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Author Topic: Questions about using Signal Generator and Signal Attenuators  (Read 323 times)

WA4ZYN

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  • Posts: 327

Good day.  I am trying to expand my radio testing and repair abilities now that I am retired.  I have some questions about signal generators.  In order to test receive sensitivity, my signal generator needs to output a signal in the uV range.  I know some receivers are rated for 1uV sensitivity, and some 0.5uV, or even 0.25uV.  If my signal generator is only capable of reducing its output to 1mV, can I use a 50-ohm signal attenuator to reduce the output sufficiently?  And can I use a series of attenuators to reduce the signal even more?  (I believe attenuation is simply additive). From the electronic knowledge I have gained over the years I believe this to be so, but for the safety of my receivers I just want to make sure.

Thank you, any and all, for your help and patience with me.
73,
Francis WA4ZYN
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WA4ZYN

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  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #1 on: April 04, 2022, 08:19:31 AM »

If the idea of serial attenuators is too cumbersome, can someone recommend a signal generator that could output uV level signals?  And if I could find a signal generator that could also sweep, so I can check bandpass filters, it would be ideal!

Tnx agn, es 73,
Francis WA4ZYN

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W1VT

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Re: Questions about using Signal Generator and Signal Attenuators
« Reply #2 on: April 04, 2022, 08:54:55 AM »

You would need to insure that common mode and radiated signal from the generator is less than the signal through the attenuators.
One idea may be to use a long coax cable to reduce the radiated signal pickup and common mode chokes on all the wires.

Zak W1VT
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N7EKU

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Re: Questions about using Signal Generator and Signal Attenuators
« Reply #3 on: April 04, 2022, 10:16:10 AM »

Yes,

Getting the needed shielding is difficult with a general purpose signal generator.  Better would be to use something that only generates a low signal from the start.  I have one similar to this and it is very handy:

https://www.multus-sdr.com/index.php?rt=product/product&path=72&product_id=129

73.
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Mark -- N7EKU/VE3

N0GV

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Re: Questions about using Signal Generator and Signal Attenuators
« Reply #4 on: April 04, 2022, 10:20:00 AM »

A 40 dB fixed 1 watt attenuator should drop you to the -86 dBm range  for 1mV in. You would need another 40 dB attenuator AND a 30 dB attenuator to drop that 1 mV down to -156 dBm..... at this point the signal leakage from your generator, coax and so on are likely to be larger than your attenuated signal....

If you are only aligning and testing to repair equipment just the single 40 dB attenuator should be fine as it gives S7 + 4 dBm signal levels. Stacking a 20 dB and the 40 dB attenuators would get you down to -106 dBm... or S4 + 4 dBm....

All the best,

Grover
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W9IQ

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  • Posts: 8866
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #5 on: April 04, 2022, 10:23:21 AM »

I know some receivers are rated for 1uV sensitivity, and some 0.5uV, or even 0.25uV.  If my signal generator is only capable of reducing its output to 1mV, can I use a 50-ohm signal attenuator to reduce the output sufficiently?

Francis,

Yes, you would do that with attenuators. To go from 1 mV to 0.25 μV, you would need about 75 dB of attenuation [20*log(V1/V2)]. Keep in mind that the attenuators are accurate only if they see the input and output impedance for which they are made which is most commonly 50 ohms.

Because of the very low signal levels, you should interconnect everything with double shielded coax (e.g. RG-142 or RG-400) to minimize leakage. It is also a good idea to put some clamp-on ferrite of the right mix and size to suppress common mode current off of the shield of these test leads

And can I use a series of attenuators to reduce the signal even more?  (I believe attenuation is simply additive). From the electronic knowledge I have gained over the years I believe this to be so, but for the safety of my receivers I just want to make sure.

Yes, that is correct. In fact, you generally get better attenuation when daisy chaining. You can also find switchable attenuators and attenuators with dials. Be careful with used switch attenuators that have slide switches unless you are prepared to do some maintenance. But watch the surplus market as there is often very nice selectable attenuators available.

Have fun with the projects!

- Glenn W9IQ
« Last Edit: April 04, 2022, 10:30:01 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WA4ZYN

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  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #6 on: April 04, 2022, 10:47:09 AM »

You would need to insure that common mode and radiated signal from the generator is less than the signal through the attenuators.
One idea may be to use a long coax cable to reduce the radiated signal pickup and common mode chokes on all the wires.

Zak W1VT

Thanks, Zak.  So, use a long coax to allow for extra distance in order to decrease the effect of spurious signals radiated from the signal generator itself? 

Thanks for your help!
73,
Francis WA4ZYN
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WB6BYU

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    • Practical Antennas
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #7 on: April 04, 2022, 10:48:50 AM »

That's often how signal generators are designed - a stable oscillator
at a fixed level, with a switchable attenuator on the output.  You
need very good shielding of the unit to get good results at the
sub-microvolt level.

I've used older HP 8656, HP 608, and Wavetek signal generators
for testing receive sensitivity, with good results, although I'm sure
there are better ones on the market now.  I've also used an
external switched attenuator, although, as others have noted,
coax shielding becomes important. 

Some ham-grade switched attenuators may have marginal
shielding.

You may find 75 ohm attenuators:  they will work, and you can
calculate the corrections for 50 ohm loads if it is critical.  But
make sure the attenuator is designed (and shielded) for RF:
I've come across some audio attenuators that are not suitable.

If you are going to build a switched attenuator, I'd suggest a
maximum step size of 20 dB, with internal shields both between
adjacent sections and across the middle of each.

WA4ZYN

  • Member
  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #8 on: April 04, 2022, 10:49:09 AM »

Yes,

Getting the needed shielding is difficult with a general purpose signal generator.  Better would be to use something that only generates a low signal from the start.  I have one similar to this and it is very handy:

https://www.multus-sdr.com/index.php?rt=product/product&path=72&product_id=129

73.



Thanks!  That looks like a great idea.  I haven't gotten to build a kit in a while, and that one looks challenging.  I may just go ahead and buy one.  Thanks so much!

73,
Francis WA4ZYN
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WA4ZYN

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  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #9 on: April 04, 2022, 10:50:37 AM »

A 40 dB fixed 1 watt attenuator should drop you to the -86 dBm range  for 1mV in. You would need another 40 dB attenuator AND a 30 dB attenuator to drop that 1 mV down to -156 dBm..... at this point the signal leakage from your generator, coax and so on are likely to be larger than your attenuated signal....

If you are only aligning and testing to repair equipment just the single 40 dB attenuator should be fine as it gives S7 + 4 dBm signal levels. Stacking a 20 dB and the 40 dB attenuators would get you down to -106 dBm... or S4 + 4 dBm....

All the best,

Grover

Thanks, Grover!
73,
Francis WA4ZYN
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WA4ZYN

  • Member
  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #10 on: April 04, 2022, 10:52:36 AM »

I know some receivers are rated for 1uV sensitivity, and some 0.5uV, or even 0.25uV.  If my signal generator is only capable of reducing its output to 1mV, can I use a 50-ohm signal attenuator to reduce the output sufficiently?

Francis,

Yes, you would do that with attenuators. To go from 1 mV to 0.25 μV, you would need about 75 dB of attenuation [20*log(V1/V2)]. Keep in mind that the attenuators are accurate only if they see the input and output impedance for which they are made which is most commonly 50 ohms.

Because of the very low signal levels, you should interconnect everything with double shielded coax (e.g. RG-142 or RG-400) to minimize leakage. It is also a good idea to put some clamp-on ferrite of the right mix and size to suppress common mode current off of the shield of these test leads

And can I use a series of attenuators to reduce the signal even more?  (I believe attenuation is simply additive). From the electronic knowledge I have gained over the years I believe this to be so, but for the safety of my receivers I just want to make sure.

Yes, that is correct. In fact, you generally get better attenuation when daisy chaining. You can also find switchable attenuators and attenuators with dials. Be careful with used switch attenuators that have slide switches unless you are prepared to do some maintenance. But watch the surplus market as there is often very nice selectable attenuators available.

Have fun with the projects!

- Glenn W9IQ

Thanks, Glenn!  You have been a wealth of information for me over the years.  I really appreciate all the advice and warnings!

73,
Francis WA4ZYN
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N4ATS

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Re: Questions about using Signal Generator and Signal Attenuators
« Reply #11 on: April 04, 2022, 12:32:06 PM »

I have several of these , cheap , accurate and well worth the money  https://www.ebay.com/itm/363760370277?hash=item54b1cec265:g:1mMAAOSwTPxiLALD  They are available all over the place and will do .1 uV calibrated
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N4ATS

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  • Posts: 1416
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #12 on: April 04, 2022, 12:40:57 PM »

I have several of these , cheap , accurate and well worth the money if you decide to make it simplistic...  https://www.ebay.com/itm/363760370277?hash=item54b1cec265:g:1mMAAOSwTPxiLALD  They are available all over the place and will do .1 uV calibrated
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HAMHOCK75

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  • Posts: 1297
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #13 on: April 05, 2022, 03:45:28 PM »

Would you be interested in an HP8640B?

https://www.hpmemoryproject.org/timeline/bob_devries/hp8640b_01.htm

It's free. You pay for shipping ( it's about 50-60 lbs boxed ). It is untested. I gave one to another ham who said it required no electrical repair but there were frozen switches, dirty switches, and switches that were forced and broken. It is capable of 0.1 uV output. It's precision attenuator alone cost more than most ham rigs of the day because it had to be accurate to 0.1 uV at 512 MHz.
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WA4ZYN

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  • Posts: 327
Re: Questions about using Signal Generator and Signal Attenuators
« Reply #14 on: May 24, 2022, 07:39:55 AM »

Hello.  I hope that you see this message.  I apologize:  I got busy here and haven’t been back to my own threads, so I am just now finding your offer.  What do you think it would cost to ship to Olive Branch, MS 38654?

Thank you for your very interesting offer.

73,
Francis WA4ZYN
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