How are pulsars detected from Earth?
the signal period of known pulsars is 1 ms-15 s (0.07-1,000 Hz since F=1/T).


That's the frequency of how often the signals occur, not the frequency of the waves they emit. Just compare it to a lighthouse, which has a signal period of a few seconds but a wave frequency of $10^{15}$ Hz.

The first known pulsars, for example "Little Green Man" or PSR B1919+21, were radio pulsars; your graph shows those waves are detectable from Earth's surface. I stumbled upon this article which lists the frequency of some of the pulsars:


  After the discovery of pulsars at 81.5 MHz (Hewish et al. 1968), the first pulsar detected at 40 MHz was PSR B1133+16 (Arecibo Observatory, 1968, unpublished). In 1969–1970 the pulsars PSR B1133+16 and B0809+74 were detected at 25 MHz (Bruck 1970).


Below is the full detection range for the telescopes:


  Therefore joint observations (simultaneous or not) will be highly beneficial, providing a continuous frequency coverage from 8–240 MHz.


which is roughly equivalent to wavelengths of 40m to 1m.

X-ray pulsars like Centaurus X-3 were discovered later, and not by telescopes on the surface:


  Centaurus X-3 was first observed during experiments of cosmic X-ray sources made on May 18, 1967. These initial X-ray spectrum and location measurements were performed using a sounding rocket.