1.8 GHz voltage doubler detector

Created: May 2019

Introduction

Although RF detectors can be realized with log amps, diode detectors are considerably cheaper and available from more manufacturers. When detecting RF power lower than -20 dBm and diode bias is either unavailable or inconvenient, the best Schottky diode variant is the Zero bias [1, 2]. Though germanium diodes such as 1N34A have been advocated for this application, they are PN diodes which are inefficient at microwave frequencies, their reverse leakage current is several magnitude orders higher and their point-contact is unreliable, microphonic-prone [3] and is not amenable to automated packaging.

Material & methods

A voltage doubler configuration (Greinacher) [4] was chosen for the detector because of improved sensitivity and lower input impedance [5]. To eliminate the need for external DC bias, a low barrier Schottky, the HSMS-2852, is chosen as the rectifying element. The HSMS-2852 incorporates two closely matched diodes in one SOT-23 package. However, it must be noted that the HSMS-285 family has poor consistency of key parameters above 900 MHz. So, board to board tuning is required to maintain the same centre frequency. The unbiased diodes have high impedances and so, an L-network (2.7 pF and 9 nH) is used to match the detector's impedance to 50 ohm (fig. 1).

Fig. 1: Voltage doubling diode detector for 1.8 GHz

The detector was modeled in a circuit simulator in order to synthesize the matching network and to predict the performances (fig. 2). A single (half-wave detector) using HSMS-2850 diode was also simulated to enable comparison of performances.

Fig. 2 Equivalent circuit model of 1.8 GHz detector

Simulation showed the voltage doubler has greater sensitivity above -30 dBm (fig. 3). Below -30 dBm, both detectors are equally sensitive (the traces overlapped) - however, this is contingent on a high impedance load (10 MOhm).

Fig. 3 Simulated sensitivity of single vs. doubler detector using HSMS-285x at 1.8 GHz

Simulation also showed the voltage doubler has twice the matched bandwidth (fig. 4).The wider bandwidth advantageously makes the detector more tolerant of uncertainties in component values.

Fig. 4 Simulated return loss versus frequency. The voltage doubling detector has twice the matched bandwidth of the single detector

The HSMS-2852 is a p-type Schottky which has a lower forward voltage [6] than the n-type commonly found in mixers.Since p-type Schottky is rather uncommon, it is tempting to replace it with the ubiquitous n-type.For example, BAT15-04W is a hybrid n-type diode with an uncharacteristically low forward voltage; i.e. comparing Vf at 1mA, BAT15-04W is 0.25V [7] versus HSMS-2852’s 0.20V. So, at a glance,BAT15-04W would appear to be a good substitute for HSMS-2852.However, simulation predicts that its matched bandwidth will be impractically narrow (fig. 5).

Fig. 5 If hybrid low barrier Schottky such as BAT15-04W is used as a substitute, the matching becomes very narrow (0.2% of of centre frequency at the -10 dB return loss points). The sharp matching may also require impractical component values

Results

Fig 6 Measured input match

Both simulated and experimental centre frequencies are ~20 MHz higher than target - representing an 1.1% error (fig. 7). The simulated return loss not only show a very similar behavior to the experimental, but even have semi-quantitative agreement. The measured bandwidth at -10 dB return loss is ~60 MHz.

Fig. 7: Modeled and measured return loss vs. frequency

Both simulated and experimental output voltage vs. frequency have very similar behavior although the former is slightly lower (fig. 8).

Fig. 8: Simulated & measured output voltage vs. frequency

Good agreement is achieved between simulated and experimental sensitivity (fig. 9). The voltage sensitivity of the 1.8 GHz detector, g3, is given by: -

Fig. 9: Modeled and measured sensitivity

References

[1] Design tip D004, “Detector selector”, 6 Jul. 1999, Agilent Technologies, [Online] Available: http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/dt_d004.pdf

[2] Application note AN969, “The Zero Bias Schottky Detector Diode",Avago

[3] R. Bayliss, E. Cabrera, and S. Howe,“Microwave Diodes.. Why a Schottky-barrier? Why a Point-contact? Mar. 1968. Microwaves & RF [Online] Available: https://www.mwrf.com/rf-classics/microwave-diodes-why-schottky-barrier-why-point-contact

[4] “Greinacher circuit” in Voltage Doubler, Wikipedia, [Online] Available:

https://en.wikipedia.org/wiki/Voltage_doubler#Greinacher_circuit

[5] Waugh, R., "Designing Detectors for RF/ID Tags", RF Expo, San Diego, February 1995

[6] B. L. Smith and E. H. Rhoderick, “Schottky barriers on p-type silicon,” Solid-State Electronics, vol. 14, no. 1, pp. 71–75, Jan. 1971.

[7] Datasheet“BAT15-04W”, Infineon,[Online] Available: https://www.infineon.com/cms/en/product/rf-wireless-control/rf-diode/rf-mixer-and-detector-schottky-diode/bat15-04w/