Production of 150MHz digital frequency meter based on ICM7216

When making high-frequency circuits, you will often encounter the problem of frequency measurement. In fact, a frequency counter is an electronic device that is relatively easy to make yourself. The performance of the frequency counter to be made below is not inferior to that of products made by general manufacturers.

The electrical indicators of the produced frequency meter are as follows:

(1) Number of digits displayed: 8-digit LED

(2) Frequency test range: 10Hz to 10MHz, using a prescaler to divide by 1/100, up to 150MHz

(3) Reference time stability: Due to the use of TCXO unit (temperature compensated crystal oscillator circuit), the temperature characteristic is ±3PPm/-20"60℃ aging rate ±1ppm/year.

The above parameters have reached or even exceeded those of frequency meters produced by general manufacturers.

Frequency Meter Block Diagram

(The main circuits are the input amplifier, prescaler, and display. In addition, the important control circuit, counting circuit, display circuit, etc. are already included in the frequency meter LSI.)

The structure of a frequency meter

Schematic diagram of the frequency meter circuit to be made

▲Counting (frequency counting) IC;

This production uses ICM7216B (Intersil). This IC contains an 8-digit BCD counter, a BCD to 7-byte segment decoder, a control circuit, etc. By adding peripheral wide-band amplifiers, LED actuators, etc., a frequency meter can be completed.

▲Reference time signal generation circuit

The main parameter indicators of the TCXO unit show that it has excellent frequency stability. The oscillation frequency of the TCXO is 12.8MHz, and the frequency division IC 74HC4060 is used to divide the frequency by 1/12 8, and then the reference frequency of 100kHz is supplied to the frequency meter LSI.

Characteristics of the temperature compensated crystal oscillator TCXO TCO·703A (Since it has a built-in temperature compensation circuit, the frequency stability is good. The oscillation frequency is 12.8MHz, and then the frequency is divided by 1/128 using the frequency division IC74HC4060 to become 100kHz) [page]

Output frequency 10MHz 12.8MHz or 14.5MHz

Power supply voltage 5V±5%

Working power consumption ≤5mA

Output level 1VP-P

Load 20KΩ//5pF

Frequency stability temperature characteristics ±3ppm/-20"60℃

Power supply variation characteristics ±0.3ppm/5V±5%

Long-term characteristics ±1ppm/(year)

Frequency adjustable range is greater than (including trimming capacitor)

The same series of products can be selected from 10MHz to 24MHz

▲Pre-divider circuit

Since the upper frequency limit of ICM 7216B is 10MHz, the upper frequency limit can be increased by using a pre-divider. Figure 41 shows the circuit diagram of the pre-divider, which is used for 1/100 frequency division.

1/100 frequency division pre-frequency division circuit

(The maximum input frequency of the frequency counter IC is only 10MHz, so using the preset divider to divide by 1/100 can expand the frequency range of the test.)

Making and Adjusting Frequency Meter

Frequency meter parts layout and printed circuit board pattern

The printed circuit board includes a rectifier circuit and a voltage stabilizing circuit.

The LED used for numerical display uses an eight-segment common cathode LED. A larger size can provide a better viewing effect. Of course, if this specification of LED is not available, the printed circuit board can be modified as necessary according to the available LED specifications.

After the substrate is completed, use the VR 10k of the wideband amplifier to adjust the collector voltage of 2SC2347 to 2.5V. There is nothing else to adjust. Find a beautiful shell to install the whole circuit, and the production of the whole frequency generator is completed.