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Antuino as a Spectrum Analyzer: Lots of birdies?

Jerry Gaffke
 

Looking at the latest schematic up on github:
    https://github.com/afarhan/antuino
it appears that Antuino does not have any filtering up front.

That would work fine as an antenna analyzer or scaler network analyzer
or to check a transmitter for harmonics and spurs.  In those cases the
incoming RF will be just a few signals, perhaps the main signal of interest
and a few harmonics.  The crystal filter will almost always knock out the false signals.

As a general purpose spectrum analyzer looking at noisy wideband incoming RF,
I'd expect a lot of false signals due to images and local oscillator harmonics.
In the discussion from Dec 28 of 2018 included below, Farhan suggests
that images could be dealt with in software.

Anybody have any comments on how well the Antuino works as a 
spectrum analyzer?

Jerry, KE7ER


On Fri, Dec 28, 2018 at 08:05 PM, Ashhar Farhan wrote:
the challenge would be to eliminate the if image. this could be done when you sweep to both sides and eliminate the image reading in software. here is an example :
 
1. lets imagine that there is a signal at 10 mhz. for this, we set the local oscillator at 35 mhz (as the IF is at 25 mhz). how can we be certain that the signal we are resolving is not at 60mhz (60mhz - 35 mhz = 25 mhz)?
2. we can take a second reading by setting the local oscillator at 85 mhz. in this csse, thr 60 mhz response (if any) should show up. else we can confirm that the signal we resolved in step 1 is indeed a 10 mhz signal.
 
one has to be aware that such a system will be slow. lets imagine that you have to sweep 30 mhz. the crystal filter is 10 khz wide. hence, we will need 30,000/5 = 6000 readings. if we can get, 100 readings a second, this will take a minute to complete the scan. an alternative would be to have two filters, a crystal filter and a 4 or even 5 section LC filter at around 12 mhz. we could switch to the lc filter for large sweeps. my specan circuit has a suitable design.
 
you are aking what happens if there was signal at 110 mhz in step two? aaaah, you tell me!
 
- f

Jerry Gaffke
 

In those cases where the birdies are a problem,
a handful of external bandpass filters would cure it.


On Thu, Aug 1, 2019 at 04:41 PM, Jerry Gaffke wrote:
As a general purpose spectrum analyzer looking at noisy wideband incoming RF,
I'd expect a lot of false signals due to images and local oscillator harmonics.
In the discussion from Dec 28 of 2018 included below, Farhan suggests
that images could be dealt with in software.

Ashhar Farhan
 

Everything instrument has limits on its accuracy. While making the Antuino, I was well aware of its deficiencies. 
I made a decision to keep it simple to a point where a radio ham could throw this thing together in an evening or two.
In order to overcome the limitations inherent in the Antuino design, the complexity could have been prohibitively complicated and expensive.
On the other hand, it is an extremely useful instrument that grows on you. I no longer use a frequency counter or the specan. I rarely use the oscilloscope. Antuino does most of my measurements.

There are two very important things you must be aware of while using the Antuino :
1. In the power measurement mode (the problem is non existent on swr or sweep mode), any reading above 25 mhz could be an image. Thus, if you see something at 35 mhz, you will have to do some mental math to figure out if it is not an image. An easy way to know is to add an external low pass filter with 25 mhz cutoff.
You have to use it like a radio in this mode. If you want to measure the harmonics from your pixie radio with 7040 crystal, tune to 14080, 21120, 28160 and measure. It is as accurate as any spectrum analyzer with more than 80 db of usable bandwidth. Raj and I struggled to get this for a month.
2. Unlike a full fledged spectrum analyzer, Antuino has just one bandwidth of about 7 KHz. This is enough to made IMD measurements at 20 KHz tone separation. The sweep plot does only 128 readings. Thus, if  you sweep a low pass filter from 0 to 20 mhz, it will measure the filter response every 120 khz. If there is something lurking between the steps, it will miss it. This is a common challenge with spectrum analyzers. So, a crystal filter should be swept at less than 100 khz. 
There is a software hack to mitigate this. First : introduce another control for step size. This can slow down the plot. A 30 Mhz sweep at 5 khz steps will involve 30 x 200 =60000 readings.It could take minutes. The Second : write a more optimal Si5351 routine that changes frequencies faster. I know that smaller jumps can be instantaneous on Si5351. I don't know that hack. If someone wants to take a stab at it, I am willing to work with them.

In a nutshell, Antuino is a very useful instrument. You don't have to buy it. You can build it. It is just as challenging as a direct conversion receiver. It does a fabulous job though. It can measure oscillator frequencies, it can measure amplifier gain, distortion, frequency response, it can measure filter response, it can show mixer behaviour, it can tune your antenna, it can measure power from a few uV to 100 mv and more with attenuators. It does all this slowly but surely. Like any precision tool, you must know its limitations and use them as an aware user. I would wager that if you have to choose just instrument for your lab, it would be this; Apart from a DVM.
And (I repeat) don't buy it, build it (grin)

- f

On Fri 2 Aug, 2019, 5:29 AM Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
In those cases where the birdies are a problem,
a handful of external bandpass filters would cure it.

On Thu, Aug 1, 2019 at 04:41 PM, Jerry Gaffke wrote:
As a general purpose spectrum analyzer looking at noisy wideband incoming RF,
I'd expect a lot of false signals due to images and local oscillator harmonics.
In the discussion from Dec 28 of 2018 included below, Farhan suggests
that images could be dealt with in software.

Jerry Gaffke
 

The Antuino does look very useful for poking at radio gear.
Those wishing to build one could borrow the Raduino from a *Bitx* rig
and not bother building the Nano/Si531/Display part of the Antuino.
     https://github.com/afarhan/antuino/blob/master/antuino_schematic.pdf
Perhaps have the top line of the LCD show the current frequency, the bottom show the reading in dBm.
And ship all data off to the Arduino IDE via the USB cable for plotting, using Serial.print() calls:
     http://www.instructables.com/id/Ultimate-Guide-to-Adruino-Serial-Plotter/
Though the Antuino's 128x64 graphic display would be very handy,
making it a nice small standalone instrument.

A $20 JYE scope could still be quite useful.
For example, watching I2C and UART traffic on the Nano pins,
or watching a rotary encoder work.  Can't do that with an Antuino.
If the signals are too fast for your scope,
you can often modify the program to slow things down.

Jerry, KE7ER



On Thu, Aug 1, 2019 at 07:19 PM, Ashhar Farhan wrote:
Everything instrument has limits on its accuracy. While making the Antuino, I was well aware of its deficiencies. 
I made a decision to keep it simple to a point where a radio ham could throw this thing together in an evening or two.
In order to overcome the limitations inherent in the Antuino design, the complexity could have been prohibitively complicated and expensive.
On the other hand, it is an extremely useful instrument that grows on you. I no longer use a frequency counter or the specan. I rarely use the oscilloscope. Antuino does most of my measurements.

Jerry Gaffke
 

If you are going to use a Raduino with its 25mhz reference oscillator,
you probably want to move the crystal filter to something other than 25mhz.

Jerry



On Thu, Aug 1, 2019 at 09:02 PM, Jerry Gaffke wrote:
Those wishing to build one could borrow the Raduino from a *Bitx* rig
and not bother building the Nano/Si531/Display part of the Antuino.

Ashhar Farhan
 

24 mhz would be a good idea. If you have borrowed the raduino. Alternatively, as I did, you can order a qrp labs Si5351 board. It is nifty, you should have a couple in your drawer.


On Fri 2 Aug, 2019, 9:50 AM Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
If you are going to use a Raduino with its 25mhz reference oscillator,
you probably want to move the crystal filter to something other than 25mhz.

Jerry



On Thu, Aug 1, 2019 at 09:02 PM, Jerry Gaffke wrote:
Those wishing to build one could borrow the Raduino from a *Bitx* rig
and not bother building the Nano/Si531/Display part of the Antuino.

Tom, wb6b
 

On Thu, Aug 1, 2019 at 09:02 PM, Jerry Gaffke wrote:
For example, watching I2C and UART traffic on the Nano pins,
I've found this little $10 logic analyzer to be very handy for looking an I2C, encoders and other digital things.
https://www.amazon.com/gp/product/B07F8C1PMQ/

I use it with this software. The software will capture fairly high speed data for seconds on end and even display many data protocols in their decoded form.
https://sigrok.org

Tom, wb6b

Jerry Gaffke
 

24mhz might be problematic for the Antuino filter.
Perhaps 27mhz would be a better choice?
Or most any oddball frequency in that ballpark?

A few users were having trouble with the uBitx back when its filter
was at 12mhz, due to interference with local oscillators on the Nano.
Though typically not, since the ceramic resonators on the Nano
are not very accurate.  The second harmonic of 12mhz is 24mhz.

Jerry, KE7ER


On Thu, Aug 1, 2019 at 10:23 PM, Ashhar Farhan wrote:
24 mhz would be a good idea. If you have borrowed the raduino. Alternatively, as I did, you can order a qrp labs Si5351 board. It is nifty, you should have a couple in your drawer.
Hide quoted text

 


On Fri 2 Aug, 2019, 9:50 AM Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
If you are going to use a Raduino with its 25mhz reference oscillator,
you probably want to move the crystal filter to something other than 25mhz.

Jerry



On Thu, Aug 1, 2019 at 09:02 PM, Jerry Gaffke wrote:
Those wishing to build one could borrow the Raduino from a *Bitx* rig
and not bother building the Nano/Si531/Display part of the Antuino.

Ashhar Farhan
 

27 mhz is tricky because a number of these crystals turn out to be 9 mhz crystals sold for their 3rd overtone.
The 27 mhz filter was also horribly lossy. More than 13 db, if i remember correctly.

On Fri 2 Aug, 2019, 8:11 PM Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
24mhz might be problematic for the Antuino filter.
Perhaps 27mhz would be a better choice?
Or most any oddball frequency in that ballpark?

A few users were having trouble with the uBitx back when its filter
was at 12mhz, due to interference with local oscillators on the Nano.
Though typically not, since the ceramic resonators on the Nano
are not very accurate.  The second harmonic of 12mhz is 24mhz.

Jerry, KE7ER


On Thu, Aug 1, 2019 at 10:23 PM, Ashhar Farhan wrote:
24 mhz would be a good idea. If you have borrowed the raduino. Alternatively, as I did, you can order a qrp labs Si5351 board. It is nifty, you should have a couple in your drawer.
Hide quoted text

 


On Fri 2 Aug, 2019, 9:50 AM Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
If you are going to use a Raduino with its 25mhz reference oscillator,
you probably want to move the crystal filter to something other than 25mhz.

Jerry



On Thu, Aug 1, 2019 at 09:02 PM, Jerry Gaffke wrote:
Those wishing to build one could borrow the Raduino from a *Bitx* rig
and not bother building the Nano/Si531/Display part of the Antuino.

Jerry Gaffke
 

Hmm, if 27mhz crystals are often 3'rd overtone, I wonder if some of the 25mhz crystals are too.
Might want to be careful about exactly what brand of crystal is being used.

Arv had an interesting notion for a hobbyist's spectrum analyzer,
even simpler than the Antuino:  https://groups.io/g/BITX20/message/55902
The conversation there brings to mind a possible hack for the Antuino:

Add a second mixer, using the third output of the si5351, to convert the 25mhz IF to audio.
Have the si5351 a bit to one side of the 25mhz IF so we get single signal reception.
Monitor that audio using a soundcard audio spectrum analyzer.

This would allow us to easily differentiate by ear between first mixer images and primary signals,
the images move in frequency the opposite direction when the thing is tuned.
Also harmonics of the VFO, they would cause an audio tone that moves about 
faster than the VFO fundamental..
And the audio spectrum analyzer makes this a very fine grained tool, able
to distinguish between RF signals that are jsut a few hz apart.
Just listening to the resultant audio would be instructive.

Jerry, KE7ER


On Fri, Aug 2, 2019 at 08:12 AM, Ashhar Farhan wrote:
27 mhz is tricky because a number of these crystals turn out to be 9 mhz crystals sold for their 3rd overtone.
The 27 mhz filter was also horribly lossy. More than 13 db, if i remember correctly.