Back in May we did a review of Japanese RTL-SDR experimenter Nobu’s products, which were his HF Upconverter, Galvanic Isolator and 14 MHz Low Pass Filter. The low pass filter was designed to be used with a direct sampling modified RTL-SDR receiver, but unfortunately we didn’t have one of those on hand at the time.
The image below shows the AM broadcast band with the filter in place. Mouse over the image to see the effect of removing the low pass filter. (If on mobile click inside the image, and outside the image to toggle the mouse over effect). We can see that there is some insertion loss from the filter, however with the LPF not connected there is severe interference from the broadcast FM band and some AM signals are completely unusable.
We repeated the same test at 9 MHz. Again, mouse over the image to see the effect of removing the low pass filter. Once more we see that without the LPF there is severe interference from the broadcast FM band, as well as in this case what looks to be a DAB signal.
Similar interference is found all through the 0 – 14 MHz frequencies without the low pass filter in place and most weak signals cannot be listened to without the filter connected. It is clear that without a low pass filter the direct sampling modification is almost useless in the presence of strong interfering signals, such as those from the FM broadcast band.
Back in April we posted about some new products made by Japanese RTL-SDR experimenter and product manufacturer Nobu. Nobu’s new products were a 1:1 galvanic isolator and a low pass filter. The galvanic isolator isolates the antenna from the RTL-SDR and PC, significantly reducing noise. The low pass filter is useful when used with direct sampling modified RTL-SDRs to filter out any strong interfering signals that are above 14 MHz.
Recently Nobu sent us at RTL-SDR.com some samples of his products. He sent us one of his HF upconverters, a galvanic isolator and a low pass filter.
Previously we posted about Japanese RTL-SDR experimenter Nobu and his work in prototyping a new 14 MHz low pass filter and galvanic isolator for use with the RTL-SDR. The low pass filter improves reception when using the RTL-SDR in direct sampling mode by reducing out of band interference and the galvanic isolator (isolation transformer) reduces computer and other noise when using the RTL-SDR with an upconverter.
Another product Nobu is working on is an isolation transformer (aka Galvanic Isolator) which can be used together with an upconverter to help reduce noise generated from common ground sources such as the PC. The isolation transformer is inserted between an upconverter and antenna.
In the image below Nobu shows the effect of inserting the LPF . An interfering FM broadcast band signal is removed after inserting the LPF.
The image below shows the effect of the isolation transformer showing a clear decrease in noise floor and increase in signal strength.