Fabrication of an 8-bit 150MHz digital frequency meter

When making high-frequency circuits, we often encounter the problem of frequency measurement. In fact, a frequency counter is an electronic device that is relatively easy to make. 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 frequency counter are as follows:
(1) Number of displayed digits: 8-digit LED
(2) Frequency test range: 10Hz~10MHz, using a preset divider (Prescaler) to do 1/100 division, up to 150MHz
(3) Reference time stability: Due to the use of TCXO unit (temperature compensated crystal oscillator circuit), the temperature characteristics are ±3PPm/-20~60℃ aging rate±1ppm/year.
The above parameters meet or even exceed the parameters of frequency counters produced by general manufacturers.

The block diagram of the frequency meter produced
(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 LSI for the frequency meter.)
The structure of the frequency meter
The block diagram of the frequency meter produced is shown in Figure 38. The following is an explanation of the block diagram.

The schematic diagram of the frequency meter circuit to be produced

▲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. Adding peripheral wide-band amplifiers, LED actuators, etc., can complete the frequency meter.
▲The main parameter indicators of the TCXO unit of the reference time signal generation circuit
show that it has excellent frequency stability. The oscillation frequency of TCXO is 12.8MHz. The frequency division IC 74HC4060 is used to divide the frequency by 1/12 8, and then the reference frequency 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)

TCXO TCO-703A performance parameters
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 capacitors)
The same series of products can provide frequency options 10MHz~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 performs 1/100 frequency division.

▲Other circuits
The frequency meter LSI input terminal switches between the output of the wideband amplifier or the output of the pre-divider by using a 3-state buffer 74HCl25 as a switch, as shown in Figure 41.

Pre-frequency divider circuit for 1/100 division
(The maximum input frequency of the counter IC is only 10MHz. Therefore, using the preset frequency divider to divide by 1/100 can expand the frequency range of the test.)
Production and adjustment of the frequency meter
Figure 42 shows the parts configuration and printed circuit board pattern of the frequency meter. The printed circuit board includes a rectifier circuit and a voltage regulator circuit.
The LED used for numerical display uses an eight-segment common cathode type (Common Cathode) LED. A larger size can provide a good observation effect. Of course, if this specification of LED is not available, the printed circuit board can also be modified as necessary according to the available LED specifications.
After the parts configuration of the pre-frequency divider and the printed circuit copper foil board are completed, the collector voltage of 2SC2347 is adjusted to 2.5V using the VR 10k of the wideband amplifier. There is nothing else to adjust. Find a beautiful shell to install the entire circuit, and the production of the entire frequency meter is completed.