About Digital Step Attenuators
Q. Suppose I don’t work at less than the specified 1 dB compression of a step attenuator, what are the consequences?
A. Should you exceed the specified compression, typically given at 0 dBm input, harmonics will be generated which may affect system operation. Also, when you measure the fundamental for attenuation, an error will appear because some of the fundamental’s power has been transferred to the harmonic products.
Q. What is meant by the “zero” value of a step attenuator?
A. Step attenuators are referenced to the minimum insertion loss of the switch. If the switch in its “zero” position and has an insertion loss of 3 dB at 10 MHz, it might be 3.5 dB at 1 GHz. All attenuation values are given relative to this “zero” reference.
Q. Why do Mini-Circuits’ ads on TTL controlled step attenuators emphasize the word “precision”?
A. Most step attenuators on the market have a relatively wide variation of attenuation over the specified frequency band; for example, a 3 dB step could be off by as much as l dB so that attenuation is anywhere from 2 to 4 dB. With Mini-Circuits’ step attenuators, the difference would be 0.3 dB or less. The same situation occurs for temperature variation.
Further, when switching internal resistors to increase attenuation from say 3 dB to 6 dB, the sum may be off by as much as 2 dB if return loss within the unit is poor. With Mini-Circuits’ precision step attenuators, designed with 24 dB return loss, the sum will be within 0.6 dB of nominal.
Q. What makes Mini-Circuits’ step attenuators so precise?
A. Along with the 24 dB return loss criterion, extremely short lead lengths, resistors trimmed with high accuracy, and switches with low parasitics, yields precision performance.
Q. What happens when step attenuators are cascaded?
A. Attenuation will be arithmetically additive provided return loss of each unit is extremely good; 20 dB or better. Otherwise, the sum may be other than the total of the units. Mini-circuits’ attenuators can be cascaded without loss of accuracy.
Q. How fast is attenuation switched from one value to another?
A. TTL switching from one value to another is specified by delay time, and is 6 μs max. Delay is defined as the time to achieve 90% of the resulting RF amplitude change.
Q. If I exceed the maximum input level, will I burn out the step attenuator?
A. It’s not recommended, but you can exceed the input power rating by 10 dB without damage. The penalty would be the presence of distortion products and a decrease in attenuation accuracy.
Q. What is the consequence of feeding pulses with excessive peak voltage into a step attenuator?
A. Excessive voltage peaks will widen the pulse width.
Q. Please sketch the configuration of a typical TTL-controlled step attenuator.
A. Each of the three steps is configured with two single-pole, double-throw switches and a fixed attenuator; see Figure. Step 1 is shown with B1 and B1′ contacting Attn #1, Step 2 with B2 and B2′ contacting Attn #2, and Step 3 with B3 and B3′ contacting Attn #3. Thus, all steps are connected to their individual attenuators and are in cascade to provide maximum attenuation.
Q. What are the TTL requirements for proper operation and how much latitude is allowable?
A. For MCL attenuators, TTL”0″ corresponds to 0 to 0.8V and TTL “1” corresponds to 2 to 5V.
Q. Must the TTL input be 50Ω? What if it is 75Ω?
A. TTL interfaces are defined in terms of voltages and currents, rather than impedance.
About Electronic Attenuators
Q. As per the schematic of an electronic attenuator, it appears the device is just an application of a conventional mixer. Why should I have to use this device, when your conventional mixer is much cheaper?
A. The electronic attenuator is a special mixer design to produce low harmonics, high compression, and better intermodulation performance.
Mini-Circuits’ electronic attenuator/switches use extremely well-matched PIN diodes, since matching of the diodes is the key to producing excellent performance.
Q. Does a switch/attenuator require a control port biasing configuration? If yes, what is a suggested configuration?
A. For improved RF performance, the control port should provide a proper 50Ω match at the RF frequency. A simple circuit is shown below.
At RF frequencies greater than 1 MHZ, the capacitors act like a short and the 50 and 500Ω resistors appear in parallel, which is almost 50Ω. For DC and low frequencies, the capacitor is nearly on open. Hence, DC current is applied through 500Ω from the DC power supply.
Q. What is the attenuation linearity vs control current?
A. Attenuation is not linear as control current is varied, as shown in the graph below.
Attenuation vs frequency is a function of the control current; the higher the control current level, the flatter the response.
Q. What is the significance of input-control isolation of the switch attenuator?
A. In-control isolation is a measure of the amount of RF signal leaking into the IF port. The amount that can be tolerated is dictated by system requirements for leakage.
Q. All of Mini-Circuits’ switch attenuators are tested and specified at 50Ω impedance, can I use them at 75Ω?
A. Yes. The electronic attenuators are designed and tested for approximately a 50Ω system. In many cases, the actual impedance is higher than 50Ω, therefore enabling a match to 75Ω termination impedances. However, due to the higher impedance, the frequency response will be reduced by approximately 30%. For situations where critical specs in a 75Ω system is required, please contact our applications department. Our engineering staff would be able to redesign the internal components of the unit to meet the specific requirements.
Q. Can Mini-Circuits provide surface-mount switch/attenuators?
A. Yes. We provide surface-mount TFAS-5 and SYAS-models. The specification sheet gives frequency range and features.
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