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Author Topic: How to tune a particular frequency with homebrew, non-calibrated tuners?  (Read 1839 times)
ZEISSIKON
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Posts: 126




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« on: January 26, 2019, 12:11:24 PM »

I'm planning to build my own vacuum tube (valve, for the British speakers) receiver and VFO or VXO transmitter for CW (40 and 80 m, primarily, possibly 20 m).  Something has occurred to me -- without a frequency counter, how would one tune a particular frequency?  If answering a CQ, one "just" has to spot the TX to the RX setting and zero beat, the exact number doesn't matter (much -- need to be sure you're in a permitted band, of course), and the same is generally true when making a random CQ -- but if you're meeting a scheduled QSO, how do you be sure you're transmitting (and receiving!) on the correct frequency with boat-anchor equipment?

Operators certainly were able to do this, even with mobile equipment, during WWII and earlier (some aircraft had radios before the War, and had to tune to tower frequencies, for instance), and those certainly lacked counters at that time -- and had some tuning ability, even if they were crystal controlled.  Did the mobile operators listen for the ground station and tune to it?  Or did they just have a crystal selector and therefor a limited set of working frequencies?  Or do I have a completely wrong conception of how "call me at 0800 GMT on 7.035" worked without crystals or with tunable crystal oscillators?
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G3RZP
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« Reply #1 on: January 26, 2019, 01:50:36 PM »

Your transmitter should have a @net' position. This puts (usually) just the VFO on, and you tune it to 'zero beat' with the station you are calling. Now in CW, there's an offset equal to the CW tone you are using to set the tX VFO to the same tone, although if you don't have a single signal rx with a crystal filter, you need to be careful that you aren't a few hundred Hertz on the other side of zero beat.
 It's easier to demonstrate than to describe - finder a local old timer!
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N8AUC
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« Reply #2 on: January 26, 2019, 03:21:32 PM »

Back in the day when I used much older equipment, and had a home built transmitter, it went something like this...

First turn on the receiver and wait 30 minutes or so to let the VFO stabilize.
Second, turn on the crystal calibrator (usually a 100KHz crystal controlled square wave generator calibrated to WWV).
Third, set the receiver to zero beat the calibrator signal, then turn off the calibrator. Your RX dial should be pretty accurate after this step.
Fourth, tune the receiver to the desired frequency.
Fifth - disconnect (or ground) the antenna connection to your receiver to prevent damage in step 6.
Sixth, tune the transmitter frequency so you could hear your signal in the receiver.

If your transmitter had a "NET" position, or "SPOT" position, you'd turn that on to only energize the oscillator so you
could find your signal with on the receiver.

Life is much easier with modern equipment. You don't have to go through all that work anymore.
Basically, the old saying "the good old days were neither" certainly applies in this case!

73 de N8AUC
Eric
« Last Edit: January 26, 2019, 03:24:15 PM by N8AUC » Logged
AC5UP
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« Reply #3 on: January 26, 2019, 04:02:01 PM »

Back when stable VFO's were rare and CW was the mode of choice it wasn't uncommon to operate duplex.

Depending on local nets or other interests, the typical op would collect a series of TX crystals for multiple band segments like Novice or General as they worked their way up.  This assured they were on a known legal frequency but there was no assurance anyone else had a crystal for the same frequency.  So, they'd CQ on the crystal most likely to get a response then listen +/- 20 kHz or more.  The replying station would return the CQ for a minute or two to allow the first station to find them and once tuned they'd exchange calls, frequency, signal report, etc. along with the rest of the Q.

It should also be noted that rockbound CW wasn't foolproof as some bands were tuned on a harmonic of the fundamental crystal frequency.  This allowed a 3.5 MC crystal to double up at 7 MC with the next harmonic near 14 MC's.  This is one reason why the traditional bands are harmonically related and an opportunity for an inexperienced operator to transmit at double the intended frequency, never hearing a reply to their CQ...

 Tongue
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ZEISSIKON
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« Reply #4 on: January 26, 2019, 04:46:07 PM »

First turn on the receiver and wait 30 minutes or so to let the VFO stabilize.
Second, turn on the crystal calibrator (usually a 100KHz crystal controlled square wave generator calibrated to WWV).
Third, set the receiver to zero beat the calibrator signal, then turn off the calibrator. Your RX dial should be pretty accurate after this step.
Fourth, tune the receiver to the desired frequency.
Fifth - disconnect (or ground) the antenna connection to your receiver to prevent damage in step 6.
Sixth, tune the transmitter frequency so you could hear your signal in the receiver.

If your transmitter had a "NET" position, or "SPOT" position, you'd turn that on to only energize the oscillator so you
could find your signal with on the receiver.

Okay, let's see here.  First, my homebrew regen receiver won't have a VFO to stabilize, though temperature effects likely will still require a warm up period to prevent it drifting after initial tuning.  There won't even be a calibrated dial on the RX, though I ought to be able to make up a log <=> frequency table by tuning known stations and noting the vernier log position (there are still a few AM broadcasts in the SW bands, and SSB operators sometimes mention their exact frequency).  I suppose I could tune WWV in the band I'm working and use that to verify my table after allowing warm up time.  BUT: I'll only be able to approximate an intended frequency (as accurately as I can read the log dial and to the limits of my conversion chart) unless I'm replying.

Also, I won't (at first) have a crystal frequency reference.  I'm starting with a soldering gun, wire cutters, etc. and a Harbor Freight multimeter, plus parts from eBay and Amazon.  Sounds like I may need to build a frequency reference alongside my transmitter project.  Might have to start prowling thrift stores for an old analog TV (colorburst crystals are all the same frequency, as I recall, and it's in our usable range -- plus, even in a modular solid state set, so long as it's a CRT type, there are other useful parts, like flybacks, capacitors, and inductors).

It does occur to me that few if any operators were able to do better than approximate before frequency counters became common equipment.  If I'm expecting a contact, I might need (as suggested in another reply) to tune a broad band around the expected frequency.  Then again, most folks I might contact will be running either digital controlled radios or VXO, so once my Morse is good enough for something beyond a basic QSO, I can probably ask for exact frequency and use that information to upgrade my frequency chart.  Duplex sounds like it was a solution to a problem that's changed into something else since.  Not that many operators are rockbound now -- most Technicians seem to operate mostly on 2 m handy-talkies.  Now that they're not forced into it, not many folks use Novice type equipment unless they just like CW or, prefer to be "retro".

Shouldn't be difficult to include a "SPOT" function when building the TX, however -- just extract the oscillator signal before it goes to the amplifier and feed it (suitably attenuated) to the RX antenna connection.  Zero beat, then retune the RX (in oscillation) to give the desired tone.
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G3RZP
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« Reply #5 on: January 27, 2019, 12:16:18 AM »

In my days of using home brew TRF receivers, I built a crystal calibrator. A simple Pierce crystal oscillator and a 500kHz crystal - but you could use  almost any low frequency crystal, it's just that the harmonics won't be so conveniently spaced in frequency. If you can get a 1 or 2 MHz crystal, you can divide the frequency down with twin triode multivibrators.
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ZEISSIKON
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Posts: 126




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« Reply #6 on: January 27, 2019, 04:29:31 AM »

So, presuming I've built a nice, stable oscillator with a 500 kHz crystal standard, how do I select for a particular harmonic?  There will be a peak every 500 KHz until they're too weak to detect (if I build for a square wave, that'll be odd multiples up to 7x or 9x at least, though they'll be pretty faint by then).  A bandpass filter?  Frequency doublers to get into the range I need (7+ MHz)?
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N4MQ
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« Reply #7 on: January 27, 2019, 05:30:17 AM »

Modern days are here and can help, for less than 20$ you can buy a frequency counter that is from 1 - 500 Mc.  This could be used to monitor your transmitter rather well.

https://www.ebay.com/itm/High-Accuracy-Frequency-Counter-RF-Meter-1-500-MHz-Tester-Module-For-ham-Radiox1/332991612077?hash=item4d87d8c4ad:g:l80AAOSwAmJcLIMP:rk:1:pf:1&frcectupt=true


If you can afford some test equipment that is useful in many ways, you can get a signal generator or antenna analyzer for 100$ or less on ebay that will provide a reference signal.  I found the 55$ signal generator useful for aligning my Johnson Invader 2000, there are many very low cost items to get you going with less pain. 
 
Ebay has helped me find Johnson thunderbolt amplifiers, 6n2 THUNDERBOLTS, and the pieces needed to get them running, there is lots of ham items - like a year round ham fest.

Enjoy, Woody

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G3RZP
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« Reply #8 on: January 27, 2019, 05:39:11 AM »

Something like a 6J5 in a Pierce will give detectable harmonics up to 30 MHz with a 1 MHz or 500kHz crystal. Some sort of wavemeter is useful to determine which harmonic you have, but you can roughly determine things from what stations you can hear - the amateur bands, the BC bands (Depending where in the world you are) at 3.2 - 3.4, 3.9 - 4.0, 4.75 - 4.995, 5.005 - 5.06, 5.9 - 6.2, 7.2 - 7.45, 9.4 - 9.9, 11.6 - 12.1, 13.57 - 13.87, 15.1 - 15.8, 17.48 - 17.9, 18.9 - 19.02, 21.45 - 21.85 and 25.67 - 26.1 MHz plus the standard frequency and time signals at 2.5, 5, 10, 15 and 20 MHz, plus some other odd STFs.

An absorption wavemeter consisting a variable capacitor and an inductor in parallel will 'suck out' energy at its resonant frequency or the parallel tuned circuit in series with the antenna will trap out a signal at resonance. You can get within 5 or 10% on inductance by using the standard formula and put fixed capacitors across the coil until stations in the known band get trapped out: by using a selection of capacitors going up in double values e.g 10, 20, 40 , 80 pF etc (use nearest standard values) you can get a rough idea of where you are, close enough for the crystal harmonics to be heard. Some highe frequcny crystals such a42 or 4 or 8 MHz can provide good rough markers, too.
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ZEISSIKON
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« Reply #9 on: January 27, 2019, 07:39:30 AM »

I hadn't realized frequency counters had gotten cheap (must be microprocessor technology doing it).  Last time I looked at them (college days, 1980 give or take a couple years) they cost as much as a low end new car.

For $20 or so, I'll probably just get one and call it good for actual operation, though I'm still interested in the methods used back in the day.

FWIW, I'm in the USA.  AM BC here runs from 540-1610 kHz; FM BC goes from 88-108 MHz.  The second harmonic of 500 kHz is just about the middle of the AM band.

So a combination of a very "dirty" Crystal oscillator and a suitable filter will give a usable frequency reference, and in the bands of interest, 500 kHz spacing of harmonics should be enough for calibration -- 7.000 is far enough from 7.500 for our purposes.  Seems simple enough.
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N8AUC
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« Reply #10 on: January 27, 2019, 11:13:18 AM »

So, presuming I've built a nice, stable oscillator with a 500 kHz crystal standard, how do I select for a particular harmonic?  There will be a peak every 500 KHz until they're too weak to detect (if I build for a square wave, that'll be odd multiples up to 7x or 9x at least, though they'll be pretty faint by then).  A bandpass filter?  Frequency doublers to get into the range I need (7+ MHz)?

They're available in kit form for a very reasonable price.
Check this out...
https://www.eham.net/reviews/detail/9480
This one actually provides switchable markers at either 1MHz or 100KHz intervals, which is kind of handy.

This one is also very reasonable, and it will give you marker signals every 50KHz, 25KHz, 10KHz or 5KHz if you want it.
https://qrpguys.com/k7qo-marker-generator
A kit for $15? Can't beat that.

Then there's the frequency counter that N4MQ mentioned, which is even more useful.

Lots of ways you can go here, and save some ca$h along the way.

And yeah, back in 1980 you couldn't touch a frequency counter like that for less than a few grand.
I remember those days. I was still an undergrad EE student back then. Things have gotten WAY better since then.

73 de N8AUC
Eric

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ZEISSIKON
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« Reply #11 on: January 27, 2019, 11:42:21 AM »

I like tubes -- but I like saving money too, and I'm capable of recognizing when something can't be done at reasonable cost (or with usable accuracy) with technology I understand.  I'm not above using some magic, if it does the job I need done...

I'm also capable of soldering through-hole components on a PCB (wouldn't want to try surface mount, though) -- I built a metal detector kit in high school (1973) and it worked (as well as a small/cheap metal detector is going to).  Looks like I'm going to have to order some kits to build some tools.  Marker generator, frequency counter (I actually found a link on another thread for a combination crystal tester and frequency counter in kit form for under $10 shipped from China), maybe a few other things.  For a hundred bucks and a few hours assembly time, I can put together a tool kit that would have cost $50k when I left college.

That'll make building 1920s-1950s radio gear a lot easier.   Cool
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G3RZP
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« Reply #12 on: January 28, 2019, 03:39:52 AM »

I seem to recall that in the mid 1980s, Sabtronic offered a 1 GHz counter for around $200.
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AC5UP
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« Reply #13 on: January 28, 2019, 08:29:01 AM »

Sure did, but 1978 is more accurate...  I had one when I was living in Dallas and it was a kit product sold through a local swapmeet and magazine advertising.  Reasonable quality for the price but I had to visit their shop as mine didn't work.  I'm experienced with kits but not so much that I'd trust my work over the vendor so I took it back, they had glued small pieces of styrofoam top & bottom over the 10 MC clock generator for temperature stability.  PC board didn't like the glue.  No clock, no count, no joy.  No problems after that.

The other hot item at the time was a DSI counter and matching DMM.  Larger boxes but similar otherwise, at the time they were a killer combo as I had yet to experience the joys of an auto-ranging meter....
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G8HQP
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« Reply #14 on: January 29, 2019, 02:08:39 PM »

Quote
There won't even be a calibrated dial on the RX, though I ought to be able to make up a log <=> frequency table by tuning known stations and noting the vernier log position (there are still a few AM broadcasts in the SW bands, and SSB operators sometimes mention their exact frequency).
You will need some sort of dial.

Quote
Sounds like I may need to build a frequency reference alongside my transmitter project.
Yes. When I was starting out I built a 1MHz crystal oscillator, which then fed divide-by-10 TTL to get 100kHz and 10kHz. I could then calibrate any receiver I built.

Quote
So, presuming I've built a nice, stable oscillator with a 500 kHz crystal standard, how do I select for a particular harmonic?
That is why you need some sort of dial on your receiver. Then when you hear a crystal harmonic you know which harmonic it is.

You will probably also need an absorption wavemeter, which you will need to calibrate somehow. Otherwise you will never know where you are transmitting.
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