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RTL-SDR is a very cheap software defined radio that uses a DVB-T TV tuner dongle based on the RTL2832U chipset.
The most common today RTL-SDR dongle contains two primary chips: the Raphael Micro R820T radio tuner and than the Realtek RTL2832U which contains an 8-bit ADC and USB data pump.
During last year the R820T chip has been substituted by the R820T2 with a little better receiver performance.
I bought some R820T2 dongles and I needed a RF generator in the GHz range for comparison.
I investigated the possibility to modify a RTL2832U+R820T dongle to use it as a Radio Frequency Generator and hereafter the results.

The R820T block diagram shows that the internal VCO has no output pin.

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The idea is to use the pin 5 (Det2) that is related to the mixer output. Of course it is not thought to be a RF output, nevertheless part of VCO signal is routed out.

WARNING the modified dongle is no more a RTL-SDR receiver.

It becames a RF generator in the 500-1500 MHz range. ( see Update 1.0 )

The hardware modification is:

  • remove the input capacitor C13 ;
  • remove the bypass capacitor C22 ;
  • remove the protection diode D6 ;
  • wire a capacitor 100pF connecting the Antenna input (D6 pad) to the pin 5 of R820T (ex C22 pad).

Options:

  • remove the IR receiver (not used)
  • replace the blue led with other color (red) to easy recognize the generator modified dongle.

Refer to the following pictures of the hardware modification.

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The following screenshots has been saved while using a RT2832U+R820T2 dongle as receiver and the modified RT2832U+R820T dongle as generator. Generator and Receiver are connected via a coax cable .


( NOTE: the RTL_SDR receiver is sensitive to harmonics of the tuning frequency. The generator output is in the VCO range 1.8 - 3 GHz. See Update1.0 )

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Notice the DVB-T signal reception on the left.

The RF generator has an usable range of 500-1500MHz and the output level is a little weak.

That is not correct! why the RTL_SDR receiver is sensitive to even harmonics of the tuning frequency.The generator output is in the VCO range 1.8 - 3 GHz. See Update 1.0


Future ideas:
The possibility to FM modulate the signal must be verified. The pll filter looks accessible. A PC audio output could be injected to the filter via an attenuator.