Thus, p‑type diodes are generally reserved for detector
applications (where very high values of RV swamp out
high RS) and n‑type diodes such as the HSMS‑282x are
used for mixer applications (where high L.O. drive levels
keep RV low). DC biased detectors and self‑biased detec‑
tors used in gain or power control circuits.
•
The two diodes are in parallel in the RF circuit, lowering
the input impedance and making the design of the RF
matching network easier.
•
The two diodes are in series in the output (video) circuit,
doubling the output voltage.
•
Some cancellation of even‑order harmonics takes place
at the input.
DC Bias
Detector Applications
Detector circuits can be divided into two types, large signal
(P
in
> ‑20 dBm) and small signal (P
in
< ‑20 dBm). In general,
the former use resistive impedance matching at the in‑
put to improve flatness over frequency — this is possible
since the input signal levels are high enough to produce
adequate output voltages without the need for a high Q
reactive input matching network. These circuits are self‑
biased (no external DC bias) and are used for gain and
power control of amplifiers.
Small signal detectors are used as very low cost receivers,
and require a reactive input impedance matching net‑
work to achieve adequate sensitivity and output voltage.
Those operating with zero bias utilize the HSMS‑ 285x
family of detector diodes. However, superior performance
over temperature can be achieved with the use of 3 to 30
µA of DC bias. Such circuits will use the HSMS‑282x family
of diodes if the operating frequency is 1.5 GHz or lower.
Typical performance of single diode detectors (using
HSMS‑2820 or HSMS‑282B) can be seen in the transfer
curves given in Figures 7 and 8. Such detectors can be re‑
alized either as series or shunt circuits, as shown in Figure
11.
DC Bias
Zero Biased Diodes
DC Biased Diodes
Figure 12. Voltage Doubler.
The most compact and lowest cost form of the doubler is
achieved when the HSMS‑2822 or HSMS‑282C series pair
is used.
Both the detection sensitivity and the DC forward voltage
of a biased Schottky detector are temperature sensitive.
Where both must be compensated over a wide range of
temperatures, the differential detector
[2]
is often used.
Such a circuit requires that the detector diode and the
reference diode exhibit identical characteristics at all DC
bias levels and at all temperatures. This is accomplished
through the use of two diodes in one package, for exam‑
ple the HSMS‑2825 in Figure 13. In the Avago assembly
facility, the two dice in a surface mount package are taken
from adjacent sites on the wafer (as illustrated in Figure
14). This assures that the characteristics of the two diodes
are more highly matched than would be possible through
individual testing and hand matching.
bias
Shunt inductor provides
video signal return
Shunt diode provides
video signal return
DC Bias
differential
amplifier
Zero Biased Diodes
DC Biased Diodes
matching
network
HSMS-2825
Figure 11. Single Diode Detectors.
The series and shunt circuits can be combined into a volt‑
age doubler
[1]
, as shown in Figure 12. The doubler offers
three advantages over the single diode circuit.
Figure 13. Differential Detector.
[1] Avago Application Note 956‑4, “Schottky Diode Voltage Doubler.”
[2] Raymond W. Waugh, “Designing Large‑Signal Detectors for Handsets
and Base Stations,” Wireless Systems Design, Vol. 2, No. 7, July 1997,
pp 42 – 48.
6
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
