Receiver testing with signal generator

General discussion and support for the Peaberry SDR V2.

Receiver testing with signal generator

Postby Jtrenalone » Fri Dec 05, 2014 7:52 pm

Greetings to the community.

I am reading through the build instructions for the V2 and have a (probably one of maNY to come) question.

Regarding the receiver section build and test. The test instructions ask to "If you have a signal generator then inject an RF signal between -40dBm and -100dBm on C27. This range is easily seen in HDSDR".

I have an oscilliscope, but do not have a signal generator. I have built many kits before, but I would like to learn more bulking this kit.

My question is can I just use a DDS from another radio to inject a signal for testing. I do not know the amplitude of the signal from the DDS (I could check in on the scope). Being a bit of a novice with test equipment, I don't quite understand what exaxtly a signal between -40dBm and -100dBm means. Also I do not know what the frequancy of the test signal should be.

I'm just trying to get by without buying a full blown signal generator at this time. But I would still like to do the test.

Thanks in advance for any help,
Joel
KD6AGW
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Re: Receiver testing with signal generator

Postby W4MMP » Sat Dec 06, 2014 4:42 pm

Hi,
I think I can help just a bit. This is from a Wikipedia page:
"IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms."

The frequency of the test signal is not important (other than it falls within the band of your choice).

Actually, the entire Wiki page is a good read: https://en.wikipedia.org/wiki/S_meter
73
Ron / W4MMP
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Re: Receiver testing with signal generator

Postby W4MMP » Mon Dec 08, 2014 4:38 am

Just one more note (FWIW): Apparently receive bandwidth has an impact on the S meter reading also. My understanding is that the receive bandwidth should be set to 500Hz when adjusting the S meter.
73
Ron / W4MMP
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Re: Receiver testing with signal generator

Postby AE9RB » Mon Dec 08, 2014 7:52 am

W4MMP wrote:receive bandwidth has an impact on the S meter reading

This should not be the case. Please report the steps to reproduce to the software authors so they can fix the problem.
73 David AE9RB
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Re: Receiver testing with signal generator

Postby Jtrenalone » Wed Dec 10, 2014 6:26 am

OK, thanks for the information.

So 50 microvolts is .05 milivolts. Pretty low output. I was thinking of just using my 5 watt 40 meter tranciever and attenuating the output (poor man's signal generator at 50 ohm impedance) but I would have to really attenuate the signal.

A stand alone DDS (Chinese ebay ones for $20) does not output at 50 ohms, so I supose that would not work.

I'm just trying to be inventive to avoid buying a regular signal generator for testing.

I'll have to think about this......
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Re: Receiver testing with signal generator

Postby kz1x » Wed Dec 10, 2014 12:18 pm

Elecraft sells small kit precision signal generators, models XG2 and XG3, designed for the purpose of setting up and diagnosing receivers.

http://www.elecraft.com/mini_module_kits/mini_modules.htm#xg2

Their earliest crystal controlled XG1 models sell used for not very much money. The newest XG3 model is synthesized, multi-band with multiple output levels.

Others also exist:

http://www.norcalqrp.org/s1s9generator.htm

One can get more granular in level control by buying a step attenuator, which Elecraft also sells in kit form, as well as Hendricks

http://www.qrpkits.com/attenuator.html

or they can be had at hamfests.

It is never cost effective to buy test gear to troubleshoot a kit. Having such devices is handy, of course, and there's a value to that. However if your intention is not not need them on a semi regular basis it is hard to justify ... buying test gear to diagnose a kit when the test gear costs more than the kit did. Instead, if you have no local elmer resource, there is a guy on this forum who will get your radio running for a reasonable fee, probably less than the cost of the test gear you'd need to do it yourself.
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Re: Receiver testing with signal generator

Postby Florida_weaksignal » Thu Dec 11, 2014 12:36 pm

Jtrenalone,
My kit is in the mail, but I have reviewed the schematics. The design uses an on-board 24 MHz ref. oscillator that should
be easy to receive as is. Some would call it a "birdie", but in this case it can be called a local signal generator. It's safe
in that no high levels are presented, or attenuators needed to generate a receive signal. This is theory only at the moment
since I don't have a unit in hand, or if it will tune to 24 MHz. Maybe help, maybe not, but requires no extra test equipment
and just a moment of your time to try.
George K4RSV
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Re: Receiver testing with signal generator

Postby Jtrenalone » Sun Dec 21, 2014 10:10 pm

Thank you for the information George. I am getting close to the installation of the SI chip, so I'm not very far along yet.

Please let me know when you get your kit and your progress, as I would be interested to follow your progress, and the possibility of using the local oscillator. I just would like it learn more about theory during this kit. And it is helpful to me to see thing on my scope to understand operation. But I don't have a full array of test equipment yet.

You can PM me at my email jtrenalone at yahoo dot com,
Thanks,
Joel
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