One of the most common reported issues is a significant drop in signal level both in the receiver as well as the transmitter of the HackRF. In this post we will explain the amplifier chain, how to identify this issue and finally how to repair the HackRF.
HackRF has three amplifiers in the receive section. The first is a low noise amplifier (LNA) with fixed gain of 14 dB. The second is an Intermediate Frequency (IF) LNA with gain that varies from 0 to 40 dB. Finally, the baseband amplifier gain can be varied from 0 to 62 dB. On the transmit side, there are two amplifiers. The first is an IF LNA with gain that can be varied from 0 to 47 dB. The second amplifier has a fixed gain of 14 dB.
The following block diagram shows the amplifier chains in the receive and transmit sections of HackRF.
The first amplifier in the receive and the last amplifier in the transmit sections can be switched in and out of the chain. The designers of HackRF picked the same component MGA-81563 with Gain 14 dB, for both these amplifiers. As it turns out, this amplifier component is very susceptible to damage – even when the maximum input signal level of +13 dBm is not exceeded. This has been reported by a number of users.
So the first question is – how do you know if this amplifier has been damaged? In the video below, adjustments are made to the three different gain settings.
Adjusting LNA Gain and VGA gain affects the input signal level. When the Amp box is checked, the MGA-81563 amplifier is switched into the chain. In the video this is the only setting that results in no change in signal or noise level. The signal level should in fact increase by around 14 dB in the FM band if this amplifier is working correctly.
Similarly if the last amplifier in the transmit chain is not working, there will be a weak signal out of the HackRF as measured on a spectrum analyzer. If you don’t have a spectrum analyzer, you can use an RTL-SDR to view the signal with software like SDR#. Note that if you are plugging the output of the HackRF Transmitter into a RTL-SDR make sure to use an attenuator of 20 dB or higher or you will damage the RTL-SDR.
Once you have determined that the MGA-81563 on either or both receive or transmit paths is damaged, you can repair it.
The schematic below shows the Transmit (TX) and Receive (RX) LNAs.
The picture below shows the location of the LNAs on the PCB.
Here is an instructional video on how to replace the parts without a hot air gun
In this article we discussed how to fix one of the most common issues with the HackRF – i.e. damage to the first RF LNA. If you have seen other hardware issues with HackRF, let us know and we will discuss them as well.