[Collins] Chirpy signals

[Dr. Gerald N. Johnson]

On Mon, 2006-03-20 at 22:17 -0500, TJRachwal wrote:
> Hi Guys,
> 	I need some advice.  In the last week all of a sudden my 75A-4 had 
> developed "chirpy-wobbles" on signals (CW and SSB).  It has been 
> working fine until now.  The actual frequency "seems" to be OK but 
> the signals (CW) are 1960's UA style!  Before I jump into discovering 
> the source I was hoping someone had also experienced this and could 
> point me in the correct direction.  Any thoughts?
> 73,
> Ted, K8AQM
> 
> 
I can think of four possibilities right away.

1. Power supply voltage regulation gone bad as filter capacitors age.
Voltage varying because of varying audio current.

2. Oscillator voltage regulator bad.

3. Too much signal voltage at the diode detector.
	Could come from RF/IF gain too high.
        Could come from lack of AVC.

In a diode detector there's always a compromise on the value of the
coupling capacitor from BFO to diode. Larger gets a better BFO voltage
to the diode, but couples more IF signal to the BFO to vary the BFO
frequency as it tries to lock to the IF signal. So we generally run the
RF gain backed off on CW/SSB to cut the signal to the diode detector
(which works best with more BFO than IF signal, otherwise it distorts)
and to limit this effect. This shouldn't be an effect with a product
detector.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer