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G8MNY > TECH 06.04.06 19:20l 71 Lines 3098 Bytes #999 (0) @ WW
BID : 24720_GB7CIP
Read: OE5RCO DL1LCA GUEST
Subj: Calibrating Frequency
Path: DB0FHN<DB0RGB<DB0AAB<F6KFT<LX0PAC<ON0LGE<DB0LJ<DB0RES<ON0AR<GB7CIP
Sent: 060406/1020Z @:GB7CIP.#32.GBR.EU #:24720 [Caterham] $:24720_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Mar 06)
(8 Bit ASCII Graphics use code page 437 or 850)
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 U can compare LSB & USB & adjust for
the same pitch. Other HF broadcast station can be used but they a not 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 newly rebuilt 2M beacon GB3VHF @ Wrotham in Kent
is currently on 144,430,000 Hz with typically only ñ 1 Hz error, as it is now
locked to GPS satellite reference, this could be 100x better than your
frequency counter!
COUNTER CALIBRATION
Many hams use a frequency counter for setting up/checking rig frequencies. Here
is a method of transfusing 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.
DRIFT
Without an 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, these give quite noticeable
warm up delay, & consume power, but drift can easily be 10x better than an
uncompensated crystal oscillator.
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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