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Transcript
IF Alignment
Using a Wobbulator
By Gerry O’Hara, VE7GUH
Objectives
• Optimize (not necessarily maximize) IF
gain
• Produce desired IF response curve(s)
– Width of ‘nose’
– Steepness of ‘skirts’
– Symmetry
• Analyze for spurious responses
Response curves
• What is an IF response curve?
• What should it look like?
– Depends on what your using it for…
Alignment Techniques
• Use a received signal and tune ‘by ear’
• Use a signal genny and ‘tune by ear’
• Use a signal genny and an output
meter
• Use a ‘Wobbulator’ and ‘scope
• Use a Wobbulator, scope and accurate
frequency source (eg. crystal oscillator
or frequency synthesizer)
What is a ‘Wobbulator’?
• A signal genny that can be made to
‘sweep’ its frequency across the IF
bandwidth of the receiver being aligned
Not a
wobbulator
• The swept output is applied to the IF
amplifier input
• The (rectified) output voltage of the IF
amplifier is viewed on a ‘scope
• The output voltage will vary depending
on the alignment of the IF tuned circuits
What is a ‘Wobbulator’?
• The oscillator in the signal genny is
made to be voltage controlled (‘VCO’)
• The VCO is controlled by the timebase
of the ‘scope – this is a sawtooth
waveform
Voltage
Time
What is a ‘Wobbulator’?
• Ok, so this circuit is solid-state (argh!),
so just imagine these are triodes…
Homebrew Wobbulator
• Very few commercial units available…
so, why not build one yourself?
The example here is from
’Radio Bygones’ magazine,
April/May, 2003. It is
combined in one box with a
DFM kit from Norcal
The wobbulator circuit board is on the
lower right, frequency counter board
upper left
So what can it do?
The controls:
In use…
• For automatic sync, your ‘scope must
be able to output a suitable ramp
output signal (‘sweep signal’)
• If it does not have this (the Hitachi
example here did not), then tap into
the ‘scope’s timebase circuit
• The ramp signal controls the sweep
width of the wobbulator’s VCO (+/from the pre-set centre frequency)
In use
• Connect the wobbulator, receiver and
‘scope as shown in slide 6
IF-in
IF-out
Y-in
Sweep in
Sweep out
• Set the wobbulator centre frequency to
the desired nominal IF frequency
• Set the ‘scope to sweep at a slow rate
In use
• Set wobbulator sweep width to be
slightly wider than the IF bandwidth
• Observe ‘scope trace and commence
adjustment of IF transformer cores
• Start with the one nearest the detector
first, working back towards the mixer
BFO
stage
3a/b
2a/b
1 Detector
AF Stages
From
Mixer
Stage
1st IF
2nd IF
In use
•
•
•
•
•
Outer peak is usually the correct one
Many IF trxfrs have two adjustments
Use the right tool(s)!!
Be careful not to break the core (‘slug’)
Remove any old elastic filament, wax
etc, replace with small dab of Rocol
high-viscosity grease
In use
Left: ‘Scope trace of
IF response of radio
before alignment
(nominal IF is
465kHz)
Right: ‘Scope trace
of same radio after
alignment
445kHz 455kHz
465kHz 475kHz 485kHz
In use
Left: Using a second signal
genny or crystal oscillator to
pinpoint a frequency along
the trace – here exactly at
the 465kHz centre
frequency. This produces a
beat frequency with the
wobbulator signal
Right: As above, but with
the second signal
genny/crystal oscillator set
to 460kHz (zero beat to left
of the curve)
In use
Left: Flattening of trace
caused by too high an
injection level resulting
in receiver overload
Right: Trace resulting from
IF strip that includes a
4kHz mechanical filter.
Receiver aligned for
maximum sensitivity
In use
Left: Trace resulting
from IF strip that
includes a 2.6kHz
ceramic filter.
Receiver aligned for
maximum sensitivity
• A flat ‘nose’ and steep ‘skirts’ are ideal
response characteristics
• Wider ‘nose’ giver better fidelity
response (+/-8kHz) for AM reception
In use
• Tailoring the IF response curve:
• May be obtained by ‘staggering’ tuning of
several IF transformers, or
• Adjusting coupling within transformers
(closer spacing gives wider curve) – used
in Super Pro and some Eddystones
• Narrower ‘nose’ better for SSB and
CW reception
• Steeper skirts to optimize selectivity
(reject adjacent signals)
Questions?
By Gerry O’Hara, VE7GUH