# How are pulse trains generated in a radio transmitter?

When simulating a PSK signal in MATLAB or Python, I often create an impulse train where each impulse is modulated with a symbol. This modulated impulse train then passes through a pulse shape filter to yield the PSK signal.

However I am wondering if this is indeed how PSK signals are generated in real radio electronics hardware. Is there an electronics module which generates modulated impulses which passes through an analog filter? If so, how narrow are such pulses typically?

I know the block diagram representation of PSK implementation in hardware often shows dataflow split into two paths, one for In-phase and one for Quadrature-Phase. Each path passes through a Root Raised Cosine filter before combining back again. However it is not clear to me how this RRC module is implemented in hardware.

## 2 Answers

What's the date of the radio?

If it's 1950 through about 1980 or 1990, then the all-analog way to do it would be to generate the carrier and a $$90^\circ$$ shifted version of it, then switch in the carrier, the carrier shifted $$180^\circ$$ (which is trivial), the $$90^\circ$$ carrier, or its $$180^\circ$$-shifted version. (Alternately, you'd always have the carrier or it's $$180^\circ$$-shifted version added to the $$90^\circ$$ shifted or $$-90^\circ$$ shifted version -- it may be easier to realize in circuitry, and you'd still get phase shifts in $$90^\circ$$ increments.)

If you wanted the result to have raised-root-cosine shaping, then you'd come up with a filter that takes rectangular pulses in, and makes an approximation to your nice raised-root-cosine pulses out.

If you're building a radio today, you'll probably do all of the signal generation in a processor (conventional or DSP) or an FPGA. You'd either pipe a baseband signal out a pair of ADCs to an up-converter, or (particularly in the FPGA case) you'd generate your signal at some RF frequency to an ADC, which either drives an antenna directly or is upconverted to your desired transmit frequency.

• My second "inside the FPGA" option is basically the method that @BalooRM outlines. May 21 '20 at 14:17

Arbitrary Waveform Generators are a special type of test equipment that can generate, as the name implies, arbitrary waveforms as output signals that can be fed into transmitters or other circuits. They vary significantly in sophistication.