6/25/2017 ~ Working Field Day (at QRP level) with the 20M Shirt Pocket SSB Transceiver. At 16 cubic inches --smaller than a Bitx ! I actually made QSO's with this transceiver and it is a testament to good things come in small packages. Definitely a DifX.
6/23/2017 ~ See video at the bottom for more MMIC Applications
Now that I was able to resolve the Crystal Filter that will be used in the second IF (it is not a Dishal nor any other form of homebrew filter) I have turned my attention to the issue of various amplifier stages that are used elsewhere in the rig. One such circuit is the bidirectional amplifier board I developed where there are two amplifier circuits comprised of a single 2N3904 in each leg. One leg is the receiver RF amplifier stage and the second leg is the Transmit Pre-driver stage. Relay switching is used to direct the signals in/out of the board -- a total of three relays.
While the 2N3904's are cheap, the relays are not! While the stage was broad band there were issues of making the gain constant over a range of 30 MHz (160 to 10 Meters). Since most of my rigs are single band units or perhaps two bands like 20/40 Meters this was not so much of a problem.
So in noodling the problem to have broad band amplifiers stages that are constant gain, are 50 Ohms, involve minimum switching (no more than one relay) and are termination insensitive. While you could use the Hayward/Kopski Termination Insensitive Amplifiers -- I wanted this to be a Pete design and not use the work of others.
My noodling took me back to 2010 and my very first article in QRP Quarterly where I used MMIC (Microwave Monolithic Integrated Circuit) amplifier blocks. The project was a 20 Meter MMIC based QRP SSB transceiver. A pair of MMIC amplifiers are diode steered so that they are bilateral (operating in two directions depending on which amp is powered on and steered for the proper mode). Yes Alice (or Virginia) it has a Digital VFO,
(Interesting note in that same issue we have G3UUR Analyzing Crystal Filters)
The MMIC device selected was the TriQuint (Watkins Johnson) AG-303-86G, This device is good to 6 GHz and has a fixed gain of 20 dB with a Z in/out of 50 Ohms and operates from 5 VDC. It also is termination insensitive, Did I also mention they work quite well.
Thus it was an easy decision to once again employ the MMIC amp and we have a board operating with a test transceiver. The alternative for those who don't want to enjoy using MMIC's in your next rig then you can always use the Hayward/Kopski TIA amps. Shown below is a board that was just installed in the transceiver that was recently used to evaluate the 128X32 OLED noise issues.
BTW I did have several conversations with TriQuint and they provided information about the use of the 1N3070 diodes as the "best choice" over the 1N914 or 1N4148. The actual building of this bilateral stage will present construction problems in that the MMIC's are surface mount and really small. It will be difficult to build this amp using Manhattan techniques. So while the benefits are superb, the construction may well beyond the skill/capability of those who have never done this type of homebrew construction. In the last 7 years the price of the MMIC's used has increased significantly so that may be a factor in your decision but my having a stock of the devices makes it a non-issue for me.
As you are perhaps gathering, the Dual Conversion DifX will have features and functionality not found in other currently popular designs.
For those sitting on the edge of your chairs regarding the detail of second IF filter --patience grasshopper (for those who remember the Kung FU TV series). Oh I will tell you it is not 9.0 MHz.
Below is a video of MMIC's (four of them) used in a 40M CW transceiver built about the same time as the 20M MMIC SSB Transceiver. I do not know of many other transceivers that employed MMIC's in various transceiver stages. Perhaps another tip of the spear from N6QW.
6/20/2017 ~ Short diversion -- OLED noise with a 128X32 OLED Blue Display
Believe it or not this test is important to an upcoming project that will soon be revealed.
6/18/2017 ~ In the original post I mentioned that I saw three German Filters on a board for sale from Israel but did not know much about them. Thanks to one of the regular blog readers (Thanks Jim), we now have info on these filters and from what I can see they appear to be symmetrical which is important if they are installed in a bilateral circuit such as the Termination Insensitive Amps and the Z in/out is 1 K which is a 20:1 match to 50 Ohms. A 9 turn to 2 turn matching transformer gets you close 9^2 = 81 and 2^2 = 4. 81/4 = 20.25.
The good news is that they are 8 pole filters which will really tighten up the skirts and resolve the "feathering" issue complained about by one of the 40 Meter SDR police and cited in the post. Thank again Jim for the info and the plot of one of the filters taken "bare" without any matching which would affect the ripple content in the pass band. If that ripple is smoothed your are looking at 60 dB
rejection and pretty narrow skirts.
Don't rely on my analysis but for some this might be a good buy and you get an AM filter too! This is definitely not a Dishal homebrew filter and would be hard to replicate in a cold, dimly lit garage with the only tools being a cheap VOM and an 80 Watt RS soldering iron.
I just bought a board with the three filters -- $34 shipped and that gives me three filters.
Having sworn off Dishal and other homebrew filters there has to be some alternatives for those who would like to homebrew a rig. The reasons are many for not homebrewing a filter and chief among these is that it is difficult to achieve consistent results from these somewhat arcane processes without the benefit of an extensive amount of test equipment. Anyone who tells you they did it with just a VOM is Fake News. Now if you just want to try your hand at it --by all means build a filter.
But with the advent of the SDR radios and those who lurk around 40 Meters looking for aberrant signals, there is a strong possibility you will get a report that some one has spotted energy above 2800 Hz (especially on 60M) or that your filter is feathering. (OK try to figure that one out.) Yes a homebrew filter will let you enjoy the experience of making a rig where you literally built everything by hand. But I would bet that in most cases the homebrew filter that results unless you are extremely lucky will not match a commercial filter.
So OK where can one buy a reasonably priced SSB filter? One place is the GQRP Club. Join GQRP, become a SPRAT subscriber and buy a high quality filter for about 12 Pound plus shipping. At a $2 to 1 Pound exchange rate --about $30 with shipping puts one of those jewels in your hands. Many of my rigs have this filter. In fact two W7ZOI designed rigs have two such filters in each rig. The IF is 9.0 MHz and works very well for most applications save 17 Meters. Below is that filter in the Zia Transceiver built in 2014 which uses the Hayward/Kopski Termination Insensitive Amplifiers. The photo right below shows another GQRP filter installed in the LM373 Rig.
For the same amount of money INRAD sells a 4 pole Filter Kit --also on 9.0 MHz. I have two of those filters and they work very well. It is their model # 351. The Crystals are color coded and come with both SMD and Leaded Caps so you can pick your method of building the kit. Below is the #351 in a 2017 project implemented with the SMD caps.
Just yesterday I toured eBay looking at commercial/surplus filters. Wow some one from downtown Serbia is selling Elecraft SSB filters for an amazing price. There are also filters from Kenwood, Yaesu and Icom radios. Some are pricey but some are really at a very good price.
There were several Heathkit filters and some listed starting at $10. I have used a Heathkit filter in a transceiver and it works very well. The only problem --it is rather large. The filter IF is a 3.395 MHz and so this lends itself to a dual conversion approach. In the photo below the signals ahead of this board were converted to frequencies in the 8.8 MHz range. The PTO (lifted from a Ten Tec Triton IV) operating a 5 MHz converted the signals to the IF at 3.395 MHz using the TUF-1 on that board. The relay on the board enabled adding AGC to the IF or ALC. The device is a DGM 3N209 in a circuit developed by G4GXO in Sprat 128 --it is bilateral! In the upper right hand corner is a diode ring modulator/demodulator that had both resistive and capacitive balance. That is not something you find in many homebrew rigs. I built this in 2009.
But just as I saw some really good filters there were some questionable ones. There were several boards from Israel showing three German filters on one board. The price was OK --it is just that I didn't immediately recognize the filters. More research needed here.
The 8.8 MHz (or so) Kenwood Filters look like a good fit with a 45 MHz 1st IF. Many of the commercial filters are 2.6 KHz wide and so would give better results on received audio as well has having a Hi Fi sounding signal on transmit with all of the benefits or presence, brightness, color, with great lows and highs.
Frequently with a homebrew filter you will get tons of complaints about having pinched or restricted audio --typically form those operators using SDR radios with 72 Inch LCD screens. So they can really see your signal!!!!
So expand your horizon's and think beyond homebrew filters -- it avoids a lot of work that frequently results in a marginal filter that unless you are extremely lucky will have a difficult time competing with a $30 or less commercial filter. Oh -- the Heathkit Z in/out = 2K Ohms, the GQRP is 500 Ohms and the INRAD #351 is 200 Ohms. Pretty easy to match and a known quantity!
BTW I bought this filter after finding out more of the specifications and this now lends more weight to my argument -- I just bought three filters for $30 and $4 shipping. These filters are better (being 8 pole) than any 4 or 6 pole Dishal filters. After purchasing the board I got a prod from eBay essentially saying buyers who purchased this board also bought Si5351's --so there are individuals out there who are already on this path. (6/18/2017 N6QW)
That is the other bit of good news. In the past having the filter without the matching BFO crystals was a huge problem. No more fellow homebrewer's. With the Si5351 --you simply change the frequency in the sketch and you are off and running with a new filter. Hooray --that is a problem no longer.
Arduinos and Si5351 PLL Clock Generators are inexpensive (well depends where you live); but for around $20 (US) you can purchase a ProMini and a si5351 Clock Generator. I have not seen many designs where two Arduino's and two PLL clock generators are employed. So why not? Yes you will hear from the lurking illuminati that you don't need to use two but has any one really explored the possibilities opened up by such an approach.
Let us think "Out of the Box" for a minute. Suppose we take a device like the Pro-Mini and loaded that with code so that CLK2 would provide the injection frequency for a second mixer in a dual conversion transceiver (the 1st conversion would be an up conversion to say 45 MHz).Thus the second conversion would be to the IF frequency and CLK0 would provide the BFO injection frequencies. You got that OK?
Now think for a minute of a two position switch mounted on the front panel so that one position would be to select USB, the second position would be for LSB. You might even have a second encoder with a PB so that by depressing the encoder push button you could have pass band tuning limited to the range of separation between the USB/LSB frequencies. Pretty cool.
Now about the second Arduino and PLL Clock Generator. Suppose that this time for the up-conversion we use the Si570 controlled by an Arduino Mega. The use of the Si570 would cure the gaff from the EMRFD Illuminati about phase noise and would use a true encoder and not a pot like in the Minima. I am thinking with the Mega you would have more digital and analog IO so that controlling band pass and low pass filters would be unencumbered by pin limitations. A bigger display (and more programming space in the Mega) would let you display to your hearts content. The two PLL Clock generators will also address the naysayers who complain about the spill over onto the third clock. The upper frequency limit of the Si570 is about 5X the Si5351 so other possibilities here.
So maybe it is time to add a few more Arduino's and PLL Clock generators in our rigs.
We must look to the past often times to see the clarity of the way forward. I am reminded of something that Thomas Edison experienced. (Don't know if this is factual but may be more of an urban legend thrown in for good measure.)
It seems there was a major fire at his Menlo Park Laboratories and with that fire many of Edison's projects were burnt to a crisp. One of his assistants was lamenting to Edison about how terrible the situation was in that many of his projects were no more. Edison, ever the visionary, said this is actually good! The assistant could not believe what he heard --Edison chimed in -- "most of what burnt up didn't work and I just didn't have the heart to take it to the dump". The fire actually solved a problem for Edison --with the clutter removed he could focus on more productive things.
So it is with all things Dishal! I had a ceremonial fire today and gone is all that wretched crap that didn't work and probably never would work. But with that fire, I have a new plan and I think a much better solution for the second filter. I am somewhat mad that I didn't noodle this before.
While I work the new approach the blog will be silent. But I am really jazzed that where I am headed will in effect be a super DifX.
There always has to be a reason why something as fundamental as 1 + 1 does not equal 2 and so it is with the results arising from building a Dishal Crystal Filter. So lets us take a few steps back and examine why 1 + 1 may not equal 2.
The Dishal software in large part requires the user to make some initial measurements outside of the software itself. Starting with the G3URR oscillator and my SDR receiver (capable of measuring to 1 Hz and verified by first locking on to WWV) I measured the crystals in the loaded and unloaded condition. Again the receiver is calibrated to be accurate to 1 Hz and the G3URR component values are 470 PF (NPO COG) in the Colpitts circuit and the load cap was 30 PF NPO COG. So the data was taken and crystals were found that were very close in frequency and thus were the basis for entering the data into the Dishal software. It is pretty automatic once you enter the data.
The Dishal software can be tweaked by factors such as the number of crystals and the ripple factor. Once these are selected then you get a plot of the filter. But the software also spits out the Filter Center Frequency, the values of the coupling capacitors and the impedance that must be matched to 50 Ohms. So far so good.
Now the problem is one of what BFO frequencies do you select. Starting with the predicted Center Frequency I chose BFO frequencies about 1.5 KHz above and below that value. This choice was one based on the stock of 9 MHz crystal Filters I have and the matching BFO crystals are just that -- 1.5 KHz above and below. That isn't gospel but merely a place to start. Those values were input to the Arduino Sketch. It became pretty obvious that those were not the right values.
My next step was to disconnect CLK2 from the SBL-1 and in its place use my FeelTech signal generator to supply the BFO signals. Two cautions here with the first being that you crank down the output to 1.4 Volts PTP before connecting to the SBL-1 and second that you have a 10 NF cap between the FeelTech and the SBL-1. What I found was that the values that seemed to work were only about 1 KHz away from the Cf. There is something wrong here.
A 1 KHz BFO would place the signal on the top of the pass band and not on the slope of the curve! Thus 1 + 1 is not 2. This bothered me and that is when the light bulb went on. Dishal is lying to me!!!! Well not entirely true but close to the truth (I must sound like Donald Trump.) Is this being taped?
This is when I tried something and this may provide an explanation for some of the crappy results I am seeing using Dishal. The first thing I did is to connect the radio to a dummy load so that no signals were incoming. Then I listened to the rig as I tuned the BFO (FeelTech) from about 2.5 kHz below the supposed Cf to about 2.5 kHz above and just listened -- I noted the change in the background noise and at one point heard a "null". Was this in fact the Center Frequency? I think it may be --tell me I am wrong. This value was close but different that the Dishal value. So I think my evaluation may have some validity. Using this value I added/subtracted 1300 Hz to set the frequencies. In looking at the location of +/- 1300 Hz this puts the frequencies about 3 dB down on the curve and so that may add some additional validity to my selection.
I have changed the Arduino sketch with the 1300 Hz values and will now await the 40M band to perk up so I can run some listening tests and get some reports on the transmitted signal. I very likely missed this part in the Dishal Tutorial (who reads that stuff anyway) but the setting of the BFO frequencies does indeed affect the filter performance.
Lurking here is the theoretical (Dishal Predictive Curve) versus what really results and that may be a shift of the Center Frequency because of imperfect components, stray capacitance, lead length and whether you have dandruff or AFF (Athlete's Foot Fungus). So many things could impact if indeed 1 + 1 = 2.
My Dishal experience has consumed way too much time and the results so far do not justify the effort. Despite what you may have read in all of the testimonials and the touted results --did those users actually install the filter in a radio and "air test it" and match their real world experience with what the curves may be telling them? I can only share what I have seen and so far it has not been pretty.
I am disappointed with the results from using the Dishal method where quite often what you get does not match the predictions or expectations. There are those reading this blog who are now quickly preparing a flaming response. There will be the usual comments: you didn't do it properly; or you did not make the measurements correctly or you built it in a haphazard manner or on and on.
My 6 pole filter sucks and is real crap. I then decided to build a four pole filter using four different crystals but ones from the same batch. I was very careful (again) to follow all of the steps and to double check the build, the test setup and the measurement process. The outcome was not unlike the six pole but somewhat better. I did find that the skirt rejection was better than the six pole and closer to the predicted curve but installed in a radio sounded "not so good" when tuning across a signal. That to me is telling --how does it work in a rig.
So below are plots of the four pole filter responses both from the predicted value and the actual measured value. Using the FeelTech signal generator for the signal source and my Rigol scope for the detector. There was a 50 Ohm series resistor in the source end and a 50 Ohm termination resistor on the output side. The readings were taken as RMS readings.
Here is how I converted the Vrms data. First I multiplied the readings by 2.828 as this converts the readings to V Peak to Peak. I thought this would be useful to know and have as a matter of record the PTP readings. I then squared that PTP reading and divided that by 400. The 400 factor accounts for 50 Ohms and does in effect take the value back into RMS. The answer is in watts. If you multiply that by 1000 you now have milliwatts. Or you can simply take the squared value and multiply by 2.5 (1000/400). This answer is now in milliwatts. If you take 10 X LOG10 of this number you will have dBm. That is what I did. So find fault as you will; but the curve will represent the relative impact of the pass band and give you a prediction.
Here is my curve based on the measurement process. I took readings every 200 Hz. There might be some advantage at this point to repeat the test using 100 Hz data points. But this curve does look like the Dishal curve for the four pole filter. Installed in a radio I can say that on receive it does not sound that good.
The Dishal Predicted Curve!
So what do you do now? You have the filter, it looks like the prediction but does not sound too good. There are opportunities for changing things. One change: make the bandwidth less. I set it at 2.5 kHz so it would have that "hi fi" or enhanced SSB sound. A filter at 2.3 KHz night be a better choice, and doing so will change the cap values and the matching transformer. So before I do that I want to do some more testing with the current configuration.
This now begs the questions: Why did the 6 pole shoot craps and Why the 4 Pole while looking like the prediction not sound so good? Maybe the sounding not so good is that the bandwidth has been opened up and I am used to hearing like about 2.1 kHz. Stay tuned.
6/6/2017 ~ Added >10 dB Attenuator to handle super strong signals. It is a "T Type" controlled by a small relay with the Series arms being 33 Ohms and the Parallel arm being 22 Ohms. A switch on the front panel controls the In/Out. Kind of amazing --with the pad Out of the circuit you can hear the noise on the signal and with it In --the noise drops like a rock. Good affirmation about RF amplifier stages and their contribution to noise. The Pad/Relay was mounted on a small circuit board which was soldered vertically to the main board.
The panel mounted toggle switch that selects Pad In/Out is located below the volume control and above the earphone jack. This is a solution to the strong signal overload.
Every so often the planets align and you end up with a really nice rig. No nice is not the right word but the correct terminology is that you end up with a Superb Rig. I have fully documented the building of the LM373 Rig at http://www.n6qw.com/LM373.html
Frequently I joke that I have two boxes of electronic projects with a giant box containing rigs/projects that worked once, or ones that don't work or ones that will never work (the 6 Pole Dishal Filter is in this group). Then I have a smaller box of rigs/projects that work very well. The LM373 Rig is definitely in that box.
Rigs can look good --or can even be called pretty; but how do they work is the real test. Typically the unofficial criteria is how do they sound on the other end, can they work DX , how do they receive, and for those who lurk the bands with their SDR store bought radios with 72 inch screens is there any energy below 200 Hz or above 2800 Hz. This latter criteria seems to be the one that is the most controversial with those of us who homebrew their rigs.
So now I would like to share some LM373 Performance information. With IRF510 for a final it will put out in excess of 5 watts (6-8 can be easily had). With my intermediate amp it will do >110 watts and with the SB200 in line following the intermediate amp it will do 700 watts (easily). Couple that with a 2 element beam (which I will talk more about a bit later) and you can break pile ups --which I have. Stations have been worked running QRP and most of the stations worked at 110 watts and of course stations worked at high power. Interestingly about 1/3 of the DX stations were worked running the 110 watts -- the beam antenna is a great field leveler.
Now how does it sound -- I get frequent rave comments about the audio (subjective from non SDR monitors). Punchy and articulate are frequent descriptors --kind of amazing since the audio stage is a single 2N3904 that was modeled in LT Spice. There have been no reports about spurs, lack of sideband suppression or pinched sounding. All this from a homebrew rig!
On the receive side it hears really well and that has been a frequent question from the other end after hearing the transmit side. Well I have found the hearing part an interesting aspect of the rig. The LM373 has built in AGC Circuitry and the data sheets include suggestions for making this adjustable versus fixed values. That will be a further experiment.
In W5BAA's design he used a second LM373 as the receiver mixer stage and riding along with that is that you do get some gain from that stage since it is an active (versus passive) mixer. In my rig I used an SBL-1 as the mixer stage which as a passive mixer (non-gain) does have a conversion loss. Ahead of the SBL-1, I added a single manually adjustable gain stage. Mind you the manually adjustable is a trim pot on the circuit board. My original intent was to set it for about 10 dB and forget it. Well I have found that on really strong signals and with the beam down the throat of the station that the rig will overload. Crank back that trim pot ( 2 to 3 dB of gain) and everything clears right up without any other adjustments. This does not happen all of the time --just on the rock crushing signals and sometimes I do see those kinds of signals. With the beam moving off the signal heading this clears up the signal too. So I smile as I now know the beam is doing its job!
The solution to the infrequent overload can take many forms and two of those would be to make the AGC a panel control as shown in the LM373 Data Sheets or add a 10 dB attenuator (a simple relay with a 10 dB pad on one side of the contacts) as I did with the KWM-4. The attenuator would be easy to implement right on the 2N3904 Rx RF Amp Board. This has perhaps a better approach as the engagement is a simple toggle switch versus a panel mounted pot. So this whole discussion boils down to that the receiver is sensitive and can be too sensitive if upstream there is not some signal limiting for extremely strong signals. A more elaborate AGC would also 'save the day'.
Now how does it work with DX -- well my DX standard is the European / VK-ZL path. I have worked the following pre-fixes S51 (Slovenia), EA3 (Spain), E51(Cook Island), LU1 (Argentina), XE1(Mexico), ZL1(New Zealand) and 9Y4 (Caribbean). Mind you that so far I have made 50 contacts total with the rig and 15% have been DX.
Sides have been added to the case (painted black) and now I am working on a top cover. One possible top cover might be a sheet of 1/8 thick Lexan plastic so you can the "innards" another is a metal cover with a speaker built into the cover. Still noodling that piece.
There is just something about being able to work DX stations (some running Flex 6700's) and to have them say nice signal --all from a homebrew rig using 1970's technology.