IF AlignmentUsing 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

• 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

Not a wobbulator

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 sawtoothwaveform

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 wobbulator circuit board is on the lower right, frequency counter board upper left

The example here is from ’Radio Bygones’ magazine, April/May, 2003. It is combined in one box with a DFM kit from Norcal

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

• Set the wobbulator centre frequency to the desired nominal IF frequency

• Set the ‘scope to sweep at a slow rate

Sweep outY-in

IF-in

Sweep in

IF-out

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 stage 12a/b3a/b Detector AF Stages

BFO

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 Rocolhigh-viscosity grease

In useLeft: ‘Scope trace of IF response of radio before alignment (nominal IF is 465kHz)

Right: ‘Scope trace of same radio after alignment

465kHz455kHz 475kHz445kHz 485kHz

In useLeft: 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