Q-Branch Mode? NEW RTL-SDR Dongel HF support via direct sampling using Q Branch mode?

[geor...@gmail.com]

rtl-sdr.com

Has released a new dongle. for $19.95. Same price as the old version 2.

One of Its big new feature's is

1) HF support via direct sampling. Connect an HF antenna directly to the SMA connector and tune from 500 kHz – 24 MHz with the direct sampling mod. (No hardware modding or soldering required)


From the help guide.

Feature 1: Direct Sampling HF Mode

This feature allows you to listen to HF signals between about 500 kHz to 28.8 MHz.

To use direct sampling mode

1. Connect an appropriate HF antenna to the SMA antenna port (this is the same port where you connect your VHF/UHF antenna). 
2. In SDR# select the Q-branch in the configure menu (the cog icon next to the play button).
3. Press Play and tune to 500 kHz – 30 MHz.

VHF antennas like small discones or short whip antennas will probably not pick up HF signals very well, if at all. If you have no such antenna you can try extend the large telescopic antenna to its maximum length of 1.5m, or use the screw nut provided with the antenna base to clamp on a long wire antenna. Ideally you should use a 9:1 unun with the long wire antenna for optimal reception.

Can  GQRX talk to a RTL dongle using the Q-Branch technique. The help sheet says that SDR# can.

I imagine this new RTL dongle will be very popular, with a lot of people wanting a GQRX  Q mode/ Q Branch option  for Linux.

[Alexandru Csete]

Gqrx supports the direct sampling mode by adding "direct_samp=..." to
the device string:

direct_samp=0|1|2
Enable direct sampling mode on the RTL chip. 0: Disable, 1: use I
channel, 2: use Q channel

So, for using the Q channel the device string would be something like:

rtl=0,direct_samp=2

You may have to check "No limits" to allow tuning below 24 MHz, I
don't remember.

Alex

On Wed, Aug 17, 2016 at 7:00 PM, <geor...@gmail.com> wrote:
>
>
> rtl-sdr.com
>
>
> Has released a new dongle. for $19.95. Same price as the old version 2.
>
>
> One of Its big new feature's is
>
>
> 1) HF support via direct sampling. Connect an HF antenna directly to the SMA connector and tune from 500 kHz – 24 MHz with the direct sampling mod. (No hardware modding or soldering required)
>
>
> From the help guide.
>
> Feature 1: Direct Sampling HF Mode
>
> This feature allows you to listen to HF signals between about 500 kHz to 28.8 MHz.
>
> To use direct sampling mode
>
> Connect an appropriate HF antenna to the SMA antenna port (this is the same port where you connect your VHF/UHF antenna).
> In SDR# select the Q-branch in the configure menu (the cog icon next to the play button).
> Press Play and tune to 500 kHz – 30 MHz.
>

> VHF antennas like small discones or short whip antennas will probably not pick up HF signals very well, if at all. If you have no such antenna you can try extend the large telescopic antenna to its maximum length of 1.5m, or use the screw nut provided with the antenna base to clamp on a long wire antenna. Ideally you should use a 9:1 unun with the long wire antenna for optimal reception.
>
>
> Can GQRX talk to a RTL dongle using the Q-Branch technique. The help sheet says that SDR# can.
>
>
> I imagine this new RTL dongle will be very popular, with a lot of people wanting a GQRX Q mode/ Q Branch option for Linux.
>
>
>
>

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[Greg Troxel]

Alexandru Csete <oz9...@gmail.com> writes:

> Gqrx supports the direct sampling mode by adding "direct_samp=..." to
> the device string:
>
> direct_samp=0|1|2
> Enable direct sampling mode on the RTL chip. 0: Disable, 1: use I
> channel, 2: use Q channel
>
> So, for using the Q channel the device string would be something like:
>
> rtl=0,direct_samp=2

What does I and Q mean for direct sampling, vs zero-IF? Is it about
whether the hardware is configured to report the samples (which are
presumably not either I or Q, really) on the I channel or the Q channel
of a a protocol designed to carry both when using a local oscillator?

[Alexandru Csete]

I am not 100% familiar with the internals of RTLSDR dongles so what I
am saying may be wrong...

I think the RTL2832 chip can run in two modes,(1) complex I/Q mode aka
zero-IF where it uses two analog to digital converters to sample the I
and Q channels respectively, and (2) Real, direct sampling mode where
it samples a real signal through either the I or the Q channel and
converts it to complex IQ internally.

I think only the E4000 tuner used zero-IF in the RTL-SDR dongles. The
R820T tuner uses bandpass sampling, so it is technically also uses
direct sampling but it acts as a downconverter in front of the ADC.
Not sure about this part though.

Alex

[geor...@gmail.com]

Gqrx supports the direct sampling mode by adding "direct_samp=..." to
the device string:

direct_samp=0|1|2
Enable direct sampling mode on the RTL chip. 0: Disable, 1: use I
channel, 2: use Q channel

So, for using the Q channel the device string would be something like:

rtl=0,direct_samp=2

Worked great thanks.

I now have my new Soft66Q working great in GQRX.

[Curt Justice]

Hi,  Forgive me for being a noob, but I am trying to learn Linux.  Using Lubuntu, how do I go about adding this to the device string to enable direct sample mode.  Still shaky on terminal commands.

Thanks

[Robin Gape]

Curt,

no terminal required! After GQRX has started, there is a configuration icon directly to the right of the on/off icon. Clicking on the configuration icon (green, might be a PCI card) brings up a dialogue box, one of whose entries is  device string. The rest is up to you!

73,

Robin, G8DQX

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[Curt Justice]

Thanks Robin,

That was far easier than I thought it was going to be.  Just wish I had seen that myself before I asked such a silly question.  Now to get it working better.  Rather glitchy and laggy in its current state.  Any settings recommendations that may help.  My PC may be a bit old to do any better than it is already also.

Thanks

Curt,  KF4OFI

[Robin Gape]

Glad it helped. Living within 20 miles of Sutton Hoo, one is tempted to suggest a Viking Funeral for the old PC. But without knowing what the PC is, it's not so easy to give a useful answer. The usual sound advice about not running other programs in the background, installing more memory, tweaking the BIOS (if there's anything useful to be found there) applies.

Also, minimise the load on the system. With an RTL dongle, select the lowest sampling rate is the customary mantra. This is not helpful when direct sampling is in use, of course.

To diagnose the problem, one can start by monitoring the load on the system. The classic tool is top, and these days there is a more complex, and likely to be more useful in this instance, htop. What htop can show you is the load on each individual processor core. (If you're trying to use an old single core processor with direct sampling that's not good.) The results from ntop or htop are probably the start of a conversation with the comptroller of the Piggy Bank!

HTH, 73,

Robin, G8DQX

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