Rubens R.

Fernandes, ex-PY2QE
37 Borodino Ct, Greenwith, SA 5125, Australia; ruferna@gmail.com

A Frequency Counter for

the Experimenter
Here is a classic workbench general purpose instrument, with better than
26 mVrms sensitivity from 30 Hz to 2.2 GHz.

Frequency counters are considered

indispensable tools for experimenters and
basic models can be found on the market for
reasonable prices nowadays. Nevertheless,
besides basic features like good sensitivity
and variable gate times, I wanted something
more, like the possibility of synchronizing
to external high-stability time bases. (I have
an old HP 10811-60164 time base and one
of these days I will gather enough courage to
build a GPS receiver and lock this unit to the
1 part per second cesium-based reference
signal.) I also wanted a counter that could
do all the boring math when connected to
the VFOs of the QRP rigs I have been build-
ing for years, independently of the mixing
strategy. It should also measure lower fre-
quencies, down to audio range. As I didn’t
find such an instrument having a reasonable
price, I started designing my own.
The final result was a two-channel coun-
ter, the first channel ranging from 30 Hz The frequency counter in this project is connected to a commercial RF generator.
to 55 MHz, with high impedance, and the
second channel from 55 MHz to 2.8 GHz,
with a 50 Ω impedance. Four gate times
were included: 0.01, 0.1, 1 and 10 seconds.
Photo A shows the front end enclosure that I processor and some components here and
It can measure period too, up to 25 kHz, and
built. Another important thing is to provide there. I suggest a shielded enclosure for the
display the result as time or frequency, at
a path as close as 50 Ω as possible from the unit, to reduce electromagnetic interference
your choice. We can also introduce an offset
Channel B connector (the high frequency (EMI) problems. The main box was built
— there are pre-defined offset frequencies
channel) to the prescaler (LMX2326). I used using one sided phenolic boards. The lead
you can choose (based on the most common
a 0.1 inch trace on the circuit side. FR4 (fiber photo shows the assembled unit.
IFs found on homebrew and basic commer-
glass) material is a good choice for the board It would be wise to orient the external
cial rigs) or you can just input the frequency
(although I have used phenolic material), sync connector to the outside — a quick
of your choice and it will be memorized.
with a ground plane on one side and the cir- connection type would be great, like a sub-
Although the main objective was sinusoidal
cuit on the other. Use surface mount devices miniature B (SMB) connector. I have used
signals, it will work with other waveforms,
(SMD) for both channels. ordinary (though good quality) BNC con-
like square or triangular waves. A low-bat-
The original unit has four printed cir- nectors for the channel inputs.
tery alarm was also included.
cuit boards: front end, power supply, time
base and control board, and all boards were
Circuitry
Construction assembled using 0805 surface mount tech-
Figure 1 shows the schematic diagram
Both channels must be very well shielded. nology (SMT) components, except for the

10 QEX – May/June 2010 Reprinted with permission © ARRL

of the front end circuit board. On the low
frequency channel (Channel A), the FET
provides high input impedance. It is followed
by two amplifiers and a Schmitt trigger
inverter that “digitizes” the signal. The high
frequency channel (Channel B) input has a
3 dB pad that helps to define the 50 Ω path
between the connector and the monolithic
amplifier. The LMX2326 is a low power
phase locked loop (PLL), but only its pres-
caler is used here. Channel A has an overall
sensitivity of about 25 mV RMS from 30 Hz
to 55 MHz, (from 0.2 Hz for square waves).
Channel B has roughly the same sensitivity,
from 55 MHz to 2.2 GHz. Above 2.2 GHz,
the sensitivity is degraded to about 150 mV
RMS at 2.8 GHz. Careful layout for the
Channel B input could probably improve this
behavior. The power supply of the channel
not selected is switched off, to avoid interac-
tion and to minimize power consumption.
Maximum levels are 50 V peak for Channel
A and +16 dBm for Channel B.
The power supply (see Figure 2) was pro-
Photo A — It is best to build the counter front end circuitry into a shielded enclosure.
vided with a protection circuit at the power

Figure 1 — Here is the schematic diagram of the counter front end circuitry.

Reprinted with permission © ARRL QEX – May/June 2010 11

 (A)

(B) (C)

Figure 2 — The schematic diagram at Part A shows the counter power supply. Part B shows the top of the supply circuit board and Part C
shows the bottom of the board, with circuit traces and SMT components.

12 QEX – May/June 2010 Reprinted with permission © ARRL

input (9-12 V dc) that can be omitted if you original counter, I used a commercial adjust- with 1 Vpp, the internal reference is auto-
have a tidy workbench and well behaved able oscillator, disassembled from a piece matically switched off.
friends. The fuse will blow if you have an of old NEC equipment. I suggest the two- The PIC16F876A processor is the heart
internal short circuit, if the supply leads are transistor circuit of Figure 5 for the internal of the counter and it also controls the display,
inverted or in the case of an overvoltage time base, noting that the capacitors that are which requires much less power for its blue
(>15 V dc). The power supply module gener- part of the oscillator are all NPØ, including back light than the older green ones.
ates 5 V dc for general use, switchable 5 V dc the trimmer — SMT fixed capacitors would
for the front end channels and an adjustable be fine. I have tested this circuit with an
Adjustments
voltage for the display back light. ordinary crystal, and the circuit worked very
There are only four adjustments in the
Figure 3 shows the schematic diagram of well. It would be a very good idea to enclose
counter. First of all, adjust the trimpot on the
the internal time base, along with photos of the circuit with a small shielding box. When
processor board for the best display contrast.
the top and bottom of my circuit board. In the you connect an external time base signal

(A)

(B) (C)

Figure 3 — Part A is the time base schematic diagram for the circuit I used. Part B shows the top of my circuit board, with the commercial
10 MHz module. Part C shows the bottom of my circuit board, with the circuit traces and SMT components.

Reprinted with permission © ARRL QEX – May/June 2010 13

 Figure 4 — The PIC 16F876A controller, shown in this schematic diagram, is the heart of the counter.

Now connect a 25 mV RMS, 50 MHz sig-

nal to Channel A and adjust the trimpot at
the input of the inverter until this frequency
(or close) is steadily shown on the display.
Increase the frequency up to 55 MHz, always
tweaking this trimpot.
Now connect a reference signal (10 MHz,
for instance) to the Channel A input. Adjust
the internal time base trimmer until the dis-
play shows the exact frequency, down to
units of Hz. Note that the precision of this
calibration adjustment will depend on the
precision of this external reference signal.
Finally adjust the power supply module
trimpot for a comfortable display back light
— if you intend to use this counter frequently
with batteries, it would be wise to adjust to
the minimum voltage that will still give good
readability.

Software and Operation

Excluding the ON-OFF switch, this is
a one-button-only instrument — pressing
and releasing the MODE button will give
the SCROLL command, and pressing and
holding the button will give the ENTER
command. If you are in the main display,
scrolling will toggle between Channel A Figure 5 — This 10 MHz reference oscillator can be used as a reference oscillator for
the counter.

14 QEX – May/June 2010 Reprinted with permission © ARRL

and B and ENTER will take you to the the dark side of the force and/or don’t care Rubens R. Fernandes earned a BS in
main menu. The following choices will be about the world above 550 MHz, replace the Electrical Engineering in 1970, and worked
shown: gate options, period measurements LMX2326 with an LMX2306 (same pinout). for about 31 years in the telecommunications
and offset. The software, in this case, has to be changed industry, in research, design and production.
Gate options: Select the gate time — and the channel B sensitivity will be around He retired about 5 years ago. He has been a
0.01, 0.1, 1 or 10 seconds. This is the time the 5 mV RMS, from 25 MHz up. I suggest, in licensed Amateur Radio operator since 1979,
and held the call sign PY2QE while living in
gate will be open for the measurement. Faster this case, that you replace the original front
Brazil. He is a CW enthusiast. He now dedi-
measurements will give less precision. end pad with a 9 dB one, resulting in an over- cates his spare time to home brewing small
Period: When you measure frequency, all sensitivity better than 10 mV RMS. The transceivers and test equipment. He has a
the gate will be kept open for a defined exten- hex program files for both versions are avail- small workshop that includes mechanical, elec-
sion of time and the events will be counted — able for download from the ARRL QEX files trical, software and silk-screening facilities.
on the other hand, when you measure period, Web site. (See Note 1.)
two subsequent events will respectively open
and close the gate, and the time between the Final words
events will be measured. For this reason, I have been using this counter for over a
measuring the period of low frequency sig- year, and it is extremely useful, together with
nals will lead to more accurate results. You other home built equipment I have made.
can measure very low frequency signals Many concepts used here were derived from
using this technique, and the result can be good technical literature and research on
shown as frequency or as time. Remember the Internet, especially concerning compo-
that Frequency = 1 / Period. To leave the nent data sheets.2, 3 The design of this coun-
Period mode and start the Frequency mode, ter was also possible due to good friends:
just choose a gate time. Delson, PY2DME, helped me with measure-
Offset: Here you can add or subtract a ments above 1 GHz. Eduardo, PY2GNZ,
frequency to the signal present at any input. assembled and tested prototypes and Jorge,
There are some predefined frequencies you PY2PVT, made the circuit boards.
can choose, related to the IF channel of sim- During the final stages of preparing this
ple rigs, or you can use the “Extern” option: issue of QEX for the printer, I discovered that
just input the offset frequency and choose National Semiconductor recently discontin-
this option. Example 1: suppose you have a ued production of the LMX2306/2316/2326
40 m (7 MHz to 7.1 MHz) CW rig, a 4 MHz family. While these chips are still available
IF filter and a 3 to 3.1 MHz VFO. In this case, from some sources, stock will be limited
connect the counter to the VFO and choose and they may become scarce in the near
plus 4 MHz. future. I learned that the Analog Devices
Example 2: Now you have the same ADF4116/4117/4118 family of ICs appear
band but a 10.7 MHz IF and the VFO rang- to be equivalent, in terms of specifica-
ing from 3.7 to 3.6 MHz. This time choose tions, functions, pinouts and packages, but
a –10.7 MHz offset, and everything will be I can’t be sure that they will work with the
just fine. The software will take care of the same software, and provide the same per-
negative values and will show the correct fre- formance without assembling a new set of
quency of the signal at the antenna. counter boards and testing them. The Analog
There are many options you can use to Devices ICs are available from Digi-Key. I
load the hex file into the processor. Some of would like to note that last year the LMX
them (both software and hardware options) family of parts were easy to find at very good
have been shared among Internet users. prices, but the market is very dynamic, and
Particularly, I have downloaded the MPLAB the manufacturers only want to produce parts
IDE software from the Microchip Web site for which there is a very large market.
(IDE stands for Integrated Development
Environment), because it has interesting Notes
tools for the developer and I have a compat-
ible programmer.
1
The author’s hex code program file is avail-
able for download from the ARRL Web site
QEX files area. Go to www.arrl.org/qex
A Word of Caution files/ and look for the file 5x10_Fernandes.
zip.
This counter was designed for amateur 2
Wes Hayward, W7ZOI, Rick Campbell,
use and I have purposely pushed the limits of KK7B, Bob Larkin, W7PUA, Experimental
the processor and prescaler to reach a desired Methods in RF Design, ARRL, ARRL Order
performance. I have tested the circuit with No. 9239, $49.95. ARRL publications are
available from your local ARRL dealer or
samples I had, analyzing the circuit behav- from the ARRL Bookstore. Telephone toll
ior with frequencies even higher than those free in the US: 888-277-5289, or call 860-
stated here and I would expect the same 594-0355, fax 860-594-0303; www.arrl.org/
shop; pubsales@arrl.org.
performance for average chips. As a matter 3
Bob Okas, W3CD, “The Norcal Frequency
of fact, two more prototypes were assembled Counter – FCC-1,” ARRL, QST, Sep 2006,
with other chips, leading to same results. pp 28-32.
Nevertheless, if you don’t want to step into

Reprinted with permission © ARRL QEX – May/June 2010 15