DEMO-HSMS285-0 Datasheet, PDF (6/13 Pages) Broadcom Corporation. – Surface Mount Zero Bias Schottky Detector Diodes

R_j=
8.33 X 10^-5
I_S + I_b
n
T
=
R_V–
R_s
0.026
=
at 25°C
R_S is perh^Ia^S p⁺s^Ibthe easiest to ­ measure accurately. The V-I
curve is measured for the diode under forward bias, and
( ) ^tvI^ha=^eluI_S^sel(^oe_opx_fp^ecuor₀^fV_re.^t_0-^hn_2Ite^R6₍^Scsuurc-^vh1^e)_aiss
taken
5 mA).
at some relatively high
This slope is converted
into a resistance R_d.
R_S = R_d –
0.026
I_f
R_V and C_J are very difficult to measure. Consider the
iiSmschapopetpdt­rak_{RonyVx,c≈iRem}2_Vo_Ia6_Sift,_s+0eC0_ilJnIy0_b=t1h0M.e16Ωrap.nFFgowerhoaef n5wmteolel a2ds5uersKeiΩgdn, aeatdn1dzMeitHroszh—boiratisst
out the junction capacitance. Moving up to a higher fre-
quency enables the measurement of the capaci­tance,
but it then shorts out the video ­resistance. The best mea-
surement technique is to mount the diode in series in a
50 Ω microstrip test ­circuit and measure its insertion loss
at low power levels (around -20 dBm) using an HP8753C
­network analyzer. The resulting display will appear as
shown in Figure 7.
Detector Circuits
When DC bias is available, Schottky diode detec-
tor circuits can be used to create low cost RF and mi-
crowave receivers with a sensitivity of -55 dBm to
-57 dBm.^[1] These circuits can take a variety of forms,
but in the most simple case they appear as shown in
Figure 8. This is the basic ­detector circuit used with the
HSMS‑285x family of diodes.
In the design of such detector ­circuits, the starting point is
the equivalent circuit of the diode, as shown in Figure 6.
Of interest in the design of the video portion of the
circuit is the diode’s video impedance — the other
four elements of the equiv­alent circuit disappear at all
­reasonable video frequencies. In general, the lower the
diode’s video impedance, the better the design.
RF
Z-MATCH
IN NETWORK
VIDEO
OUT
-10
0.16 pF
50 Ω
-15
50 Ω
-20
-25
50 Ω 9 KΩ
-30
50 Ω
-35
-40
3
10
100
1000 3000
FREQUENCY (MHz)
Figure 7. Measuring C_J and R_V.
At frequenci_He_Ss_Mb_Se_-2lo₈₅w_A/1_6A0_fM_ig H₁₀z, the video resistance dom-
inates the loss and can easily be calcu­lated from it. At
frequencies above 300 MHz, the junction capacitance
sets the loss, which plots out as a straight line when
frequency is plotted on a log scale. Again, ­calculation is
straightforward.
L_P and C_P are best measured on the HP8753C, with the
diode ­terminating a 50 Ω line on the ­input port. The re-
sulting tabulation of S₁₁ can be put into a ­ microwave
linear analysis ­ program having the five element equiv-
alent circuit with R_V, C_J and R_S fixed. The optimizer can
then adjust the values of L_P and C_P ­until the ­calculated
S₁₁ matches the measured values. Note that extreme
care must be taken to ­ de‑embed the parasitics of the
50 Ω test fixture.
RI_{RNFj =}
8^Z.^-3^M3^ATX^C1^H0^-5
NETWORK
I_S + I_b
n
T
=
VIDEO
R_OVU–_TR_s
0.026
Figur⁼e 8. Bas_Ii_Sc ₊De_It_bec^ato^tr ²Â­Ci⁵rc^°u^Cits.
The situation is somewhat more complicated in the
( ) ^di(I_nwe=_chsliI_uigScd(^nhex_csop_{afntthhb0Veee.0-t2}^RI_pu^R6FnaS_ciemkd­a-^pog1^eue)^{dt ­a),nintchdeeu ­mscetaaritneccsherinegsaisnntdaent­wcceao,prtkah,ceiwtjauhnniccche-}
tion ­capacitance and the video ­resistance. Of these five
esilteicms_Re_{Sanr=etsRcodo–}fntsht0_ae._n0Id_f2_tis6oadned’s
equiv­alent circuit, the
the video resistance is
four para-
a ­function
of the current flowing through the diode.
R_V
≈
26,000
I_S + I_b
where
  I_S = diode saturation current in µA
  I_b = bias current in µA
Saturation current is a function of the diode’s design,^[2] and
it is a constant at a given tempera­ture. For the HSMS-285x
series, it is typically 3 to 5 µA at 25°C.
Saturation current sets the detection sensitivity, video re-
sistance and input RF impedance of the zero bias Schottky
detector diode. Since no external bias is used with the
HSMS-285x series, a single transfer curve at any given fre-
quency is obtained, as shown in Figure 2.
[1] _{Avago Application Note 923, Schottky Barrier Diode Video Detectors.}