Wednesday, September 27, 2017

When Band-Pass/Band-Reject (Bp/Br) duplexers really aren't band-pass


Since this article was originally published, the availability of low-cost test equipment like the NanoVNA has allowed more-thorough testing of their RF signal paths.

With the advent of inexpensive and "good" test equipment like a NanoVNA, there is little excuse these days for not knowing if one's duplexer has the needed band-pass characteristics described in this article.

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In the repeater world there is a misconception that just because the duplexer may say "Band Pass, Band Reject" on its label - or even in its 'spec sheet - that it really does offer a proper band-pass response over a wide range of frequencies - but this is usually NOT the case.

A close-in look at a typical Band-Pass/Band-Reject duplexer:

Take Figure 1, below, as an example.

Figure 1:
The magenta trace is that of a proper band-pass cavity, the yellow trace is that of a one side (3 cavities) of a 6-cavity Phelps-Dodge "Band-Pass/Band-Reject" duplexer while the cyan trace is the combination of the two.  The top of the yellow peak (with the "1" marker) represents the center frequency of the duplexer with a bit over 1dB loss.

In the analyzer trace above, the YELLOW is the response of one of half of a typical amateur "Band-Pass/Band-Reject" 6-can Phelps-Dodge duplexer tuned in the 2-meter amateur band and from this trace we can see several things happening:
  • As there should be, there is a peak at the pass frequency corresponding to the "band-pass" of the duplexer - in this case, a bit over 1dB loss.
  • Just above the peak - 600 kHz, to be precise - is a very deep notch - corresponding to the frequency to "band-reject" part of the name.  In reality, the depth of the notch depicted in Figure 1 is about 100dB, but the true depth is not apparent from the trace.
  • Once one moves about 1 division (1.5 MHz) either side of the peak/notch frequency, the attenuation isn't that great - only about 20-30dB, and the trace above the center seems to be on a asymptotic trajectory upwards (lower attenuation) as frequency increases.
From the above we can see that while this duplexer offers a "Band-Pass/Band-Reject" response, this occurs only at frequencies very near the input/output frequencies of our hypothetical repeater.  Once you get "farther away", this "band-pass" response diminishes.

On the other hand the MAGENTA trace shows a single band-pass cavity filter.  While its attenuation is not very high at the notch frequency - on the order of 10-15dB - it is apparent that by 2 MHz above the center frequency it is offering greater attenuation than the so-called "Band-Pass/Band-Reject" filter and that below the center, the trend indicates that they might cross over at a point just to the left of the trace.

Comment:  This "Bp/Br" nomenclature is widely applied amongst many manufacturers to duplexers that have the same response as the Phelps-Dodge duplexer above, including Motorola and Wacom - to name but a few.  It is the rare exception to find a "Band-pass/Band-Reject" duplexer that does NOT exhibit the properties described on this page!

Unless you have already installed some band-pass cavities on each leg of your duplexer and/or have done proper sweep responses at frequencies far removed from the designed frequency, you should not assume that your "Bp/Br" duplexer is truly Band-Pass/Band-Reject over a very wide frequency range!

Taking a wider view:

Figure 1 only spans about 7.5 MHz on either side of 2 meters, so let us widen it a bit as shown in Figure 2, below:

Figure 2:
Spanning from 30 MHz to 1 GHz, the same cavities/filters as noted above.  Again, the yellow trace is one half of a 6-cavity "Band-Pass/Band-Reject" duplexer, the magenta trace is the pass cavity alone and the cyan trace is the result of the bandpass cavity and the Bp/Br duplexer cascaded.  It should be noted that at odd-numbered harmonics the pass cavity
will present a narrow bandpass response that can be eliminated with the addition of a simple low-pass filter.

When looking over a much wider frequency range - 30 MHz to 1 GHz - the picture is quite different.  Based on this sweep we can see that our typical "6 can" duplexer - of which 3 "cans" of the transmit or receive side - are represented above in YELLOW and that for the majority of the frequency range there is relatively little attenuation offered overall!  Paying particular attention we see that the attenuation in much of the VHF-low TV band (channels 2-6) and the FM broadcast band is quite poor - on the order of 3-10dB - as is the case over much of the VHF-high (channels 7-13) and large sections of the UHF TV band.

What we can see from this picture is that if we rely on only our so-called "band-pass/band-reject" duplexer on a site with other services such as FM or TV broadcast, or even land-mobile, those frequencies just above the amateur band, such a duplexer offers relatively little protection against those signals getting into the transmitter or receiver.

Why it matters:

One might wonder why it would matter whether or not a duplexer offered good "far-off-frequency" rejection.

In many cases, particularly in mountainous areas, amateur repeaters are co-located at sites with other transmitters and if adequate filtering is not implemented those "other" signals can get into the repeater's receiver and/or transmitter.

The effects of these other signals' ingress into the receiver is easier to envision:  Many of us have observed that, while driving about, our mobile radios have occasionally been overloaded with other signals - the effect being that we are hearing signals on frequencies where they are not.  This phenomenon is the inevitable result of the receiver's mixer - a device that is designed specifically to make new signals out of multiple signals in the first place - synthesizing entirely new ones out of the several that get in via its antenna.

Several decades ago it was common for land-mobile VHF and UHF radios to have receivers that had very tight filtering as there were typically only a few, closely-spaced channels that were used.  By virtue of this extensive filtering it was unlikely that other signals' somewhat-removed frequencies could even get in and cause undesired signals to be generated.  These days most radios have very broad filtering in their receiver inputs - this, to allow a wide range of frequencies to be accommodated.  While convenient, this also has a down side:  Those formerly widely-spaced frequencies from other services now have little impediment and it is more likely that they will get into the receiver and produce undesired, spurious signals.

Many years ago it was also the case that many repeaters used modified land-mobile radios with their extensive filtering, but nowadays many "store bought" repeaters (such as the Icom D-Star and Yaesu Fusion lines) are simply beefed-up mobile radios with "broad as the proverbial barn door" filtering on their receivers.  While this is convenient for the repeater owner to not have to dig up some test equipment and tune up these receivers' narrow filters, this also means is that there are many instances where a club has replaced their old, crystal-controlled analog repeater with a new one - only to find out that it did not work well at all when these off-frequency signals - formerly blocked by the old receiver's narrow front-end filter - clobbered the new receiver.  Worse still, some of these repeaters (namely the Icom D-Star) provided no analog test points where the receiver performance could be directly analyzed to determine if there was a problem, much less its extent!

What's worse is it is often the case that at many sites this sort of interference may be intermittent in nature - occurring only when a certain combination of transmitters happened to be online at once:  With most repeaters using subaudible tones for access, this degradation is often masked since the repeater may stay silent when it is being impacted, the only clue being that some users may suddenly find it difficult to get in to the repeater with a good signal at random times.  In other words, unless one uses the proper test equipment to take and record repeatable measurements at or away from the site, gradual or occasional degradation of the receiver's performance may not be so apparent.

An insidious problem:

While the overloading of a receiver is a familiar problem to many of us, it may not be as obvious that a similar thing can happen in a transmitter.  Like a receiver, a transmitter has the ability to take two signals and produce others via mixing.  For this to happen it usually requires that the "other" signals are very strong - but this is something that can happen at a busy radio site!

As a demonstration of what can happen, it was noted that via a VHF antenna atop Farnsworth Peak near Salt Lake City, Utah - a very busy broadcast site - one could read 100-150 milliwatts of RF on the coaxial cable at the input to the duplexer.  When this energy was analyzed it was found to be a combination of FM broadcast and UHF TV signals - the same transmitters that produce several megawatts of effective radiated power, combined.  If the same 6-cavity duplexer depicted in Figure 1 and Figure 2 was inserted in the line, this power would reduced - but only to the 20-50 milliwatt level!

This power was measured on the feedline of what would be a D-Star repeater, but prior to the installation of that repeater an analog Kenwood TK-740 repeater had been used for several months to assess coverage and performance prior to the installation of the Icom D-Star repeater.

On the day that the D-Star repeater was installed it was discovered that no-one could get into it, despite their running 50 watts.  Upon analysis it was discovered that the 20-50 milliwatts coming back into the coax was causing the Icom repeater's receiver to be deafened (desensed) by about 40dB - a factor of 10,000-fold!  Upon reconnecting the TK-740, no problems were noted and it was realized that the Kenwood repeater had a more traditional, narrow-band helical resonator filter assembly in its front end and compared to the more modern "broad-band" front end of the Icom repeater - which used parts of modified mobile radios - that the power in from the antenna was completely demolishing its receiver!

Figure 3:
A typical Motorola  4-can duplexer for UHF.  Just like its VHF counterparts
it easily passes energy at frequencies above and below its tuned frequency.
Click on the image for a larger version.

The installation of two bandpass cavities on the receive side allowed the Icom repeater to work as well as the old Kenwood analog repeater with its superior filtering - but this brings up the question about what might happen on transmit?

The transmitter can also act as a mixer:  Multiple signals - one of which might be the repeater's output frequency - can combine within the circuitry and instead of only the transmit frequency being emitted, some conglomeration of signals can appear!

In the example of a VHF transmitter we know that while the low-pass filter may remove the frequencies above the 2-meter band - say, UHF land-mobile and UHF TV - it will do nothing to remove energy from FM broadcast stations.  Similarly, if this were a UHF transmitter, its low-pass filter might remove some of the UHF land-mobile and UHF TV energy, but it would have little effect on signals from FM broadcast and VHF high and low band TV.

It might be suggested at this point that the use of an isolator - a device that, while allowing the transmitter's energy to go to the antenna, it directs any power coming back down the coax into a dummy load so that it cannot even get to the transmitter, might be appropriate here - and this would be correct...  Mostly.  While these devices are invaluable - and even required equipment at many radio sites - to both prevent RF from nearby-frequency transmitters from getting into your transmitter - and then re-radiated again and also to insulate your transmitter from a bad VSWR - it is far less-effective when the frequencies that are coming back down the coaxial cable are away from its design frequency.  In other words, while your VHF isolator may work okay from 140 to 160 MHz, it will probably do comparatively little at the FM broadcast band and in the UHF range.

Adding a pass cavity:

It is, therefore, a very good idea to equip any repeater with at least two pass cavities:  One on the receiver, tuned to the input frequency and another on the transmitter, after the isolator, tuned to the output frequency.

If one examines both Figures 1 and 2 you can see the Magenta trace showing the response of a single pass cavity.  When compared to the response of a typical Bp/Br duplexer (the YELLOW trace) the general trend is that the farther away one gets from the pass frequency, the more attenuation it offers.  One quirk of band-pass cavities is that they also have a response at odd multiples of their pass frequency, which means that a 2-meter pass cavity will also pass energy around the low end of 70cm, around 700 MHz, and so-on.  In the case of 2 meters, this spurious response could be eliminated by the addition of a low-pass filter.

Both figures 1 and 2 also show something else:  What happens if you cascade a Bp/Br duplexer with a single pass cavity (the CYAN trace)?  For the most part the overall attenuation of the two sets of filters is complementary - that is, the "best of both worlds."  As can be seen the simple addition of a pass cavity knocks out almost everything that is off-frequency from that which is desired.
Figure 4:
A typical "4 can" (2 on transmit, 2 on receive)
2-meter duplexer.  Even though it is labeled
as a "band-pass/band reject" unit, this refers only
to the two frequencies of interest - the transmit
and receive - and not to the RF spectrum overall!
The plots in Figures 1 and 2 are from a similar,
"6-can" (3 on tx, 3 on rx) duplexer, but the
rejection of frequencies "far removed" from
where they are tuned is comparable.
Click on the image for a larger version.

Bandpass cavities have another important property as well:  Lightning protection.  Because lightning is a broad-band energy spike, it would make sense that if you reduce the passband of the signal path from the antenna, less RF energy, overall, will get in - and the use of a passband cavity also guarantees that there is NO DC path from the center pin of the coax from the antenna to the center pin of the coax going to the radio.  One radio club - the Utah Amateur Radio Club - has several mountain top repeaters and there have been a number of instances where the repeater antenna has taken a direct lightning hit, sometimes destroying the antenna, but never has the attached receiver or transmitter ever been damaged.


If you are installing a repeater or other radio at a site with any other transmitters you should not assume that just because the label or specifications of the duplexer say that it is "Band-Pass/Band-Reject" that it will actually do so over a wide range of frequencies.  Again, most brands of duplexers will simply pass, with relatively little attenuation, those frequencies that are far removed from the operating frequencies and the "band-pass/band-reject" nature is limited to the specific frequencies of interest - such as the transmit side of the duplexer passing the transmit signal but rejecting energy at the receive frequency.

Such a duplexer should always be supplemented with at least one bandpass cavity for the transmit frequency and another for the receive frequency to provide additional off-frequency rejection - and adding a simple low-pass filter on each leg won't hurt, either.  While these added elements result in higher signal loss, this need only be 1dB or less in most cases.  Adding this extra cavity will increase the effectiveness of an isolator on the transmitter - which works only well near its design frequency anyway - but it will also prevent excess, off-frequency energy from getting into the repeater's receiver which, these days, is more typically a "mobile" unit with a very broad front end that has been converted.  Finally, the humble band-pass cavity provides good lightning protection, just by its very nature!


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