Return Loss bridge Vs AIM

[w4dnr]

As I understand it, a good Return Loss Bridge can have a port isolation of around 30 db
before the forward power masks the "return."

Is this correct and how does the AIM 4170 compare to that 30 db range number ?


The reference device under test is  WaComm BpBr 145mhz repeater duplexer. 

Thanks

Don W4DNR

[Bob]

Hi Don,

The 30 dB value applies to isolation achieved by good components and careful construction.
If it is not possible to correct for the circuit imperfections with a computer, this is a pretty good number.

The AIM can measure a return loss greater than 60 dB after calibration. That's the advantage of having a computer to compensate for imperfections in the circuit. You can see the return loss by measuring the cal resistor right after doing the calibration.
This may not be enough to show the notch depth of a good duplexer but you can measure the center frequency of the notch and get an idea of it's bandwidth.

-73/Bob

[w4dnr]

Thanks Bob,

I've always used a Tektronix 492AP SA and TG to align 146 Mhz duplexers, and I could always change the reference on the 492 to see the bottom of the notches if I couldn't see both the pass and the notch on the same screen.


I haven't tried an AIM 4170 on the transmit port with the antenna terminal terminated with 50 ohms.

Would the notch be at the bottom of the screen like the 492, or would I be seeing an Inverted notch that indicates a huge return and is there a way to invert the graph in software if so ?

[smokchsr]

I'm doing some can filter design work right now using the AIM-120, and it's interesting since the measurements are somewhat different then using a tracking generator with a spectrum analyzer. The main thing to keep in mind is that the AIM shows what the device connected to the port would see. That doesn't necessarily indicate the true pass reject points of the filter, as you would easily see with the TG.

As soon as I finish typing this I'll be bringing in the SA&TG to see if what I've set up with the AIM is really working the way I think it is. In my case I'm trying to filter out a broadcast signal from a VHF 2Way. With the AIM everything looks perfect. My thinking since I'm using a single port on the can which basically turns it into a shunt trap, that I was looking for Zmin (the green line) to dip, at the frequency I want to get rid of. After that I looked in the 150 band to see how the return loss looked, which is running about -30db across the band, after adding a bit of capacitance to the can input connector. So with the power AIM everything look great. Now if the SA will show me anything better than 20db isolation at the broadcast frequency, and less than 1db loss in the 2way band I'll be a happy camper.

I'll try to post back in a bit to see what the SA thought of what I have.

[smokchsr]

After looking with the SA & TG I was off a bit, I should have adjusted for the highest return loss and lowest R value for a reject can.

This is after adjusting for a maximum rejection of 22db on 92.7MHz. The can is set up in reject mode using a T connection on a single port. AIM is connected to one side of the T and a 50 load to the other side.