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Author Topic: transporting 50uv  (Read 2721 times)
W7EJT
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Posts: 132




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« on: December 20, 2013, 04:06:48 AM »

I have a good Wavetek RF generator and am trying to align my S-Meter. The manual says inject/place 50uv (that's microvolts) to the antenna, and adjust the S-Meter to S-9. OK, no problem.

Problem - I have a cheap charlie "probe" (just a wire) and I think perhaps the 50uv is not making it to the antenna. I don't have a scope or anything else to measure the generator output. The "wire probe" works fine at 50mv (the adjustment for 60 over S9).

What is an acceptable cable/probe for transporting 50uv?

thanks

Alan, W7EJT
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N4NYY
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Posts: 4742




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« Reply #1 on: December 20, 2013, 05:15:05 AM »

Unless you are using a 1:1 scope probe, you are attenuating the signal. I use a standard shielded coax BNC, on one end, clipped the BNC in favor of short alligator clips on the other.
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AD4U
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« Reply #2 on: December 20, 2013, 05:32:49 AM »

In this application an "acceptable" probe would be a short piece of good coax with a connector on one end that matches the Wavetek and a connector on the other end that matches the rig.

Dick  AD4U
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KA4POL
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Posts: 1901




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« Reply #3 on: December 20, 2013, 06:22:33 AM »

The output of the generator will be 50 with a coax connector. So the easiest way to get the signal to your antenna connector is 50 Ohm coax.
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WB6BYU
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« Reply #4 on: December 20, 2013, 08:28:27 AM »

Quote from: W7EJT

...The manual says inject/place 50uv ... to the antenna, and adjust the S-Meter to S-9.



Just to clarify, I would expect that to mean apply the 50uV to the antenna jack
rather than to the antenna itself.  A coax jumper between the signal generator and the
antenna connector on the receiver is the easiest way to do this.

Any common type of coax will be fine for short lengths at HF.
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KA4POL
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« Reply #5 on: December 20, 2013, 08:47:34 AM »

That is quite obvious  Grin
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AA4HA
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Posts: 1377




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« Reply #6 on: December 20, 2013, 09:30:47 AM »

Using shielded cable all the way between the signal generator and the radio will also provide shielding so you can accurately align the receiver. At very low levels you can encounter problems with other signal sources causing the radio to go into AGC action that lowers the gain of the RF amplifier stage.

We would typically do alignment with lower and lower generator signal levels.

50 uV is around -70 dBm, that is still pretty hot (strong). On some receivers we have aligned them down to around 225 nV (-120 dBm). That would be impossible to do if we did not use shielded connections and even in-line attenuators to lower the signal even more from the generator.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W7EJT
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Posts: 132




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« Reply #7 on: December 21, 2013, 06:32:22 AM »

OK, thanks to all for the great tips!! I'll be making a Coax cable - BNC to PL259 should do it..

73 (and Happy Holidays to all)

Alan, W7EJT
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WB4SPT
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« Reply #8 on: December 21, 2013, 06:43:07 PM »

Alan;  You may already know this, but a RF sig gen would generate a signal 6dB higher (two times voltage) than the indicated output.  This is because the output level is calibrated with only a 50 ohm load.   So, the actual voltage supply in the generator is 100uV, fed thru a 50 ohm internal resistor, and then to the sig gen output connector.  If you were to measure with an accurate hi-Z meter, you would read 100uV.  If you attach something like a Boonton 92 or a 50 ohm spectrum analyzer you should measure 50uV. 
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G3RZP
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« Reply #9 on: December 22, 2013, 01:52:22 AM »

Just to complicate matters, some generators are calibrated in terms of EMF, and not the PD in 50 ohms! So if they say 100 microvolts, they mean 100 microvolts EMF....
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KH2G
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« Reply #10 on: December 22, 2013, 12:57:31 PM »

You should also measure the voltage at the end of the coax your going to plug into the radio. There will always be some cable loss.
Regards, Dick KH2G
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W7EJT
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Posts: 132




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« Reply #11 on: December 23, 2013, 08:47:04 AM »

OK, didn't know this. I will try it at 100uv and see what happens.

I just now procured a 2 foot long RG58 (50 ohm) coax with direct connection from the RF Gen to the antenna jack.

Also, another Gentleman asked about the antenna jack. I failed to mention, yes I am transporting the 50uv directly to the antenna jack... (Not to the Antenna)

thanks

Alan, W7EJT


Alan;  You may already know this, but a RF sig gen would generate a signal 6dB higher (two times voltage) than the indicated output.  This is because the output level is calibrated with only a 50 ohm load.   So, the actual voltage supply in the generator is 100uV, fed thru a 50 ohm internal resistor, and then to the sig gen output connector.  If you were to measure with an accurate hi-Z meter, you would read 100uV.  If you attach something like a Boonton 92 or a 50 ohm spectrum analyzer you should measure 50uV. 

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