Budget Frequency Counter

Budget Frequency Counter

This project was sort of last-minute.  I bought this frequency counter a few years ago just to see if I could really use one from a vendor in Hong Kong.  The unit is a PLJ-9VFD-A and features 9 digits via a vacuum florescent display.  LED versions with eight digits are more common but I've always like the VFD's more.  My sister has a handheld space shooter that uses the VFD as does my Heathkit GC-1107 clock.  The odometer in my daughter's 2000 Wrangler is a VFD too.  Lot's of small displays use VFD's we don't even think about.  Microwave ovens, VU meters on stereos, gauges and so on.  They work on the same principal as nixie tubes but use a lot less power.  The PLJ-9VFD-A runs on a 9V battery if you need it to.  For example: Portable operation in the field without a house current outlet nearby.  It could even be equipped with power poles and use battery clamps from a car battery to operate on.  Here are the specs:


TECHNICAL SPECIFICATIONS

Mfr: SanJian Studio
GATE TIME0.01 sec0.10 sec1.0 secCHANNEL MEASUREMENT PERFORMANCE (CHANNEL LEVEL HIGH IMPEDANCE)Low channelMeasuring range: 0.1 MHz - 60 MHz
Accuracy: ± 100Hz (0.01 sec gate time)
± 10Hz (0.1 sec gate time)
± 1Hz (1.0 sec gate time)

Low channel sensitivity:
0.1 MHz - 10 MHz: Better than 60mVpp
10 MHz - 60 MHz: Better than 60mVpp
60 MHz - 75 MHz: Not specifiedHigh channel (Divide by 64)Measuring range: 20 MHz - 2.4 GHz
Accuracy: ± 6400Hz (0.01 sec gate time)
± 640Hz (0.1 sec gate time)
± 64Hz (1.0 sec gate time)

High channel sensitivity:
20 MHz - 30 MHz: Better than 100mVpp
30 MHz - 60 MHz: Better than 50mVpp
60 MHz - 2.4GHz: Not specifiedAuto ChannelDepending on the frequency of the input signal, the counter will automatically select the
high or low channel (Change occurs at 60 MHz). If the input signal is greater than 60 MHz but
a weak signal level prevents automatic high channel selection, it is possible to manually
select the high frequency channel.

IF SETTINGSIndependent double-IF design allows the IF settings to be adjusted in minimum increments of 100 Hz.Intermediate frequency range: 0 - 99.9999 MHzOffset: Plus or minus IF mode can be configured

FREQUENCY REFERENCE13.000MHz temperature compensated voltage controlled crystal oscillator (VC-TCXO) in 5032 packageFrequency stability: ± 2.5 ppm

SUPPLYDC Input: 9V - 15V (Reverse polarity protection)
Current: 170 mA max
 
INTERFACESDC IN (Power Interface): HX2.54-2P socket
RF IN (signal input): HX2.54-2P socket
ICSP (programming interface): 2.54-6P Pin
 

SanJian Studio - PLJ-9VFD-A -                $16 (Included Shipping back in 2014.  No longer available)

From Allied Electronics
Hammond Enclosure - 1598BBK -                        $12.30
NTE Electronics - 57-PJ30 Power Jack -               $  1.94
RS Pro - 487854 Straight DC Pwr Plug -               $  1.21
NKK Switches - M2013SS1W03 ON-OFF-ON -  $  3.04
                                                                    Total - $34.49

There was tax assessed due to this state's legislation requiring us to pay tax on internet purchases.  I ordered two switches, two DC jacks and two plugs.  It's nice to have these on hand for another "water closet" idea.  Shipping was only $6.18 which is better than Digikey, Mouser, Jameco or even Marlen P. Jones.

I've looked for another one but haven't found it readily available anywhere.  I paid $16 for it back in 2014.  Incidentally, the manual was never translated to English.  Their PLJ-8LED model did have an English manual available.  Their 7VFD model was also in English but the device is no longer available.  I used the specs from the 8 LED model and raised the current rating to 170mA max to take into account the two extra digits and the fact it uses a VFD.  Other than that, it operates identically.

So back when I first obtained the 9VFD in 2014, I soldered a BNC chassis connector to the RF IN leads and left the power leads bare ended.  Connecting a homebrew carrier detector/demodulator for use with my oscilloscope, I was able to test the frequency output on my Kenwood TS-450S/AT.  I set the power to the lowest setting and keyed up the radio into a dummy load.  It showed that the transmit frequency was off by 200Hz.  I connected a borrowed frequency counter that had been recently calibrated to a rubidium standard.  This inexpensive little unit from SanJian Studio did not disappoint.  Stability of the output was high and allowed me to see down to 6 decimals on 40m.  I aligned the radio using the 9VFD and have enjoyed on-freq performance.  After that, it was put away until I needed it again.

Fast forward to a few nights ago and I returned it to the bench to check the output of the IG-102 signal generator I'm working on.  The borrowed meter has long since been returned and thus I have only this unit to work with.  I purchased a Heathkit IM-4120 frequency/period counter that I hope arrives by the weekend.

To make a long story longer, I ordered a small enclosure, switched, jacks and plugs to build the 9VFD into a proper benchtop/portable frequency counter.  I plan to have it selectable for battery power and an auxiliary 12V port on the back.  It would be a basic featured counter.  IF offsets would have to be set every time it's powered up.  There wasn't a provision for a battery backup or memory save feature to my knowledge.  I will most likely try it out on the Hallicrafters S-40B with the 455kHz IF to see how it performs.  Straight out of the box, as it were, it's accurate for alignment work and perhaps more exacting needs.

That's it for tonight's posting.  I'll leave you with these photos.

73!

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April 12, 2019

Tonight was fun.  I began the work on the enclosure to house the counter and interfaces to the outside world.  The more difficult part was getting the counter supported.  I originally wanted it to be mounted to a plate with a window for the display.  I don't have the right tools to make that piece so I compromised and mounted it to the front of the plate using stand-offs.  Doing that will offer a shield of sorts from interference coming from inside the box on the DC line or the signal input.  I will either paint the inside with conductive paint or line the inside with foil bonded to the signal ground.

Speaking of grounds, I tested the signal ground and found that it is connected to the DC ground on the unit.  I think I'll use some Ferrite beads to suppress any interference on a twisted-pair signal lead instead of locking myself into a fixed impedance cable like RG-58 or RG-62.  It would be possible to have a switch selectable impedance of 50 or 75 ohms at a future date if needed.

Starting off with a blank slate.  I had to cut about 3mm off the
end to fit in one of the PCB slots.  After filing it for the final
fit, I began drilling for the stand-offs for mounting the counter.

The display will sit a tad to the left as I wanted to make sure
there was enough room to use the buttons when needed.

Here's a top view of the stack.

Right side showing the 2P power connection.  Drilling for the
leads was a requirement as you can see here.

Notching the end to allow the assembled cable to be
disconnected in the future will be done on the other
side before final assembly.  No point in having to fire up
the soldering iron just to get the counter out for replacement
or programming.  If programming will be required, there isn't
any access to the pins located rear-center on the board.

How it will sit in the enclosure.

Rear panel work.

From left to right:
DC IN - EXT/OFF/INT power source - Signal IN

Last fitment check.

 It is late and I have to go to work in the morning.  I hope to have this completed and working tomorrow night.

73!
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April 13, 2019

Tonight I finished up the wiring.  I don't have a dedicated probe for checking circuits so I'll use an oscilloscope probe for that.  I do have a double ended BNC cable for use with a transmitter but the IG-102 uses the old style microphone jack.  I'll change it over to a BNC once the old connector fails.  Wait.  That won't work.  It hasn't failed in the last 50 years so I guess it won't fail.  Well, I'll make an adapter if I need to instead.  ;-)

Currently there isn't a front glass or acrylic panel on the box.  I'm looking for a source for smoked or tinted small panels that can be used for the front.  Tinted is preferred as it will hide the circuit board and reduce the glare from the VFD.  Below are the final pics of this stage of the project.

Wiring complete.  Note that I haven't installed
the Ferrite beads.  That's because I don't
have any as yet.  I'm shopping around and
have found a few suppliers that sell a large
amount of various sizes cheap.  I'm suspicious
as these might not be the correct type of
Ferrite and can make the problem worse.
Research, research, research.  I try to be lazy
and do it right the first time.

Used some brass PCB stand-offs to act as nuts for the front stand-offs.  It seems
these are metric and I didn't have the correct parts so I perused my PC parts
bin and found the brass ones.

As luck would have it, I found a wall-wart solid state switching power supply
in my junk box to charge a puck style LED flash light.  I need to replace the
batteries in it anyway so the supply is free for use here.  It puts out
9VDC 700mA (6.3VA).  Plenty for the counter.  The power switch is an
ON-OFF-ON switch so I can use an internal battery in the future.

And here it is all powered up and nothing to attach to.  At least for now. 

I put a dummy load on the transceiver and connected the CleanRF box.  It appears the Kenwood has drifted off again.  Might mean it's time to replace some E-caps.

Here's a short video on its operation.

Please don't mind the lighting.  I use an amber LED strip light to illuminate my keyboard and have the rest of the lights off here in the shack.  I prefer amber over any other color.

The only things left I want to do with this is to install dual 9V batteroes in parallel, a stand that will let me prop it up and the tinted glass/acrylic front panel.  Other than that, this will be a nice addition to my workbench.

73!
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April 14, 2019

Sooooo yeah.  I had to make a front panel.  The pics show the process.  The color was flat black enamel.

The plastic sheet is Plaskolite and measured 8 x 10 x .050 inches.  It's a non-glare plastic used typically as picture frame 'glass'.  The display needs some tinting so I'll look to the automotive market and find some window tint I can apply to plate.  This should help in the contrast of the display.

To make the mask I laid down a strip of shipping tape then cut around the center to peel away the mask.  This left the plastic outside the display area exposed.  It's kind of like a film negative I suppose.  Spraying a couple of coats of paint down to create the black out mask was easy, waiting for it to dry wasn't.  I'm not sure why it took so long to dry even with air flow over it.  It could be the high humidity since it rained a lot over the last two days.  Most of the circuit board is hidden.  I could narrow the viewing area a bit later.  The two holes for the button require a rod or tool but I hope to find button caps I can use that will protrude from the faceplate to operate them.

I haven't decided on a name for this piece of equipment yet.  FC-9VFD might be too complicated or ambiguous but I'm sure something will come up.

73!
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