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Author Topic: RF Signal Generators  (Read 1415 times)
WB4WOT
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Posts: 13




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« on: March 24, 2005, 05:41:02 AM »

Hi, guys,
I'm interested in buying a descent, used signal generator that won't break the bank.  It's for personal station use only on the HF bands.  Any thoughts?

73's
Thom
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W7DJM
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Posts: 1




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« Reply #1 on: March 24, 2005, 07:34:44 AM »

Hard to say, how much money do you have?

What do you call "decent?"


  IF you can get to a hamfest, and therefore don't have to ship it, one of old HP  606(?)  I think, about as big and heavy as a Heathkit Apache,  these are still good units, but use tubes, are big and heavy.

Lot's of Military URM-25D's available, again, tubes, but a lot smaller.

One thing you must be careful of, is that some of the very early synthethizer stuff--typically thumbwheel--can be pretty dirty at the output.

A good one, if not cheap, is the HP 8640 series.  You might just spend upwards over 400.00, for one of these, and even then, it won't be "certified."

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WB6BYU
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Posts: 13578




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« Reply #2 on: March 24, 2005, 01:21:22 PM »

I like the HP608 that was given to me by a friend, but
as was mentioned before, it is "portable" only in the
sense that it has handles.  And it only goes down to
10 MHz, so I have to get creative for the lower frequencies.

Probably the first question is, what do you need the
signal generator to do for you?  How stable does it need
to be?  How tunable?  Does the output level need to be
adjustable?  or calibrated?  What frequency range will
you be using?

I've been experimenting with 80m receivers and ended up
with a number of different solutions: a dip meter or SWR
analyzer will generate a signal, but frequency calibration
isn't good, and output coupling isn't always convenient.
An old Heathkit crystal oscillator got a lot of use, but
relies on having some useful crystals in the junkbox.
And I used to use an old 100 kHz crystal calibrator as
a signal generator for aligning receivers.

None of these are as versatile as a good synthesized
signal generator, but they got the job done for a lot
less money.  And often you can find inexpensive RF
generators at hamfests for $10 to $25 that may or may
not meet your needs, depending on what you need it to
do.
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WB2WIK
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Posts: 20666




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« Reply #3 on: March 28, 2005, 09:52:44 AM »

There are signal generators and then there are laboratory instruments which are signal generators.  The latter types can be extremely useful and generally trusted, while the former types include everything -- including a bunch of junk not worth the space it occupies.

As Dale said, the HP 608 family (which went up to at least an "F" version, maybe higher) are versatile, lab-quality instruments but only go down to 10 MHz at the low end (the high end goes to 400 or 480 MHz).  It's very heavy and uses a lot of tubes, but a "working" 608 is surely a workhorse.  I have a 608F which I purchased used in 1974 and it's worked perfectly ever since then.  Although not quite a stable as a more modern synthesized (PLL) generator, the 608, being a cavity oscillator, is extremely "clean," with harmonics, spurs and overall "noise" performance that's better than many modern and much more expensive RF signal sources.

It also has an RF amplifier, RF modulator and ALC circuit to keep its output relatively constant over substantial frequency changes.  And its piston attenuator, a waveguide-below-cutoff design, is foolproof, long-lasting and very accurate.

You can pick up a 608 model in good working condition for $100-$200 nowadays, and this is about the "minimum" generator worth the space it occupies.  But it won't go below 10 MHz without an external frequency divider or mixer.

To get my 608F down below 10 MHz, I use an external 10 MHz crystal oscillator and doubly balanced mixer to create signals from 10 MHz down to virtually "DC."  That works well, but isn't as "clean" as the original signal source, and of course is also not as powerful: The mixer has about 10 dB insertion loss, so all the calibrations on the generator are wrong.  To check what's actually happening, I use my Boonton 42A RF power meter, which is good down to about -50 dBm with 0.5 dB accuracy.  To create weaker signals than that, I use an HP 355D step attenuator (10 dB steps, to 120 dB) and call it a day.

I also have a more modern HP 8640B (vintage 1977, all solid state and PLL) which goes down below 1 MHz and up to 520 MHz (1040 MHz with an external doubler) and is also quite good, but not as "clean" as the older 608; so for near-channel measurements (measuring stuff within a few kHz of the generator frequency) it's actually not as good as the older, vacuum tube unit.

WB2WIK/6
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NS6Y_
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« Reply #4 on: March 29, 2005, 03:38:58 AM »

The HP 8601A is a good one, has a crank on the front, always a sign of trustworthiness! It has a wide range, both in freq (100kHz to 110 MHz) and amplitude (-110 to +20dBm) and does some fancy sweep and modulation stuff. But it's limited on the low end in freq. My HP 3314 goes down to .001 Hz (!) I know, I just fiddled with it to see. It will also do 10V p-p. Also does a bunch of neat tricks and "arbitrary waveform generation, and has a loudish fan to remind me I've left it on.  It's more modern and neither are cheap.

You might want to look into the BK Precision, Elenco, etc type instruments, there are even some that are kits you can build, I think most modern ones will go down to 10 Hz although they may only do say 10MHz on the high end. But if you generate a square wave you have all those nice harmonics.
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WB6BYU
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Posts: 13578




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« Reply #5 on: March 29, 2005, 01:16:24 PM »

.."this is about the 'minimum' signal generator worth the
space it occupies"...

And that makes it pretty valuable, since it sure occupies
a lot of space!  Certainly won't fit the small instrument
shelf above my bench - it has to live in the steel shelves
instead.

Mine died several months ago with a bad tube in the HV
regulator.  Fortunatly a friend had some spares, but by
the time I got that replaced one of the RF buffer tubes
had an open filament (interesting interactions among the
regulators for the HF and filament voltages!)  There are
only 3 tubes in the RF section, and the two pencil triodes
are very difficult to find.  But it was the third tube,
and a replacement is on its way.  Then I'll be able to
put it back in the case and back in the shelves.

This is not a piece of equipment that is easy to turn
over while troubleshooting.  I ended up taking it out
of the case and putting it on my workshop stool with
the rotating seat.  This allowed me to spin it around
while checking voltages.  (Besides, I couldn't clear
enough room on the bench for it.)
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WB2WIK
Member

Posts: 20666




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« Reply #6 on: March 30, 2005, 01:19:12 PM »

Dale, I agree with you: It certainly occupies a lot of space.

You probably noticed the 608 has "feet" not only on its bottom, but also on its rear (back side of the enclosure), so it can be used while sitting on the floor on its "back," facing upwards.  I use mine that way, most of the time.  It won't buckle the floor!

73

Steve
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G4AON
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Posts: 545




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« Reply #7 on: April 04, 2005, 10:44:56 AM »

Hi Thom

While the manufacturers over this side of the pond are a little different, the basic ideas and potential problems are the same. I acquired a really nice synthesized Marconi 2018 generator that was faulty. I was lucky as the 2018 came with a service manual, and an easily fixed minor fault.

If one of the more complex units fails, could you either fix it or get it fixed for a reasonable sum? If not the purchase price would have to reflect the fact it could be  on the scrap heap if fails! At least if you go for a basic unit, the chances are you or someone nearby could keep it working for years.

Whatever model you decide on, being able to obtain a service manual for it seems like a very good idea.

Dave
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WB4WOT
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Posts: 13




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« Reply #8 on: April 06, 2005, 03:19:21 PM »

Thanks for the valuable insight, gentlemen.  Good luck and 73's.
Thom
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N0TONE
Member

Posts: 173




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« Reply #9 on: April 12, 2005, 12:54:50 PM »

I use a general-coverage HF rig "opened up" for transmit.  Disable both the PA and the driver, and send the pre-driver output through an old step attenuator.

The signal from such a radio is a whole lot cleaner than most cheap sig gens, and as a bonus, you can modulate the signal with any flavor you wish.

AM
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N3JWN
Member

Posts: 103




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« Reply #10 on: May 15, 2005, 05:18:35 AM »

Signal generators: goto ebay and get a fairly recently built heathkit form the 80's not the 50-60's versions and a nice Frequency counter for reasonable prices and 99% on reliability Heath made great equip using standard parts of the day and very good circuitry and easy to repair with the manuals of course a sorce I feel cant be beet. have fun good luck and be safe.....Dick / n3jwn on ebay rgbn3jwn
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