Thursday, March 9, 2017

A Band Scope for the BITX




The use of an RTL-SDR dongle with a discarded Android phone as the display seems to me to be a very BITX-appropriate use of road-kill technology.  Bill N2CQR

Thoughts and experiments on BITX Band Scope
by Ken Marshall G4IIB

I have explored several very simple ways of implementing a band scope on my BITX. To test the concept I am using HDSDR on a PC and an HF converted RTL SDR dongle (a la Sprat 162). We can turn the sound off on our SDR software as we are only interested in the display. I have also tested it in demo mode on an android phone using SDRTouch as seen above.

Method 1 implementing a band scope on the first IF stage (output of Q2). This gives a classic tunable Pan-adapter. You tune your SDR software (HDSDR or SDRTouch) to the IF frequency 12Mzh and you will see a band scope and waterfall display.  You tune stations in the normal way on the BITX and you will see that once you resole the LSB signal on the BITX it resides within the 12Mhz pass band near 12Mhz on the waterfall.  See below you will also see that the lower portion of the 40M band is to the right note the CW section 1260 – 1280 and the high end of 40M is to the left  this is inversion due to my use of high side vfo. This will not be he case if you are using the conventional low side VFO ( the waterfall will change if you use high side VFO to change to USB as implemented on the Raduino using the fixed BFO crystal, if that’s what you use). What you see on the scope is not what you get in that every thing is referenced to 12Mhz you have to do a conversion in your head to know where you are within the 40M band, but it does show you the activity on the band which you can tune to via the BITX.

The next image shows the mess the waterfall gets in to when you tune the band. The trick when tuning is to ignore the waterfall and concentrate on the peak in the band scope of the signal you want to listen to as it moves into the 12Mhz pass band when tuning the BITX. 




As previously indicated the hardware mod itself is very simple, tack a 10 or 20pF cap (whatever low value you have) onto the crystal side of C23 and connect this to your dongles antenna connection via screened lead.

Method 2  implementing a band scope on the RF amp stage (output of Q1). This gives an untunable Pan-adapter/band scope. You tune your SDR software (HDSDR or SDRTouch) to 7.1Mzh and you will see a band scope and waterfall display of the whole of 40M.  You tune stations in the normal way on the BITX the band scope does not move as you tune all of the signals you see are in the correct place on the band. If you see a signal on the band scope at say 7140 tune the BITX to 7140 to listen to it, the band scope does not change. As the main purpose of the band scope is just to give a visual indication of what activity is on the band you may find this approach simpler to use.

Again the hardware mod itself is very simple, this time  tack a higher value cap 10nF onto C13 and connect this to your dongles antenna connection via screened lead.

The main downside to methods 1 & 2 are the amount of spurs that can be seen in either of the displays as above. This is a known issue with these cheap RTL SDR dongles and not easy to overcome. However, most of the spurs seem to be generated within the BITX. This is hardly surprising given that I'm running an arduino, which has its own internal clock, which is driving an SI5351 producing VFO and BFO frequencies, harmonics from these and those produced by the RTL devise itself will be generating and mixing all sorts of other harmonics. Contrast the above with the SDR on its own antenna not connected to the BITX. Hardly any spurs see below. This would be another option (method 3) to use the dongle with either a PC or android device as a band scope using a separate RX antenna.

 I have loaded SDRTouch on my android phone and it works but only for 60 seconds in demo mode. Not really long enough to test and fully evaluate the functionality. I have to pay£7 for a key to use the full spectrum functionality. I'm not sure at this stage which android devise I want to run it on or the use I'll make of it. But I may consider it in the future. This is why most of my testing has been carried out via HDSDR on a PC. I have tried other android SDR apps but SDRTouch seems to be the only one to offer I or Q channel inputs necessary to operate HF modded dongles.

Whilst methods 1 & 2 work they would benefit from further experimentation. Reducing or eliminating the spurs would be nice but difficult. I have since found that screening the Arduino and using coax instead of just screened lead to connect to the RTL helps. As it stands even with the spurs it still gives useful band information. A band scope found on $2000 plus rigs it is not! But for the outlay of $13 for an SDR dongle and an android device it could be a useful addition to the BITX.

In summary, both methods are very easy to implement but I feel that method 2, although not a classic tunable band scope, is probable the easiest option from a user perspective. Try it on your PC or android and see what you think.

Post Script check out these V3 dongles from the RTL-SDR.com people. They have improved screening, spur suppression, lower noise floor and are already modded for HF. It's only 19 of your American bucks.
http://www.rtl-sdr.com/new-rtl-sdr-blog-units-now-available-in-store-hf-via-direct-sampling-software-switchable-bias-tee-less-noisespurs/

Saturday, February 25, 2017

Putting the BITX Raduino on CW


Clean CW on the Raduino
 by Don Cantrell, ND6T

Although the original Raduino v 1.01 sketch included a CW mode I wasn't pleased with the signal produced from the side tone insertion into audio. Even when filtered to where it sounded acceptable there were multiple artifacts accompanying the tone. Since I like a 700 Hz tone there were three additional harmonics spread across the spectrum within the 2.8 KHz spectrum. These are attenuated by the degree of filtering but unless I used active filtering they were quite evident. Additionally, there was the suppressed carrier and the opposite side band both showing at several milliwatts. All this would not be detectable if my signal was quite weak, but there are many times when nearby stations will be annoyed by these “companions”.


I decided to use the CLK1 output of the Raduino module (P3 pin 11) inserted, at operating frequency, into the RF Power Amplifier predriver (Q13). I used a 470 ohm resistor soldered to C132 (either side, CLK1 has a blocking capacitor) fed with a shielded RG-174/u cable. I grounded the shield at the synthesizer at P3 pin 12 but left the other end open, dressed back ½” , and covered with tubing. The 470 ohm resistor throttled the CW output to 5 watts. If you want to chance it, a 220 ohm resistor at that point will output 10 watts. I prefer 5.

 The T/R relays are keyed by placing a 2N7000 NMOS transistor from ground to the PTT pin on the microphone jack. Source to ground, Drain to PTT, and Gate to P3 pin 1 of the Raduino.

Another 2N7000 is placed across R44, the base of Q4 to ground. Source to ground, Drain to the base side of R44, and the Gate tied directly to the gate on the 2N7000 T/R driver that was just installed. This transistor disables the post-mixer transmit buffer in order to eliminate the remaining carrier and anything coming in from the microphone.

The result is a clean and clear CW tone.

In my software I use a side tone that matches my transmit offset. A 700 Hz offset will produce a 700 Hz side tone, making it easy to spot my transmit signal in the receiver. This side tone appears on P3 pin 2 on my build and is injected into the speaker jack through a 1 uF capacitor. Those users who have chosen to remove C113 from the audio output amplifier can use that capacitor: Just lay it on top of the jack and solder one end to the “tip” connector pin (the brown wire). Want a different level? Then either change the capacitor to a different value or put a resistor in series with the 1 uF. Another variable resistor on the front panel, perhaps?

I wrote a primitive little iambic keyer routine and placed it in a “while” loop. The structure is timed by a simple iterative count and jumps back out to normal operation after about one and a half seconds of inactivity. That is the “semi-break in” time out and is changeable in software to meet your needs. The key inputs are fed into P3; pin 3 for the dits, pin 4 for the dahs.

Two transistors, one resistor, and a capacitor. Less than $1 USD in parts!

The CW routine is called by holding the paddle to either side. The relays are operated and the fun begins. Speed is set in the start of the program as a variable, “wpm”, and is fixed on my simple sketch. You can set up a simple pot (just like the tuning control) to generate a number to be directly scaled to the word-per-minute rate and place that control right on the front panel for speed control. Place the WPM rate on the second line of your display. Class!

You could do the same thing with the offset/side tone frequency. Go crazy and amaze your friends!

I can provide a current copy of the 60m sketch that includes the CW program. It is written in simple Arduino, no fancy C++ calls. I like simple. Also included is the added tuning of 5 to 6 MHz. Nice to calibrate with WWV, listen to trans-oceanic aircraft and short-wave broadcasts. Write me at ND6T@ARRL.net.  An adaptation for 40 meters will also be available when I get to it.

It certainly serves as a successful proof of concept. It is not, however, a great CW transceiver. Yet.

*       Activation of the CW function is awkward and slow. It is delayed by the iteration of  the main loop, including the display. Solvable by an interrupt routine, perhaps?

*          It needs a straight key mode.

*          Keying waveform is too abrupt. Perhaps an keyed integrator in the PA power plug circuit?

*          Receive bandpass it pretty wide for CW. Audio filter?

Plenty of room for improvement and more modifications. Isn't that the whole point of the project? Incremental improvements and improvisations, learning (both hardware and software), collaboration and open-source hardware and software. These are grand times.

de ND6T

Tuesday, February 21, 2017

Cap Stack Hack: Putting the BITX40 on 60 Meters


Putting the BITX40 Module on 60 Meters
by  Don Cantrell, ND6T

I have been enjoying the BITX40 with  the AD9850 DDS VFO but when the Raduino was announced I was overwhelmed with curiosity. I ordered a new BITX complete with Raduino. What would I do with another 40 meter BITX? How about 60 meters?

60 meters in the U.S. is pretty limited, currently only 5 fixed channels and limited to 100 watts effective radiated power, and so it is seldom addressed in commercially built gear. It has good propagation for nearby communication and there is a possibility that a new segment (not channels!) will soon be authorized for QRP operation. The BITX would be perfect since it is easy to program for new allocations.

While waiting for the new BITX I considered conversion strategies by using the AADE filter modeling program. First, I modeled the existing 40 meter RF bandpass filter and then modified it for 60 meters. My first idea was to change just the series capacitors in the original. It worked!

The nice curve to the right (in black) is the original RF band pass response. Changing the three series-tuned capacitances produced the response to the left (in red). Yes, it is twice as broad, but that is what I wanted since it would include WWV and a number of frequencies that I enjoy.

The conversion does not require removing any hardware. Just add four common value capacitors! Parallel the three 100 pF capacitors (C2, C4, and C6) in the RF bandpass filter with 100 pF capacitors. I  just stacked them. The fourth additional capacitor is a 220 pF capacitor across L7 (in the transmit low-pass network) to attenuate to second harmonic (now > 50 dBc). Voltage ratings are not a problem.

Stacking is done simply if you have some 1206 sized capacitors. Just sweeten the solder fillets on each end of the target capacitors with just a touch of extra solder. Place the additional part atop the target part. Hold it down while re-heating the solder at each end and you will see the capillary action pull the solder up onto the ends to complete the joint. Ta-DA!

No SMD capacitors in your junk box? Just use ceramic disks, then. The 220 pF needs to have leads  in order for it to reach across the L7 terminals and it goes underneath the board. You can use a ceramic disk like I did, but a silver mica or any other RF type would work well.

That was too easy. The only thing left to do is to load new operating software and put it on frequency.

I wrote an entirely new sketch for the Raduino. Instead of tuning a band, we now have (currently) just five channels. The new program has you tuning the knob to the far clockwise end to start the channels  changing from #1 to #5 and then starting at #1 again. I will add other channels for WWV and the broadcasters to suit me and will institute tuning when the allocation is approved in this country. This way I don't need to modify more hardware. As it now stands, I can just turn the knob to where it slowly  scans all of the channels and walk away until I hear activity. To stop channel selection, turn the knob counter-clockwise anywhere out of the channel select zone.

I will provide the 60 meter sketch to anyone that is interested free (of course) if they will write me at ND6T@ARRL.net unless Bill or Pete can think of another way for me to post it. You can use it as a basis for any kind of experimental platform that you would like since it is very small and simple.

To put it right on frequency it would be best to use a frequency counter. This is due to the variations in BFO frequencies and the reference oscillator on the Raduino module. You could do it without  equipment by “change and re-try” but a counter (even a borrowed one) would be easier.

This calibration is the subject of a previous article.

de ND6T

Monday, February 20, 2017

VU2XE's CAD files for a BITX Box


Kiran VU2XE built a nice BITX box using newly developed CAD skills. 
He suggests that you could take these files to a local laser/CAD shop and have them reproduce  box in aluminum. 

Here (I hope!) are the CAD files. 

http://soldersmoke.com/GP_Metalbox_top.dxf

http://soldersmoke.com/lcd_Metalbox_bottom.dxf

Sunday, February 19, 2017

Calibrating the Raduino

Don's discussion of the calibration problem is especially relevant for the "channelized" 60 meter band in the USA -- he has been working on a Module modified for that band.   But I think accurate calibration is also important on 40 SSB.  The vast majority of phone QSOs on 40 take place on whole integer kHz frequencies:  7164 kHz, NOT 7164.3 kHz!  If you call CQ on a non-integer frequency, people get confused and irritated. Often they will tell you that your signal is "distorted."  I have the Si5351 on my DigiTia well calibrated (thanks to Tom AK2B) and I keep the increment at 1 kHz.  This keeps people happy.   Bill N2CQR
Raduino Calibration
by  Don Cantrell, ND6T

The BITX and Raduino combination can be used as shipped without calibration as long as you do not operate right at the boundaries of the assigned frequency allocations. The old saw “Good 'nuff fer gum'mit work” applies for most use. Certainly it is more accurate (and stable) than the stuff we old geezers used “back in the day”. But what if you want all of the accuracy that the equipment is capable of producing? That digital readout is tempting.

Ashar Farhan composed a brilliant solution in his Raduino sketch. He included a software calibration that uses a push button press to enable the operator to tune a known frequency signal “by ear” to establish an offset to be applied to the VFO frequency. This puts it very close, much better than the uncalibrated result, since it accounts for both  BFO and Si5351 reference variations. There remains a small intrinsic error (insignificant in a single band application) in that the correction is frequency proportional. If you were to use the scheme for wide frequency ranges it could be a small problem.

When I converted my new BITX40 to 60m I realized that my channels were several kilohertz off frequency. In this country these channels are only 2.8 Khz wide. I couldn't even hear them. I measured the Beat Frequency Oscillator frequency at C106, the blocking capacitor to the balanced modulator/product detector, and found that it was 11.999045 MHz. Other BITXs will be significantly different since the crystals are carefully selected to match those in the crystal IF filter.

I wrote this value into my operating program file as the “BFO frequency” but found that I was still not on frequency. The only other reason for this discrepancy, aside from a math error, would be the reference oscillator for the Si5351 Phase Locked Loop.

I wrote my 60m operating system from scratch. It is simple since I am a novice at such things. No internal calibration routine. Instead I rely upon the accuracy of the Raduino reference oscillator value.

To find what that is, without disturbing the oscillator loading with a probe, I wrote a short and simple sketch for the Raduino.

/*

 Calibration program for Raduino

 Don Cantrell,ND6T  v 1.0           7 Feb 2017

 Compiles under etherkit Si5351 library v 2.0.1

 This source file is under General Public License version 3.

 Generates the reference clock frequency so that it can be

 measured and substituted as the corrected frequency of the

 particular oscillator.

 */

#include <si5351.h>

Si5351 si5351;



void setup() {

   

    si5351.init(SI5351_CRYSTAL_LOAD_8PF,25e6L,0);

    si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);

    si5351.output_enable(SI5351_CLK2, 1);

    si5351.set_freq(25e8 , SI5351_CLK2);

}

void loop() {

}

The crystal frequency is assumed to be exactly 25 MHz, at least that is what the Si5351 thinks. So we ask it to generate a 25 MHz signal and then read what it actually is. Simple.

If you are comfortable with re-loading the original Raduino sketch (or whatever sketch that you have been running) and are familiar with the different versions of the libraries and procedures that are needed to do this then, and only then, you are ready to continue. If you aren't comfortable with that then stop right here! You can easily “upgrade yourself out of service” as most of us have realized, much to our chagrin. If you find yourself in that corner then you may not have anyone to rescue you.

That said, the next step is to replace the operating system with the little calibration sketch and let it run for a few minutes so that the oscillator will stabilize (it can drift 10 Hz or so while warming up). There is no display routine for the Raduino so ignore the display (I told you this was a simple program). Measure the Raduino output frequency where it feeds the “DDS1”jack at pin number 1 there on the BITX board. Record your result from the frequency counter and use that value in the setup line for the Si5351 in place of the 25000000 value. Don't forget to add the “L” (for Long integer) if the old value had it.

In the original Raduino sketch v 1.0.1 this appears at line number 589.

“ si5351.init(SI5351_CRYSTAL_LOAD_8PF,25000000l); “

Reload your revised operating sketch and enjoy. It certainly solved my problem. My 60m BITX now seems to stay within 1 Hz, or so, of where it should be (after a few minutes “warm up”, of course).

Keep this value to use whenever you use this particular reference oscillator. Your frequencies will now be as accurate as your frequency counter and component drift will allow.

de ND6T

Saturday, February 18, 2017

Bitx40 Spectral Output



Wayne, NB6M has written in. he says :

I find that its second harmonic suppression doesn’t meet our current (since 2003) standard that all spurious output must be at least 43 dB below the level of the fundamental.  At the bottom of the band the second harmonic was only 38 dB below the fundamental, and the rig just barely met the standard at the high end of the band.

There is a very easy fix for this, simply parallel L7 with a 100 pF C0G (NP0) cap of suitable voltage rating.  After doing so in my unit, the worst case scenario, at the bottom of the band, had the second harmonic some 56 dB below the fundamental and it was now some 60 dB below the fundamental at the top of the band.

Thanks for the hack Wayne, we'll see how we can incorporate this in the next batch of the PCBs. In th meantime the amateurs in the USA can make this fix.

Another Approach to RF Gain Control (on ND6T's BITX60 Module)


Another Approach to RF Gain Control
by  Don Cantrell, ND6T

I am lazy. So on my first BITX I devised the simplest way that I could think of to install a usable RF gain control.  With so little effort, I was pleasantly surprised that it worked so well. Still, the range was not spectacular, just 17 dB. Could it be improved upon with more (please excuse the following four-letter word!) work?

Most of the best QRP designs have used a simple potentiometer at the receiver input. Simple is good, and the arrangement has proven itself to be both effective and cheap. The problem is that the only place available for it in the BITX design that is not shared with the transmit function is the connection between  pins number 14 on the two relays. Without any components in the path or any solder pads (other than the connections to the relays beneath the board) I had to do surgery on that beautiful board. Still, I had to try the idea.

Locating the conductor run on the top surface of the board, I used a hobby knife to carefully scrape the solder mask from about 5 mm of that trace  toward the front panel from R143. Not the trace that connects to R135 and R144, but the adjacent long one, the one that disappears beneath the relays. Be careful not to scrape too deeply, just to where the copper is clean and bright enough to accept solder. In the center of that cleared zone carefully remove a 1mm section of the trace, separating the path. Tin both sides of the cut and use an ohm meter to ensure a clean separation.

As homage to Ashhar Farhan's legacy of plug-in connections on the BITX40,  I removed a two pin portion of right-angle pin connector stock to fabricate a make-shift header. With small pliers I bent the short pins down to make contact with the foil when the plastic part of the connector was flush to the flat surface of the board. A drop of gel cyanoacrylate glue helped hold it in place. I soldered the previously formed short pins to the trace and was ready for the control installation.

Unlike my original RF control project that used just DC supply current, this design should use shielded cables to and from the control. I used RG174/u miniature coax, grounded at the low side lug of the 1K ohm potentiometer (both runs), the lug on the other side will go to the center conductor of the cable going toward K1 (that's toward R142). The other coax will connect to the wiper on the pot. The connector ends of those coax runs will solder to a two conductor female plug like the BITX uses for power, antenna, and speaker. That plug connects to your new header.

This arrangement gives nice, smooth control of 26 dB on my build. That is a lot of effort for just the additional 9 dB of control. However, now for the good news: You can still connect a switch between R15 and R16 (my previous RF gain control project) and use that as a 17 dB attenuator. That would give a 43 dB total attenuation!

If you choose to remove the control you can use a shorting plug on your new header to restore the radio to normal receive. If you want to install a proper Automatic Gain Control in the future, this jack might prove quite useful.

It could be worth all the trouble after all.
de ND6T

Don's BITX60