LED Frequency Counter
Circuit Details

This counter was first known as the Modular Dial developed by Bill Carver, W7AAZ, and published in the Spring 1998 issue of Communications Quarterly.

When I took a serious look at the Modular Dial in 2006 it seemed a good match for the Electroluminescent Receiver Kit (ELR). I was looking for a counter with an LED display and flexible add/subtract offset frequency functions.

When I contacted Bill, W7AAZ, he handed over the project and told me to run with it. There was a lot of work to do. The original PIC16C62 chip had become obsolete and the code needed some rewriting to work with the modern updated PIC16F72. There had been a lot of updating on the way code was written since the original PIC chip. A lot of help, study, and rewriting was needed to get it working.

Luckily, there was a PIC programmer who worked across the street from my work place, a huge surprise, since I never expected to find anyone who had even heard of PICs in my little town of Pampa, Tx. He had been a programmer for over 10 years and understood Bill's code. We met at lunch, working on paper napkins, until we got it working.

I also had the help of another dyed-in-the-wool homebrewer who had considerable PCB design work under his belt. His suggestions on layout and some circuit design was invaluable. He followed along with the design of the PCB helping with final decisions on the layout.

The original board was redesigned with most of the interface circuits integrated on the PCB so it would be easy for ELR and general purpose use in other receivers/projects as well.

The goal of the new design was to make it as universally usable as possible with the option of using any LED display of the builder's choice.

The Frequency Inputs

The counter has two signal inputs, the black squares labeled "VFO IN" (left side) and "OSC IN" (right side). The "OSC IN" is counted once when first turned on for two seconds, briefly displayed as a diagnostic tool and saved. "VFO IN" is counted 5 times a second, either adding or subtracting the "OSC IN" value and displaying the result to the nearest 10 Hz on seven LEDs.

Oscillator Input	VFO Input

Both frequency input circuits are identical using a 74HC4046. The input circuit comes from AADE. Neil uses this circuit on all his frequency counters. It does very well at low drive levels (input range 50 mv to 5V p-p) which makes it very easy to work with any VFO/oscillator circuit. Connecting this counter to the VFO in the ELR does not make any noticeable difference in the output. For ideas interfacing to other projects, check out Neil's Applications Page.

The 74HC4046 can be overloaded causing improper counting and, in some cases, shuts down counting altogether. If any problems occur when interfacing the counter to a VFO, lower the drive level with a lower value input capacitor.

In the ELR, the input circuits are tied into a Vackar VFO and an FET crystal oscillator with 3.3pf capacitors.

The prescalar inside the PIC is rated to 50 MHz. The math to add, subtract, and convert counts to decimal numbers for display overflows at 83.886 MHz. For example, summing the frequency of a 70 MHz oscillator and a 20 MHz oscillator will not display properly even though they both count perfectly.

Reset and Recalibration Function

When power (or a reset/recal) is applied to the counter, a +5 volt signal is generated on Pin 28 of the PIC which drives a 2N3906 and supplies 12 Volts to the 74HC4046 of the "OSC IN" input circuit. The 2N3906 sends a signal to the crystal oscillator switching circuit to hold the current frequency for uploading into the "OSC IN" frequency input.

A 47 mfd capacitor and 56K resistor at the crystal oscillator switching circuit provides a delay for the counter to get the "OSC IN" reading. The "To Freq Counter" in the above schematic goes to the "XTAL OSC" square on the counter next to the 2N3906.

Pin 1 of the PIC16F72 is the reset pin and is labeled on the PCB as "RECAL", meaning "recalibrate" the counter. When the crystal filter switch on the ELR is moved, it provides a down-to-up or up-to-down 12 volt/ground change (from the 40/20 side of the crystal filter switch) in which the 4077 (detecting an edge change) uses to reset the PIC with a quick +5V on Pin 1.

The "40/20" box connects to the crystal filter switch (40/20 side) on the ELR to provide the trigger for a recalibration when the crystal oscillator changes frequency.

Add/Subtract Function

The Add/Subtract function is controlled by the "20/17" box interfaced to the PIC with a 4066 (Quad Bilateral Switch) CMOS IC. The schematic is below:

The 4066 isolates the PIC from outside currents and voltages that may accidentally kill the chip.

Grounding the "20/17" box adds the two frequencies input into the counter and +12V or +5V volts at the box subtracts. Use +5V if this function is used outside the ELR receiver. With no connection at the "20/17" box the counter adds.

A wire from the "20/17" box goes to the 20/17 side of the bandpass filter switch to provide the signal (12V or ground) to activate the plus/minus offset function.

Zero Function

The Zero function in the schematic above stores the current display reading and puts all zeros on the 7 digit display and counts, either positive or negative, are displayed relative to the stored count.

This is very handy for evaluating drift in VFOs, net operation, DXing and even casual operating.

The function is activated by a short +12 volt pulse on the "Switch" box (with a push button switch) and then a second short +12 volt pulse restores the display.

Leading Zero Suppression

The counter suppresses leading zeros. The display will show "7040.10" rather than "07040.10".

Time Base

On single conversion receivers when the "OSC IN" is reading the BFO, the counter will be reading the exact frequency of the signal.

The ELR is a dual conversion receiver with the "OSC IN" reading the first conversion oscillator (either 3.547 or 4.000 MHz), but not the BFO. So the reading will be off by 500 Hz to 1.2 kHz, depending on whether the signal is CW or SSB. So the readout accuracy is approximately plus/minus 1 kHz.

The display is multiplexed at a high rate for easy viewing. The processor applies +5 volts to the segments and a ground for the digits The digits (displays) are common cathode high-efficiency orange.

Multiplexing is achieved by grounding the digit lines to pull the emitter of a digit transistor (PNP 2N3906) to 0.7 volts, turning on the segments of each display. The digit lines are labeled 1 through 7.

Number 1 is the Least Significant Digit (LSD). The LSD is the 10 Hz value. Number 7 is the Most Significant Digit (MSD). The MSD is the 10 MHz value.

The segments are current limited by 220 ohm resistors in the segment lines. These resistors determine the brightness of the display and can be lowered or raised to suit individual preferences. It is recommended not to go below 47 ohms to keep the segments from burning out over time.

The displays in the kit are high-efficiency which give high brightness with little current. Other displays will require lower resistor values to get readable displays in bright room lighting.

Display Noise

Display noise is always a problem with a frequency counter. With the mounting of the counter below the First Mixer when using stacked boards in the ELR, the display noise has an easy route into the receiver.

An extensive ground plane was incorporated on top of the board. The ground lead between the PIC and LED sections is between the segment and display driver leads. The theory on the ground plane is that capacitance between the display driver lines and the ground plane would help suppress display noise.

Additional suppression can be obtained by placing a shield on the bottom of Board 1 at the front above the counter.

A box made out of the single sided PCB included with the counter is another option. Using the PCB for a single shield on Board 1 or building a box may depend on recommendations later when tests have been done to compare the results. Until then, the method to use is personal preference or experience.

The black arrows point to the connections to the ground plane of Board 1 of the ELR. Short pieces of bare wire are soldered to the PCB and then bent down to the ground plane of the receiver. NOTE: Double check you are soldering to the ground plane and not a trace!

Notice that the board at the top of Board 1 in the above picture has cutouts on each corner of the board. This allows the board to be soldered even with the front of the board where the counter mounts.