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Function or signal generator for ham radio?

Jul 28th 2011, 12:23

W1VT

Joined: Apr 4th 1998, 00:00
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A member asks:

For ham radio work up to 50 MHz would it make sense to buy a 20 MHz function generator ? I ask as 20 is my limit financially ($500), and I see no sense in buying one if it won't meet most of my needs.

I know there are used ones out there for supposedly low prices, but I have yet to find one on the web at 50 MHz for less than a couple of grand.


I recommended that he get the classic HP8640B signal generator, which features a calibrated output that covers all bands up to 70cm, AM/FM modulation, and enough shielding to allow receiver sensitivity measurements. Granted, a function generator is better for learning basic electronics, having a sweep function, pulse, triangle, and square wave output.

But, what would you recommend? Why?

http://reviews.ebay.com/The-HP-8640B-RF-Signal-Generator_W0QQugidZ10000000010610658
An ebay review on what to look for when buying an HP-8640 signal generator.

Jul 28th 2011, 13:12

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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He may not have known that there is a difference between a function generator and a signal generator. The function generators he may buy used would probably not be stable enough to to most frequency measurements. Your recommendation is exactly the one that came to my mind when I first read his question and his intended purpose. If he really needs the pulse, square, sawtooth and triangle waves a function generator can create, that's another story, but if he wants a good signal generator, it's hard to beat those old HP8640s for the price. The Lab bought one on eBay for $300, as we wanted to have a parts unit in case the ones we still occasionally use in the Lab failed.

But there are probably a lot of other options, and HP8640s dont' show up at flea markets often, so I, too, would love to hear other opinions to help make me smarter the next time we get the question. :-)

73.
Ed Hare, W1RFI
ARRL Lab
Technical forums moderator
Jul 28th 2011, 20:11

ka9wgn

Joined: Apr 4th 1998, 00:00
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What about the possibility of turning a software defined radio into a signal generator? Would it, could it, be stable enough? I've never tried these, but it would seem to be at least a way to have a signal generator that is computer controlled (if you are into that).

Jul 29th 2011, 02:44

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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I'd think that the SDR would be frequency- and amplitude-stable enough, but most signal generators are able to go down to less than 0.1 uV into 50 ohms, or output levels expressed in dBm of -130 to -145 dBm or so. I don't know of any SDR that can go that low in power.

You could get some range with an external step attenuator, but most provide less than 100 dB of attenuation, and at 100 dB of attenuation, a step attenuator needs to be almost perfect.

The SDR as a test instrument could have some uses, but I'd still be on the lookout for a signal generator.

73.
Ed Hare, W1RFI
Jul 30th 2011, 04:13

KE8DO

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
I got a URM-25 signal generator on eBay because that was what I used in the Navy. They are old, heavy and have tubes, so it may not be the best buy in today's solid state market. I think that it was under $100, but that was 8 years ago.
If you have the time, a solid state signal generator would make a good homebrew project.
73 Don, First Class FCC Radio-telephone License 38 years

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