field tuning instructions

FIELD TUNING INSTRUCTIONS
SYMMETRY FERRITE ISOLATORS
DUAL STAGE - INTEGRAL LOADS
COVERS MODEL NUMBERS – SPxxxx-3215-11
Symmetry dual stage isolator - field tuning instructions
Page 2 of 14
GENERAL
Figure 1 shows the location of the individual variable capacitors that are accessible through holes in
the top cover of the isolator. Capacitor adjustment alone will allow minor tuning adjustments to be
made, however more precise tuning will require adjustment of the tuning inductors in the matching
circuits. Access to these inductors (coils) is gained by removing the top cover, which is secured by the
8 cover screws. The isolator with the top cover removed is shown in figure 2.
Figure 3 shows the internal layout of a UHF isolator and in particular the circuit board on which the
matching circuits are arranged. Other frequency bands have a similar layout. This is shown for
information only, since tuning is not possible with the top gold zinc plated cover (magnetic shield plate)
removed since this forms part of the magnetic circuit and hence affects the tuning.
Always ensure that both the input and output ports are terminated into 50 ohm loads during all tuning
steps. A network analyser fitted with S-parameter measurement facilities is highly recommended for
all tuning, or if unavailable, a spectrum analyser with tracking generator and return loss bridge.
Tuning needs to be carried out in the sequence described in these instructions, with the network
analyser set to the following spans corresponding to the isolator frequency bands:
Frequency band
Span
66 -100 MHz
20 MHz
100 – 700 MHz
40 MHz
700 – 1000 MHz
50 MHz
The wide span width facilitates tuning the return loss and in particular the isolation responses, since
the positions of the relevant notches are more easily seen from which the appropriate adjustment may
be deduced. The span width may be reduced for finer adjustment of tuning once the isolator is
satisfactorily tuned across the recommended spans.
The following tuning procedure also assumes that the isolator has already been factory tuned to a
frequency within its sub-band limits (refer to product literature). The procedure is only relevant for
retunes to frequencies that lie within the sub-band limits.
Please note that these tuning instructions do not apply to earlier versions of VHF isolators (108-174
MHz), identified by PCB model number T-PCB-DV version 2. Please contact RF Industries for
guidance in retuning these models.
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 3 of 14
LOCATION OF TUNING COMPONENTS
Input return loss
Variable capacitor
Inter-stage variable
Output return loss
capacitors
variable capacitor
Cover screws
(8 off)
First stage
Second stage
isolation variable
isolation variable
capacitor
capacitor
Figure 1
Input return loss
Output return loss
inductor
inductor
First stage
Second stage
isolation
isolation
inductor
inductor
Figure 2
Doc Ref: INS-38293-1
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Symmetry dual stage isolator - field tuning instructions
Page 4 of 14
Input return loss
Inter-stage
matching circuit
matching circuit
matching circuit
Variable capacitor
Variable capacitors
Variable capacitor
Inductor
Inductor
Variable capacitor
Output return loss
Inductor
Variable capacitor
Inductor
Inductor
Load
Load
First stage isolation
Second stage isolation
matching circuit & integral load
matching circuit & integral load
Figure 3
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 5 of 14
TUNING PROCEDURE
It is assumed that a network analyser is being used to re-tune the isolator. Set the span on the
analyser to the appropriate span width (defined in the previous section), and set a marker to the new
desired frequency. All of the following adjustments refer to this frequency. Remove the isolator’s top
cover by removing the 8 securing screws.
STEP 1: ADJUSTING INITIAL INSERTION LOSS
In all subsequent tuning steps, note that clockwise rotation of the variable capacitors shifts the
response peaks lower in frequency, and vice versa.
Adjust the input return loss variable capacitor for minimum insertion loss through the isolator. Repeat
this with the two inter-stage variable capacitors, and the output return loss variable capacitor.
Monitor the reverse isolation and adjust the first and second stage isolation variable capacitors to give
the best approximate isolation at the desired frequency. This is just a coarse adjustment at this stage.
Repeat the above steps a second time to eliminate the effects of any interaction between the
individual adjustments.
The inter-stage variable capacitors should be further adjusted to give as close to a symmetrical
insertion loss response as possible, consistent with their minimum insertion loss setting. There is no
access to the inter-stage inductor as this will not require adjustment.
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 6 of 14
STEP 2: ADJUSTING INPUT PORT RETURN LOSS
Monitor the input port return loss.
Adjust the input return loss variable capacitor until the return loss peak is centred at the required
frequency. Note that clockwise rotation moves the peak lower in frequency and vice-versa.
Adjust the input return loss inductor (using an insulated probe) by moving the coils either closer
together or further apart, until the maximum peak in the return loss response occurs at the desired
frequency. The variable capacitor will also need to be readjusted slightly as the inductor is adjusted.
Note that a higher inductance (coils closer together) will shift the position of the return loss peak lower
in frequency, and vice-versa.
A typical UHF input return loss response is shown in figure 4.
Figure 4
Doc Ref: INS-38293-1
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Symmetry dual stage isolator - field tuning instructions
Page 7 of 14
STEP 3: ADJUSTING OUTPUT PORT RETURN LOSS
Monitor the output port return loss.
Adjust the output return loss variable capacitor until the return loss peak is centred at the required
frequency. Note that clockwise rotation moves the peak lower in frequency and vice-versa.
Adjust the output return loss inductor (using an insulated probe) by moving the coils either closer
together or further apart, until the maximum peak in the return loss response occurs at the desired
frequency. The variable capacitor will also need to be readjusted slightly as the inductor is adjusted.
Note that a higher inductance (coils closer together) will shift the position of the return loss peak lower
in frequency, and vice-versa.
A typical UHF output return loss response is shown in figure 5.
Figure 5
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Symmetry dual stage isolator - field tuning instructions
Page 8 of 14
STEP 4: FINAL INSERTION LOSS ADJUSTMENT
Re-adjust the inter-stage variable capacitors to minimise the isolator’s insertion loss.
Check and re-adjust (if necessary) the input and output return losses as per the preceding
procedures.
The insertion loss should now be at a minimum. A typical UHF insertion loss response is shown in
figure 6.
Figure 6
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 9 of 14
STEP 5: ADJUSTING REVERSE ISOLATION
Monitor the reverse isolation through the isolator.
Adjust the first stage isolation variable capacitor until the isolation peak is centred at the required
frequency. Note that clockwise rotation moves the peak lower in frequency, and vice-versa.
Adjust the first stage isolation inductor (using an insulated probe) by moving the coils either closer
together or further apart, until the maximum peak in the isolation response occurs at the desired
frequency. The variable capacitor will also need to be readjusted slightly.
Note that a higher inductance (coils closer together) will shift the position of the isolation peak lower in
frequency, and vice-versa.
Adjust the second stage isolation variable capacitor and inductor in a similar manner.
Repeat this process until the isolation response has reached its maximum peak at the required
frequency. This iteration is necessary since the adjustments do interact slightly.
Once the isolation peaks coincide at the desired frequency, re-adjust the first and second stage
isolation inductors only very slightly in opposite directions (that is, increase the inductance on one
coil and decrease the inductance on the other coil), whilst also slightly adjusting the first and second
stage isolation variable capacitors so as to spread the two isolation peaks slightly apart whilst still
maintaining 70dB minimum isolation at the tuned frequency. This gives a broader isolation response
curve. A typical UHF isolation response curve is shown in figure 7 on the following page.
Doc Ref: INS-38293-1
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Symmetry dual stage isolator - field tuning instructions
Page 10 of 14
Figure 7
STEP 6: FINAL RETURN LOSS ADJUSTMENT
Adjustment of the reverse isolation matching circuits may slightly affect the input and output return
loss responses.
Check and re-adjust (if necessary) the input and output return losses as per the preceding
procedures. Only a minor adjustment should be required at this stage.
The insertion loss and isolation responses should not be affected by these adjustments.
THIS COMPLETES THE RE-TUNING PROCEDURE.
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 11 of 14
TEST INSTRUMENT SETUP
INSERTION LOSS MEASUREMENT – STEPS 1 & 4
Doc Ref: INS-38293-1
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Symmetry dual stage isolator - field tuning instructions
Page 12 of 14
TEST INSTRUMENT SETUP
INPUT PORT RETURN LOSS MEASUREMENT – STEPS 2, 4 & 6
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 13 of 14
TEST INSTRUMENT SETUP
OUTPUT PORT RETURN LOSS MEASUREMENT – STEPS 3, 4 & 6
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004
Symmetry dual stage isolator - field tuning instructions
Page 14 of 14
TEST INSTRUMENT SETUP
REVERSE ISOLATION MEASUREMENT – STEP 5
Doc Ref: INS-38293-1
 RF Industries Pty Limited 2004