[IRCA] Brief RTL-SDR review/modification

["T. Hills" <thills@xxxxxxxxxxxxx>]

I decided to go cheap getting my feet wet with SDR. Saw the RTL-SDR on 
Ebay for less that $50 so I ordered one.

It was advertised as having 100KHz to 1.7GHz coverage and I've been 
playing with it using SDR# and HDSDR.  I much prefer the SDR# interface, 
seems more intuitive to me.

I can't say much about the upper end but it does quite well with FMBC 
and VHF Air/Weather. It outperformed my Grundig S450DLX for sensitivity 
and selectivity on FM with a 31" rod.

On SW it outperforms my R-71A both for sensitivity and with the digital 
filtering adjacent channel interference and noise. Both were tested 
using a 30' vertical feeding a 4:1 BALUN and RG6QS feedline. The RTL-SDR 
does suffer from overloading and a preselector helps there a lot. It's 
dynamic range of 50dB seems to be the main limitation on SW. It can 
display/record up to a 2MHz bandwidth so I thought it would be handy for 
MWDX.
I was wrong. In the upper part of the MW band the sensitivity leaves 
much to be desired and becomes nearly non existent below 1MHz. I decided 
some exploratory surgery was in order.

The RTL-SDR  has a LPF on the HF input to keep strong FMBC/TV signals 
out but the internal BALUN was found to be the limiting factor for lower 
frequencies. It's a very small binocular core wound in a 1:1:1 single 
sided input and balanced output. After removing it and replacing it with 
a home-rolled transformer with the same ratio on an Amidon FT-37-75 core 
I decided to remove the LPF elements since the 75 material would choke 
them anyway. The upper end of the SW band suffered a bit from the 
transplant and the MW band was still unusable, hetrodynes and 
overloading all over the band, again a preselector cleared up the 
problem and a tuned loop would probably be best (next on my to-try 
list). I also put in an F connector since SMA cables/ends are very 
$pendy.

The sensitivity is now rather impressive, a 6" wire shoved into the F 
connector gets the usual daytime locals and semi locals sitting on my 
bench in the basement, it starts to overload with the 31" FM rod. It 
takes about 10" of wire on the R-71A to equal the reception I get from 
the 6" wire. With no antenna it doesn't pick up the 10KW station 6 Miles 
away at all and the spectrum is flat all the way down to DC.

Tapping the of the 2nd mixer (9MHz) on the R-71A works surprisingly 
well, coupling with a 33pF disk provides the right level to the SDR and 
doesn't interfere with the operation of the Icom. The radio provides the 
tuning, preselection and AGC and the SDR /software do the demodulation. 
The main drawback is the 20KHz maximum useable bandwidth with this 
setup. For broadband MW recording this is not the SDR to have. I now 
have a DSP IF that doubles as an FM radio. Still not too bad for the 
price. The 455KHz IF out on the Grundig is the right level feeding it 
directly from jack to jack with RG6.

Overall I think it would make a good FMDX receiver for tropo openings 
with it's 2MHz display/recording capability or monitoring VHF/Air.

For SWL it works very well for the money but your going to want to put a 
HPF in line with it to keep the MW band from overloading it.

For MWDX and Longwave as it is out of the box, forget it. I plan on 
hooking it up to a high Q loop and will post the results of that when I 
get the time. Maybe with a laptop and an FSL on a cliffside? ;)

Not sure about the UHF performance but it does pick up what I assume are 
digital cell, TV, etc. but I have no other receivers that pick up those 
frequencies to compare it to.

I would suggest a dual core processor and 4GB memory at a minimum if you 
want use the 2MHz sample rate and USB2 is a must. Windows7 does not 
recognize it when plugged into a USB3 port. According to the performance 
monitor Processor usage on a 3.4GHz quad core is about 23% total 
recording at 2.048 MS/S. Using it as an IF at 250KS/S the processor is 
barely ticking over even when recording. It crashes when I try sample 
rates above 2.048MS/S, just like the manual said it would.

I plan on experimenting with it some more and will post the results.

Tim Hills
Sioux Falls, SD
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