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Has anyone built Ashhar's Specan?

Craig Wadsworth <cwadsworth@...>
 

I'm preparing to build the spectrum analyzer board designed by Ashhar Farhan.  See this link if you are not familiar with the Specan:
If I understood Ashhar correctly, a spectrum analyzer will be needed to balance the uBitX's output amplifiers. Building a working, legal, uBitX is my primary goal.  Building a working Specan would be part of the journey to a uBitX.  What has been your experience?  Comments? Advice? (Constructive) criticism?
Thanks and regards,
Craig
w9ctw

Jerry Gaffke
 

The uBitx as currently drawn gets by with just two output filters.  For example, a 40m low pass filter also gets used when transmitting on 80m.  For that to be legal, the second harmonic of your 80m signal had better be awfully low coming out of the IRF510's, since the 40m low pass filter isn't going to do anything about it.

That's possible to do with the push-pull IRF510 arrangement of the uBitx.  But requires a spectrum analyzer, and you would have to keep PA voltage and modulation levels in check, and consider effects of aging and temperature around those IRF510's.  Farhan has backed away from that strategy, the final design will likely have more output filters, enough to address the second harmonic at each freq of interest.  Like every other rig built since about 1925 that had any sense of responsibility.

I applaud Farhan for trying, don't get me wrong.  These rigs have more functionality per part than anything that has come before, and it is by trying stuff like this that you move in that direction.  Unfortunately, you simply need more than the two output low pass filters shown to get good repeatable results across all those bands.  If I scratch build a uBitx, the low pass filter will probably be a lump in the coax out to the antenna.

Jerry, KE7ER


On Tue, Jul 25, 2017 at 07:33 am, Craig Wadsworth wrote:

If I understood Ashhar correctly, a spectrum analyzer will be needed to balance the uBitX's output amplifiers.

Jerry Gaffke
 

Not something I would do if shipping thousands of units.  Too many people would cheat, not using the appropriate filter.  And the rig would get a bad reputation.


On Tue, Jul 25, 2017 at 08:19 am, Jerry Gaffke wrote:
 If I scratch build a uBitx, the low pass filter will probably be a lump in the coax out to the antenna.

Ashhar Farhan
 

There has been a problem, the transmit out (let's say on 14 MHz) would find it's way into the seccond mixer (the one that mixes 12 MHz up to 45 MHz). As second local oscillator was t 57 MHz. While the 12 MHz mixed to 45 MHz, the 14 MHz would mix to 57-14 = 43 MHz. 
The LC filter would pass the 43 Mhz and the 45 MHz together gladly, producing spurs at 2 MHz away (at 16 MHz and 12 MHz). The cure was to use 45 MHz crystal filters. Thankfully, they are available cheaply and easily through mouser, digikey, etc.
The second issue was that as there was no balance control between the push and the pull legs of the power chain, the even harmonics were not cancelling out. hence, i had to put additional low pass filters as well. i will update the circuit with these changes after testing it out on air for a day or two.

- f

On Tue, Jul 25, 2017 at 9:00 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
Not something I would do if shipping thousands of units.  Too many people would cheat, not using the appropriate filter.  And the rig would get a bad reputation.

On Tue, Jul 25, 2017 at 08:19 am, Jerry Gaffke wrote:
 If I scratch build a uBitx, the low pass filter will probably be a lump in the coax out to the antenna.


ohwenzelph
 

My first concern: Getting some of the good authentic and correct parts at a reasonable price or suitable substitute is my initial concern. In particular the MAV11, ADE-1 and the correct version of the Si570, also suitable crystals. 
Have you built a Sweeperino?
jerry aa1of

Jerry Gaffke
 

If I were to create a pcb for the uBitx, it would have pads for an optional ad8307 (or ad8310) after the crystal filter, just before the demodulator.  Drives an ADC pin on whatever processor you happen to be using, 10 bits of ADC is plenty.  So in addition to having an all band HF xcvr, would have a low grade Spectrum Analyzer.  That ad8307 could also be used as an S meter.  Removing a jumper at the input to the ad8307 and plugging in something like a scope probe it could be used as an extremely wide dynamic range RF probe for trouble shooting.  

Or maybe instead sample the output of the demodulator using a 24 bit ADC, use DSP techniques on one of the cheap ArmCortex Arduino compatible processors to do an FFT for fine grained spectrum analysis.  This would be more difficult to do right.

Of course, you would have to build two if you want to check emissions on your new uBitx.

Jerry, KE7ER

Ashhar Farhan
 

you could just do rms of the audio. the main challenge would be that given 3 khz bandwidth, the 30 mhz scan would need a reading every 1.5 khz. that means, 20,000 readings for the full scan. it will take a long time. this ia why specans have variable bandwidths.
- f

On 26 Jul 2017 10:35 p.m., "Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:
If I were to create a pcb for the uBitx, it would have pads for an optional ad8307 (or ad8310) after the crystal filter, just before the demodulator.  Drives an ADC pin on whatever processor you happen to be using, 10 bits of ADC is plenty.  So in addition to having an all band HF xcvr, would have a low grade Spectrum Analyzer.  That ad8307 could also be used as an S meter.  Removing a jumper at the input to the ad8307 and plugging in something like a scope probe it could be used as an extremely wide dynamic range RF probe for trouble shooting.  

Or maybe instead sample the output of the demodulator using a 24 bit ADC, use DSP techniques on one of the cheap ArmCortex Arduino compatible processors to do an FFT for fine grained spectrum analysis.  This would be more difficult to do right.

Of course, you would have to build two if you want to check emissions on your new uBitx.

Jerry, KE7ER

Jerry Gaffke
 

Maybe switch between the 45khz crystal filter, the 12mhz crystal filter, and analyzing the audio with an ADC.
Jerry, KE7ER


On Wed, Jul 26, 2017 at 10:17 am, Ashhar Farhan wrote:
you could just do rms of the audio. the main challenge would be that given 3 khz bandwidth, the 30 mhz scan would need a reading every 1.5 khz. that means, 20,000 readings for the full scan. it will take a long time. this ia why specans have variable bandwidths.

Jerry Gaffke
 

An ADC on the audio with a simple program to do peak detection might do about as well as the ad8307 on the 12mhz IF.
But the ad8307 will give better dynamic range (unless you have an awfully nice ADC),
and the ADC would give the possibility of doing an FFT in software for very fine grained resolution.
I think.


On Wed, Jul 26, 2017 at 10:36 am, Jerry Gaffke wrote:
Maybe switch between the 45khz crystal filter, the 12mhz crystal filter, and analyzing the audio with an ADC.

Jerry Gaffke
 

The 45mhz crystal filter is likely rather narrow as well.
Perhaps have an option to replace the 45mhz crystal filter with something like this:  http://www.mouser.com/ProductDetail/Murata-Electronics/PX1002/?qs=Xb8IjHhkxj400FumWvF4dQ%3D%3D
Mouser summary says it's 25khz wide, but datasheet says the 3db passband is typically +/- 25khz from center, so 50khz wide.

Don't have to stuff this sort of thing on all uBitx boards, not everybody will want a spectrum analyzer.
But would be nice if it was easy to hack in, with pads on the board for the PX1002 and AD8310.

Jerry


On Wed, Jul 26, 2017 at 10:36 am, Jerry Gaffke wrote:
Maybe switch between the 45khz crystal filter, the 12mhz crystal filter, and analyzing the audio with an ADC.
Jerry, KE7ER
. . .

ashok.das81@...
 

Sorry for bumping up an old thread, but this was my last resort for this Specan project. I am planning to built one and I am requesting all to respond on this topic as W9CTW originally asked. Craig have you done some progress on this project so far. What is your outcome. Please let us know.

Thanks & Regards
Ashok