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A few newbie questions on connecting an RF Generator

Cody

Dec 31, 2016
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Hi,

I know a decent amount about designing electronic circuits, mostly digital, some analog, but not much with RF-related designs.

I recently purchased a used HP 8656B Signal Generator because it was at a good price.

I also bought a Heathkit GC-1000 "most accurate clock" that receives the WWV atomic time signals for accuracy.

I want to "tune" the 5/10/15Mhz AM receivers in the GC-1000, and I had a few basic questions about how to connect the RF Generator to it - I just don't want to assume anything and blow something up:

1) Do I need a dummy load or an Attenuator on the output of the RF generator or can I simply directly wire the output of the generator to the BNC antenna input of the Clock, but just make sure the generator output's level is set low to simulate the signal level of an over-the-air antenna input signal?

2) I'm also confused about the use of an "Attenuator". The 8557B version of my generator has a built-in "Electronic Attenuator" (but my model does not have it). So, if the 8557B and my generator are already able to adjust the output level/voltage of the RF Generator, what is the purpose/need/usefulness of also having an "Electronic Attenuator" anyway?

I told you these were newbie questions... any help would be greatly appreciated :)
 
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davenn

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Sep 5, 2009
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1) Do I need a dummy load or an Attenuator on the output of the RF generator or can I simply directly wire the output of the generator to the BNC antenna input of the Clock, but just make sure the generator output's level is set low to simulate the signal level of an over-the-air antenna input signal?

No

Yes


2) I'm also confused about the use of an "Attenuator". The 8557B version of my generator has a built-in "Electronic Attenuator" (but my model does not have it). So, if the 8557B and my generator are already able to adjust the output level/voltage of the RF Generator, what is the purpose/need/usefulness of also having an "Electronic Attenuator" anyway?

not sure
 

shrtrnd

Jan 15, 2010
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I typically use a 50 Ohm 2W feed thru resistor on the output of my signal generator. I assume that's what they're
talking about. Some devices have an internal one, some have an internal one you can switch into the output circuit or switch out of it, and some need an external 50 Ohm feed-thru resistor for some applications.
I'd Google it for more information about the uses.
 

Cody

Dec 31, 2016
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Thanks for the info.

I bought the "Technical Manual" for the GC-1000 and it looks like Heathkit is suggesting to make up a PL256 coax cable and add an alligator clip to the center conductor at the other end. Then they want me to set the RF generator to 2200uv at 456khz and then physically attach this alligator clip to the outside of a cap (C336) - no electrical connection, just a physical connection. I think they want the signal to be "transmitted" through the air (but over a very short distance) instead of a direct electrical connection.

Lets see how well the setup works to tune everything.
 

Cody

Dec 31, 2016
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Well, its true that I don't "need" the 50 ohm dummy load for the generator to work. But, what I did find out is that in order to be able to accurately set the output level (ie "20mv"), you need to have the dummy load connected or else the output voltage will be much greater then what you set it to. The Generator doesn't actually monitor the output voltage level to make sure it is what I set it to. The generator's output was simply calibrated with a 50 ohm load present, so it will only be accurate when there is a 50 ohm load.
 

davenn

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But, what I did find out is that in order to be able to accurately set the output level (ie "20mv"), you need to have the dummy load connected or else the output voltage will be much greater then what you set it to. The Generator doesn't actually monitor the output voltage level to make sure it is what I set it to. The generator's output was simply calibrated with a 50 ohm load present, so it will only be accurate when there is a 50 ohm load.

correct ... the stated output on the dial will be what it is when terminated in 50 Ohms
 

ERNEST ERICKSON

Apr 4, 2017
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Here's a handy dBm to Microvolt scale you can use to adjust your receiver.
Depending on manufacturer, and the scale they use, yu can determine sensitivity(to some degree) but the chart is a handy reference when reading the specs. included with the factory manual.
An 'attenuator' is term given to a non-inductive, 50 Ohm load, terminated with some type of RF connection on bth ends, such as BNC, or tye 'N', SMA, Etc. The attenuation is noted in Db, with 3 dB being 1/2 of the signal, such as a 5 Watt signal now becomes a 2.5 Watt signal.
Most attenuators are not designed for RF power past 2 Watts, then you move into the power attenuator models.
You can either directly couple the signal generator to your receiver's antenna port, or simply generate a signal using an antenna connected to the output of the generator, and increase the output of the generator until the receiver begins to respond.
The attenuator as used in the output of the generator helps in three ways...
1. Limits RF power transferred TO the unit under test
2. Limits RF signal levels FROM the generator to the unit under test
3. Maintains a normalized 50 Ohm characteristic impedance of the signal at the designed level of attenuation.

I have always been a proponent of real world level testing...off-air,, not directly connected to the unit under test.
This helps me create a baseline for sensitivity levels you can expect in most instances.
Direct connections do not provide this, as the signal being fed to the unit, has no outside interference to deal with, no IMD products, no distortion, and no good means to test dynamic range of the receiver.
This is only my opinion, ymmv.
 

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