reducing 2mhz bandwidth on Terratec T stick plus e4000


diyer
 

got it. tks for all the responses.

Here's the reason I was asking. Considering the fact that adjacent strong signals can drive agc down or create overload thus lessening sensitivity to nearby weak signals of interest, I think that this could be mitigated by pushing the strong signal outside the window, its effect on agc should be less.

Has someone already tried that?

73 wa5ngp


Paul NN4F
 

Don,

What sort of signals are we talking about? Ham? something else??

Paul - NN4F

--- In SDRSharp@..., "don_wa5ngp" <m240zz@...> wrote:

got it. tks for all the responses.

Here's the reason I was asking. Considering the fact that adjacent strong signals can drive agc down or create overload thus lessening sensitivity to nearby weak signals of interest, I think that this could be mitigated by pushing the strong signal outside the window, its effect on agc should be less.

Has someone already tried that?

73 wa5ngp


hb9ajg <hb9ajg@...>
 

Hi Don,

you might want to have a look at my measurements on the effects of strong signals within and outside the spectrum window: go to "Files" > "Some Measurements on E4000 and R820 Tuners.pdf" of August 6, 2013. You might find some answers to your question.

73 de Walter HB9AJG

--- In SDRSharp@..., "don_wa5ngp" <m240zz@...> wrote:

got it. tks for all the responses.

Here's the reason I was asking. Considering the fact that adjacent strong signals can drive agc down or create overload thus lessening sensitivity to nearby weak signals of interest, I think that this could be mitigated by pushing the strong signal outside the window, its effect on agc should be less.

Has someone already tried that?

73 wa5ngp


diyer
 

Hi Paul
ham and SWL. I see some discussions about putting in external front end tuning to narrow the response of the radio to strong signals outside the frequency of interest when the signal of interest is very weak. or similarly turning down gain to keep front end from overloading.

to be honest I haven't played with it enough to know if this is really a problem. just wondering.


73 don

--- In SDRSharp@..., "paulnn4f" <paul@...> wrote:

Don,

What sort of signals are we talking about? Ham? something else??

Paul - NN4F


uhh_huhh <uhh_huhh@...>
 

Hi Don

Depends on how far apart the signals are and whether you need a low pass, high pass, band pass or cutoff filter.

The broadcast FM radios stations are a problem here, as they show up almost everywhere if the gain is set high. I use a low pass filter to reduce the FM signals on he HF amateur bands that are acceptable, not perfect. I do not use a filter above the FM band as the frequencies I use there just happen to not fall into the FM overload sections.

I guess that the type of filter and its characteristics vary from station to station.

bobbie
kd4lv

--- In SDRSharp@..., "don_wa5ngp" <m240zz@...> wrote:

Hi Paul
ham and SWL. I see some discussions about putting in external front end tuning to narrow the response of the radio to strong signals outside the frequency of interest when the signal of interest is very weak. or similarly turning down gain to keep front end from overloading.

to be honest I haven't played with it enough to know if this is really a problem. just wondering.


73 don

--- In SDRSharp@..., "paulnn4f" <paul@> wrote:

Don,

What sort of signals are we talking about? Ham? something else??

Paul - NN4F


diyer
 

I have that problem as well despite the fact that I have a 5 element chebychev filter that cuts off at 40mhz. I have my upconverter in a metal box but I think other that cables bring in the FM broadcast and I suspect that there is stray coupling inside the box.

I'm going to try to clean this up but in the end adjusting sensitivity may be the best solution. The basic problem is that it is doing a mixing subtraction
LO-FM=SW, eg in my case 104mhz(LO) -93(FM) = 11 mhz(SW) and that 93 mhz station is very strong. I think I will try a specific trap for that 93mhz station.

I spend more time tinkering with it than using it, but then that gives me an excuse to buy interesting test equipment.

I also see periodic PC noise showing up too. My laptop is the best behaved computer I have with its screen being the largest offender.

73 don



hb9ajg <hb9ajg@...>
 

Hi Don,

you might want to have a look at my measurements on the effects of strong signals within and outside the spectrum window: go to "Files" and look for "Some Measurements on E4000 and R820 Tuners.pdf" of August 6, 2013. Section 6 will provide some answers to your questions.
You will see that within the filter bandwidth of +/-0.8MHz from the center frequency for the E4000 (+/-3MHz for the R820) signals of about –65dBm (-70...-65dBm for the R820) will cause overload or 1dB compression of a desired signal. At +/-3MHz the E4000 can tolerate signals of about –40...-35dBm before overload or 1dB compression sets in (-30...-25dBm at +/-3.5MHz for the R820). At +/-50MHz the corresponding figures are about –25...-20dBm. This filter effect is independent of the width of the spectrum display (i.e. the MSPS set in Configuration).
The dBm figures given here are for CW signals with Filter Bandwidth set to 500Hz. If you have several equally strong signals present at the input of your dongle, add 6dB per signal to avoid overload or 1dB compression.
In short: If you have strong signals within the filter bandbidth of the dongle, use the Gain slider in Configuration to avoid overload or 1dB compression (both effects are close together, within about 1...2dBs). If your strong signals are outside the filter bandwidth of the dongle, I suggest to use filters to keep them below about –40dBm to be on the safe side.
Do not activate RTL AGC and/or Tuner AGC in Configuration: it may make things a lot worse! (see my paper for details).

Best 73 de Walter HB9AJG

PS1: In my paper I used –80dBm and –120dBm for the desired signal. I made some tests this morning with a very weak desired signal of only 6dB above the noise floor (i.e. 3dB above MDS): The results were the same.

PS2: I made two shorter posts as a reply to Don before this one, but Yahoo seems to have ignored them. In case they show up later, I apologize for the wasted bandwidth.

--- In SDRSharp@..., "don_wa5ngp" <m240zz@...> wrote:

got it. tks for all the responses.

Here's the reason I was asking. Considering the fact that adjacent strong signals can drive agc down or create overload thus lessening sensitivity to nearby weak signals of interest, I think that this could be mitigated by pushing the strong signal outside the window, its effect on agc should be less.

Has someone already tried that?

73 wa5ngp


diyer
 

Hey Walter,
that's a great write up. I wish we could make it a "sticky" since it answers so many questions.

So I come away with 2 conclusions with E4000 version.

1. If I reduce sample rate with SDR# although the observed window narrows the filter width is unchanged.

2. With the E4000 if the SDR# window is greater than 1.6Mhz then signals on the edges of the window are being attenuated by the filter inside the dongle.

I am assuming that the filter bandwidth of neither dongle is controllable by software?

73 don


Paul NN4F
 

Don,

I don't think you will have any issues, I run the R820T with the Ham it Upconverter, have it connected to my 4 element 6 band Mosley or my 160m full size dipole, if it's too strong I just back off the gain a little, found on HF, I run about 20.7db with the RTL-AGC on and get great results, weak signals are there, and as long as I tighten up the filtering, i can pull them out, still amazes me what you can pick up on this cheap setup...I also run a dual band 2/440 antenna which works well on all the vhf/uhf bands, I run the gain at about 30-35db higher up

Paul - NN4F

--- In SDRSharp@..., "don_wa5ngp" <m240zz@...> wrote:

Hi Paul
ham and SWL. I see some discussions about putting in external front end tuning to narrow the response of the radio to strong signals outside the frequency of interest when the signal of interest is very weak. or similarly turning down gain to keep front end from overloading.

to be honest I haven't played with it enough to know if this is really a problem. just wondering.


73 don

--- In SDRSharp@..., "paulnn4f" <paul@> wrote:

Don,

What sort of signals are we talking about? Ham? something else??

Paul - NN4F