Duplexer Tuning using the Amtronix SW2012N Return Loss Bridge

 The Amtronix SW2012N Return Loss Bridge can be a valuable aid in duplexer tuning. The pass section of a duplexer will be properly tuned when there is maximum return loss (minimum SWR) from the transmitter to the antenna and from the antenna to the receiver. Manufacturers recommend NOT to use RF power to tune a duplexer. RF Power applied to an out of tune duplexer could cause a rapid increase in temperature and damage the unit.
    There are several different types of duplexers including "reject", "pass" and "pass / reject". A reject duplexer has a receive section (2 cavaties) that are tuned to reject the transmit frequency and a transmit section (2 cavaties) tuned to reject the receive frequency.  The "Pass" duplexer does the opposite tuning the transmit section to pass the transmit frequency and the receive section to pass the receive frequency. A higher quality model is the "Pass / Reject". This duplexer has both pass and reject properties in both sections.
    The Pass / Reject Duplexer can be tuned by using a return loss bridge to tune the Rx and Tx sections for maximum return loss at their pass frequencies. To tune this duplexer, connect the Tx port to the return loss bridge DUT (device under test) Port. Connect a 50 ohm load to the antenna port. Adjust the duplexer main tuning plunger for maximum RL (return loss).  TX RX Systems indicates the pass band will be 15 dB or more. The rejection notch is tuned after the pass notch. The rejection notch is not tuned wit the return loss bridge but with a tracking generator or similar means to measure maximum rejection of the unwanted frequency.  TX RX Systems has excellent instructions for tuning their 6.625" duplexer. **The HP 8920A/B can be used as the Spectrum Analyzer / Tracking Generator and the Amtronix SW2012N as the return loss bridge. There is a section in the Agilent / HP 8920B Applications Manual on duplexer tuning:   Agilent / HP 8920B Applications Manual


 The following is the instructions for tuning the TX RX Systems 6.625 Pass Reject duplexer:

Due to the sensitivity of the adjustments, it is strongly recommended that the proper equipment

be used when tuning the individual filters, otherwise the filter should be sent to the factory or an

authorized representative for retuning. The following equipment or its equivalent is recommended in order to properly perform the tuning adjustments for the Vari-Notch duplexer.

 

1. IFR-2975 spectrum analyzer with optional tracking generator installed. **(HP 8920A/B, 8924C, E8285A, 8935 can be substituted for Aeroflex -IFR 2975)

2. 5/32" hex wrench.

3. Double shielded coaxial cable test leads (RG214).

4. 50 ohm load with at least -35 dB return loss (1.10:1 VSWR).

5. Female union (UG29-N or UG914-BNC).

6. Return Loss Bridge (Eagle model# RLB150N3A). ** (Amtronix SW2012N can be substituted for RLB)

7. Insulated tuning tool (TX RX Systems Inc. part# 95-00-01).

 

 

Figure 1: Block diagram of a typical four-cavity Vari-Notch Duplexer (6.625" diameter cavities)

Tuning Procedure

Tuning of the filter requires adjustment of the pass band and the rejection notch. The pass band

is adjusted while observing the return loss response and the rejection notch is adjusted by

monitoring the output of a tracking generator after it passes through the filter. All Vari-Notch filters should be temporarily removed from the system and tuned on the bench using test instrumentation only. Do not adjust the filters while they are under transmitting power. To insure

proper tuning of the 6.625" Vari-Notch filter, all adjustments should be performed in the following

order:

1. Rough tune the pass band.

2. Rough tune the rejection notch.

3. Final tune the pass band.

4. Final tune the rejection notch, always the last adjustment made.

 

WARNING - Tuning while under transmit power can result in damage to the duplexer.

PASS BAND

The peak of the pass band will correspond very closely to the point of minimum reflected energy

from the filter and maximum forward power through it. A transmitter connected to the filter will

operate best when the reflected energy is lowest, therefore the return loss response will be used to set the pass band. The pass band can be checked and adjusted using the following procedure.

 

Checking the pass band

1. A zero reference for return loss must be established at the IFR-2975 prior to checking the

    pass band frequency, this is done by connecting the return loss bridge to the

    analyzer / generator as shown in figure 3.

 

 

Figure 3: Setting the return loss reference

 

2. Set-up the analyzer / generator for the desired frequency (center of display) and for a vertical
    scale of 10 dB/div.

 

3. Do not connect the return loss bridge (RLB) to the cavity, leave the "load" port on the bridge

    open. This will supply the maximum amount of reflected energy to the analyzer input.

 

4. Insure that the IFR-2975 menus are set as follows:

 

    DISPLAY - line

    MODE - live

    FILTER - none

    SETUP - 50 ohm/dBm/gen1

 

5. The flat line across the screen is the return loss curve. Select the "MODE" main menu item and
     then choose the "STORE " command.

 

6. Next select the "DISPLAY" main menu item and choose the "REFERENCE" command.

   This will cause the stored value to be displayed at the center of the screen as the 0 dBm  

    reference value.

 

7. Connect the "DUT" port on the SW2012N RLB to the input of the loop plate, make sure the output of the

    loop plate is connected to a 50 ohm load, refer to figure 4.

 

 

 

Figure 4: Checking the pass band

  

The display will now present the return loss curve for the 6.625" Vari-Notch filter being

measured.

 

The pass band is that frequency range over which the return loss is 15 dB or greater.

Adjusting the pass band

 

Set the tuning rod at its mid-point. Adjust the pass band by setting the peak

(maximum negative value) of the return loss curve at the desired pass band frequency

(should be the center-vertical graticule line on the IFR-2975's display).

 

Refer to figure 4.

The resonant frequency is adjusted by using the coarse tuning rod, which is a sliding adjustment

(invar rod) that rapidly tunes the response curve across the frequency range of the filter. Resonant frequency is increased by pulling the rod out of the cavity and is decreased by pushing the rod into the cavity.

 

Cavity Tuning Tip

When tuning a cavity that has been in service for some time it is not unusual to find the main tuning rod hard to move in or out. This occurs because TX RX Systems Inc. uses construction techniques borrowed from microwave technology that provide large area contact surfaces on our tuning probes. These silver plated surfaces will actually form pressure welds which maintain excellent conductivity. The pressure weld develops over time and must be broken in order for the tuning rod to move. This is easily accomplished by gently tapping the tuning rod with a plastic screwdriver handle or small hammer so it moves into the cavity. The pressure weld will be broken

with no damage to the cavity. Once the desired response is obtained using the coarse and fine tuning rods, they are "locked" into place. The coarse rod is secured by tightening the 10-32 cap screw and the fine tuning rod is held in place by tightening the knurled thumb nut.

 

Failure to lock the tuning rods will cause a loss of temperature compensation and detuning of the cavity.

 

REJECTION NOTCH

 

The rejection notch will track with the tuning of the pass band and therefore should be the last adjustment made to the 6.625" Vari-Notch filter. The rejection notch is adjusted by changing the amount of capacitance in the loop assembly. The capacitor is variable and is either an air-plate or tubular piston type depending upon the frequency range of the filter. The air-plate type has a red mark painted on the access barrel and one-half of the adjusting screw, when the red marks line up the maximum capacitance is achieved. On UHF models (400 MHz and over) the capacitor access barrel is omitted and a 10-32 inch screw must then be removed from the loop plate assembly to gain access to the piston trimmer under the plate.        

Checking the rejection notch

 

1. The rejection notch is checked by connecting the tracking generator to the input of the cavity

    filter while the spectrum analyzer is connected to the output, as illustrated in figure 5.

 

Figure 5: Checking the rejection notch

 

2. Insure that the IFR-2975 menus are set as follows:

 

    DISPLAY - line

    MODE - live

    FILTER – none

    SETUP - 50 ohm/dBm/gen1.

 

Adjusting the rejection notch

 

The notch is adjusted by turning the variable capacitor. Because of the filters sensitivity to tool

contact, an insulated tuning tool must be used to make the adjustment.

 

FINE TUNING THE CHANNELS

 

Once all of the individual filters have been tuned, each of the channels as a whole must be fine

tuned. First the pass band for both channels and then the rejection notches. The following is a step by step procedure for fine tuning the channels and completes the re-tuning of the duplexer.

 

1. Reassemble the duplexer by reinstalling the cavities and interconnect cables in their original

    locations.

 

2. The pass band for the channels are fine tuned first, in a manner very similar to tuning a single

    cavity.

3. A zero reference for return loss must be established at the IFR-2975. Connect the RLB to the

    analyzer / generator as shown in figure 3.

 

4. Set-up the analyzer / generator to the desired frequency (center of display) and for a vertical

    scale of 10 dB/div.

 

5. Do not connect the RLB to the duplexer at this time; leave the "load" port on the bridge open.

    This will supply the maximum amount of reflected energy to the analyzer input.

 

6. Insure that the IFR-2975 menus are set as follows:

 

    DISPLAY - line

    MODE - live

    FILTER - none

    SETUP - 50 ohm/dBm/gen1

 

7. The flat line across the screen is the return loss curve. Select the "MODE" main menu item and

     then choose the "STORE " command.

 

8. Next select the "DISPLAY" main menu item and choose the "REFERENCE" command.

    This will cause the stored value to be displayed at the center of the screen as the 0 dB

    reference value.

 

9. Connect the "load" port on the RLB to the equipment port of the channel to be fine tuned.

    Terminate the duplexer’s antenna connector with the 50 ohm load. The equipment port of

     the remaining duplexer channel is left disconnected, refer to figure 6.

 

Figure 6: Equipment setup for fine tuning the pass band of each channel

10. The display will now present the combined return loss curve for all of the cavities in the

      channel. The channels pass band is that frequency range over which the return loss is

     15 dB or greater.

 

11. Fine tune the pass band for the entire channel (for maximum return loss) by gently adjusting

      the positions of the fine tuning rods (coarse rods if needed) moving between cavities as

      required. Once the desired response is obtained "lock" the tuning rods into place by

      tightening the 1/4" shaft lock nuts and the knurled thumb nuts on each filter.

 

12. Move the cable from the RLB's "DUT" port to the equipment port of the other channel. This

      will allow the remaining duplexer channel to be fine tuned. Reset the analyzer / generator

      center frequency. Repeat steps 10 and 11.

 

13. The rejection notch for each of the channels must be fine tuned next.

 

14. Terminate the antenna connector with a 50 ohm load. Connect the output of the tracking

      generator to the equipment port of one of the duplexer channels and the spectrum analyzer

      input to the equipment port of the remaining channel as shown in figure 7.

 

Figure 7: Equipment setup for fine tuning the rejection notch of each channel

15. Set-up the analyzer / generator to sweep across the rejection notch frequency of the

      channel being tuned. The center of the display should be set to the desired center frequency

      of the rejection notch being adjusted. Set the vertical scale of the analyzer / generator to

      10 dB/div. Keep in mind that the high frequency channel has its rejection notch set to reject

      the low frequency signal and vice-versa for the rejection notch of the low frequency channel.

16. Insure that the IFR-2975 menus are set as follows:

 

      DISPLAY - line

      MODE - live

      FILTER - none

      SETUP - 50 ohm/dBm/gen1

 

17. Set the analyzers attenuation control so that the 0 dBm level is at the top of the display.

      The display will now show most of the rejection notch. Using the analyzer's attenuation

      control adjust the amount of attenuation so that the "peak" or lowest value on the rejection

      notch is displayed.

 

18. The cavities rejection notches are adjusted (for maximum rejection) by gently turning the

      variable capacitors in the loop plate assemblies. Move between filters as needed.

      Because of the filters sensitivity to tool contact, an insulated tuning tool must be used to

      make the adjustment.

 

19. Adjust the rejection notch of the remaining cavities by changing the sweep frequency of

      the analyzer / generator to match the new rejection notch frequency. The equipment stays

      connected as it is.

 

20. Repeat step 17 and 18 for the remaining channel.

     (cables and equipment stay connected where they are).

 

21. With the tuning completed, reconnect the equipment cables and antenna feed line.

     Test the system for proper operation.