# Flatness vs Frequency parameter in RF zero bias detectors

I require the measurement of RF power from a port, and I am considering using a zero-bias RF detector in order the measure the power being delivered through the port.

These devices give a voltage output which is roughly linear to the logarithm of the power being received at the port, ie., power in dBm.

All these detectors have a parameter known as Flatness vs. Frequency, and they mention the values in dB. I think that this may be referring to the amount of change of the output voltage as the RF frequency of the input power is changed, but I am unable to figure out how it works quantitatively, and whether the deviation is sufficient for my application.

For example, one of the detectors which I am considering shows a flatness vs. Frequency of (+/-)0.5 dB max. What does this mean, and how can I make sense of these numbers?

• pasternack.com? ask Tech Support . generally it means flat response at some power level from f1 to f2 at 25'C when terminated with 50 ohms. Power in RF is usually measured in dBm with tolerance in dB – Tony Stewart Sunnyskyguy EE75 Jan 11 '18 at 22:54

When a Schottky diode is used as an RF peak detector to measure mV/mW or RF power with some high impedance like 100k on the diode detected over a 50 Ohm load.

Sensitivity is usually rated in mV/mW of RF power over a frequency range such as 10MHz to 4GHz.

Since Power is 10 log (Pd/Pr) where Pr reference 1mW = 0dBm
and -10dBm = 0.1mW
and +10dBm = 10 mW

Tolerance 0.5 dB for power = ten to the power of 0.5dB/10
10^0.05=1.12 thus +12% voltage out
10^-0.05=0.89 thus -11% voltage out For example, one of the detectors which I am considering shows a flatness vs. Frequency of (+/-)0.5 dB max. What does this mean, and how can I make sense of these numbers?

A +/- 0.5 dB flatness means that at one point in the spectrum a detected amplitude could be 1 dB lower than that detected at another point in the spectrum i.e. one signal is 0.5 dB down and another is high by 0.5 dB giving a difference of 1 dB and that 1 dB is an error in measurement between two points in the spectrum.

In terms of real numbers, a 1 dB change in voltage is a change of 12.2%. In terms of power it's a 25.9% change.