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G8MNY > TECH 13.06.08 07:20l 200 Lines 9221 Bytes #999 (0) @ WW
BID : 58842_GB7CIP
Read: GUEST
Subj: CW/SSB Rig Frequency
Path: IZ3LSV<IW0QNL<ON4HU<ON0BEL<GB7CIP
Sent: 080613/0027Z @:GB7CIP.#32.GBR.EU #:58842 [Caterham] $:58842_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Nov 07)
(8 Bit ASCII Graphics use code page 437 or 850)
Rigs frequency readouts are not all what they seem. Here is an explanation of
why they can be wrong. With the older pre PLL rigs, tiny frequency errors of
a few 100Hz considered was irrelevant, but with 10Hz digital readouts it looks
so bad.
MASTER OSCILLATOR
On modern PLL rigs this is the oscillator that is used to lock the VFO to.
Changing this will alter all the Tx/Rx frequencies.
ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄ¿
³MASTERÃÄÄÄÄÄÄ´MIXERÃÄÄÄÄÄ´ LOOP ³ The PLL needs to do the whole
³ OSC ³ ÀÄÄÂÄÄÙ ³FILTER³ of HF down to say 10Hz for
ÀÄÄÄÄÄÄÙ ³ ÀÄÄÄÂÄÄÙ good tuning, however correcting
ÚÄÄÄÄÄÄÁÄÄÄÄÄ¿ ³ a VFO only 10 times a second
DIGITAL ð³PROGRAMMABLE³ ³DC to get 10Hz loop steps is not
FREQUENCYð³ COUNTER ³ ³Control a good strategy, as there is
INPUT ÀÄÄÄÄÄÄÂÄÄÄÄÄÙ ³ slow tuning response & higher
ÃÄÄÄÄÄÄ>OSC ³ noise sidebands (wobble)
ÚÄÄÁÄÄ¿ ³ than a faster loop.
³ VFO ÃÄÄÄ<ÄÄÄÄÄÙ
ÀÄÄÄÄÄÙ
To solve this problem another method is used...
INTERPOLATION
This is a pre calibrated offsets of a crystal oscillator. Sometimes used for
just a 5kHz FM rig shift, but more sophisticated may be up to 256 fine steps.
More typically 100x 10Hz steps to fill in frequency gaps of a 1kHz PLL step
oscillator. Correctly set up the result can be perfect, but incorrect it will
give uneven jumps for each PLL step.
Just right Gain to high Gain too low
interpolation steps interpolation steps interpolation steps
ÁÅÁÁÁÁÁÁÁÁÁÅÁÁÁÁÁÁÁÁÁÅ ÅÁÁÁÁÁÁÁÁÁÅÄÁÁÁÁÁÁÁÁÁÅ ÁÐÅÁÁÁÁÁÁÁÁÐÅÁÁÁÁÁÁÁÁÐÅ
Course PLL steps Course PLL steps Course PLL steps
So the gain of the interpolation offsets needs to be just right, so that at the
PLL step the interpolation frequency jump provides a step is the same. An
error here may CORRUPT the PLL Master Oscillator calibration!
IF OFFSET
For direct conversion rigs the PLL is used directly as the Rx & Tx frequency in
SSB but has a Rx offset to generate a CW tone on Tx.
SSB Direct CW Direct FM Only Multimode
conversion Conversion Rx Rigs Tx Tx/Rx
³ ³Tone³ LO ³ LO ³
ÄÄÅÄÄ ÄÅÄÄÄÄÅÄ ÄÅÄÄÄÄÄÄÄÄÄÄÄÄÅÄ ÅÄÄÄÄÄÄÄÄÄÄÄÁÄ
PLL Rx Tx <----IF----> <----IF---->
Dial Rx Dial PLL Jump---->Dial PLL Dial
For superhets the Rx LO frequency is offset by the IF, so the PLL is setup to
do that. In an FM only system then the PLL will be asked to jump to the Tx
frequency on Tx (N.B. not to Tx the jump QSY sweep!). In a multimode the Tx
signals are made at the IF frequency & may share the IF filter for bandwidth
limiting.
AM & FM
With these modes the Tx frequency is the same as the dial. If the rig uses a Tx
IF for these, its frequency will need setting up to generate the dial
frequency. If this is not done after the PLL is set up, & the Tx freq used for
PLL calibration a frequency scaling error will occur & the PLL will be out at
other frequencies!
SSB FREQUENCY
As well as the LO being offset by the IF frequency the modulation &
demodulation uses an IF BFO in the product detector/mixer. So there is another
frequency to set up. With USB & LSB modes it is placed above or below the IF
filter. The PLL system is programed with these mode offsets....
USB Dial LSB Dial
Freq Filter Freq
0 _³ ______________________________ |
-6dB_³ /~ ~\ |
³ / \ |
-40dB_³ _.ú' 'ú._ |
ÅÄÄÄÂÄÁÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÂÄÁÄÂÄÄÄÅ
0 100 200 300 USB 2k6 2k7 2k8 2k9 Hz
2k8 2k7 2k6 LSB 300 200 100 0
On non PLL rigs the BFO is normally set up to place the IF filter at it's best
place for good communications frequencies e.g. 300-2600Hz. But with a PLL
design a BFO frequency error will put the dial out!
Dial
Freq
³BFO ___________________________
³ ³ / \
³ ³ | |
ÅÄÅÄÄÄÄÁÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÁÄÄ
0 300 2k6 Hz
> <
error
An exacting way of checking this is to tune into an accurate AM broadcast
signal & select AM, USB, & LSB, modes & see if there is any pitch difference,
there should be none. (If the rig display is one that jumps in frequency
between modes, it will need resetting to the station's frequency when U change
mode.)
Dial
Freq
________________________ ³ ________________________AM signal
/ /~~~~~~~~~~~\ \³/ /~~~~~~~~~~~\ \
| | LSB | ³ | USB | |
ÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÅÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄ
-4k5 -2k6 -300 0 300 2k6 4k5 Hz
<----------------------AM Filter----------------------->
Using the RIT to eliminate pitch difference.. Fixed RIT difference may indicate
the PLL is off, a different amount of RIT for USB than LSB indicates the IF
BFOs are off.
IF SHIFTS/WIDTHS
Rigs using 2 IFs it is possible to overlap filters of different frequencies to
change the BFO to IF frequency without an apparent frequency shift or to get
variable IF widths.
Dial IF Shifts
Freq LOW HIGH
³ ___________________________________
³ / ú~~~~~~~~~~~~~~~~~~~\~~~~~~~~~~~~~~~~~ú
³ | : | :
ÅÄÄÄÂÄÄÁÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÁÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÁÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÁÂÄ
0 100 200 300 400 500 600 2k2 2k4 2k5 2k6 2k7 2k8 Hz
To do this the BFOs/LO are mixed together so that the overall IF difference is
nil.
Dial
Freq W I D E
³ ____________________________________________
³ / ú~~~~~~~~~~~~~~~~~~ú \
³ | : NARROW : |
ÅÄÄÄÂÄÄÄÂÄÁÄÂÄÄÄÂÄÄÄÂÄÄÁÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÁÄÄÂÄÄÄÂÄÄÄÂÄÁÄÂÄ
0 100 200 300 400 500 600 2k2 2k4 2k5 2k6 2k7 Hz
Different makers use different controls for these functions. Simple ones let
you narrow the width by removing top or bottom bandwidth in a single control,
more complex ones use 2 controls to give a full combination of the 2 functions.
If set up properly IF shift & bandwidth should not affect calibration, but they
will need calibration in them selves, so that the filters do overlap fully for
the widest IF, say when the control is in the middle. This may have
consequences for the Tx IF/SSB tone.
NOTCH FILTER
This is a crystal that is tunable across the IF to filter out unwanted signals
on spot frequencies.
Dial
Freq TUNABLE NOTCH
³ ________________ _______________
³ / \ / \
³ | <- U -> |
ÅÄÄÄÂÄÄÄÂÄÄÁÂÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÂÄÁÄÂÄÄÄÂÄ
0 100 200 300 400 500 2k4 2k5 2k6 2k7 2k8 Hz
It will not affect frequency calibration, except to itself so the centre of
control is centre of IF.
CW MODE
Note this is a split frequency mode! In Rx the offset PLL is set up to produce
a CW tone ideally equal to the CW Tx sidetone say 700Hz.
Tx
Rx ---- BFO offset ---- ³ Like the SSB mode to do
ÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄ this the PLL or IF BFOs
Ã----- Rx CW tone ---->³ are shifted between Tx
Dial & Rx & the 2 frequencies
<Ä CW AF Sidetone Osc ´ will need setting up if the
Fixed 700Hz rig is being calibrated.
Then tuning in to a CW station at the same pitch has your sidetone you will be
able to Tx EXACTLY on the same frequency & not a few 100 Hz out!
If there is a narrow CW IF filter then the BFO will need to be calibrated for
that to do the same.
CONCLUSION
Of course the above is all true for DSP as well, except that all the
calibrations are in the software, so if the master PLL is OK all should be OK.
But generally to calibrate a Rig's frequency there are several BFO oscillators
to set up.. USB, LSB, CW Tx, CW Rx, AM Tx & FM Tx, as well as the PLL master
oscillator. VHF/UHF sets may have other mixer oscillators as well.
See also my TECH buls on "Calibrating Frequency", & "Off air lock ref osc".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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