Tuning of the hardware freq

[Rob PE1CHL]

I am looking for the correct way to tune the hardware freq in gqrx.  So far I have not been able to
determine how gqrx decides the hardware freq to use when tuning to a frequency, and how to have
more control of that process.

Let me show a use case: for example, I type 145.350 in the freqency field to tune to a local channel
in the 2 meter band.  The SDR is set to 2 MHz sample rate and bandwidth.  I now get a "random"
hardware freq (sometimes 145.350 and sometimes different) that allows me to receive that frequency,
but I would like it to be 145.000 to see the entire (european) 2 meter band and notice possible other
signals that are present.  However, tuning the receive frequency normally does not change the hardware
freq anymore (understandable).  Even when I key in 145.000 to begin with, it will not always select
145.000 for the hardware freq (I think it changes the hardware freq in larger increments when the desired
receive frequency is within the selected bandwidth anyway).

I thought it would be intuitive when dragging the center line on the scale would perform this operation,
but it only makes the receive frequency jump to there (nulling the offset), no way to change it directly.

It would be very nice when there was some way to do this (moving the hardware freq, if possible without
changing the receive frequency), so you can look what is happening around the frequency you are
receiving in a band that you decide yourself (instead of having the software decide it for you).

Rob

[Alexandru Csete]

Hi Rob,

I don't quite understand why you need to change the hardware frequency
without changing the receiver frequency. To be honest I think there is
just some misunderstandings, so I will try to explain how tuning works
in gqrx.

There are two things to tune in an SDR: The hardware oscillator
(hardware frequency) and the DDC offset, aka. filter offset in gqrx.
The filter offset is a software tuning and provides instantaneous
tuning as opposed to the hardware tuning, which can take as long as
several seconds (usually only few 100 msec though). So it is clearly
an advantage to tune using the DDC, but it is limited to tune within
~80% of the sampled bandwidth.

The hardware frequency will always be in the center of your spectrum
as indicated by the FIXED blue line on the panadapter.

The DDC offset is the offset of the filter box / red line from the
center of the spectrum, which was the hardware frequency. If you drag
the line or click on the spectrum it will tune by changing offset. The
exact offset is available in the "receiver options" panel. You can
reset it to 0 with a right click on the highest digit.

The sum of the hardware frequency and the filter offset is the actual
receiver frequency that is being demodulated. This is shown on the big
frequency display in the upper left side. If you change this using the
mouse, or by entering a frequency, it will actually change the
hardware frequency. However, the filter offset will stay the same and
consequently the actual receiver frequency will change to be the sum
of the filter offset and the new hardware frequency.

The frequency entry widget you are using was intended for visually
impaired people who can not use the graphical frequency entries. It
determines the actual receiver frequency which may involve change the
hardware frequency and/or the filter offset.

If you want to see the entire 2 meter band, reset the filter offset to
0 and tune to 145 MHz.

I hope this helps because I think we have a usable tuning concept and
I would prefer not changing it now.

Alex

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[Rob PE1CHL]

Op zondag 27 mei 2018 12:53:08 UTC+2 schreef Alexandru Csete:

Hi Rob,

I don't quite understand why you need to change the hardware frequency
without changing the receiver frequency. To be honest I think there is
just some misunderstandings, so I will try to explain how tuning works
in gqrx.

I know how SDR works.

The frequency entry widget you are using was intended for visually
impaired people who can not use the graphical frequency entries. It
determines the actual receiver frequency which may involve change the
hardware frequency and/or the filter offset.

If you want to see the entire 2 meter band, reset the filter offset to
0 and tune to 145 MHz.

My problem is that it is unpredictable (to me, because I do not understand the algorithm)
if tuning will change the offset or the hardware freq.  When I do what you write above,
usually the hardware freq will not change but the offset will change to tune, in this case
to 145 MHz.  I have no grip on what the hardware freq does, also not when tuning in
the frequency entry widget.  Sometimes it changes only the offset, sometimes it changes
the hardware freq.
In case of tuning within a band I would in fact prefer to lock the hardware freq entirely and
tune only using the offset.

 

I hope this helps because I think we have a usable tuning concept and
I would prefer not changing it now.

Alex

What you should recognize is that besides tuning (for which the current function is fine) there
is also the function of "panoramic display" (to see what else is happening in the band) and for
that it would be nice to have some more control over- and stability of the hardware freq.

But maybe I am biased because I have more experience with the wellknown "WebSDR" where
the operator determines the hardware freq and it never changes, and the users tune using the
offset only.   This also reflects in the waterfall history: when the software decides to unnecessarily
change the hardware freq, the waterfall changes into a curvy pattern.  I would prefer when the
waterfall is always straight when tuning, until I decide that the hardware freq has to change, or
when I should tune so far away that it has to change to accomodate tuning to the new frequency.

[Alexandru Csete]

Rob,

I don't wish to insult your intelligence, but I need to provide
certain amount of details to properly explain how tuning in gqrx
works.

I have attached a screenshot with numerical indicators to make sure it
is clear which widgets I am talking about.

Receiver frequency (1, 4, 6) = hardware frequency (2, 5) + filter offset (3)

Changing the receiver frequency using (1, 4) changes the hardware
frequency (2, 5) but leaves the filter offset (3) unchanged. The above
equation still applies.

The easiest way to change the hardware frequency (2, 5) to a specific
value is to first reset the filter offset (3, 6) to 0, then tune the
receiver frequency using (1, 4).

To tune within the sampled bandwidth without changing the hardware
frequency (2), i.e. websdr mode, do one of the following:

1. Click on the signal on the spectrum
2. Use the mouse wheel on the spectrum (except on the axes)
3. Change the filter offset using the mouse on the filter offset digits (3)
4. Drag the red receiver frequency indicator (6) to the desired signal
on the spectrum

When you do this, the receive frequency (1, 4) will update. I think
this works pretty much as websdr, i.e. click on a signal, then use the
mouse wheel to fine tune.

If you change the receive frequency using (1, 4) it will in turn
change the hardware frequency (2). It will leave the filter offset (3)
as is.

I understand it would be convenient to have a checkbox to lock the
hardware frequency.


Alex

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[Rob PE1CHL]

Alex,

It is now more clear to me how it works, and my confusion is probably caused by the fact that I
often tune coarsely by dragging the red cursor using the mouse on the scale (like in websdr) and then
set it to the correct channel using the frequency digits e.g. when I tuned to 145.575 but the display
says 145.573.4 I make up the final difference clicking some digit up or down, especially when I
am in whole band view so the accurate tuning using the mouse is more difficult, especially on NBFM
where the mousewheel increments of 100 Hz would mean still 16 clicks to go in this case, rather
than 2 or 3.

What has puzzeled me is that the hardware freq is changing all the time under these conditions,
but now it is more clear to me.  Of course in WebSDR the hardware freq is not going to change
when typing in the frequency field, but that is because the hardware freq just never changes.
(it can only do a band-selection in a WebSDR that has multiple receive bands)

I think the way it now works is not really aligned to your earlier concern that changing of the offset
is smooth and changing the hardware freq isn't.  With that in mind it should change the hardware freq
as little as possible.  I also do not understand how that "offset" is relevant to the user.  It would
seem more natural when the hardware freq appeared in a user accessible field instead of the offset,
and the offset would be the display-only field below it.  The hardware freq would be auto-set when
the user tunes using the main tuning mechanisms (e.g. it would be set the same as the entered
receive frequency when a frequency outside the receiver bandwidth is selected), but when tuning
well inside the bandwidth it can be untouched.  There could be buttons to set the hardware
freq equal to the current receiver frequency (centering it on the dial) and/or to lock it when you would
still want it to auto-change under some circumstances (like clicking on the frequency digits).
It would be preferable when changing the hardware freq would not change the receive freq, it would
just re-calculate the offset, unless of course a big change is made that would move the receiver
frequency out of the bandwidth, in which case the receiver frequency would be set the same.

I understand your concern that it may confuse the first-time user.  In fact my confusion is also
mostly caused by being a first-time user.  I understood what the two fields do immediately, but I
had a hard time understanding how the controls work.  When I start CubicSDR instead, it is a
whole new world.  It does support the change of hardware freq by just dragging, but as in all
programs (Gqrx, WebSDR or CubicSDR) it is very easy to change something you would not have
wanted to change (like a passband edge) and which is tricky to undo.  All of them require some
proficiency...

Rob