433MHz Diode Detector Impedance Matching

[awmt102]

Hi All,

I am trying to design a very low power, high sensitivity 433MHz detector. I initially used a series of diode voltage doublers which worked very well with input voltages of above about 100mV, but I want to detect voltages around 10mV. Adding a bias voltage to the diode detector works well in simulation with low impedance source so the next step is to impedance match it to the 50 Ohm antenna. When I simulate with an ac sweep with a 1A current source I get what look like very strange phase and mag plots (see attached). In particular I get a phase <-1.5rad which does not seem right. I'm new to using spice to impedance match so would appreciate any tips.

Thanks

Andy

[Neganur]

Does that spice feature harmonic balance simulation?
Your matching depends on input amplitude as well, I will see if I can look at it later.

Ps, you need a load after the diodes, the cap itself is not enough. To maximize power in it set it to R_L=1/(alpha*I_s) (diode parameters)

Why do you use a current source btw, and 1A, that’s not “very low power”

[awmt102]

Thanks for the reply. I'm not sure about the harmonic balance simulation. I use the website partsim.com for simulation... I'll see if I can find out.

I did not think that matching depended on amplitude as the purpose is to provide optimum energy transfer... can you elaborate?

Thanks for the load tip, I will look into this.

The reason I use 1A current source is simply to make an assessment of characteristic impedance, it is not part of the final circuit - the voltage magnitude after simulation represents the real component in ohms (simple example: 1A into 50R would show 50V magnitude). This may fall foul of your amplitude point earlier of course! If you can recommend a better way then I am all ears.

Thanks

[German_EE]

Why not add an amplifier in front of the assembly that has a 50 ohm input impedance?

[awmt102]

The problem I can see is finding an opamp with low quiescent power but reasonable gain bandwidth product... if you have any suggestions I am all ears but a quick google did not show anything useful.

[Neganur]

I did not think that matching depended on amplitude as the purpose is to provide optimum energy transfer... can you elaborate?

You're using the circuit as rectifier to detect a 433 MHz signal.

Have a look at the datasheet of the diode you're using (PDF, and attached PNG, package parasitic effects are omitted) where you also can see the AC circuit model of a Schottky diode. The datasheet actually calls it "equivalent linear circuit model" but really, the junction capacitance is also a function of junction voltage (so matching depends on the junction voltage and other stuff like the SOT package parasitics).



The current through Rj is voltage dependent Id(Vj)=Is*(e^(α*Vj)-1).  (no latex addon?)
Further, the diode junction capacitance (varactor) is Cj(Vj)=C0/((1-Vj/Vo)^gamma)

This is what the spice model is doing for you (compare the values from the datasheet with the spice model):
(HSMS285X.PNG)

Code: [Select]

*SPICE model for HSMS-285x
*The parameters are for a single diode (HSMS-2850). Parameters also apply
*to the individual diodes within multiple diode configurations.
*
.SUBCKT hsms 1 2
DCD1    1     2      DMOD   
.MODEL  DMOD D(IS=3E-6, CJO=0.18E-12, VJ=.35, BV=3.8, IBV=3E-4
+ EG=0.69, N=1.06, RS=25, XTI=2, M=0.5)
.ENDS

But you also need to simulate the package parasitics and the layout will also affect the matching.

I don't have the time right now to do redo an old design for 433MHz but here's what the dependency looks like for a different design frequency. You can clearly see how the optimum detection frequency changes with different input power:



Harmonic balance is necessary in order to simulate the rectified voltage (and all the harmonics) correctly. It may not make too much of a difference but since you're interested in very low powers I would say it is good to include harmonic balance in addition to just the linear solver. Especially when you want to plot voltage over the load resistor vs. input power to the detector.

[Wolfgang]

Just a hint:

have you tried the AD8307 log detector chip ? it goes down to -50dBm at least