Tag Archives: insertion loss

Equipment selection: reflection bridge

Attenuation is defined as insertion loss minus reflection loss.

The insertion loss measurements – that’s quite straightforward, with signal genarator and receiver. We will deal with the particulars later.

For the reflection loss, we still need another device, a directional device. Either a directional coupler, or a return loss bridge.

After careful review, I selected a Narda 5082 “Precision high directivity bridge”. Several reasons:

(1) It is a fairly robust device, and offers N and APC-7 connectors. Luckily, APC-7 adaptors were included. Also included was a combined short/open, APC-7 style. That’s really great.

(2) It is very broadband, 2-18 Ghz full range with one device. This eliminates connections – there are hardly and couplers available that offer 35+ dB directivity, over the full band.

(3) The bridge has a bit more insertion loss compared to a coupler/multi-coupler solution, about 6 dB, but the loss is well defined and flat, will be calibrated out.

(4) It was available, at a few cents for the list price in dollars, and in pristine condition.

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Next step: need to connect the “reflected” port to a switch matrix, via an APC-7 to SMA adapter (which I don’t have in my collection).

What is an attenuator calibrator?

Q: What do you want to do? A: I would like to measure attenuation (insertion loss minus reflection loss) of various devices, in particular, programmable attenuators, over the 2 to 18 GHz range (for other ranges, I have other equipment). Measurement should be traceable, and as accurate and precise as technically possible, with some kind of reasonable effort (say, a 3 kUSD budget). Range should be 0-60 dB, usable 0-110 dB with reduced accuracy.

Q: Why don’t you use a VNA? other Q: A what?
A: A VNA (vector network analyzer) – an instrument used to characterize networks, of all kinds. It is very versatile, but has disadvantages:

(1) Extremely expensive, especially, for above 3 GHz. We need 18 GHz.

(2) Not so accurate for attenuation – sure, it is fairly accurate, but just not quite enough for calibration standard type work.

(3) Even more expensive, and if you get used equipment, it might work for some time, but due to the complex design, not easy to troubleshoot.

Various ways exist to measure attenuation. See Alan Coster’s review, of the IEE.

Well, for the “attenuator calibrator”, there are some main parts:

(1) A signal source, it needs to be of stable amplitude, in a useful range (a least 10 dBm), and 2-18 GHz range.

(2) A receiver – needs to highly linear, preferably fundamental-mixing, with a calibrated IF chain, preferably, at 30 MHz. 30 MHz is still the reference frequency for power meter calibration, and many traceable attenuators are available, for 30 MHz, and I have other equipment that allows very accuarate attenuation measurements at 30 MHz.

(3) A switch matrix, to allow “through” calibration without handling any connectors. At the levels we are talking about 0.01 dB, even slight movement of (precision microwave) cables can cause significant measurement errors. The switches can have some little losses, which don’t matter, but they need to be of very high repeatability.

(4) A high directivity bridge, preferably, 35 dB or better directivity. This will allow measurement of reflection loss. Attenuation will be calculated by measuring insertion loss and reflection loss. Attenuation is then calculated. All the measurements are taken relative to the “through” calibration signal.

(5) All the necessary interfaces and control circuits to handle signal source, received and switch matrix.

Sure enough, an attenuator calibrator can also measure gain of amplifiers, SWR of devices (antennas!), and many other useful things. It is more or less a high-precision scalar network analyzers.

Q: Why not use an “attenuator calibrator” all the time, rather than typical scalar network analyzers? A: The calibrator is slow, about 5-6 seconds minimum for each frequency. At each frequency, IF attanuation is selected, and the signal integrated, to get optimal S/N ratio, and to ensure high IF detector linearity.