Tag Archives: return loss

Terminations: 50 Ohm, +-1%

Proper termination of all high frequency transmissions lines is critical; unless you want to get some signals reflected, which is not the topic of this little post. For most of the items we deal with, systems are operated at 50 Ohm characteristic impedance, at least when it comes to instrument-to-instrument and sub-system/module connnections, and GHz frequencies.

There are many types of terminations around, from all reputable corporation manufacturing RF parts. Here, we want to look at 5 devices:

(1) Huber+Suhner, Switzerland, N-type termination, rated to 6 GHz – specified maximum SWR: 1.05@1 GHz, 1.1@4 GHz, 1.2@6 GHz

(2) Midwest Microwave, USA, N-type termination Model 2070, rated to 18 GHz – specified maximum SWR: 1.25 up to 18 GHz

(3) Aeroflex, USA, N-type 6 dB attenuator AH-06N, rated to 18 GHz – SWR 1.35 to 12.4 GHz, SWR 1.5 to 18 GHz

(4) and (5) No-name BNC 50 Ohm termination, labeled “+-1%”, connected to the test system with a reasonable quality N to BNC adapter, the two terminations are of the same kind, just two samples to check part-to-part variability

The warriors: Left to right-items 1 to 5

The test setup: Micro-Tel SG-811 Signal Source, Micro-Tel 1295 Precision Attenuation Measurement Receiver, Narda Precision High Directivity Bridge Model 5082, Narda APC-7 to N adapters.

The test procedure: At each frequency, full reflection (open) is used to set the “0 dB” level. Return loss can then be measured directly as attenuation of the reflected signal, when the termination is attached to the test port. To determine the error limits of the SWR measurement, the directivity is known, the insertion loss of the bridge has been measured, and the attenuation measurement error itself (by the Micro-Tel 1295) is negligible.

140827 swr of various terminations d0
140827 terminations data

SWR vs. frequency, all terminations
140827 terminations1

There are clearly two groups, the terminations made for GHz service, and the others – BNC, items (4) and (5), that were not. Interestingly enough, at 6 GHz, these terminations are more or less ideal, with SWR close to 1. So you can use these at exactly 6 GHz, and integer-multiples of 6 GHz, but still, I would suggest not to. They are also working great at DC, because of their “+-1%” accuracy which is mentioned on the label.

SWR vs. frequency, only the items (1) through (3)
140827 terminations2

A closer look at the real performers – the Huber+Suhner, although only specified to 6 GHz is working well within its SWR max. 1.2@6 GHz specification, even considering the error limits of this measurement, and can be used up to 12 GHz, no problem.
The Midwest Microwave 2070 – I like this model very much, because of the rugged stainless steel construction (nothing gold-plated and fancy; you can get these for a few dollars, from surplus). It is way better than specified (SWR max. 1.25, measured: 1.12) – and, at 18 GHz, within the limits of the HPAK 909F Precision Coaxial Termination, intended as calibration standard, and listed for close to 1 k$.

A little hint – how to distinguish really high frequency N-type parts (18 GHz), from regular, say, several GHz (below 12 GHz) parts: the shield of the “precision” N-type connector is a non-slotted (see picture-right termination), where as the regular N connectors have a slotted shield, typically, 4 slots (see picture-left termination):

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.


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.