Fixing Up An Old Homebrew Rig -- Barebones Superhet and VXO 6 Watter

 

I'm not exactly sure why I pulled this old rig off the shelf, but I'll write up what I did -- I often use this blog as a kind of notebook.  I can look back and easily see what I did on my last encounter with the rig. 

The receiver is Doug DeMaw's Barebones (aka Barbados) Superhet.  This was my first superhet receiver. I built in in 1997.   The transmitter was my first real homebrew project -- it is the VXO 6 watter from QRP classics.  I built it in the Dominican Republic, probably in 1993 or 1994.  I built the power supply so that I could say that the entire rig is homebrew. 

This rig is getting a bit long in the tooth:   The receiver is built with 40673 Dual-Gate MOSFETs, an some of the transistor cans have gotten rusty.  The frequency readout on the receiver is the top of a coffee can fitted onto the reduction drive behind the tuning knob from a Drake 2-B (not MY 2B!). 

Here are two 2013 videos that I did on this receiver: 

https://soldersmoke.blogspot.com/2013/03/video-of-my-barebones-superhet.html

-- I put the crystal filter back in CW mode.  I had widened it so that I could listen to 20 meter SSB, but I decided to go back to its original configuration.  When I built the receiver in 1987, I didn't characterize the crystals -- I just used the capacitor values that Doug DeMaw had in his article.  I pretty much did that again this time, just putting caps that are close in value to what Doug had.  DeMaw used color burst crystals at 3.579 MHz.  So I guess this would be a GREAT receiver for the Color Burst Liberation Army!  

-- I used My Antuino (thanks Farhan!) to check the passband.  Here is what it looks like.  I just put the Antuino across the 10k resistors on either side of the input and output transformers.  The coil cores had become very loose -- I just tried put them in the right place.  I may need to put some wax in there to allow them to better stay in place. I think they could have used toroids instead -- that would have been easier. One of the transformer connections was open -- they don't work well that way,  once I fixed that, the passband looks like this: 

-- Each of the horizontal divisions is 500 Hz.  The passband is not pretty, but it is OK, and I  didn't feel like doing too much work on this to get it in better shape. 

-- The filter peak was a bit lower in frequency than expected.  I found that trimmer cap C3 in series with the BFO crystal would not allow me to lower its frequency sufficiently.  So I moved C3 to a position in parallel with the crystal.  With this mod, I could get the BFO frequency to 3578.69.  This produces a 690 Hz tone when the received signal is at the peak of the IF passband.  Opposite sideband rejection is quite good. 

March 2013 Rebuild of the VXO 6 watter

-- I didn't have to do any real work on the transmitter.  The RF amplifier in the transmitter had served for a time as the RF amp in by 17 meter DSB rig (I had added a bias circuit, which I removed when I put the amplifier back in Class C).  Some time ago I rebuilt the oscillator circuit (which had been literally cut off the board when I used the amplifier in the DSB rig). 

-- I did have to reconfigure the muting circuit -- the T/R switch in the transmitter switches the antenna and also -- through a two wire circuit -- cuts off 12 V DC to the transmitter when in receive mode. 

-- For sidetone I just put a small piezo buzzer through a 1k resistor between 12 V DC and the key line. 

It all worked fine -- I talked to three stations on the high end of  the 20 meter CW band. 

Putting a Ceramic Filter in the "Mate for the Mighty Midget" Receiver

It is really simple.  I had one of the +/- 3kHz (6 kHz wide) 455HT filters on hand.  The spec sheets call for 2000 ohms at both ends, but looking at the schematic it appeared that I already had high impedance on both sides of the filter.  I put a .001 uF cap on the input side to keep the DC voltage off the filter (see above and below). This capacitor allows us to avoid the dreaded problem of electro-migration that is so nicely described by SV8YM here: 

http://sv8ym.blogspot.com/2010/07/mysterious-case-of-withering-filters.html. 

Tasos also provides a good description of the innards of those little black boxes that contain ceramic filters. 

Once you get the filter in your receiver, you have to carefully place the BFO signal in relation to the filter passband.  I have trouble properly sweeping 455 kHz filters -- my HP8640B will not go that low.  Nor will my Antuino (I need to modify the code -- someone help me please).  I know the NanoVNA will do the job, but I just couldn't seem to get it to work.  So I went "old-school" and manually swept the filter using my FeelTech sig gen and my Rigol scope.  This gave me a rough idea of where the passband was.  I put the BFO on the low end of the filter passband, at 451 kHz.

   

With this filter the MMM RX has become a real asset.   The 6 kHz bandwidth allows for nice reception of both SSB signals and AM sigs.   I may try to use one of the +/- 2 kHz filters  (4 KHz wide), but so far I have not been able to find a source for this part. 

Farhan and the SBITX on Hack-A-Day

 https://hackaday.com/2021/08/06/sbitx-hackable-hf-sdr-for-the-raspberry-pi/

Mythbuster Video #12 -- Bandpass Filters

The really cool part comes at the end when I put the scope probe on the output, then on the input of the bandpass filter.  Exciting stuff my friends! 

Mythbuster Video #2 -- 10 Pole Crystal Filter

This is a 5.2 MHz crystal filter. I used the G3UUR method for determining the crystal's motional parameters. I then used Dishal and AADE software to design a 10 pole Cohn Min-Loss filter. I tested the bandwidth with an Antuino Scalar Network analyzer (thanks Farhan!) and a NanoVNA. I found the passband to be a bit tight for SSB, so I replaced the capacitors with caps of a slightly lower value -- this broadened the passband. It is still a bit tight, but the SSB audio -- while not enhanced or Hi-Fi -- sounds fine.  

Passband filter shape as seen in the Antuino and in the NanoVNA. The -20 db line in the Antuino actually corresponds to no loss. 

The schematic provided by the AADE software.  Dishal software may have come up with better, more correct values for the capacitors. 

The passband as predicted by AADE.  Skirts so nearly vertical as to strike fear in the hearts of SDR owners! 

Filter under construction -- waiting for the caps from Mouser. 

First scan with the NanoVNA.  Insertion loss looks very high but that is only because I am terminating the filter with resistors -- I just wanted to see the passband shape. 

10 Pole Crystal Filter Passband as Seen in Antuino and NanoVNA

I continue to work on the "Mythbuster" rig,  but I am taking it slow, trying to learn something from each stage.  I'm especially trying to master the used of the great test gear that has arrived in my shack in recent years:  The Antuino, the NanoVNA, and the TinySA.  

Above you can see the passband of the 10 pole crystal filter as measured across the 50 ohm terminations on the filter.  I use simple FT37-43 transformers to match the filter impedance down to 50 ohms.  I used the Antuino first -- it scanned the passband and held the image on its screen.  I then disconnected the Antuino and connected the NanoVNA.  So in this shot you can see the passband on both devices.  

You will notice that the Antuino says there is a 20db insertion loss.  That's only because in the Antuino 20db is really 0 db loss.  I think the NanoVNA gives a more accurate insertion loss reading -- about 3-5 db.  The cool thing is how similar the shapes of the passband are.  

Feedback on Farhan's FB Feedback Amp Video

As I said a couple of days ago, Farhan has put out a very informative video on amplifier design. During the video we can see him determine bias, feedback and load levels, then select component values. We then see him actually build the amplifier "ugly style" and use his Antuino to test it.  Fantastic.   

Watch Farhan's video here:  https://www.vu2ese.com/index.php/2020/06/18/feedback-amplifier/


I sent Farhan some of my reactions to the video.  In the hope of stimulating some discussion, I repeat them here: 

___________________

Wow Farhan, I really enjoyed your video and learned a lot.  You definitely have the Knack for explaining this stuff. 

I have been struggling to understand feedback amps for a long time.  I took up this topic on pages 187-190 of the "SolderSmoke -- Global Adventures" book.  I was more focused on the benefits of FB amps, and on how and why feedback affected input impedance.  

 I took notes as I watched: 

-- I liked the fulcrum analogy. 

-- Really wonderful how you describe the selection of components to get desired bias. 

-- Even better when you explain tapping down of RF coil to get 50 ohms to look like 200 ohms. Great stuff. 

-- Standardized on BNC.  FB! 

-- .1uF caps.  Love them!  No argument here! 

 

-- I copied your resistor list.  Will use it with Mouser. 

-- Saving the pigtails.  Indeed!

 

-- I like your ugly "resistors first" technique.  I may have to move away from Manhattan. 

--  Wow.  Ugly circuits IN SPAAAAACE!  FB!

 

--  2N3904   I always think it is EBC when looking at the flat end of transistor with leads facing down. Only had reverse pinout with PN2N2222.  EBC is my usual assumption.

 

-- I still have the big box of trifilars you left me.  I use them.  Thanks!

-- I like that VTVM in your Antuino.   Very handy. Want one!  Also, We need to get the Antuino to go down to below 455 kHz. 

-- I also like your dual Return Loss and SWR display.  I think in terms of SWR.  Return Loss sometimes messes me up. 

-- Didn't know about the Hold and Zoom feature on Antuino. FB.

 

-- Your measurement of the Q of the crystal was awesome.

  

-- I checked my soldering iron temp:  I'm at 480 C   A bit too hot. Will back off. 

 

-- Good description of need for an attenuator at amp output to keep it in Antuino's range.

 

-- As I watched you tug on the components after the solder cooled, I remember an old and silly admonishment from the ARRL Handbook:  NEVER  use solder for mechanical connections.  Ha!  They were WRONG!

 

-- Demonstration of the flatness of feedback was great.

 

-- I found your measurement of impedance using the SWR feature of the Antuino to be very useful.  Is there a chart relating the SWR/Return Loss to actual impedance values? 

-- Loved your description of how output impedance affects input impedance.  That is why you advised use of TIA amps in my DIGI-TIA.   But now I'm thinking that if I can accurately measure impedances of non-TIA amps, I can design L networks that will keep the crystal filter passbands ripple-free, right? 

-- Great explanation of the benefits of the 6 db pads at amp output.  Allison often recommends this. Now I know why. 

-- Wow!  Now I KNOW what that two-tone box you left with me is for!  Now I understand how it can be used to measure IMD on FB amps.  I pulled mine out just as you began to discuss yours.  Really cool. 

Thanks a lot Farhan for doing this.   These videos will be of long-lasting use to homebrewers around the world.  I hope we will see many more VU2ESE videos like this one. 

73  Bill 

Excellent Video from Farhan on Amplifier Design

Farhan has produced a really excellent video explaining the theory behind the feedback amplifiers that we use in so many of our circuits.  He takes us through the design and construction of these amplifiers, then uses his Antuino network analyzer to test an amplifier  and to measure input and output impedances.  

There is a lot of tribal knowledge and wisdom in this video! 

Check it out here:  

https://www.vu2ese.com/index.php/2020/06/18/feedback-amplifier/

Thanks Farhan! 

SolderSmoke Podcast #216 Is Available: BITXs, Paesano, Paraset, ET2, Antuino, Mailbag

Bill's uBITX with HB keys and the mic that used to be the podcast mic! 

SolderSmoke Podcast #216 is available 

21 December 2019

HAPPY HOLIDAYS TO ALL! 

http://soldersmoke.com/soldersmoke216.mp3

Pete's Bench 

Sprat Article

BITX 40 Fun

ZL2BMI Rig

Paraset.   Three tubes!  Almost an ET-3!

Bill's Bench

ET-2 Adventure over,  Rig on the wall

Final QSO count. 20   3 "random" 

Last QSO with AA8OZ

Lessons learned:  100 mW not the problem. Crystal Control cramps your style. 

N0WVA regen amazingly effective.

Tried for the Sunrise Net.  Walter sent me some crystals.  

On to the uBITX. 

Accidentally wiped out calibration and BFO settings. 

Had to do recalibration and reset BFO. 

Learned a lot about the rig. 

How they did CW and how they do it now. Shift TX?  Or shift RX?  Or just shift BFO on RX? 

How all the signals end up as upper sideband.  Only one BFO freq.  Very cool. 

TalentCell 12 V Lithium Ion 3 AmpHour battery. Size of a deck of cards. 

Inspired by Peregrino -- I ordered EFHW Tuner from QRP Guys. 

Homebrew Straight Keys

Antuino upgrade 

SMT soldering. 

Back to Arduino Nano and the IDE.  

A very cool tool. 

Antuino filter analysis.

Version 6 of uBITX out. 

The "Watt Meter" DC power meter.  Very useful.  8 bucks.  LINK:

https://www.ebay.com/itm/Digital-DC-combo-Meter-LCD-Watt-Power-Volt-Amp-RC-Battery-charging-Analyzer-M/152339793114?fbclid=IwAR0u9SlZi2Dm6zOJyZt4fDTu7w_pjBIEYD_FiNfLymxtQUbMjcNHjfB17P0

SPRAT, balloons and hardcore homebrew hydrogen. 

MAILBAG

VK3HN's AM receiver.  I ordered 6kHz 455 kHz filters from Australia. 

Dean's MMM

Peter VK2EMU   Hertz not hertz. 

Ben KC9DLM LTSpice YouTube Videos

Steve Silverman  Electroluminescent Receiver Kit

Lyndon N0LFX back to listening.  FB OM

Steve M0KOV built a pill bottle variometer.  Did you get the regen going? 

Anthony VU3JVX   FB HB uBitx.  

Allison KB1GMX  Great to hear from her

Leif WB9IWT -- helped me trouble shoot my uBITX (BFO was low)

Mike EI0CL  old buddy from Azores days.  Recognized his voice on 20.  Great QSO.  

Antuino Filter Analysis

Farhan had given me one of the early Dayton Hamvention models of the Antuino SWR/PWR/SNA RF test lab.  He later identified the need for a few mods to improve performance on that early model.  So I brushed up on my surface mount soldering, got the needed (tiny!) parts and made the mods.  I also put the battery pack inside the box and put some feet on the Antuino cabinet (it just seemed like the right thing to do). 

Antuino has already proven to be very useful as an SWR analyzer.  I know have a much better understanding of the SWR bandwidth of my wire antennas. 

And it is very useful in evaluating the passbands of filters.  I had an old 11.273 MHz filter from an old CB radio in my junk box.  I have no specs on this device -- I didn't know what impedances it was designed for.   So it was time for some Antuino technical detective work. 

First, take a look at the filter with nothing between it and the Antuino.  Input and output on the Antuino are 50 ohms, so here is what the passband looked like with 50 ohms: 

Next I put in two 47 ohm resistors, one in series with the input, the other in series with the output.  Antuino connected at the other side of each resistor.  Here is what it looked like.  Note the improvement in skirt shape.  But there is still a lot of ripple in the passband: 

Then I went to 100 ohms.   The passband ripple was reduced noticeably: 

Then up to 330 ohms.  Here the passband doesn't seem quite as flat as it was with 100 ohms: 

Finally, 1000 ohms.  Definitely too much.  Note the ripple. 

 

Farhan prefers the passband with the 100 ohm resistors.  I agree.  

BTW the filter is from TEW of Tokyo, Japan.  Model FEC-113-2  11.2735 MHz  No. 2   A    2

It had three crystals on the board with it:  11.275 and 11.272 -- these are obviously for LSB and USB.    The third crystal is at 11.730 MHz, indicating to me that they had a second IF of 455 kHz in this rig.   If I use it, I think I'd stay with single conversion. At 11.273 MHz the filter is of ideal passband width for SSB.  I do feel the urge to build something around this filter. 

Doing the mods on the Antuino was fun, and having worked on the device at least a little bit I feel more of a connection to it. 

Antuino Mods and Updates from Farhan

Both messages from the BITX20.io email group.

26 July 2019  

Peeps:

The Antuino has been in use for over a few months now. We had produced 100 of them that were sold at the FDIM. These work quite well for an SWR meter and Antenna Analyzer, but they were sub-optimal for serious RF work. For those who bought it for SWR measurements, you can continue to use it. For those who want to improve it, read on.

It was noticed that the db scale was not accurate. Antuino was designed to be an accurate and precise instrument. The db readings should be within +/- 1 db accuracy. However, they tended to vary by as much as 2 db on the upper range of power measurments. This was finally tracked down to having too much gain. I had prototyped the original with 2N3904 transistors but in production we used BFR93W as I guessed it would have 'better' performance. It turned to be a bad choice. The higher gain resulted in compression of signals above -30 dbm. This restricted the useful range of the Antuino to about 50 db. 

I am attaching the new (Version 2.1, it is a minor edit of the V2 that was sold at the FDIM). In summary, these are the changes:

1. The new software with a cleaner way to calibrate is on https://github.com/afarhan/antuinov2.1. Please upgrade to it even if you don't want to carry out the other hardware modifications.

2. Replace the Q1 and Q3 transistors to 2N3904. Although the PCB pads are SMT, you can solder the leaded type by twisting the legs around.

3. In the first IF amplifier (that immediately follows the mixer), we decrease the emitter resistor (R18) from 100 ohms to 51 ohms (you can also parallel a regular 1/4 watt 100 ohms with the original 100 ohms to get to 51 ohms). We also parallel the (R19) 220 ohms collector load with a 10 uh inductor (you can use 10 turns on FT 37-43), the exact value is not important.

4. In the second and third IF amplifiers, we replace the 100 ohms emitter resistors (R7, R34) with 10 ohms and remove the 4.7 ohm resistors (R32, R4).

5. In the last IF amplifier, we change the load resistor(R33) from 51 ohms to 220 ohms.

You need a few 10 ohms resistors and a 220 ohm resistor. You can resuse the R31 at R18.

I have detailed the modifications at http://www.hfsignals.com/index.php/2019/07/26/modifications-to-the-daytonfdim-2019-antuino/

Attached are the images and the circuit.

73, f 

_._,_._,_

---

August 3, 2019

Every 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