[Hackrf-dev] [hackrf-dev] Shape of output waveform

[Michael Ossmann]

On Sun, Sep 02, 2018 at 08:47:12PM +0200, kamilkarpacz at interia.eu wrote:
>
> What is shape of output RF signal (carrier wave without modulation).
> Is it sine or square?It shoud be sine in 2.3-2.7ghz because its
> outputed directly from MAX transceiver, but what about other
> frequencies?

The mixer bypass band is from 2.15 to 2.75 GHz on HackRF One.  In that
band there are fewer images, but there is some power at the second
harmonic (4.3 to 5.5 GHz).  See the MAX2837 datasheet for details.

> IF frequency is mixed in RFC synthesiser, but the output from this
> synthesiser is square so the mixed output should be mix of many
> harmonics from sqare wave.I would like to use hackrf also as a RF
> signal generator so i would need a somewhat nice sinewave.

Yes, there are multiple images produced as mixer products.  The exact
nature of these depends on both the selected RF and IF.  In general, the
strongest undesired image should be at least 10 dB below the intended
frequency.

Neither frequency synthesizer is perfect, and neither mixer is perfect,
and there can be products of multiple imperfections.  Oh, and there are
various amplifiers that can cause extra images if they are overdriven.
In order to know how much power there is at what image frequencies you
would have to measure it for a specific configuration.

Additionally it is possible to have notable aliases produced in the
analog baseband, but those can typically be avoided by configuration.

General guidance: If having random image frequencies at -10 dBc isn't
clean enough for your application, you'll need external filtering.

Mike