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setup for analysis of before/after measurements of v4 spectrum purity

Mark Hatch
 

Hi,

Have all the "recommended" parts (inductors + relays) that is supposed to make a "typical" v4 either FCC compliant or very close. But before I did the surgery, wanted to do a before/after measurement.

I am going to try to use the sdrplay sa software. I have seen where several people have used it to at least get a ranging shot on the spectrum. Will also send in a two tone signal thru microphone port using a windows soundboard app that was suggested on these forums.

So I do know I need to send the ubitx into a dummy load.... duh

And that I want to put lots of attenuation in front of the sdrplay (people seem to be typically using 40db, but I will put more than that to start until I figure it out.)

But nobody seems to mention how the sdrplay will pick up the signal? I am guessing that I don't use *any* antenna and just let the attenuators sit open.  Can someone validate this? Also, any experience in using the sdrplay SA software will be appreciated. Would prefer not to blow it up if possible. :-)

73
Mark
AJ6CU

Robert D. Bowers
 

I'd start with 60db at the least, and go up from there.  60db is 1000x voltage (loss or gain).  Those SDR dongles are quite sensitive (mine is down in the microvolts, maybe fractional microvolt range), and if the Bitx puts out 7 watts, that's roughly 53 v p-p in a 50 ohm setup.  

80 to 100 db would be better IMO.  That would help to not overload the dongle... and should get the signal level down to the point where you wouldn't be encountering any limiting in the signal.   Put a t fitting on your dummy load - one side goes to the radio, the other to your attenuator.

If you have access to a step attenuator, even better.  That's the way I'd do it (I've got one 120db to 0).  Start with 120 and reduce the attenuation until you have enough signal to measure - I think the specs call for better than 40 or 45db down for unwanted signals, so you'd want your carrier to be higher than that.

I've done the "coax into dummy load, and then used a dongle to measure near the coax" method.  That also worked well enough for what I was doing.  It's a quick way to do it... without all of the extra components.  You just start moving the dongle closer and closer until you get enough signal.

Bob

N4FBZ

On 5/17/19 10:15 AM, Mark Hatch wrote:
Hi,

Have all the "recommended" parts (inductors + relays) that is supposed to make a "typical" v4 either FCC compliant or very close. But before I did the surgery, wanted to do a before/after measurement.

I am going to try to use the sdrplay sa software. I have seen where several people have used it to at least get a ranging shot on the spectrum. Will also send in a two tone signal thru microphone port using a windows soundboard app that was suggested on these forums.

So I do know I need to send the ubitx into a dummy load.... duh

And that I want to put lots of attenuation in front of the sdrplay (people seem to be typically using 40db, but I will put more than that to start until I figure it out.)

But nobody seems to mention how the sdrplay will pick up the signal? I am guessing that I don't use *any* antenna and just let the attenuators sit open.  Can someone validate this? Also, any experience in using the sdrplay SA software will be appreciated. Would prefer not to blow it up if possible. :-)

73
Mark
AJ6CU

Evan Hand
 

I have used the SDRPlay as a SA, and had some challenges.

1 - First of all, the frequency span was not big enough to measure the harmonics on 40 meters in one test.  You need to change the center frequency to do the test in two separate measurements.  I was not sure that the gain stayed the same, so I my experience, the harmonics test was inconclusive.
2 - It was not clear how the gain controls operated in the SA software.  I still am not sure, so maybe someone can be specific on how to defeat the AGC and adjust the gain for a good capture.
3 - I built the following 50 ohm "tap":
http://www.ad5x.com/images/Articles/Tap50dbRevA.pdf
I still needed a step antenuator feeding the SDRPlay from the tap.

uBitx = TAP = Watt meter = Dummy Load
               |
               L= Step Antenuator = SDRPlay

It has been awhile, but I believe I had to adjust the step antenuator and gain to insure that when I added or removed 10 db, the SA software reported the same amount of change (this was how I overcame the AGC issues)

4 - The SDRPlay SA software will warn when you are getting close to overload.  Even if you stay below the warning message, there is signal compression even without the AGC engaged (reason for the set testing) 

My experience was it was good for close in purity testing, not so good for the harmonics

Above is my experience, things may have changed, or there may be better ways to accomplish the task.  Yours may vary.

73
Evan
AC9TU

Mark Hatch
 

Robert and Evan,  thank you for sharing! I have some heavy duty hp variable attenuators that I can front to the fixed ones when they arrive. But the 50db tap looks easy to make and probably can put it together before the fixed set arrives from China. 

Did either of you use the two tone window sound generator?  How did you set the input volume?

73
Mark
AJ6CU

Richard Spohn
 

Evan, thanks so much for the article from Phil Salas about the -50db
tap. Great advice -- and Mr. Salas is one of my gurus. 73 - Rich
WB2GXM

On 5/17/19, Evan Hand <elhandjr@...> wrote:
I have used the SDRPlay as a SA, and had some challenges.

1 - First of all, the frequency span was not big enough to measure the
harmonics on 40 meters in one test.  You need to change the center frequency
to do the test in two separate measurements.  I was not sure that the gain
stayed the same, so I my experience, the harmonics test was inconclusive.
2 - It was not clear how the gain controls operated in the SA software.  I
still am not sure, so maybe someone can be specific on how to defeat the AGC
and adjust the gain for a good capture.
3 - I built the following 50 ohm "tap":
http://www.ad5x.com/images/Articles/Tap50dbRevA.pdf
I still needed a step antenuator feeding the SDRPlay from the tap.

uBitx = TAP = Watt meter = Dummy Load
|
L= Step Antenuator = SDRPlay

It has been awhile, but I believe I had to adjust the step antenuator and
gain to insure that when I added or removed 10 db, the SA software reported
the same amount of change (this was how I overcame the AGC issues)

4 - The SDRPlay SA software will warn when you are getting close to
overload.  Even if you stay below the warning message, there is signal
compression even without the AGC engaged (reason for the set testing)

My experience was it was good for close in purity testing, not so good for
the harmonics

Above is my experience, things may have changed, or there may be better ways
to accomplish the task.  Yours may vary.

73
Evan
AC9TU