Mythbuster Video #6 -- On to 20 Meters (But With Bandpass Filter Woes). Please help solve the mystery!

Here's how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH). 

But Elsie lets you specify the coil value.  So I then I went with 1 uH.  But with this value you don't get 50 ohms at either end.  You need a matching network.  Elsie provides this too!  

I asked Elsie to match my BP filter to 50 ohms.  It provided several options to do this --  I went with a simple capacitive impedance divider.  But alas, I was now bumping up against the 7 limit of the free version of Elsie so I had to reduce the number of LC elements from 4 to 3.  Bummer. 

With 3 LC tuned circuits and matched to 50 ohms the plot looks OK.  But I would have preferred 4 LC circuits. 

 

The rftools website created a BP filter for me with 4 LC elements, and matched to 50 ohms.  Very useful. https://rf-tools.com/lc-filter/

But here's my problem:  With both the filter designed by Elsie and the one designed by rftools, I found that the filter passband was too low.  It was in the 12 - 13 MHz range.   I found that by removing 3 turns from the 1 uH coils I could move the passband up to the desired range.  But why the discrepancy?  I was measuring the coils and the caps with an AADE meter.  I was testing the passband both with a NanoVNA and with a combination of an HP8640B sig gen and a Rigol oscilloscope (with the filter terminated into a 50 ohm resistor).  Any suggestions on why these filters should have passbands lower than predicted would be appreciated. 

Mythbuster Video #5 -- More Listening on 75

Mythbuster Video #4 -- First Signals, 75 meter Bandpass Filter, Yaesu VFO output

This receiver required almost no coaxing or tweaking, probably because I had been so careful about testing and measuring each of the stages. 

I have been pleasantly surprised at how well the receiver works without an RF amplifier ahead of the first mixer.  But I need to know how much AF gain I have in order to understand how/why the entire receiver works so well.  I think I have about 35 db of gain (combined) through the two TIAs and the crystal filter.  That would mean that all of the remaining gain is provided by the AF amplifiers (with some loss in the product detector).   I haven't really measured the gain of the AF preamp/LM386 combo, and I had some trouble measuring the input impedance of the pre-amp with the NanoVNA.    

The 75 meter LC filter to the left of the VFO is actually a bandpass filter, not the lowpass filter. And what I call "the mixer" to the right of the VFO is really the Product Detector/BFO.

For the 75 meter bandpass filter, I used the ELSIE program. 

75 meter Bandpass Filter designed in Elsie.  10 turns on a T50-2 toroid yield .46uH.

Here's the plot from Elsie on the 75 meter BP filter.  

Alan W2AEW asked for a picture of the VFO output. 

On this shot I had the probe between theVFO and the 

outboard booster amp that I built to bring it to 7dbm. 

Mythbuster Video #3 -- Using the VFO from a Yaesu FT-101

FT-101 VFO

I used LTSpice and Wes's FBA program (from LADPAC) to come up with a circuit that would provide the needed gain. I needed to get the 290 mV rms signal (across a 50 0hm load) up to the 500 mV rms signal needed by the ADE1 mixer.  Above is the amplifier that I came up with.  The key here is to adjust R2 and R1 to get the required gain. 

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. 

A Video Series on the Mythbuster 75/20 Rig -- Video #1

I am happy to report great progress on the Mythbuster project.  I have the receiver working on both 75/80 and 20 meters.  And it in fact inverts  the 75 meter LSB signals, turning them into 5.2 MHz USB signals for passage through my 5.2 MHz USB filter/BFO combo.  No switching or shifting of the BFO is needed. 

I am following Farhan's BITX20 advice -- I have paused in the construction and am enjoying the receiver that I have built.  I'll build the transmit circuitry later. 

Inspired by Frank Jones (you really should be reading the FMLA articles) I have this rig prototyped "Al Fresco" on a pine board that I found discarded on a neighbors front stoop.  

There is no RF amplifier in this rig.   Following the advice of multiple receiver gurus, I ran the BP filters right into the ADE-1 diode ring mixer.   I have the TIA amps set at about 24 dbm.  There is a lot of audio gain from the LM386 and the audio pre-amp.  This seems to be enough, even on 20.  I hear the band noise when I connect the antenna on both 75 and 20.  

Here is the first video in the series.   I'm posting them first on Patreon, then, a few days later, here and on the YouTube channel. 

The Woz on Homebrewing and The Right to Repair

There is a lot of wisdom and history in this Cameo video by Steve Wozniak.  

Thanks to Chuck KE5SPY for alerting us to this.  

Here a good BBC article that summarizes the Right to Repair issue and Woz's involvement.

https://www.bbc.com/news/technology-57763037

The Woz has The Knack.  

A Great Line from the FMLA Articles

The narrator was in CW contact on 6 meters with a guy who turns out to be... Frank Jones. 

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 I threw the switch on my AEA keyer and sent the station description while I deciphered my notes. When the keyer signed, Frank said: QSL THE COMMERCIAL GEAR - EVER THINK OF TAKING UP AMATEUR RADIO? QRQ?  

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More to follow.  

The complete FMLA collection can be found here: 

http://soldersmoke.blogspot.com/2021/07/summer-reading-for-homebrewers-frank.html

Romanian Mighty Mite and Ciprian's Knack Story

Another wonderful video from Ciprian in Romania. 

Any suggestions on his Mighty Mite power output problem? 

Here is Ciprian's YouTube channel. His intro video presents his Knack Story.  

https://www.youtube.com/channel/UCgaWKPt1P-4D6YI-Opd15_Q

I really hope Ciprian can get a license very soon.  His homebrew projects alone should qualify him. Ciprian has The Knack. It would be great if the IBEW (especially the European branch) could help Ciprian get some more parts and test gear. 

M0NTV's Latest Breadbox Rig -- The Radio Gods Have Spoken (TRGHS)!

Tony G4WIF sent me this video from Nick M0NTV.  It presents Nick's latest Bread Bin project -- "The Optimizer." 

-- I really like the Bread Box enclosures.  And leaving the b and the d on the box is just brilliant.  These letters now stand for BiDirectional!  They even appear symmetrical.  TRGHS!

-- The switch for a tuning tone is a great idea.  I still have to plug my Maplin AF sig gen into the mic jack to do this.  FB. 

-- I too have the connector on the back for keying the outboard linear amplifier.  (Shhh! Don't tell G-QRP!) 

-- As for the bidirectional TIA amps.  I'm really glad that someone else is using these circuits.  Wes's article came out in 2009 and concluded with a call for someone to build a complete rig with these circuits.  I wonder how many rigs like this have been made.  It is a great circuit.   One thing I would suggest for Nick:  Wes's article points out that you CAN have higher gain in one direction than you have in the other.  Just use resistor values in the chart provided in the 2009 article.  You could have an amp with 15 db in the transmit direction and 24 db in the receive direction.  BTW:  I have been getting a lot of help from Alan W2AEW and Farhan VU2ESE on how to use the NanoVNA to confirm the input and output impedances on solid state amplifiers. 

-- For many years I had the same map of the Moon in my shack.  I hope that map makes it to the new house Nick. 

-- Finally, I was really surprised to hear EI0CL calling CQ during Nick's demo of the receiver.  That is Michael Higgins out in Galway.   Michael was one of my regular contacts when I was out in the Azores.  He is a truly amazing guy.  He is mentioned frequently in my "SolderSmoke -- Global Adventures in Wireless Electronics" book.  TRGHS.