Re-tuning Duplexers for Bandpass Filter Mode

Introduction

One of the obstacles to combating radio frequency interference (RFI) presented to the amateur radio astronomer is sourcing filters suitable for the frequency of observation. Getting custom-made filters is an expensive solution and besides, quite often some frequency agility is required to adapt to changing RFI environments.

A case in point is the 430 MHz Vela pulsar observations carried out here at HawkRAO.

While 430 MHz as an observation frequency compared to higher frequencies has definite benefits for observing Vela (e.g., higher flux density, availability of ready-made amateur radio antennas) the level of RFI in this band is high - not only from amateur transmissions but also various LIPD transmissions. Due to the location of HawkRAO being close to a city of some 5 million inhabitants the level of RFI is quite high - and is increasing day-by-day.

Such increasing levels of RFI has required the observation frequency to be moved around inside the operating bandwidth of the antennas to find the current quietest spot - there is no quiet spot. It's become a game of 'reverse whacka-mole' - where the observation frequency is the 'mole' popping up at different places and the RFI being the 'whacker'. Moving the RFI filters to the same frequency is required.

Note: the filters described here are only suitable for placement in the receiving chain after the point where the system temperature has been established as they exhibit significant loss. That is, they need sufficient gain ahead of them to prevent such loss raising the system temperature significantly.

Configuration

In order to arrive at the required filtering characteristics a number of filters can be cascaded such that the overall passband shape is the product of each filter's bandpass. A typical arrangement might be to place a wide bandpass filter at the head of the filter chain to remove signals far removed from the desired observation bandwidth. These can then be followed by narrower bandwidth filters to arrive at the overall bandpass characteristic. As can be seen later this approach can be quite useful.

Test Gear

Usually I use a broadband noise source and a spectrum analyser to align filters - but as a spectrum analyser is not a cheap instrument to buy, the following has been done by using an example of the inexpensive VNAs now readily available.

SAW RFI Filters

There are fairly inexpensive SAW filters available for 430 MHz - and these have been employed at HawkRAO. These are nominally 430 - 440 MHz bandwidth - but in practice are much wider. An example 3 dB passband is shown below as extending from ~424.5 MHz to ~447.5 MHz = ~23 MHz.

Cascading a series of these helps by making the out-of-band attenuation higher (but also increasing in-band loss). The response of three SAW filters in series is shown below. The extra attenuation comes not only from the SAW filter losses, but also from attenuator pads between filters to reduce interaction. The passband on the right below is the response without attenuator pads which shows a higher level of ripple across the passband.

Cavity Filters

Online can be found some reasonably inexpensive 'cavity filters'. The author has come across basically two types...

Standard bandpass filters - identified by only having 2 connectors: input and output
Duplexers - identified by having 3 connectors (marked 'LOW', 'ANT' and 'HIGH').

The standard bandpass filters are straight-forward to align as they are designed to be bandpass filters. The skirts of these filters are not especially steep - but are nonetheless useful as will be shown later.

By contrast the duplexers have been designed to work with repeaters - where simultaneous reception and transmission is done using the same antenna. The filters described here have a passband and notch characteristic. These notches are produced by bypassing the cavities by an inductance - in this case a length of coaxial line as can be seen in the image below. The notch is sharp and so can be used to achieve a sharper roll-off in the passband skirts. Re-tuning them as bandpass filters involves configuring and tuning for the desired bandpass response .

Note: there are various flavours of duplexers - some are bandpass, some are pass and notch, and some are a combination of these. The duplexers described here are 'pass and notch'.

Normal Duplexer Operation

The type of duplexer described here is quoted to operate between 380 - 520 MHz (so 430 MHz is conveniently central to this range). Its purpose is to isolate a receiver and transmitter operating close in frequency. In Australia the normal 'split' between receive and transmit frequencies is 5 MHz in the 430 MHz amateur band - with the repeater transmit frequency output being the higher. The minimum separation quoted for the duplexer is ~ 3.5 MHz - so this would be useful for that application. In the case of re-purposing the duplexer for bandpass operation the 3.5 MHz specification gives an indication of the minimum bandpass width.

There are 3 connectors labelled 'LOW', 'ANT' and 'HIGH'. When viewed through the 'ANT' port (antenna) the 'HIGH' port displays a high-pass characteristic (with a notch at the roll-off), and the 'LOW' port displays a low-pass characteristic (with a notch at the roll-off). The 'HIGH' port passes the transmit frequency while its notch sits on the receive frequency. The 'HIGH' is the reverse - passing the receive frequency while having a notch at the transmit frequency.

The supplier of the duplexers described here assume the unit is destined to be used for repeater use and so need to be supplied with HIGH and LOW frequencies so they can tune them. The duplexers are not used for this purpose - so arbitrary frequencies in the 430 MHz band were chosen; whilst ensuring the spacing was about 5 MHz to satisfy the 3.5 MHz spacing minimum.

Bandpass Operation

To press the duplexer into bandpass mode the connection is simply having the 'HIGH' and 'LOW' as the input and output ports instead of having 'HIGH', 'LOW' and 'ANT' port pairs, As the duplexer is a passive device either port can be input or output - although the return loss might be different for each port. The 'ANT' port is unused - although slightly better passband shape has been observed if the 'ANT' port is terminated by a 50 ohm load - and in principle it's best to not have the centre conductor of the 'ANT' floating in the air - but shielded with a load connector.

The default transfer function between the 'LOW' and 'HIGH' ports for a duplexer tuned for repeater operation is shown on the right. It can be seen that because the 'LOW' notch is higher in frequency than the 'HIGH' notch the transfer function between those two ports' notches is a bandpass shape. The notches are ~6.5 MHz apart and ~70 dB down producing a steep skirt slope.

Of course the attenuation drops again below the lower frequency notch and above the higher frequency notch and so the passband at this stage is not ideal. This is where the wider 'normal' bandpass filters come into play.

Improving the Bandpass Shape

By placing the above-mentioned SAW bandpass filters in series with the duplexer the 'batwings' in the response can be reduced as shown on the right - but this still far from ideal.

To get rid of the 'batwings' more effectively a filter with a narrower passband is required.

As mentioned previously the 'normal' cavity bandpass filters don't provide the required steep skirt roll-off. However when placed in series with the sharp roll-off duplexer the marriage produces quite a nice bandpass characteristic.

'Normal' Cavity Bandpass Filters

These filters just have input and output ports. Missing also are the bridging coaxial lines which produce the notches in the duplexers.

The passband is narrower than the SAW filters - but the slope of the skirts is much less steep than the duplexer.

By placing two of these 'normal' bandpass filters in series with the duplexer the composite bandpass is quite nice. Where the response is rising outside the notches in the duplexer the response is falling in the 'normal' bandpass filters.

Composite Bandpass Characteristic

The duplexer is placed in series between two 'normal' bandpass filters as shown below (note: the short cable is only temporary until an adaptor is procured)...

The composite bandpass is shown below. Note that the span has been reduced from the 50 MHz used in the preceding displays to 20 MHz in the left hand image and expanded to 100 MHz on the right hand image. This is done to show more detail in the first instance and to show the now singular response in the second instance.

Tuning Filters to a Specific Frequency

The above description focused on the general principle of configuration. As increasing levels of RFI exist there is a need to be frequency agile to find the current quietest frequency slot. To change frequency slots requires re-tuning the two 'normal' bandpass cavity filters and the single duplexer. This involves 12 tuning screw adjustments and so obviously is not a trivial task - but once setup is done can be completed within 15 minutes - but I have had a lot of practice...:-(

The example following is for a centre frequency of 428.5 MHz and an observation bandwidth of 2.4 MHz.

Tune each 'normal' cavity filter separately to be centred on 428.5 MHz. Adjustment of the three cavities should aim for a balance between maximum response, flatness of passband and steepness of roll-off. No need to worry about making it too narrow for 2.4 MHz as the 3-cavity filters won't get to that.
Tune the duplexer by itself to the new frequency of 428.5 MHz starting with adjusting the 'LOW' tuning screws if the new frequency is higher than the current setting - as these affect the higher frequency skirt. Or - conversely - start with the 'HIGH' tuning screws if the new frequency is lower - as these affect the lower frequency skirt.
After getting in the ballpark of the new frequency adjust the 'LOW' and 'HIGH' tuning screws for the best compromise between response, flatness, closeness to 2.4 MHz bandwidth and steepness of roll-off.

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