Digital Frequency Display Applications Notes

Digital Frequency Display Interfaces

DFD will operate to 45 MHz LO with 2V p-p or more drive.
If you have trouble with input frequencies above 20MHz it is recommended that you bypass, or short out, the 1K input resistor. (see schematics)

At 40MHz it normally takes 450mv p-p.
Bypassing the resistor it takes only 225mv p-p.

Use an external coupling capacitor as small as possible to minimize capacitive loading of the local oscillator.

The back to back diodes are there because some radios present high voltage transients when switching bands which will lock up the DFD requiring it to be turned off and back on to unlock.


Several customers have reported success getting local oscillator signals from tube radios by wrapping a few turn of insulated wire around the tube.
Make sure the insulation can take the heat.


20db gain 30MHz bandwidth preamplifier.

provides useful gain to 50Mhz.

For mounting inside radio.
Can be powered from an

IN capacitor would typically be in the order of 5pf to100pf. (As small as possible while providing sufficient coupling)
OUT capacitor would be on the order of 100pf depending on type of cable used to carry signal to counter.

.01 ufd capacitors are supplied with the kit but the input capacitor should be kept as small as possible if connected across the oscillator tank circuit.

.01 is OK if signal is taken from a buffered signal source.

Circuit board and parts layout.
Board is 1" X 2" finished size.

This item is available as a kit.

Click above for ordering

$6 plus $1.50 shipping (no shipping if ordered with a DFD kit)

I5TDJ developed the following very sensitive preamplifier which replaces the 74HC4046

This circuit gives 50mv sensitivity to 50MHz. The output of the 74AC00 drives the PIC directly bypassing the 74HC4046.

Piero used a DFD with the IC-202S two meter rig. It triples a 15MHz VXO to 45 MHz and then triples it again to 145MHz. Piero took the signal after the first tripler and set DFD to multiply by 3. The signal was taken from the junction of R25, R26 and C39 through a 100pf capacitor and about 12" of RG174U.

For those applications that do not require this much gain I played with just the final stage of I5TDJ's design.

Using a 2N3904 this little amplifier gave a gain of about 15 at 6MHz and dropped to about 3 at 50MHz.

It has a rather low input impedence so for those who need to decrease the load on their VFOs an emitter follower can be added.

It is probably best to put this preamplifier in the radio to buffer the radio from the cable capacitance.

The 5V DC can be borrowed from the DFD at the output of the 78L05.

I purchased your DFD and it works great. Although to get it to work with my Kenwood TS-820. I had to add an amp to the front end to beef up the VCO signal from my radio. I thought you would like to know that I used a Signetics NE5205 wide band 20db amp. I used two 1000pf caps, one on the input and one on the output. Also a .01 bypass cap. I taped into the 5volts on your board and I was in business! I did a bench test and the DFD will convert a 50mvp-p signal up to 47Mhz. Which is well above where my radio VCO tops out at. I thought you would like this info and find it useful, if you didn't already know about this chip. Thanks for bringing my radio up to modern standards.

Regards, Steve KE6WOH

You can also use an NE5204. These devices have a 50 ohm input impedence. An emitter or source follower can be added to the input to increase the input impedence.


How to use the optional backlit display module.


Direct conversion units are simple. Connect to the LO (VFO) output and adjust the IF offset to zero. The NE/SE602 is a popular device in such units. The counter should be connected to pin 7 (the emitter of the oscillator transistor)


Single conversion superhets are common in QRP transceivers and older vacuum tube receivers. When connecting to a vacuum tube unit a resistor attenuator may be required. Put a resistor from the input to ground on the Zt pads provided on the PCB. Connect the unit through a series resistor and capacitor to the oscillator plate or cathode (if unbypassed).




ATLAS 210/215 Using DFD1 (A custom chip is available DFD1-Atlas)

The oscillator pre-mix type unit mixes a VFO with a crystal oscillator. Either the sum or difference frequency of that is filtered and sent to the RF mixer. The pre-mix output frequency is the main LO frequency. The IF offset should be set to the units IF frequency. ADD/SUBTRACT depends on if the main LO is above the RF frequency (SUBTRACT) or below (ADD). In some units it may be a function of which band it is set on. In that case it is necessary to have a switched input to the ADD/SUBTRACT input of the DFD.

This type uses a frequency translation phase lock loop to add the VFO to the crystal oscillator. This is better than the pre-mix approach because it lacks the spurious frequencies generated by the LO mixer.

This type is a crystal controlled band switching converter in front of a tunable IF.

I have developed a special version of the DFD (DFD2)which will measure three different frequency simultaneously, the crystal OSC, the VFO and the BFO. It will then compute the carrier frequency of the RF for LSB, USB, AM and the zero beat carrier frequency of CW. It will automatically determine the operation mode as a function of BFO frequency and display LSB, USB, CW or AM.

There are custom chips for the following which offer jumper selectable 10/100Hz resolution and display using either "MHz" or dummy zeros to fill out 8 digits. ie: 14.234.56MHz or 14.234.560 (10Hz resolution) or 14.234.5 MHz or 14.234.500 (100Hz resolution).

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Suggest connecting DFD3 to the 3-5MHz VFO at the jack leading to HF LO mixer as shown above.


This type uses a frequency translation phase lock loop to generate the local oscillator. The VCO is locked onto the sum or difference of the VFO and a harmonic of the reference crystal.


This is a novel design approach intended to provide a stable MHz per band approach without using phase lock loops. A 1MHz xtal oscillator is run through a diode harmonic generator to generate harmonics from 3 to 32 MHz. The "MHz oscillator" tunes continuously from 55.5 to 84.5 MHz. Whenever it is close to one of the harmonics from the harmonic generator one of the many frequencies will be 52.5 MHz. This is selected out by a bandpass filter. A level detector on the output of the filter usually drives some sort of front panel "lock" indicator.

The "MHz oscillator" also mixes with the incoming RF frequency and is passed through a 1 MHz wide bandpass filter centered around 55 MHz. The output of that filter is mixed with the 52.5 MHz filter output causing a 1MHz band of frequencies centered at 2.5 MHz at the output of the 2-3 MHz filter.Any positive drift in the MHz oscillator causes a negative drift in the 52.5 MHz frequency effectively canceling out the drift so far as the 2-3 MHz band is concerned.

A 3.455 to 2.455 MHz VFO tunes the 2-3 MHz into a 455 KHz IF section.

At present the  DFD1  must connected to the 3.455-2.455MHz VFO and only the bandspread frequency (0-.9999MHz) will be displayed. The MHz portion must be read off the front panel when the lock indication is on.

You can also connect  DFD3  in the same way and obtain complete frequency display using the band selector function of DFD3.


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