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G8MNY > TECHNI 28.01.22 09:04l 122 Lines 5193 Bytes #999 (0) @ WW
BID : 59385_GB7CIP
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Subj: Calibrating Frequency
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 20)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
HF
For broadband Rx it is fairly straight forward, you just select a standard
frequency station at say 5 10 or 15MHz can be heard (weak at times) & adjust
the dial tuning to read that frequency, then adjust a calibrate control
(internal) until a zero beat is heard or estimated in SSB mode. Sometimes a
known offset such as 1kHz is needed, then you can compare LSB & USB & adjust
for the same pitch. Other HF broadcast stations can be used, but they are not
all so accurate. For more accurate work calibration of a HF Tx harmonic against
a VHF beacon can be used, see below.
VHF
The accuracy becomes more important for some modes & the same procedure can be
done with a good beacon. The rebuilt UK 2M beacon GB3VHF starts its 60th year
of operation in located at Fairseat on the North Downs in Kent, the beacon
is on 144.430MHz at a height of 205m ASL. See www.gb3vhf.co.uk. It is typically
only ñ 1 Hz error off air, as it is locked to GPS satellite reference, this
could easily be more than 100x better than your frequency counter! It Tx 17
seconds of blank carrier every 2 mins, enough to zero beat to 1 Hz by ear
(allowing for VHF flutter).
144.430000 \³/ ÚÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿
MHz ³ ³ VHF SSB ³ AF ³Frequency³
ÀÄÄ´144.429 RxÃÄÄÄÄ´ Counter ³
ÀÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÙ
Rx Calibrated to give 1kHz
COUNTER CALIBRATION
Many hams use a frequency counter for setting up/checking rig frequencies. Here
is a method of transfuring the high calibration accuracy of a beacon like
GB3VHF to a frequency counter.
Ref Signal
2m Stray Tx
\³/ \³/
ÚÄÄÁÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ RF ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ VHF ³ ³ VHF ÃÄ´DUMMY³ pick up ³ FREQ COUNTER ³
³ Rx ³ ³ CW Tx ³ ³LOAD ÃÄÄÄÄÄÄÄÄÄ´ BEING CALIBRATED ³
ÀÄÄÂÄÄÙ ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÙ /ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
| AF [or a good Sig gen] | for AF Note
À - - - - - - - - - - - - - - - - - Ù frequency
This assumes you have access to a Rx (ideally AM mode, but FM/CW/SSB will do)
that can Rx the beacon. And a CW Tx or a harmonic of one!
This is a 2 step method, so if the Rx is always running the error in the middle
CW Tx kit can be monitored & kept to a minimum.
STEP1
1/ Adjust the stray signal emission if you can to be a similar strength to the
beacon.
2/ Adjust the CW Tx to zero beat with the beacon, or if not, to a known
measured AF beat note high or low off the beacon.
STEP2
3/ With the counter now measuring the CW Tx, adjust the counter's cal trimmer
to read either the beacon frequency or the beacon +/- AF offset. If using a
harmonic of a CW Tx, do the maths to calculate the true Tx frequency &
calibrate the counter to read that.
EXAMPLE
Using say a 28MHz 1kHz step synth Rig as the CW Tx that the 5th harmonic can't
be zero beated on the 144.430MHz beacon....
Ref Signal
2m 144.430 Stray Tx x5
\³/ 144.43
ÚÄÄÄÁÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ RF ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ VHF Rx³ ³ 10m HF ÃÄ´DUMMY³ pick up ³ FREQ COUNTER ³
³ AM/FM ³ ³ CW Tx ³ ³LOAD ÃÄÄÄÄÄÄÄÄÄ´ BEING CALIBRATED ³
ÀÄÄÄÂÄÄÄÙ ³ 28.886 ³ ÀÄÄÄÄÄÙ /ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
| AF ÀÄÄÄÄÄÄÄÄÙ | AF Note frequency
À - - - - - - - - - - - - - - - - - Ù say + 450Hz.
A/ Determin if the beat note is due to Tx being too high or too low, by tuning
the Tx higher does the beat note increase, the Tx is high, if it lowers in
pitch Tx is lower than beacon.
B/ Measure the beat note on the counter, say it is + 450Hz
C/ Calculate actual Tx frequency, 28.886 +(450/5) = 28,886,090Hz
D/ Measure the Tx freq on counter & adjust it's trimmer to read 28.886090 MHz
E/ Recheck the beat frequency has not altered. Repeat if it has.
DRIFT
Without a PLL to an external reference most crystal controlled equipment is
only about ñ 10ppm over a modest temperature range or long term. Temperature
compensated oscillators are better, where temperature variation on other
components is allowed to compensate for the crystal oscillator temperature
drift. Short term improvements of 5x are possible (ñ 2 ppm). Some kit may even
have a crystal oven to eliminate temperature drift altogether, these give quite
a noticeable warm up delay, & consume power, but drift can easily be 10x - 100x
better than an uncompensated crystal oscillator.
See my related buls on "Crystal Drift Compensation", "Simple Crystal Oven",
"198kHz Off Air Standard", "Off air lock for ref osc" "Comparing Off Air Freq
Standards" & "Marconi Counter Type 2432A".
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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