[time-nuts] If there a FAQ

jmfranke jmfranke at cox.net
Mon Nov 29 14:26:49 UTC 2010


Accurate Zero Beating, another perspective.
When trimming an oscillator so it or one of its harmonics zero beats with 
WWV or other standard frequency transmission, much comment has been made 
over the ability to approach true zero beat.  When the harmonic is directly 
zero beat, the stated accuracy is generally in the 1-5Hz range.  There is a 
technique that allows one to repeatedly zero beat to a much higher accuracy. 
The method is called the "Three-Oscillator Method" and dates back to the 
1930's, or earlier.  The earliest discussion I have found was on page 47 of 
Bulletin 10, "Frequency Measurements at Radio Frequencies," published by the 
General Radio Company in February 1933.  The bulletin states that the 
"method has been in use for a number of years."  The technique is also 
presented in sections II and XII of the 1956 Technical Manual (TM11-2665) 
for the AN/URM-18 Frequency Calibrator Set, the military version of the 
General Radio Type 1100-A Frequency Standard.  More recently, Alan Melia, 
G3NYK, reports an accuracy of 0.1 Hertz using the same technique, 
http://www.alan.melia.btinternet.co.uk/freqmeas.htm .
The three oscillators are the standard, the unknown, and either another 
lesser accuracy oscillator or a receiver BFO.  The AN/URM-18 and the General 
Radio 1100-A frequency standards utilize regenerative receivers.  Using 
reception of WWV as an example; in normal practice the unknown or a harmonic 
of the unknown is adjusted to zero beat with WWV by injecting a sample of 
the unknown source into the antenna of an AM receiver tuned to one of the 
WWV transmissions.  As the unknown is trimmed or adjusted to match WWV, a 
beat frequency will be heard that approaches 0 Hz or zero beat with the WWV 
transmission.  Unfortunately, the audio bandpass of the receiver and the 
observer's ear limit hearing a beat frequency much below ten Hz.  It is 
possible to reach closer beat frequencies by listening to the background 
noise wax and wane, but the results are not readily repeatable.  Now, a 
third source is introduced when the receiver BFO is turned on or the 
regenerative receiver is adjusted to oscillate.  With the unknown source 
temporarily disconnected, the receiver is tuned to give a nominal 1 kHz beat 
frequency while receiving the WWV transmission.  When the unknown source is 
once again added, the 1 kHz beat will wax and wane at a rate equal to the 
beat between the unknown source and the WWV transmission.  Changing the BFO 
or receiver tuning only changes the frequency of the tone that waxes and 
wanes.  The waxing and waning rate is determined solely by the beat between 
the WWV transmission and the unknown source.  It is now easy to reliably 
adjust the unknown, or its harmonic, to within a fraction of a Hertz of the 
WWV transmission.

John M. Franke  WA4WDL

--------------------------------------------------
From: "Hal Murray" <hmurray at megapathdsl.net>
Sent: Monday, November 29, 2010 3:31 AM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Subject: Re: [time-nuts] If there a FAQ

>
>> I know I can zero-beat  it by ear and get within a couple Hz out of 
>> 10MHz.
>> That is better then 1E-6 simply by hand, ear and screwdriver. No 
>> computer.
>
> How do you get down to "a couple Hz"?  I thought most (young?) ears 
> dropped
> out at about 20 Hz and I expect lots of radios chop off more than that 
> just
> to get rid of noise.
>
> Can you center the screw between two I-hear-it spots?
>
>
>> I figure my first upgrade is to replace the crystal with a temperature
>> compensated oscilator chip.   Now to go find one.
>
> Don't get too focused on TCXOs.  You are just as likely to find a low cost
> OCXO.
>
> While you are looking, keep an eye open for a TBolt or Z3801A.  (or one of
> the Z38xxA variations)  If you find one at a reasonable price, they have 
> an
> incredible price/performance ratio.  If you hang around here for long, you
> will probably get one anyway.  Unless you like the sort of fiddling you 
> have
> described, getting one now will save you a lot of time/effort.
>
>
>
>
>
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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