User Superphish over on YouTube has posted a video of a discovery made over on Reddit by Anonofish involving tuning your RTL-SDR to a high frequency below the range normally accessible by the dongles tuner. It does appear this is for dongles with the E4000 tuner.
“By using a E4000 RTL-SDR and tuning to 3686.6 MHz and above, you can receive a small part of the AM band without the need for an upconverter or direct sampling mod.” – Superphish
From Reddit: “I discovered awhile ago that tuning to a frequency where the PLL doesn’t lock and using AM modulation on the centre peak produces an audible hum (the dongle I’m using has an e4000 tuner). This hum is audible in the L-band gap and by tuning past 2.2GHz. This doesn’t work when tuning below 50MHz however due to the tuner seemingly repeating a section of the band above 50MHz.
Curiously, tuning past 2.2GHz there are some bands where the hum stops. These are:
3686.6MHz – 3730MHz 4022MHz – 4295MHz
At 4295Mhz the tuner seems to loop back to 0Hz and gives all the same signals as if you were tuning up from 0Hz. This can be tested by using the rtl_sdr command line program. Tuning to 4295MHz gives back “[E4K] PLL not locked for 32704 Hz!” whereas tuning to 4294MHz gives “Tuned to 4294000000 Hz.”. Interestingly rtl_sdr will show a PLL lock for any frequency between 4Ghz and 4.295Ghz but not below in the 3686.6MHz – 3730MHz band where there is no hum.
I’ve tried over the past while to see if I could pick up anything at these frequencies, either actual 4Ghz signals or more likely lower frequency signals which would show that the tuner was just looping as it does for sub 50Mhz and above 4295Mhz.
Today I tried scanning these frequencies while connected to an old yagi TV antenna on my roof. To my surprise I saw signals between 3686.6MHz – 3730MHz that were never there before. The signals did not change when changing frequency but using AM demodulation revealed that they were in-fact AM radio broadcast signals. It would seem that when tuned between these frequencies the tuner is passing the rf signal straight through to the RTL2832U chip without any filters, in the same way as the direct sampling mode. Enabling the recently added offset_tune mode changes the received signals.
The best reception I’ve gotten has been by plugging by laptop’s power supply in (but switched off) and connecting the centre rf pin of my dongle to the centre rf pin of the tv antenna.
Hopefully someone can figure out what the e4000 tuner is doing here so that direct sampling mode can be used without modifying the dongle.” –