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Author Topic: Input Boost for Vintage Frequency Counter  (Read 51643 times)
KB1WSY
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Posts: 812




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« on: July 18, 2013, 01:29:53 PM »

I just picked up a vintage Heathkit IB-1101 frequency counter. It dates from about 1970 and has cool nixie tubes. Tested with my signal generator and grid-dip meter, it works very nicely. With the GDO all I have to do is place a "pickup coil" of a couple of turns of wire close to the GDO's own coil, and the IB-1101 gives a stable reading of the frequency.

My aim is to use the IB-1101 as the core of an otherwise homebrewed "frequency monitor" for my station. Probably I will remove it from its case and place it inside a larger one, giving pride of place to the Nixies and adding some additional circuits to increase its usefulness:
(1) Heterodyne oscillator to inject the correct frequency offset when used with homebrew superhet receivers.
(2) Switching circuitry so that it can display transmit/receive frequencies.

Initially I want to use it with my recently built 3-transistor regenerative receiver. One of the neat things about regen sets (and direct-conversion sets) is that they oscillate at the same frequency as the receive frequency therefore one should be able to use a frequency counter as an accurate "dial" for the radio without having to mess with oscillator offsets.

However, because of the IB-1101's relatively low sensitivity of 50mV, I have only been able to read the regen radio's frequency by (a) driving the set into superregeneration, and (b) connecting the frequency meter's probe directly to the drain of the FET regenerative detector. This is useless because (a) I don't want superregeneration and (b) connecting the counter directly to the FET drain kills the audio.

So I'm looking for a simple circuit consisting of a couple of loops of wire as a "pickup" for the oscillator output, then a signal booster of some kind, to bring it above the sensitivity threshold of the frequency counter. I tried using a 2N2907 as a simple grounded-base "RF stage" between the coil and the frequency counter but it is not boosting the signal enough. Then I started looking at various "RF preamplifier" possibilities but realized that I was a bit out of my depth.

Suggestions? I'm looking for a simple homebrew solution, preferably using discrete components (transistors or tubes) and it needs to be wideband enough to encompass 80m through 20m.

Of course, I could have avoided all of this by obtaining one of the numerous modern digital frequency displays, in kit form or otherwise. But I want to try to keep things within this "era" of about 1970 or earlier. Besides, by building a little RF "booster" I'm sure I'll learn something. (I've already learned quite a lot by finding out what *doesn't* work.)

73 de Martin, KB1WSY
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WB6BYU
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Posts: 13481




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« Reply #1 on: July 18, 2013, 02:08:12 PM »

A grounded-gate amplifier has a low input impedance, which will tend to load the
circuit more.  A J-FET in a standard common-source configuration would give you
a high input impedance for less circuit loading, and could be driven through a small
capacitor.  A source-follower would have an even higher input impedance, but does
not deliver any voltage gain (only power gain) so would need to be followed by a
second stage (and/or a wide-band step-up transformer, depending on the input
impedance of the counter.)
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KB1GMX
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Posts: 815




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« Reply #2 on: July 18, 2013, 03:04:08 PM »

first you need more way gain than a grounded gate design would yield.

The other is input impedance to avoid loading the circuit under test.
If you using a small loop of wire then low input impedance is fine.

But likely you want something much higher for direct connection
and here is the rub.  My Yaesu 355D (same era with nixies) has
the option of 1meg ohm input, but 3ft of coax also adds about
60-75PF of parallel capcitance across the circuit.  So instead I
use a scope 10X probe that holds it to about 9pf and requires
10x more signal.  So counter loading can both detune and even
stop oscillators.  FYI I still use it as back in the 70s I added a
very good TCXO for stability and accuracy.  I also have a low
cost hand held unit good to a few GHZ that works better.

A regen oscillating is a very weak signal sometimes in the very low
millivolts.  When you go near, it will detune.  Loose coupling means
very weak signals.

Solution is gain, a MIMIC based amplifier (useful from a few khz to Ghz)
with a FET based active probe to keep the coax out of the connection
picture to the circuit under test.  Also the probe input needs to be well
protected as you might use it to measure a signal out of a tube.

FYI if your 1101 works I'd consider that a near amazing as most I'd
seen were dead within years of being  built from lead corrosion and
the crappy molex connectors.  a few that I'd fixed were killed by CBers
using it directly on the outputs to measure frequency.


That's your options.


Allison
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KB1WSY
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Posts: 812




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« Reply #3 on: July 18, 2013, 04:18:06 PM »

... depending on the input impedance of the counter.

It's "1 megohm, shunted by less than 15pF" according to the spec sheet.

...counter loading can both detune and even stop oscillators.... A regen oscillating is a very weak signal sometimes in the very low millivolts.  When you go near, it will detune.  Loose coupling means very weak signals.<<

I've experienced both issues. Simply introducing a coupling coil within an inch or two of the main tuning coil is detuning, and even stopping the oscillation altogether. Therefore the coupling will have to be very loose.

Solution is gain, a MIMIC based amplifier (useful from a few khz to Ghz) with a FET based active probe to keep the coax out of the connection picture to the circuit under test.  Also the probe input needs to be well protected as you might use it to measure a signal out of a tube.

So as a newbie I will ask some elementary questions:
--By "FET based active probe" I assume you mean that the FET should be at the end of the cable, near the coupling coil??
--This would feed back to a MIMIC  a couple of feet back, near the FT-1101?
--Can you be more specific about the probe input needing to be "well protected"? Is coax terminated in a BNC enough?

FYI if your 1101 works I'd consider that a near amazing as most I'd seen were dead within years of being  built from lead corrosion and the crappy molex connectors.<<

It's in great shape inside. The electrolytics show no sign of swelling etc. so I'm considering leaving them alone. On the outside, slight corrosion on the chrome-style fascia around the front panel -- or perhaps just crap that can be washed off, I'm not sure yet, and it's not an issue since I am likely to discard the cabinet. Otherwise it's perfect. I bought that particular model because it has socketed nixies (as opposed to wired) and I figured that would provide more flexibility to move the nixies away from the PCB (or replace a defective nixie). I bought it on eBay. The seller said that the counter "came from an old school computer guy who still used them up to recently" and I would have to say that this seems plausible, having now received and tested the item. Which is more than can be said for many eBay items!

There are some pictures here:

http://tinyurl.com/klzo4h8

73 de Martin, KB1WSY
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KA4POL
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Posts: 2094




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« Reply #4 on: July 18, 2013, 10:03:37 PM »

I'd use a circuit like this one: http://www.qsl.net/z/zl1bpu//MICRO/COUNTER/page3.gif or http://wb9kzy.com/preamp2.htm
For protection you could use two antiparallel diodes like in the second link schematic.
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KB1WSY
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Posts: 812




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« Reply #5 on: July 19, 2013, 05:43:14 AM »

Thank you everyone for your suggestions, I am now hopeful about putting something together that should do the trick. At this point the main challenge is that I don't have any experience homebrewing solid state circuits from scratch (except using prehistoric point to point construction) so I need to figure out how to do this. Some of the suggested circuits, such as the one from wb9kzy which looks very interesting, have a corresponding pre-etched PCB but these are out of stock. Perhaps at this stage I can get away with cobbling something together on a perforated board -- I'm only planning to use this on HF frequencies.

By the way, let's suppose I was feeling lazy and built a kit instead; I don't suppose this one would do the trick?

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=SA7

Among other things I noticed that one of the "reviewers" on Amazon.com was using it as a preamp for his frequency counter and having a good result. It has a low input impedance -- but does that matter if it is connected only to a pickup loop i.e. not to the receiver itself?

73 de Martin, KB1WSY
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KE3WD
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Posts: 5689




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« Reply #6 on: July 19, 2013, 07:02:13 AM »

You don't have to use discrete component design here, there are some single IC options available, low parts count, easy assembly. 

Do a websearch for "monolithic RF amplifier IC" and pay attention to those fine ones offered by an outfit called, "Mini-Circuits" among others. 

I'm thinking of the MiniCircuits IC amp that tops off around the 60MHz mark, but there may be others as well. 

High input impedance, low output impedance, very simple hookup, as few as four connections on the IC itself, gain can be selected by the user, typically by changing one resistor. 


73
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KB1GMX
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Posts: 815




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« Reply #7 on: July 19, 2013, 05:16:19 PM »

... depending on the input impedance of the counter.

It's "1 megohm, shunted by less than 15pF" according to the spec sheet.

Then add cable at typically 20pf/foot.




Solution is gain, a MIMIC based amplifier (useful from a few khz to Ghz) with a FET based active probe to keep the coax out of the connection picture to the circuit under test.  Also the probe input needs to be well protected as you might use it to measure a signal out of a tube.

So as a newbie I will ask some elementary questions:
--By "FET based active probe" I assume you mean that the FET should be at the end of the cable, near the coupling coil??

if you using a coupling coil the FET amp is not needed as the coil is low impedance just
a 50ohm gain stage like a MIMIC (monolythic integrated microwave IC).  MIMICs are available from Avago and mInicircuits and and are basic gain blocks of 4 leads.  Two of the leads are ground and one the input (add capactor here) and the output is to a DC source though a resistor.  A part like MAR11 (minicircuits.com) will provide 20db of gain from near DC (use a .1uf input cap) to 1000mhz
and is cheap.

With a coupling loop cable and its ccapacitance is unimportant.

--This would feed back to a MIMIC  a couple of feet back, near the FT-1101?

Yes.

--Can you be more specific about the probe input needing to be "well protected"? Is coax terminated in a BNC enough?

Usually back to back diodes to prevent excessive RF from burning out the amplifier.
An example is you need to measure a weak signal on the plate of a heterodyne mixer tube.

A fet based amp is used where you want to load the circuit the least and you make a probe
with the amp right there in it at the front of the cable rather than the tail.

Someone else published address this and its a common circuit.
http://www.qsl.net/z/zl1bpu//MICRO/COUNTER/page3.gif

FYI if your 1101 works I'd consider that a near amazing as most I'd seen were dead within years of being  built from lead corrosion and the crappy molex connectors.<<

It's in great shape inside. The electrolytics show no sign of swelling etc. so I'm considering leaving them alone. On the outside, slight corrosion on the chrome-style fascia around the front panel -- or perhaps just crap that can be washed off, I'm not sure yet, and it's not an issue since I am likely to discard the cabinet. Otherwise it's perfect. I bought that particular model because it has socketed nixies (as opposed to wired) and I figured that would provide more flexibility to move the nixies away from the PCB (or replace a defective nixie). I bought it on eBay. The seller said that the counter "came from an old school computer guy who still used them up to recently" and I would have to say that this seems plausible, having now received and tested the item. Which is more than can be said for many eBay items!

Lead corrosion was a TI part problem and the molex socket were crap.  I rescued one
by removing the outer case and running the whole thing sans transformer thorough the
dishwasher.  Before that any vibration it would lock up due to intermittent connections
at the leads. After wards it was solid and ran for years.

Everyone socketed the nixies, it was the normal thing. Obviously it was well kept. FYI shock
can make the nixie elements move and short, be careful.  Also the decoder driver ICs are
getting scarce.   My Yaesu YC355D is about 40 years old and served me for two years
in the '70s as a landmobile tech as it was good to about 380mhz and ran on both 12V DC and
115V AC, it's still on the bench.  That and my Bird 43, Heath Cantenna and the NLS MS15
miniscope have been around with me since then and handy for field troubleshooting.

You can dial in the time base against WWV at 5,10 or 15mhz to insure its accurate to
the limits of the digits available.


Allison
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KA4POL
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Posts: 2094




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« Reply #8 on: July 19, 2013, 09:52:11 PM »

At these frequencies you can use the dead bug method, no problem. MMICs are clearly overkill for 50 MHz.
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KB1WSY
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Posts: 812




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« Reply #9 on: August 11, 2013, 04:41:59 PM »

Success! (But still some issues to sort out.) I built a preamp circuit similar to the one suggest by KA4POL (JFET and buffer). Here are some pictures:

Schematic: http://tinyurl.com/lkkzw2n.

The preamp in action: http://tinyurl.com/khbrad6.

Circuit board top view: http://tinyurl.com/m4qppdl.

Circuit board bottom view: http://tinyurl.com/m4qppdl.

Getting a direct frequency readout from my 40-year-old Nixie tube frequency counter is very cool. There are however some issues that you experts may be able to help me with. Initially I tried to use a coupling coil inside the regenerative receiver. This did work, but was much too fiddly for routine usage (the coil's position had to be adjusted within a millimeter or two, and this varied with the band chosen and various other factors). So instead, I used a direct, high-impedance connection to the receiver's RF stage.

Issue One: On 80m and 40m the frequency readout is stable. However on 20m it is unstable: it flickers over a range of + or - 50kHz from the center frequency.

Issue Two: There is a background audio tone whenever the frequency counter is switched on. It is somewhere between 400 Hz and 500 Hz, I think. It is not really loud but it does interfere with weak-signal reception (at which point I can get rid of the problem by turning the frequency counter off). The tone gets louder, the higher the RF frequency i.e. it is a bigger issue on 20m than on 80m (on 80m it is barely audible).

Suggestions? Thanks!

73 de Martin, KB1WSY
« Last Edit: August 11, 2013, 05:00:52 PM by KB1WSY » Logged
KB1WSY
Member

Posts: 812




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« Reply #10 on: August 11, 2013, 06:16:15 PM »

Quick correction: the frequency meter is connected to the receiver's detector/regenerative stage (not RF stage). Hence, the ability to get a direct frequency readout.

73 de Martin, KB1WSY
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K8AXW
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« Reply #11 on: August 11, 2013, 08:31:37 PM »

Martin:  Congratulations on a job well done! 

I can use this circuit with my 30 year old HB frequency counter.

I'd also like to know about your regenerative receiver.  You can post that info here or to K8AXW@ARRL.net.

I'm interested in your experience(s) with it.  My last regen. experience was disappointing.

Al - K8AXW
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KA4POL
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« Reply #12 on: August 11, 2013, 09:35:45 PM »

Al, seems you got a nice counter there. Is it made by the second from top under this link http://www.google.de/imgres?imgurl=http://www.beerdrinkersguide.com/BDGWebsite/Images/General/Hblogo.gif&imgrefurl=http://www.beerdrinkersguide.com/BDGWebsite/MunichBeer/BigSix.htm&h=323&w=300&sz=20&tbnid=ty1gamuWGD2l7M:&tbnh=93&tbnw=86&zoom=1&usg=__YiYVG7jb-j7yo6KpsIzKjN4n0JM=&docid=PgWVGsvLr9HOBM&sa=X&ei=82QIUuf6FYLRtQbuuIEI&ved=0CFYQ9QEwCw&dur=6865   Grin
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KA4POL
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Posts: 2094




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« Reply #13 on: August 11, 2013, 10:08:06 PM »

Thanks for the feedback, Martin. Good to see you got a solution.

The counter uses a 1 MHz xtal and divides this frequency to get the time base. All those rectangular signals can cause problems if not fully shielded. So that is probably what you hear. The flickering readout could come from either a to strong or to weak signal. I already had both cases.
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KB1WSY
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« Reply #14 on: August 12, 2013, 06:32:25 AM »

I'd also like to know about your regenerative receiver.  You can post that info here or to K8AXW@ARRL.net.

The project, which is a 3-transistor ARRL design from 1968 (and also my first major homebrew/non-kit effort in 40 years), is described at length here: http://www.eham.net/ehamforum/smf/index.php/topic,90273.0.html.

It is still not "finished" (is anything homebrewed ever so, apart from beer?) and I am in the process of making some additional improvements, including building a cabinet to shield from AC hum and other noise. This is complex because I am adding a custom "trap door" in the top to enable coil-changing, and also because any regenerative receiver is prone to hand capacity (and "cabinet capacity!') effects so I am making the case extra-rigid and adding lots of brackets and screw fasteners to avoid torsion or slippage of metal parts. I'm also adding a period-appropriate additional audio stage and a speaker, not really for regular use but mainly to show off the radio to family and other non-hams. A lot of metalwork for a little receiver!!!

Here are photos of the cabinet work in progress: http://tinyurl.com/mgynwtd.

My last regen. experience was disappointing.

Regen receivers are tricky! Suffice it to say that I had low expectations for that project but came out pleasantly surprised. The set is extremely sensitive and, with the (crucial) modifications described in the above thread, it is working nicely on 80m, 40m and 20m. Selectivity leaves something to be desired but under the right conditions, single-signal reception is possible even on a congested day (it helps to have that huge Jackson dial and my custom, narrow-bandspread coils for the CW band portions) and I expect to be able to conduct at least a few QSOs with this very modest radio.

My next possible project is the solid-state version of the ARRL "SelectoJet" -- a relatively elaborate CW audio filter (notch or amplify, adjustable passband center pitch and width). I've also got my first transmitter project in the wings, but would like to finish conquering Morse Code first. Nothing more frustrating than having a TX one can't use, I suspect!

73 de Martin, KB1WSY
« Last Edit: August 12, 2013, 07:09:49 AM by KB1WSY » Logged
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