Frequency counter operation with block diagram

A frequency counter measures the frequency of a signal by counting the number of its cycles during an accurately known time interval (the gate time) and displaying the count:

$$f = \frac{N}{T_{gate}}$$

Block diagram

 Input   +-----------+  pulses  +-----+   +-----------+
 signal->| Amplifier |--------->|     |-->| Decade    |
         | & Schmitt |          | AND |   | counters  |
         | trigger   |    +---->|gate |   | (BCD)     |
         +-----------+    |     +-----+   +-----+-----+
                          |                     |
 +---------+  +---------+ | gate     +----------v----+
 | Crystal |->| Divider |-+ pulse    | Latch         |
 | osc.    |  | chain   |---+        +-------+-------+
 +---------+  +---------+   |                |
                       +----v-----+  +-------v-------+
                       | Control  |  | BCD to 7-seg  |
                       | (reset,  |  | decoders and  |
                       |  latch)  |  | displays      |
                       +----------+  +---------------+

Working of each block

1. Amplifier and Schmitt trigger (wave shaper): amplifies the unknown input (sine, triangle, etc.) and converts it into clean rectangular pulses of the same frequency, one pulse per cycle.
2. Time base: a crystal oscillator (e.g. 1 MHz) followed by a chain of divide-by-10 counters gives a very accurate gate pulse of 1 s, 0.1 s, 10 ms, … selected by the range switch.
3. AND (main) gate: one input receives the shaped pulses, the other the gate pulse. Pulses pass to the counter only while the gate pulse is HIGH, i.e. for exactly $T_{gate}$.
4. Decade counters: cascaded BCD counters (e.g. 7490) count the pulses that pass the gate; each counter is one decimal digit.
5. Control logic: before each gate period it resets the counters; when the gate closes it sends a latch (transfer) pulse, then resets again for the next measurement.
6. Latch, decoder and display: the latch holds the final count so the display does not flicker while counting; BCD-to-7-segment decoders (e.g. 7447) drive the LED displays.

Example

With $T_{gate} = 1$ s, if 5000 pulses pass the gate, $f = 5000 / 1 = 5000$ Hz = 5 kHz. With a 0.1 s gate the same signal gives 500 counts, displayed as 5.00 kHz with the decimal point shifted. A longer gate gives better resolution (1 Hz for a 1 s gate); the accuracy depends on the crystal time base, with an error of ±1 count.