Microwave Circuit Rules of Thumb

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This article is translated from Microwaves101 | Microwave Rules of Thumb, which summarizes some rules of thumb in microwave circuits. The so-called rules of thumb are some rules summarized over a long period of time. They are applicable in most cases, but not all the time. Please use your own discretion.

1. For 1-mil gold wirebonds, inductance of the bond wire in nanohenries is roughly equal to its length in millimeters... an advantage of the Metric System that was brought to our attention by a French engineer named Yves (merci!) Let's restate this rule-of-thumb so that baseball fans can use it: 1 mil of bond wire is equal to 25 pico-Henries of inductance, or 40 mils of bondwire is equal to one nanohenry.

2. To be considered a "lumped element", no feature of a structure can exceed 1/10 of a wavelength at the maximum frequency of its usage .

3. To be a useful substrate, the height of a microstrip board should never exceed 1/10 of a wavelength at the maximum frequency of it usage. We've made a table for you on this subject !

4. How do you know what WR number a waveguide is just by looking at it? The WR number is simply the dimension of the broad wall in mils, divided by 10. How do you know what WR number a waveguide is just by looking at it? The number after WR is the length of the broad wall of the waveguide in mils, divided by 10. Author's note: For example, WR34 means the broad wall length of the waveguide is 340mil; the naming rule of standard rectangular waveguides in China is BJ##, and the number after it represents the main mode center frequency in GHz multiplied by 10, for example, BJ260 means a standard rectangular waveguide with a main mode center frequency of 26GHz.

5. A good way to remember which is the E-plane and which is the H-plane in rectangular waveguide is when you bend it, bends in the E-plane are the "easy way", while bends in the H-plane are the "hard way" .

6. For silicon or SiGe, 110 degrees C is the maximum junction temperature for reliable operation (1,000,000 hours is typical median time to failure criteria). With the exception of silicon LDMOS, which can operate up to 175C for 800 years, according to Leonard who works for a major LDMOS supplier (thanks!) GaAs FET (or HEMT) channel temperature should not exceed 150 C for long-term reliable operation. For gallium nitride HEMT (GaN), 175 C is a good rule for maximum channel temperature. For GaN HEMTs, a maximum channel temperature of 175°C is a reasonable value.

7. In order to cutoff spurious modes, the width of a package should generally not exceed one-half of a wavelength in free space at the maximum operating frequency .

8. For microstrip and stripline curved lines, use a minimum radius of three line widths at X-band and below. At higher frequencies, use five line widths for minimum radius. Even better, use an optimum miter instead of a curve !

9. The 10% to 90% rise time of a pulsed signal, in nanoseconds, will be approximately equal to 0.35 divided by the bandwidth of the network, in GHz. The author gives an example: Assuming the cutoff frequency of a low-pass network is 1GHz, the 10%-90% rise time of a rectangular pulse is approximately 0.35ns.

10. If you are trying to effect an RF short circuit using a quarter-wave stub, use a low impedance line, or better still, use a radial stub .