So, I thought that determining the topocentric period was sufficient to extract the pulsar from the noise; I was disappointed.
Filling in all the parameters into the software did not give the best pulsar profile. It gave a good result when I used the very strong signal captured with the Dwingeloo radio telescope.
But even here I could see in the bin over time graph that the bins shifted over time. I had to adjust the period time a little.
This is actually not right. The calculated pulsar period should be exact.
Well, the 3pt sw can be set to search for the best period time or the best sample rate.
If you are sure the pulsar period is correct, then you can decide to search for the best sample rate; the 3pt program came with the value of 2000100....
When using the rtl_sdr.exe program, it states when activated the 'real' sample rate. So we could use that value.
It turns out that is not solving the problem neither; it still is too low.
Also We used several dongles and noted that they all displayed the same 'real' sample rate deviation value of 2000000.52982.
That is not likely, because the tolerances of these simple crystals do vary.
Another way to determine the correct sample rate is by measuring the actual crystal frequency.
The nominal frequency is 28800000Hz. From this the sample rate is generated (it boils down to this; the actual method is more complex).
The devision factor for SR of 2MSps is 28800000/2000000= 14.4.
Now we measure the oscillator frequency on the dongle.
The results are
Dongle nr Frequency
1 RTL 28800300
2 RTL 28800800
3 RTL 28801400
4 RTL 28800400
5 RTL 28801200
6 RTL 28801000
7 RTL 28801600
8 Nooelec 28800000
When we calculate the sample rate for dongle nr 7: 28801600/14.4=2000111 Sps.
This is exactly the value we also deduced by the automatic searching function of 3pt.
So the conclusion is; when using a RTL dongle with an HC49 shape xtal, then measure the frequency (MF) also.
The corrected sample rates for the selected sample rate will be;
chosen SR real SR
Also temperature drift can add to the problem; so avoid measuring in the start up temperature transient.
There are conversion kits to replace the crystal, and with an accurate and temperature stable xtal in later dongle versions, no correction is needed.
Att are the graphs for the S/N results with the two different sample rates; note the S/N values.