# Quick RF Tips for General Reference

I have been in meetings in the past and through knowing a few RF guidelines I was able to quickly know if someone was as smart as they acted or if a product was feasible.

##### Know these rules to quickly assess the feasibility of a product, or just to look good in a meeting:

• Electromagnetic energy such as microwave radiation travels one foot in one nanosecond in free space
• Linear passive devices have noise figure equal to their loss. Expressed in dB, the NF is equal to -S21(dB).
• Noise figure is dependent on the first gain stage. If there is 20 dB gain in a receiver LNA, the noise figure contribution of the subsequent stage will likely be insignificant.
• The third-order intercept point of an amplifier is generally 10 dB higher than its one-dB compression point, when measured at the output (9 dB higher at the input).
• Nepers? To convert neperts to decibels, multiply by ~8.68. (just use 9 for really quick conversions)
• The isolation of a Wilkinson is limited to 6 dB better that the return loss of the common port match.
• Metal loss is proportional to square-root frequency.
• Dielectric loss is proportion to frequency.
• The P1dB point of a mixer at its RF input is often about 6 dB less than its LO drive level.
• The noise figure of a mixer is approximately equal to the magnitude of its conversion loss.
• N-way resistor power dividers are very lossy, power is transferred as (1/N)^2 compared to a lossless power divider which transfers power at (1/N).
• The number of elements in a phased array antenna can be used to estimate the total gain of the array. A 30 dB gain array needs ~1000 elements and a 20 dB gain array needs ~100, 10 dB needs ~10.
• If your circuit does funny things when you close up the box, it’s oscillating!
• A Wilkinson power splitter can be made in sections to increase bandwidth. To determine how many sections are needed divide the center frequency by the lowest frequency; i.e. 1-10 GHz, plan on 5 to 6 sections (5.5 is not an integer).
• You can electrically measure the approximate length of a transmission line by analyzing the frequency separation between the dips in VSWR.
• Make sure you have more gain than you need, it is easier to change an attenuator than to add more gain to the final design. (With good designs oscillations should not be to much of a problem, but be weary of just adding gain, do a proper loss budget first!)
• For microstrip, you can cut metal losses by ~1/2 by doubling the dielectric thickness.
• The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a backward wave coupler.
• For stripline and microstrip, the attenuation factor always decreases almost linearly with characteristic impedance is reduced, lower impedance = lower loss.
• A microstrip board height should never exceed 1/10 of a wavelength at the maximum frequency of it usage.
• Metal enclosures should have a height of less than 1/4 wavelength to attenuate radiated signals and avoid other weird things from happening. You can also buy materials designed to absorb RF signals, we prefer magnetic RAM but there are several other methods.
• Metal covers should be greater than 6 times the height the substrate thickness.
• An acceptable voltage droop for a power amplifier during pulsed operation is 5%, which will drop the power by a similar amount (5%, or about a quarter of a dB). This should be used to determine the amount of capacitance needed.
• When designing a phase shifter multiple stages should be used for bits which induce more than 90° of delay. If designing a bit with more than a 90° phase shift use two elements, it is best to use one high pass element and one low pass element.
• E-plane of a wave-guide is “easy to bend” and the H-plane in rectangular waveguide is hard to bend.
• The WR number a waveguide is simply the dimension of the broad wall in mils, divided by 10.
• The Bragg frequency is when a artificial transmission line stops acting like a transmission line. This often occurs when the artificial transmission lines unit cell is longer than 1/3 of a wavelength.
• For silicon, 110 degrees C is the maximum junction temperature for reliable operation.
• For SiGe, 110 degrees C is the maximum junction temperature for reliable operation.
• The return loss of a circulator is very nearly equal to its isolation.