[R-390] Alignment

[Jim Temple]

Hi Bill,

The signal generator is a Heathkit IG-102, the next generation from the
SG-8.

My main concrern was how critical setting the -7 volts @150uV, IF Gain was
in completing the alignment.  It now occurs to me that I only have the test
equipment that I have..... and I must estimate to the best of my abilit with
what I have.  In the future, perhaps I can accumulate calibrated test
equipment.  I recently received a good suggestion, in response to an
observation by me that my generator would not attenuate enough when directly
attached, to not directly couple the generator to the antenna input, which
would attenuate the signal through AIR SPACE.  A good compromise for a
calibrated, small signal generator.  (My signal generator would not
attenuate enough without reducing the RF GAIN control).

So, the bottom line is to use what I have to the best of my ability and
align with the best guesses I can, while reviewing some of the excellent web
pages available.  And this lists' opinions.

Thanks for all the help.

Jim

----- Original Message -----
From: "Bill Smith" <billsmith@ispwest.com>
To: "Jim Temple" <jetemp@insightbb.com>; <r-390@mailman.qth.net>
Sent: Monday, April 01, 2002 8:50 PM
Subject: Re: [R-390] Alignment


> Hi, Jim
>
> A couple of observations.  150uV is .000150 volts, which could be
difficult
> to observe on a VOM, at least on an analog meter type.   Typically a one
> volt signal is fed through a switched attenuator to obtain a calibrated
> signal.
>
> Regarding the Heathkit, is it a SG-8?  I used one for years and used it to
> tune up all sorts of equipment, from AC-DC radios to 450MHz two-way radio.
> For a general purpose signal source, or a local AM transmitter (play music
> through your radio) they are great.  However, they really aren't
calibrated.
> Here are a couple of things to consider.  First, the oscillator is a
> modified multivibrator, with a pulse-shaped output.  Much of the output
> energy is at harmonics of the fundamental, so even if you were able to
> measure the voltage, the reading will probably be misleading because much
of
> the energy is at frequencies other than the fundamental.  To be
meaningful,
> the generator should produce a low-distortion sine-wave.  Remember that a
> diode detector is a peak reading device, that is the detector rectifies
the
> waveshape, and a following capacitor stores the resultant voltage.  With
> little or no load, the voltage will probably represent the peak voltage of
> the waveshape (less 0.7 or 0.2 volts lost in the diode).  Actually there
are
> a number of variables in a RF diode detector, but that is another story.
>
> Another issue is shielding.  Although you need a relatively strong signal
> (150uV), the SG-8 and similar generators leak signal through the cabinet,
> out through the line cord and through the mounting feet.  :-)   Just
moving
> around can affect the signal pickup in the receiver.
>
> Also, although probably not an issue here, the output impedance of the
SG-8
> is a total unknown, which manifests itself in SWR (radiation) on the
output
> cable as well as generating an unknown match to whatever you connect the
> cable to.  The attenuator (switch and pot) are for all purposes useless,
as
> they are not shielded, and are not an effective design to attenuate RF
> signals.
>
> In short, the generator is fine for applications where you can place the
> unit next to the device you are working on, and tune with whatever signal
> the equipment picks up, but it is a difficult device with which to make
any
> sort of measurements.  Any attempt to modify it or overcome the SG-8's
> serious weaknesses make me reflect, today on April 1, of all the times I
> have embarked on a modification project to make a pearl out of a rock.  I
> ended up with a broken, bruised rock every time.  ;-)
>
> 73 de Bill, AB6MT
> billsmith@ispwest.com
>
>
> > >
> > > I did the alignment, before my latest escapade, and it went well.
> > > However, I estimated the 150uV needed to obtain the 7volts diode load.
> > >
> > > The signal generator is a Heathkit that has no meter, so the output is
> not
> > > calibrated.  I measure the frequency with an HP freq counter, so I
have
> > > accurate frequencies.  I know that a calibrated output is not needed
for
> > > any of the alignment, except to set the 7volts at 150uV.
> > >
> > > Can the 150uV output be accurately measured with a VOM??  I think the
> > > output to be measured is RF voltage, so would a small RF detector,
such
> as what
> >    is in the ARRL handbook be suitable and accurate enough for the
> purpose.
> > >
> > > Jim.
> > >
>
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