11.8 Counters, Prescalers, BCD & Data Converters

11.8 Counters, Prescalers, BCD & Data Converters

Quick Answer: A prescaler divides an HF signal so a low-frequency counter can display the operating frequency. A decade counter gives one output pulse per ten input pulses. BCD = binary coded decimal. ADC = analog to digital; DAC = digital to analog. Binary two = 0010.

Sub-topic 3-E-040 (Counters, Dividers, Converters) is the bridge between the digital chapter and the test-equipment chapter, because the prescaler question is really a frequency-counter question.

The prescaler

What is the purpose of a prescaler circuit? It divides an HF signal so that a low-frequency counter can display the operating frequency.

The problem it solves is concrete. A frequency counter's internal logic has a maximum toggle rate — an older TTL counter might top out around 50 MHz. Measure a 450 MHz transmitter with it directly and you get nonsense. Put a divide-by-10 or divide-by-100 prescaler in front, built from a fast logic family, and 450 MHz becomes 45 MHz or 4.5 MHz, comfortably inside the counter's range.

Two practical consequences a technician must remember:

1. Multiply the reading back. A ÷100 prescaler showing 4.500000 MHz means 450.0000 MHz. Some counters do this automatically when a prescaler is selected; a standalone prescaler does not.
2. Resolution is divided too. Dividing by 100 costs you two digits of resolution for the same gate time. To recover them you must extend the gate time proportionally.

This links directly to section 7.5: the prescaler extends range, while the time base sets accuracy. A prescaler does not make a ±10 ppm counter any more accurate.

The decade counter

What is the function of a decade counter digital IC? Produce one output pulse for every ten input pulses.

"Decade" means ten. A decade counter counts 0 through 9 and rolls over, emitting one carry pulse per cycle — so it is a ÷10 stage. Cascade them and each stage divides by another ten, which is exactly how a frequency counter's display chain is built: units, tens, hundreds, thousands.

Note the contrast in the table: a plain 4-bit binary counter divides by 16, not 10. A decade counter is a binary counter with feedback that forces it to reset after nine, which is what makes decimal displays possible.

BCD

What does the term "BCD" mean? Binary coded decimal.

In BCD each decimal digit is encoded separately in four bits:

The difference matters. 25 in BCD is 0010 0101 — the digit 2 then the digit 5 — whereas 25 in pure binary is 11001. BCD wastes code space, since the four-bit combinations 1010 through 1111 are never used, but it converts to a decimal display with no arithmetic at all. That is why frequency counters, digital multimeters and channel displays use it: each BCD nibble drives one seven-segment digit directly through a decoder.

ADC and DAC

What integrated circuit device converts an analog signal to a digital signal? ADC. What integrated circuit device converts digital signals to analog signals? DAC.

Read the acronym left to right and the direction is unambiguous:

The DAC appears again in section 10.2 as part of the direct digital synthesiser signal chain — phase accumulator, look-up table, DAC, then a low-pass anti-alias filter to remove the sampling images. That anti-alias filter is the specific block the pool names, so do not stop the chain at the DAC.

Binary notation

In binary numbers, how would you note the quantity TWO? 0010.

Place values run right to left as powers of two:

Two sets the 2s column and nothing else: 0010. The distractors are 0001 (that is one), 0100 (four), and 1000 (eight) — all off by a power of two, so count the columns from the right rather than trusting the shape of the pattern.