Digital Frequency Display Applications Notes

Digital Frequency Display Interfaces

the display can be remotely located using wires or a flat ribbon cable such as

Digi-Key number A9BAG-14xx where xx is the length in inches from 2 to 8.

cable.jpg (7484 bytes)

DFDs will operate to 45 MHz LO with 2V p-p or more drive.
All DFDs now use 100ohms for R1 which doubles the sensitivity.

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

Zt is to terminate the input coax if necessary, it rarely is used.

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.


sensitivity 50 ohms.jpg (84909 bytes)

Customer supplied measurements.

He put a 50 ohm termination at the input of the DFDs and used a signal generator with 50 ohm source connected by 50 ohm coax.

Note it shows more sensitivity than the application notes minimum input level curve above.


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.

(I have several of these if you need one $5)


Hi Neil,

Again, thanks for sending the MC12079. It did solve my problem which was essentially a very weak input signal at 2.614GHz.

Based on some information around the web, I think the MB506 can support frequencies above 2.4GHz, but it does require a stronger signal. In my application, I have not been successful using it.

I am not at all sure that it is necessary, but once I got playing, I decided to address possible UHF signal attenuation in the input to the pre-scaler chip.

I moved the MC12079 and associated support off the main board to a small PCB attached to a Bomar Edge Mount BNC connector (361V504EFT - Digikey #991-1035-ND).

The .031" PCB features a 50O microstrip direct to the MC12079D (SO-8).

DFD4A-UHFpcb.jpg (82471 bytes)

This is a simple and easily reversible modification:

Remove the MC12079 from socket
Remove R2(1kO) from the main board
Connect the new PCB RF directly to the UHF-VHF switch
Pickup VCC at the location for the unused 2 pin header to pin 3
Connect GND to main board (make sure to reconnect the HF BNC ground)

DFD4A-UHF Input.jpg (361833 bytes)

Due to the cost of the BNC connector, it is not a cheap change so definitely not for everyone.
I have attached a couple of pictures of my prototype and would be happy to provide more information if you desire.
I have no way of measuring any real improvement in performance but doubt that it is significant. I do, however, feel better having eliminated the long point to point BNC to prescaler connection for MW signals.

Again, thanks for an absolutely great product and great support.

Best regards,


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)

circuits suggested on the NE602 data sheet are

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).


Here is how Tapio, KF7TY added a DFD1 inside his Swan 250.  Use (DFD1-Swan)

"Well I'm finally getting around to finishing up the Swan 250 project, and what a difference that display makes, going to put it on the air in the next few days and start using it alot when band conditions allow. Sending along a few pics to show you how I set it up. I had to separate the display from the board with some 14 conductor ribbon because of room constraints. I was able to also get the display and frame to fit into the existing analog display opening without hacking up the front panel so incase I or someone else wants to return it back to stock they can, I kept all the parts for this. I am having no problems with noise at all and the display is as solid as can be, no wavering on transmit either. Currently still running the display with a 9 volt battery which is also powering the backlight but will build a small regulated circuit run off the rigs filiment power supply. Thanks again.

73's Tapio, KF7TY"

2504.JPG (22845 bytes)

He put the display in the hole that the dial was in using one of my bezels.

2503.JPG (23097 bytes)

The counter board was mounted in the back of the unit and a flat ribbon cable

2502.JPG (29219 bytes)

was used to remote the display module to the front panel.

I can be the “point” man when others inquire about how to install it.  Just point them to me.  Also, I’ve taken a closer look at the Corsair I.  It is slightly different.  In order to create BAND, two signals have to be combined.   One is labeled 10/18 (for 10 and 18 meters) and the other bands are called HF.   By combining HF and 10/18 (through a couple of diodes), you create BAND.   SBR is ok and same as the Corsair II.  The display IC used in the Corsair I is a MK50398N.  I am not familiar with that device.  I need to find someone who has a display gone bad on their Corsair I to try out the fix.   That really shouldn’t be hard since I have seen a number of Corsair I radios with the problem.  J

Jerry W5JH

Corsair I

time base.jpg (144163 bytes)


osc mxr.jpg (229539 bytes)


PCB connections.jpg (33338 bytes)


Clarks homebrew case for DFD2, fantastic craftsmanship.

HRO-50 DFD-2 018.jpg (75581 bytes)

HRO-50 DFD-2 006.jpg (30566 bytes)

Connections to the HRO-50 shown in the photo below.

HRO-50 DFD-2 008.jpg (68315 bytes)

DFD coupled via two one-inch lengths of pvc-covered hookup wire twisted together, at the oscillator tuning gang, connected through 2 ft of RG-58 to the DFD. The twisted wires read about 2pF on my AADE LC meter.
(I suspect this means right on the main tuning capacitor, section for the local oscillator.)
SX-71 shows correct on all bands except I can not vouch for its accuracy on 6 meters as my radio has a broken dial string on the bandspread capacitor – this is the cap which tunes 6 meters. Also this radio is not calibrated or aligned on this band. However the DFD showed “believable” readouts, ie, a bit over 50mHz.
SX-71_DFD_1.jpg (65538 bytes)
SX-71_DFD_2.jpg (61389 bytes)
DFD1A (using my plug-n-play model, with optional two line blue display, in this example but applies to DFD1A kit as well) connected to my SX100 (IF frequency = 1650KHz)
using the universal interface connected to the tuning gang.  I stuck the interface PCB to the shield plate using double back foam tape.  The plug-n-play DFD1A order number is C-DFD1A.
It can be used with most any Hallicrafters except the SX115 or SX117 which are triple conversion.  It is also useable with any single conversion radio (crystal controlled second conversions don't count as they do not change the mathmatics of calculating the RF frequency).

sx100c.JPG (20475 bytes) sx100b.JPG (11721 bytes) sx100a.JPG (20766 bytes)  (Use

Pin 7 of U1 is the emitter of the emitter follower inside U1.  The base, pin 6,   is driven by the local oscillator so pin 7 is a buffered LO signal

sw40.jpg (19714 bytes)

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).

FT-101 (using DFD2-101)

ft101a.jpg (31735 bytes)

Here is the finished product.  He has not powered the backlite yet so it will be more readble when he does.

FT101b.jpg (27474 bytes)

Picture just before final testing.

FT101c.jpg (21367 bytes)

The counter board is remotely located from the display module using a flat ribbon cable from digikey

goto and plug A9BAG-140 into the search window

Power was taken from pin 12 of PCB-1547 (regulator PCB)  14 volts DC.

Howard, N4DXX,  made a nice internal installation and a second one using the blue display.

 Picture 171.jpg (155299 bytes)Picture.jpg (158759 bytes)

FT101EX.JPG (43027 bytes)

Bill, WA0VGD added a (DFD2-101) to his FR-101 to replace the existing readout.

FR-101.jpg (26468 bytes)


McKay.jpg (22159 bytes)

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

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  DFD1A  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.

Back to DFD spec page

Back to homepage