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.
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).
- 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.
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 )
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
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.