Coaxial attenuators are resistive networks, pi ot networks, connector to RF/Microwave connectors. These attenuators are commonly used to adjust signal levels in military and commercial RF and microwave systems. When choosing the right coaxial attenuator for your application, certain key parameters must be considered.
Connector Type: SMA dimmers, BNC dimmers, and N-type dimmers are very common. You can also buy dimmers quite easily with TNC, 2.92mm and 2.4mm connectors. There are a few manufacturers that offer QMA, 1.85, SMB, F, and reverse polarity (N, SMA, TNC) dimmers. Of course, coax adapters can always be used with any attenuator connector, but we all prefer to avoid them unless necessary.
Connector material: both brass and stainless steel are common. You have to watch the associated torque values; a brass sma requires 3 to 5 in-lbs, a stainless steel sma requires 7 to 10 in-lbs, depending on the manufacturer. Using a 10 in-lb torque wrench on a brass sma will usually torque the sma nut right off the part! SMA, BNC, Type N, and TNC attenuators are available in both materials, with brass being used for many commercial applications and stainless steel outlasting increased mating/unmating. Insufficient torque of certain attenuators, such as the sma, can cause degraded performance at higher frequencies, often 15ghz and above.
Power CW – For small signal applications at 18 ghz, 2 watts is the most commonly available attenuator, although some 0.5 watt and 1 watt attenuators are also available. Above 18 ghz the options are fewer, often just 0.5 watts at 50 ghz. Likewise, below 18 ghz, higher wattages are available, with 500 watt uncooled units running at 3 ghz in general. CW power is usually specified at room temperature, but reduces as the temperature increases; a 2 watt unit at 25 C can only handle 0.5 watts at 125 C
Frequency – As frequency increases, resistive chips must be manufactured with more precision and therefore cost more. Commonly available bands are 0-6 ghz, 0-18 ghz, 0-26 ghz, 0-40 ghz, and 0-50 ghz, 0-65 ghz. Since the construction is simply a resistive network, the lower frequency range will always be 0. At the upper frequency limit is where one can expect the largest ripple in the passband response and the largest deviation from the desired attenuator.
Directionality: Small signal attenuators are bi-directional, either port can be used as an input. Most very high power dimmers are unidirectional, have one input and one output, and hooking the DUT back is usually fatal. This is because high power dimmers use cascading dimmer chips; maybe 2-3 db on the first chip, 3-5 on the second, 6-30 on the third, effectively spreading the heat to be dissipated along the dimmer. Applying power to the output results in virtually all of the power being dissipated into a chip which then overheats and fails.
DC Handling – Being resistive networks, coaxial dimmers are not designed to handle DC and will shift DC. Often the dimmer will dissipate too much heat into its resistive elements and fail. One can bypass the dimmer with dc blocks and bias t. Some bias step dimmers are available incorporating the DC blocks and bias tees.
Contact Materials: Almost all available dimmers use BeCu female contacts and brass male contacts. A few consumer applications will substitute brass for female contacts to reduce cost, but these should be avoided for all but consumer applications.
Electrical Specifications: These specifications can be expected: VSWR, Dimming Value, Dimming Accuracy (Tolerance), Frequency (Upper), CW Power Handling, Peak Power Handling, Operating Temperature Range, Impedance, Connector Material, and contact material.
