[Test-Equipment] Using HP-8640-b RF Signal Generator

[Edward B Richards]

Hi, Blair;

You don't mention what kind of radios, i.e. AM, SSB, FM, CW, etc. I
assume you want to use it for all of the above. Here are some hints. For
AM, SSB, and CW set your modulation for AM at 30%, 400 cps is the
standard (or used to be). For FM set you modulation for FM. Set the
deviation according to the system; Amateur and commercial (police, fire,
etc) is +/- 5KC, TV sound is +/- 25 kc, broadcast FM is +/-75 kc. I would
use about 1/2 of that to start with. For trouble shooting it is actually
better to start at the speaker and work back. For most radios, if you
turn the volume control up and touch the center terminal of the volume
control, you should get a loud buzz. You can also connect the audio
output of your generator or any line output audio device (CD player, tape
deck, etc. here. Use a .05 or .1 mfd capacitor to prevent DC conduction.
Ground the return (shield)(not for AC/DC radios). If you get good quality
sound with adequate volume, the audio section is working ok. If not, you
need to trouble shoot the audio circuit. Insert your tone to the grid or
base of the AF output stage and work back.

Once you are certain the AF section is working ok, proceed to the I.F.
stages. Here you will need to know the I.F. frequency of your receiver.
We will leave dual and triple conversion for another time. A common I.F.
frequency for AM/CW is 455 kc. For FM it is 10.7 mc. Your mileage may
vary. Insert your generator through a .05 or .1 mfd capacitor to the
mixer input, often the stator plates of the second tuning capacitor from
the front. The first one being the oscillator section. Ground the return
to the chassis. (not for AC/DC radios). Adjust the generator to the I.F.
frequency and set the output level to be just above the noise. Use an AC
voltmeter set to around 5 volts across the speaker and you can tune the
I.F. strip for the highest reading on the voltmeter. As the reading goes
up, reduce the output of the generator to keep the reading on the meter
scale. You should get a strong signal with just a few microvolts of
input. If no signal, trouble shoot the I.F. and detector stages by
working back from the detector.

Once you are happy with the I.F. response, move your generator to the
antenna input of your radio. No blocking capacitor is needed. Set the
radio and generator for the same frequency, for example 1,000 kc, in the
broadcast band, You should hear your generator. If not tune around and
see if you can find it. If you find it on the wrong frequency your radio
needs alignment. Another subject. Once you have centered the generator on
the radio frequency, turn the attenuator down to just above the noise. To
measure your receiver's sensitivity, disconnect the generator and set the
volume control to some easy value on the AC voltmeter. Example is .5
volt. Now connect the generator and adjust the attenuator for a voltmeter
reading 10 times the noise value. In the example this would be 5 volts
AC. Read the attenuator and that is the sensitivity of your receiver. It
is common for it to be a few microvolts for the broadcast band and 160
and 80 meters where the ambient noise level is high, and about 1
microvolt or less for the higher bands. If no response, trouble shoot the
RF amplifier(s) as above. I hope this helps.

73, Ed Richards K6UUZ
Simi Valley, Ca 93065
Home of the Air Force 1 pavilion




On Sun, 23 Apr 2006 12:32:25 -0700 (PDT) Rasputin Novgorod
<priapulus at yahoo.com> writes:
> Hi Guys:
> 
> I'm a self taught, electronics hobbyist. I've accumulated
> some HP test gear, and acquired some "book knowledge"
> but lack practical experience.
> 
> I'd like to use my HP-8640-b Signal Generator as a source
> for signal tracing thru some radios, but I have no idea
> what to use for settings. For example, I'd connect the generator
> to the antenna and walk an O'scope thru the circuit
> looking for the fault (this is mostly a learning and
> fun exercise).
> 
> However, I don't know the correct settings for the Generator,
> or technique to use. I'd be grateful for some suggestions
> so I don't smoke anything. For example:
> 
> 1) Do I need a DC blocking capacitor? What value?
> 2) How strong a signal (attenuator setting)?
> 3) Should I modulate for AM, FM, SSB or CW receivers?
>     What settings?
> 
> I realize this isn't a forum for "technician training",
> But your advice might prevent the death of a very nice
> HP-8640 due to operator incompetence. I saved for a 
> long time to buy it...
> 
> If I'm off topic, please reply directly to me.
>   (priapulus (AT) yahoo (dot) com)
> 
> Sincerely
> Blair VE3CZY