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I will take a few minutes to show you how I've learned to
to tune one of these notch type duplexer filters.
These are notch type. These are not pass cavities. These are
notch cavities. And, essentially what we do is we.......
My transmit on this particular unit is going to be the low side and my
receive is going to be the high side. Higher in frequency. This
transmit cavity I'm going to tune so that the transmitter does not transmit
any noise at the receiver frequency and
this receiver side notch I'm going to tune
so that none of the transmitter power
can pass through this filter into the receiver. It will make more sense as we go
along.
This is a notch type
duplexer, usually used on mobile radio repeaters.
Just to reiterate, we're going to notch
the receive frequency out of the transmit side. And we're going to notch the transmit
frequency out of the
receiver side. I'm going to do that using my Motorola
communication systems analyzer that has a
built-in tracking generator.
There are several ways, of course, to tune one of these duplexers.
I'm going to show you the way that I know how to do it, using the test equipment I
have
and get maximum performance within this
piece of equipment's capabilities. So, if I take a look at this
service monitor, I can go to
the special function screen
and look at the options that are installed and I can see that
it has a track generator installed. So, here at the RF section
I'm going to select track generator
and over here in the display section I'm also going to
select track generator. Here at RF control I'm going to
select the range for my track generator
my track generator output is being detected at this
antenna input. And what a track generator is , its a
frequency generator that sweeps across the selected spectrum,
and this is a spectrum analyzer and the spectrum analyzer and the track
generator are in sync
so at the exact moment that the track generator is
generating that frequency that is where the spectrum
analyzer is sampling that frequency, and they
track each other across the spectrum. Here we can see some anomaly that happens at
every division. I'm selected at
25 megaherz, plus and minus. So, a 50 megahertz
span and at every five megahertz indication there some
anomaly there, and if we reduce our span we can
zoom in on that we see every five megahertz there's some
anomaly. Here we have plus and minus five megahertz we have this
anomaly. It won't impact our measurements any
but, we just want to be aware of that anomaly that shows up there
So, at this point what I've done is I've connected my
source, my RF source to my transmitter input
and have connected my antenna port to my
antenna port here to detect that power. And, I'm going to add this
50 ohm termination over here to the
receiver port to give us some impedance matching.
So, in this process I'll be adjusting this
frequency quite often and this output level as well as
as well as the
range here, the range that we're looking we're looking at. So, here we're looking at plus or minus five
megahertz.
Centered around our receive frequency. Which in this case is higher
that are transmit frequency. On our duplexer here we always want to use the
high-side and low-side
as the manufacturer had built it.
And, we can crank up our output level a little bit.
If I go too high, I get a warning
increase attenuation. We'll be limited to our dynamic range
here. But, what I'm going to do now is start tuning
this notch filter
to 466 200 and that is my receive
frequency. I'm notching out
any transmitter noise that may be immited
at my receiver frequency. And, I'm just going to start by moving this
screw over here and
observe my spectrum
and I can see this particular point moving
as I turn this rod in and out
I can see that point seems to be associated with that
particular notch part of the filter. I want to get
that point over to my center frequency
of interest. I'm going to move on to the next one and begin to tune that.
Now I'm getting my notch
at my desired frequency. Moving on to the next rod
I'll begin to move this last
notch.
And, now I'm getting my notch at my
desired frequency. So, I'm outside of the range,
my dynamic range of my instrument. Here I can
increase power output some to try and see the bottom of that notch. The best I can.
Can't really see it there and I want more
accuracy in frequency. What I can do is to begin to reduce the range
and I can see that the center frequency of my notch
looks like it's shifted over but my
frequency accuracy increases as we zoom in the range there
now I can...... I'm already at
my maximum output levels of 0 dbm and that's about the best in image
I'm going to get. But, I can still
continue to tweak this to get that notch to fall
very deeply at my desired
frequency.
Moving that around.
Now I can go up and change my range again. Here I'm plus and minus 1 megahertz we can
see we have this nice
deep notch here. Plus and minus 2.5. Plus and minus five megahertz
remember that part of this little curve over here is that anomily that
we had looked at, at first.
And, there's our notch, deep into that frequency that we had set it at.
And, that's transmitting the
receive frequency into the transmit side,
detecting it at the antenna port, notching out
any noise that might come out of the transmitter at the receive frequency.
Now, we're going to switch our cables over to do the other side. So, I've connected this up now
with our our source going into the antenna, because that's where receive
frequency is going to come into the device. Right?
And then we're going have our detector, our antenna
input here over at the receive port, and we put our dummy load on here,
our 50 ohm impedance matching device.
And now, let's take a look at what we have. Our service monitor is saying
increase receiver attenuation. Or in this case...
and we could do that down here, we could add twenty db of
attenuation. But what we're going to do instead is we're going to
reduce our power output. Because this is
the power coming from our tracking generator. So, lets line up what we have.
We are still dialed into that receive frequency.
And, we can see that we don't have a notch lining up with our receive
frequency. Our frequency that we're going to notch out of here is going to be
our transmit frequency
We don't want to notch our receive frequency out of the
receiver side of the duplexer. We want to notch out our
transmit frequency which in this case is 461.2 megahertz.
And, we want to tune these
cavities now to maximize. or
tune them all to that same frequency. If I look here I can see,
even at a wider plus or minus twenty five megahertz
here's our notches that are created by
these cavties. We can see how they move.
That lower one there as I turn this back and forth, that seems to moving,
that seems to be the notch associated...the this resonant frequency
associated with this cavity. Now get that
down close to our frequency is in the center there. This next one
lets see where that one is.
That notch appears to be
associated with this cavity.
Lets see if that is the notch associated with this last cavity.
Indeed.
And, carefully tune that there.
Now, we would like more resolution. We would like to zoom in on that better. We wnat to see the
bottom of that peak.
We want to make sure that we're getting maximum
performance. So, we're going to narrow our range in increase our
are power a little bit. Now, we can begin to
fine-tune that. See we can move this back and forth.
Move that one cavity back and forth.
Trying to get a deep dip. But the dip
has to be right on the very center. Right
where our frequency is.
And, the cavities interact with each other.
I'll go back and forth on the screws.
A little bit at a time. I'm kind of making gross movements now.
Not too much fine tuning.
How can we do more fine-tuning? Get that
exactly at the rate frequency? So, if we change our
range again to a smaller range. Plus or minus 1 megahertz for example,
we can get that we have better resolution, we have better frequency accuracy of
what's being displayed and we can make smaller and smaller adjustments. Here I'll
increase our power out again. See the bottom of
that dip. Make smaller and smaller adjustments.
Going to move that over to the center right there. Going back and forth on the three screws.
Now we want even more accuracy. We want that tuned the best that we can go.
We can reduce our range
we get a fighter level detail of exactly what our frequency is with
less error. Right to a hundred kilohertz. So it seems that were shifted off from center
we're off
a hundred kilohertz. So, we're going to make minute adjustments now.
That dip is as low as it can go, right down there.
There. That's actually better than I had it the first time. Zoom in on that.
Zoom in again.
Very fine adjustments.
Zoom in again. Zoom in again.
I think we have maximized that notch.
Okay, so here we have tuned both sides.
And now, I have it set up to put a transmit frequency into the transmit side. I have a
dummy load on the
antenna port for some impedance matching, and I have our output from the
receive side
going into our input on our analyzer. Now we should be able to see both notches
because these are conncted together remember at the antenna point
and indeed we do see both notches.
You can see that we're injecting a frequency at the center
right here 463.7
but its sweeping from a plus and minus five megahertz
so to 10 megahertz spread across here. And, the first notch that we see
is our.... in this case are ....... our Transmit frequency
Our transmit frequency has a notch. We see our receive frequecny
in this case our receive frequency is higher, here is our receive frequency notch.
So, at plus and minus five megahertz, about two and half divisions up is our
notch there are about two and a half divisions down is our notch there.
We've notched the receive see frequency out of the transmit side
and we notched the transmit frequency out of the receive side.
Now let's get in here and make an insertion loss measurement.
So, for this measurement, what I've done, I've just connected my cables
together using this barrel connector. And I've
put in my center frequency of my transmit side. I going to measure the
insertion loss
the transmitter will see. How much wasted power I'm
having from this duplexer here. And, I set my range up to the minimum range here
and I'm going to set a reference level
now we'll quickly see that this analyzer will tell me
that anything over minus 50 dbm
to the input is too much
so I'm going to adjust this to
let's see, can I make it 51 ?
I have an input level of 51 dbm
and if we go to our level measurement
here we can see we went from 51, negative 51
dbm to a minus 51.7, we have a .7 db
insertion loss on this transmit side of this filter. Let's measure the receive side
to do that i'm going to put my barrel connector again in there.
and dial that up to the receive side frequency.
So, I got my barrel connector back in here again.
I want to dial-up my
receive site frequency
set my power level to give me
50 dbm.... no 51
didn't we say 51...51 on my input level.
Now I'll connect to the receive side And so, we have a
point 8 dbm or point 8 db
loss on the receipt side
And so, once again. Going input into the transmit side,
output out of the receive side, here's our pair of notches.