I'm playing around in ngspice & trying to understand envelope detectors for ham radio.

I'm pumping 150Mhz into the circuit described below - I'm expecting it to go positive in proportion to amplitude + frequency. After a bit, it goes negative. Whats happening here?

Consider this circuit:

diode detector

  • 150Mhz 2V sine wave input, 50ohms impedance
  • D1N4148 diode
  • 100pF smoother capacitor
  • 500k bleed/load resistance

Spice input I'm using / working on:

Detector CIRCUIT
.model D1N4148 D (IS=0.1pA, RS=16 CJO=2pF TT=12N BV=100 IBV=0.1pA)

v1 1 0 sin(0 2 150MEG)

rSOURCE 2 1 50

d1 2 3 D1N4148

c1 3 0 100p

rLOAD 3 0 500k

tran 5us 50ms
write kek.raw v(3)

Gives me this:

transient analysis

Note the negative sections.

How does a diode detector go negative like this? how do I stop it?

  • 3
    \$\begingroup\$ The 1N4148 diode has a frequency limit of about 50 MHZ at 50% duty cycle. You need a UHF diode for this to work. \$\endgroup\$
    – user105652
    Sep 7, 2018 at 1:56
  • \$\begingroup\$ Yes but he was getting < 1nV output > Warning Will Rogers. \$\endgroup\$ Sep 7, 2018 at 3:48
  • \$\begingroup\$ Don't forget to check the scale of the Y-axis: it's between 1e-9 and 8e-10. That should tell you something. \$\endgroup\$ Sep 7, 2018 at 7:19

1 Answer 1


v1 1 0 sin(0 2 150MEG)
tran 5us 50ms


What's wrong with this picture?


The sampling time is violating Shannon's Law. Measurement error.

What is the diode spec?

  • Rectification efficiency \$V_{HF} = 2 V, f = 100 MHz ~~~ η_r = 45 \text{%}\$

It won't work well because recovery time is 4ns with 100 Ohm load and yours is only 500k.

Although Cj=2pF is ok. ( even though max in the datasheet is 4pF )

As @Sparky256 said try a UHF diode.

These have faster recovery time than 8ns on a signal with a 6.7ns cycle time. This is why efficiency is poor at 100MHz and worse at 150MHz.

Try a Falstad simulator. Limited parameters no custom parts. You make the passive part models and LOGIC, OA's have 0 Ohms out. Same with Voltage sources.

Press Reset anytime.

  • 1
    \$\begingroup\$ It was indeed Shannon's Law, thanks! I upped the resolution of the simulation and let it run for a few minutes and the waveform looks much better. \$\endgroup\$ Sep 7, 2018 at 9:16
  • \$\begingroup\$ Falstad's sim is much faster real time , fast or slow motion > options> other options> sampling time or mouse wheel over scope trace \$\endgroup\$ Sep 7, 2018 at 12:54

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