Updates to Removing the 1.2V Switching Power Supply on the RTL-SDR

A few months ago we posted about how a Japanese RTL-SDR modder disconnected the noisy 1.2v switching power supply on the RTL-SDR and replaced it with three diodes. The diodes reduced the voltage from the 3.3v line to 1.2v. It seems that most of the improvements seen from the previous post were inconclusive, as soon after he realized that there are minor dongle to dongle variations in sensitivity, so he could not be certain if the improvement was due to his modifications.

However, he has continued to modify the RTL-SDR using the same principal (Japanese, use Google Translate). He discovered that to fully suppress the 1.2v switching regulator pin 27 on the RTL2832U chip must be connected to ground. Once this is done the switching noise present at 1 MHz disappears, although there does not seem to be any significant improvement in noise anywhere else.

The first screenshot shows an unmodded dongle with the switching regulator still connected. There is some noise present at around 1.026 MHz. The second image shows a modded dongle, the noise at 1.026 MHz is gone.

Switching regulator on. Noise can be seen at around 1.026 MHz.
Switching regulator on. Noise can be seen at around 1.026 MHz.
Switching regulator off. Noise at 1.026 MHz is gone.
Switching regulator off. Noise at 1.026 MHz is gone.
The 3 diode mod.
The three diode mod.

9 comments

  1. Robert

    For what it’s worth , I don’t have the problem that he has . I have gone to 1.098 mhz and looked over at the 1.026 mhz space and their isn’t anything their . The only difference between his setup and mine is that I use I input and he uses Q input .

  2. TOSHI

    Hello,This is my last post.
    If I need a constant voltage, I choose the three-terminal regulator.
    However, I need to make sure that I satisfy the power sequence.
    By the way, he is, in order to make a simplified manner experiment, He used three diode, I think that’s a good choice.
    In general, simple equivalent circuit of the forward characteristic of diode is a voltage source with resistance(ESR).
    ESR of the rectifier diode is around 0.4 ohms.
    I think that he has selected the appropriate rectifier diode.
    In my calculations, constant voltage characteristics, the load current is around 300 mA, I think delta was 150mV below.

  3. TOSHI

    Hello,This is my last post
    If I need a constant voltage, I choose the three-terminal regulator.
    However, I need to make sure that I satisfy the power sequence.
    By the way, he is, in order to make a simplified manner experiment, He used three diode, I think that’s a good choice.
    In general, simple equivalent circuit of the forward characteristic of diode is a voltage source with resistance(ESR).
    ESR of the rectifier diode is around 0.4 ohms.
    I think that he has selected the appropriate rectifier diode.
    In my calculations, constant voltage characteristics, the load current is around 300 mA, I think delta was 150mV below.

  4. Andyj

    I’m no great genius on this matter so forgive any errors.
    Isn’t 2 x 0.6V = 1.2V? Two diodes. If they get warm, the diodes are too small. Heat is a noise.source
    Zener diodes are not a noise free option from what I’ve read.
    With the 1.2V switch mode circuit active it could be just as bad open circuit due to local radiation.

  5. TOSHI

    I donot know the product of 2.1v / 300mA rating of the Zener diode.
    And, more than power 0.6W becomes the heating above 50 ℃.
    I believe low-cost solution of three diode.
    It is the following heat 20 degrees, and possible to supply a stable 1.2V.

    • Jay Moore

      My only concern with the three diodes is they themselves provide no protection; you’re just using the forward voltage drop. So, if there isn’t any voltage regulation persay, it’s the same as just dropping it with a resistor. You could always combine multiple zeners to do the right voltage.

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