FIELD TUNING INSTRUCTIONS SYMMETRY FERRITE ISOLATORS DUAL STAGE - INTEGRAL LOADS COVERS MODEL NUMBERS SPxxxx-3215-11
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.
Page 3 of 14 LOCATION OF TUNING COMPONENTS Input return loss Inter-stage variable Output return loss Variable capacitor capacitors variable capacitor Cover screws (8 off) First stage isolation variable capacitor Figure 1 Second stage isolation variable capacitor Input return loss inductor Output return loss inductor First stage isolation inductor Second stage isolation inductor Figure 2
Page 4 of 14 Input return loss Inter-stage Output return loss matching circuit matching circuit matching circuit Variable capacitor Variable capacitors Variable capacitor Inductor Inductor Inductor Variable capacitor Variable capacitor Inductor Inductor Load Load First stage isolation matching circuit & integral load Second stage isolation matching circuit & integral load Figure 3
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.
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
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
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
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.
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.
Page 11 of 14 TEST INSTRUMENT SETUP INSERTION LOSS MEASUREMENT STEPS 1 & 4
Page 12 of 14 TEST INSTRUMENT SETUP INPUT PORT RETURN LOSS MEASUREMENT STEPS 2, 4 & 6
Page 13 of 14 TEST INSTRUMENT SETUP OUTPUT PORT RETURN LOSS MEASUREMENT STEPS 3, 4 & 6
Page 14 of 14 TEST INSTRUMENT SETUP REVERSE ISOLATION MEASUREMENT STEP 5