--------------------[ Slowing down signals with sampling: ]---------------------
Publication date: 1701392781


Mixers are the classic way to convert a signal to a lower frequency so it
can be processed or measured easier. But mixing only works on narrowband
signals, ones that are very nearly a sine wave. 

Great for radio, but useless for anything else. 

Sampling can slow down any repeating signal, including square waves and
narrow pulses. The trick is to measure the voltage at a slightly lower frequency
then the signal's repetition rate, and hold that voltage until the next
measurement. 

Because each sample is taken at a slightly later time in the waveform, the
resulting signal is a slowed down copy of the original. The new frequency
equal to the difference of the original's and the sampling frequency. 

[IMG sampled.png ]

Blue: Original waveform. Orange: Samples. Green: Reconstructed waveform.


The classic way to do this is with diode switches: 

[IMG sampler1.png ]

Most of the time, the diodes are reversed biased, blocking the signal. 
When a sample pulse arrives, it briefly forward biases the diodes, charging
the capacitors to the input signal voltage plus/minus the threshold voltage.
These 2 voltages are then added together with a resistor divider to get
the final voltage. 

A fully symmetrical configuration helps clean up the output: 

[IMG sampler2.png ]

This more complex sampler that charges a capacitor to the sampled voltage,
isolating the sample pulses from the output as well as the input. MOSFET
transistors can be used, and have the advantage that a short sample pulse
is not required: 

[IMG sampler3.png ]

One problem of this configuration is that the output can only change 50%
of the way to the input, but this is not a problem for measuring periodic
signals. 

Symetrical MOSFETs are hard to find, but analog switch ICs are common: 

[IMG sampler4.png ]