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 Post subject: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 6:26 pm 
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Hi, I'm a new member. I have a big interest in vintage/antique radio. I try to cover a fairly wide spectrum so I included some amateur radio theory and American design. Engineering-wise, all my hands-on experience has been with British tube radios and these usually are not overly complex. Having said that, I was also working on a rather unusual East German mid sixties tube radio that offers FM.

Enough of that. I have a question that I'm sure any old Hams will be able to clarify quite easily. The sort of question maybe best clarified by genuine involvement with amateur radio. I don't have any amateur radio kit so clearly all I have is AARL (1960).

This concerns the 100 Kilocycle calibrator (which I've never seen or used). So far as I can understand from AARL, the "spotter" generates a 100 Kilocycle harmonic which the Ham operator will hear between intervals on the bands. This helps the operator keep track of the band edges and know where he (or she) is.

The use of WWV to zero the spotter is really something quite alien to me. It says the operator must first tune in a clear signal on WWV and then zero the crystal 100 Kilocycle oscillator. Has anyone done this and have anything to add? I'm then informed that every 100 Kilocycles, you will hear this harmonic. I can only assume this works the same as a BFO used to demodulate CW? That is a difference frequency that beats with a carrier?

Sorry if this sounds a strange question but most hobbyists over here tend to focus on domestic AM and LW receiver restoration and repair. There is less interest in CW, BFO's and the actual transmission side. I figured maybe a few American members would be a lot more familiar with that side of the interest.


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 7:35 pm 
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The calibrator is a simple oscillator with a crystal--usually 100KHz. There is a ceramic trimmer capacitor that can pull the frequency a small amount in either direction. This is mounted so that a person can access it from the top of the radio chassis but not from the front panel. It is normally a one-time adjustment. The 100 KHz calibrator signal is usually fed into the radio at or near the antenna circuits. It needs a BFO to hear the crystal calibrator when no station is present at the 100 KHz interval. The BFO pitch would be set to 0. For calibrating the calibrator, its signal is beat against the WWV station when WWV is broadcasting an unmodulated signal. The calibrator trimmer is then adjusted for O beat against WWV. Some radios had the calibrator as an option and a separate module could be plugged in (octal socket).

Norm

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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 7:39 pm 
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Hi and welcome to the forum.

The 100 Khz. spotter, typically referred to as a crystal calibrator, is a simple oscillator circuit controlled by a 100 Khz. crystal and it is configured to generate copious harmonics so that it can be used up through the shortwave bands to spot or calibrate a band.

Because the standard WWV frequencies are all at multiple harmonics of 100 Khz. you can use WWV in order to calibrate the calibrator. To do so put the 100 Khz. calibrator into operation and tune the radio in AM mode to one of the WWV frequencies. During part of the WWV transmission loop they are transmitting only the carrier plus the "tick" and this is the best part of the cycle to accurately adjust the calibrator. As the calibrator is adjusted close to WWV's frequency you will hear an audio tone which is the difference between the calibrator output and the WWV carrier and you want to adjust it so that the tone goes lower in frequency and then disappears. This tone generated from the difference or "the beating together" of the calibrator and the WWV carrier is referred to as the "beat note" and zero beating means the note or tone is reduced to zero.

Typically there is a small trimmer capacitor in the calibrator that is used to adjust its frequency. If the receiver has an S meter you can use it to more accurately adjust the calibrator to WWV since once you get very close the tone will go below the audio bandpass of the receiver. To use the S meter for most accurate calibration turn it on with the AGC in fast position if it has a separately selectable release time and as you get very close to zero beat between WWV and your calibrator harmonic you will see the S meter following the beat note.

For best results you want the calibrator signal to be neither extremely strong nor extremely weak compared to WWV so you may have to adjust receiver antenna coupling or the calibrator location. The two signal levels do not need to match but you don't want either to overwhelm the receiver by itself.

You can also use any broadcast station that accurately transmits on a 100 Khz. interval to zero the calibrator but it is easier with a station like WWV since it does transmit an unmodulated carrier for part of the time cycle.

Because you are calibrating at a harmonic of the crystal the best accuracy is achieved by using the highest frequency of WWV you can receive adequately. If you use the 2.5 Mhz. WWV and are off by 100 hertz then the error at 25 Mhz. will be 1 Khz. since any error is also multiplied. If you can receive WWV well at 15 Mhz. and do a reasonably close zero beat there your accuracy will be extremely high.

Many older calibrators had both a 1 Mhz. and a 100 Khz. output position because the calibration accuracy of many general coverage sets was so poor that it would be difficult to determine which mark to adjust to when using only the 100 Khz. calibrator. In that case you would first calibrate to the nearest 1 Mhz. interval using the 1 Mhz. calibrator and then switch to 100 Khz. for fine calibration. Other calibrators had multiple outputs; at one point I used a Tektronix Time Mark generator (intended for calibrating oscilloscope time bases) as the calibrator for gear in one room and it worked very well for that purpose. Hallicrafters sold a calibrator under their brand as the HT-7 and it provided selectable 1 mhz, 100 Khz, and 10 Khz. outputs and its amplified output made it usable for any radio in a nearby space; I have one but I cannot recall who actually made the unit.

I hope this helps and again welcome to the board.

Rodger WQ9E


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 7:59 pm 
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Location: Champaign IL 61822
First, you should know about the very active British ham/military forum at
https://www.vintage-radio.net/forum/index.php

they would certainly be happy to talk about this


As to you question, many ham and military radios have calibrators. Some generate
signals at all harmonics of 1.0 MHz and some at all harmonics of 100 kHz. AS you surmise,
you simply tune a harmonic, any harmonic, with your radio, and you then know exactly where
its tuned. If the radio has a BFO as you tune the radio you will hear the difference tine change,
and when you hear it go to zero frequency, the IF frequency (downconverted from the calibrator)
agrees with the BFO frequency. Most high end ham/military radios have adjustable
frequency BFOs, while some have actual crystal controlled BFOs.

As to WWV, its frequency is exact, by definition. That is, it is generated directly from
the world primary frequency standard, a hydrogen fountain clock. You can tune crystals a small amount
by varying a small capacitor in parallel with one (assuming you use the crystal parallel
resonance, which almost all oscillators do.) So you first tune in WWV on your radio
at 2.5, 5, 10, 15, 20, and (apparently on once again) 25 MHz. You don't turn on the BFO
as WWV is (mostly) AM. Then you turn on the calibrator. It will be received too, through the front
end of the radio, just like WWV, not directly in the IF. You then just adjust that trimmer cap for zero beat.
Ideally you have a signal level meter and adjust until you see it oscillating up and down every
second or less, but this means you have to get the WWV and calibrator level similar. Otherwise
you just adjust until the trimmer is half way between say 40 Hz on either side of zero beat.
It behooves you to adjust when there is no side tone on WWV or you might adjust to that!

You don't need a 100 kHz crystal to get 100 kHz harmonics. There are circuits that will
downcount 1MHz or even 2 MHz. I posted a transistorized/IC and very very cheap circuit that
will do well on a very cheap and readily available 2 MHz crystal some time ago on this forum.
Some tube radios even counted down 1 MHz crystals using a tube parametric oscillator
(Hallicrafters SX-88).

Of course you can also just count down a 1 MHz standard from a GPS receiver.
I'm not aware of any equivalent standard in Europe ... Moscow has them, but they are
not on a multiple of 100 kHz.


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 8:27 pm 
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Not to hijack but dtvmcdonald please check your PM, over the weekend I sent you a link about the SX-25 you were looking for earlier. And the forum email seems to be hit or mostly miss.

Rodger


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 8:46 pm 
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I hate to burst everyone's bubble but....

Welcome to the forum for a start! Most of the info here is good but.... it's very difficult to get a good stable signal from WWV or WWVH in the UK - if that's where the OP is. Any signal via the sky waves is subject to doppler shifts in frequency, probably not really significant enough to worry about but it is there all the same and does tend to mask the zero beat even for me in NZ. Unfortunately in Europe the frequency standards are mainly long wave, MSF and DCF, 60kHz and 77kHz etc. The BBC long wave Radio 2 was guaranteed to be on 200kHz but then they shifted to 198kHz to comply with the 9kHz band plan. Broadcast AM stations in the 550 to 1620 band have to be pretty well on frequency, but your choice is limited - again due to the 9kHz channel spacing, so if you can hear a station on 900kHz then that will do - preferably in the day time.

To add another fly to the ointment - I don't like the 'zero beat' method. It's not easy to actually hear the zero unless you have a very good receiver and can turn the AGC off so that you can hear the noise coming and going. Better to tune to WWV or WWVH then select CW or SSB and detune to produce a 1kHz tone or thereabouts - it's not important. Contrive to get your calibrator to about the same signal level and then beat the two tones together - a much more 'hearable' beat. Even better - apply the audio to a spectrum analyser in your computer - "Spectran" is good - and you can "see" the two tones - just lay one on top of the other!

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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 10:14 pm 
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Like Martin, I have always had trouble telling exactly where the zero beat is. I usually find a close range and guess the middle. However, if a person has an output meter connected across the speaker terminals, then the zero on the output meter takes it much closer than what the human ear can hear--at least my ear.

Norm

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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Mon 22, 2018 10:51 pm 
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By ear you should easily be able to get it within 50 hz and I am not really sure how you are going to use this level of accuracy in a vintage receiver. It certainly won't hold that level of accuracy once you move a few steps away from the calibrated point.

My Racal 6790 and SRT CR-91 both will tune and display in 1 hz steps but I don't have a real need for that level of resolution.

Rodger WQ9E


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Tue 23, 2018 4:46 pm 
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Thanks to all for the feedback and the welcome. It's helped me get a better idea. The thing is I use a 1960 AARL textbook to apply to all the radios I come across. The American terminology never really confused me so I just got used to "C Batteries" (instead of grid bias batteries) and so on. So, although I'm not actively involved with amateur radio, I tend to apply what I learned to the MW, LW, SW receivers I now have.
Typically, the hobbyist who tends to repair 1950s tube radios doesn't encounter BFOs or calibrators but I think you tend to get curious the more you encounter some of these homebrew sets typical in the USA. I found myself wandering from the British domestic receivers to the wider topic of amateur radio and maybe in the future I may get actively involved.
By the way, what triggered my question was a schematic I have in AARL for a two stage IF receiver that includes C.W., S.S.B., crystal filtering, amplified AVC and, of course, a 100 Kilocycle crystal calibrator (transistorised). This is not the sort of engineering I typically encounter. Pretty much all radios I worked on were pretty basic 1950's British design.
If I ever get around to actually building a Ham radio, most people tend to tackle the Simplex X Super.
Oh, last point: I'm a Russian speaker. I'm not actually Russian but used to be a language student and I now try to swat up on USSR radio jargon.


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 Post subject: Re: !00 Kilocycle Spotters
PostPosted: Jan Sat 27, 2018 8:46 am 
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My old Yaesu FT-101ZD has a neat calibrator circuit. I liked it so much
that I copied it for use in a HB QRP radio I built back in the early 1980's.
It uses a crystal oscillator at 3.200MHz followed by a CMOS divider chain.
You can choose to divide down to 100KHz or keep going on down to
25KHz. The harmonics are quite audible even up through 10 meters, as I
remember.

Coincidentally, I just found a pretty good deal on some 3.200MHz HC-49/U
crystals on Ebay. Works out to about 60 cents per crystal. I'll very likely
build up a few more of these handy circuits...

David

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