller
value.
NB if compression trimmers are used and a trimmer capacitance has been reduced by
excessive opening of the mica leaves the trimmer should be replaced by a smaller value
as mica leaves of a well opened trimmer will tend to move (flap around) and this will
affect the value and the match.
The matching capacitors interact with each other so adjusting one may move towards a
better magnitude (50ohms) but the phase may move away from zero or adjusting one
may give a better phase (0 degrees) but a worse magnitude. So an intuitive walking in
method should be employed to achieve a good match ie one capacitor may have to be
adjusted to move the magnitude past 50ohms and 0 degrees so that the other capacitor
can then be adjusted to bring both magnitude and phase back towards 50ohms @
0degrees.