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Monday, October 16, 2023
Does Matching Matter? (Diode Matching for Diode Ring Mixers) -- Nick M0NTV Finds the Answer (Video)
Friday, July 28, 2023
Phase Noise and the Radio Amateur
http://www.sherweng.com/documents/TermsExplainedSherwoodTableofReceiverPerformance-RevF.pdf
Phase Noise: Old radios (Collins, Drake, Hammarlund, National) used a VFO or PTO and crystal oscillators to tune the bands. Any noise in the local oscillator (LO) chain was minimal. When synthesized radios came along in the 70s, the LO had noise on it. It is caused by phase jitter in the circuit, and puts significant noise sidebands on the LO. This can mix with a strong signal outside the passband of the radio and put noise on top of the weak signal you are trying to copy. This is a significant problem in some cases: You have a neighboring ham close by, during Field Day when there are multiple transmitters at the same site, and certainly in a multi-multi contest station. You would like the number to be better that 130 dBc / Hz at 10 kHz. A non-synthesized radio, such as a Drake or Collins, has so little local oscillator noise the measurements were made closer-in between 2 and 5 kHz.
http://qrp-labs.com/qcxp/
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DC4KU appears to be using the crystal filter method used by Hans:
https://dc4ku.darc.de/Transmitter-Sideband-Noise_DC4KU.pdf
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Martien PA3AKE has done a lot of great work on this topic. See:
https://martein.home.
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Thursday, April 13, 2023
The Franklin Oscillator: A Super-Stable VFO. Why No Attention? Why So Little Use?
Lee KD4RE of the Vienna Wireless Society has been talking about the Franklin oscillator. He has been telling us that it is very stable, and capable of stable operation up through the ten meter band. Lee wants to build an direct conversion receiver for all of the HF bands with one of these circuits.
I was skeptical. First, I'd never heard of this circuit. I'd grown up in ham radio on a steady diet of Hartley and Colpitts and Pierce. Vackar or Clapp were about as exotic as I got. And second, I'd come to accept that it is just not possible to build a good, stable, simple, analog VFO for frequencies above around 10 MHz. For example, in his Design Notebook, Doug DeMaw wrote, "VFOs that operate on fundamental frequencies above, say, 10 MHz are generally impractical for use in communications circuits that have receivers with narrow filters." DeMaw was known for resorting to variable crystal oscillators.
But then this month Mike Murphy WU2D put out two videos about his use of the Franklin oscillator circuit in a direct conversion receiver at 21 MHz. The VFO was shockingly stable. I began to believe Lee. I fired up my soldering iron and built one.
Lee was right, it is in fact remarkably stable, even at higher frequencies. My build (see picture above) was a bit slap-dash and could be improved a bit, but even in these circumstances here is what I got. This was with a stable 6 Volt Supply and with only a cardboard box covering the circuit:
Local time Frequency
0543 19.1114 MHz (cold start)
0636 19.1116
0804 19.1117
1034 19.1118
1144 19.1117
I started digging around for references to the Franklin. There was nothing about it in Solid State Design for the Radio Amateur, nor in Experimental Methods of RF Design. Pat Hawker G3VA (SK) did discuss it in his Technical Topics column in RADCOM, February 1990. Pat gave a great bio on Charles S. Franklin (born in 1879 and a colleague of Guillermo Marconi). But tellingly, Pat writes that, "Despite its many advantages, the Franklin oscillator remains virtually unknown to the bulk of American amateurs."
It wasn't always unknown. In the 1940s, we see articles about the Franklin oscillator circuit. There is a good one in the January 1940 issue of "Radio." The author W6CEM notes that the circuit "is probably familiar to only a few amateurs." It shows up in the "How's DX" column (above). And the 1958/1959 issue of Don Stoner's New Sideband Handbook we see a lengthy description of the Franklin oscillator. Stoner wrote: "The author's favorite oscillator is the 'old time' Franklin, and it is believed to be the most stable of them all! This rock-solid device can put a quartz crystal to shame! Because it represents the ultimate in stability, it is the ideal VFO for sideband applications." And we see a PTO-tuned Franklin oscillator in the July 1964 QST. And it is in the fifth edition of the RSGB Handbook (1976).
Monday, April 3, 2023
Technology and Methods from Wes Hayward W7ZOI
http://w7zoi.net/oldtech/ponder.html
Thanks to Tony G4WIF for spotting this gem and alerting us. And thanks to Wes for all of this.
Monday, August 8, 2022
Polyakov (RA3AAE) Direct Conversion Receiver: 40 meter DC RX with VFO at 3.5 - 3.6 MHz (with video)
I've been reading about Polyakov (or "sub-harmonic") Detectors for a long time:
https://soldersmoke.blogspot.com/search/label/Polyakov--Vladimir
But until now, I never built one. Recently, Dean KK4DAS and the Vienna Wireless Makers group have been building a Direct Conversion receiver. Their receiver uses an Si5351 as the VFO, but of course Dean and I have decided to try to do things the hard way by building non-digital VFOs. At first we just came to the conclusion that my earlier Ceramic Resonator VFO wasn't much good (it drifted too much). This led us into standard Colpitts and Armstrong VFOs, and the fascinating world of temperature compensation. Then I remembered the Polyakov circuit -- this would allow us to use a 3.5 MHz VFO on the 7 MHz band. Lower frequency VFOs are easier to stabilize, so I started building my first Polyakov receiver. You can see the results (on 40 meters) in the video above.
I started working with a circuit from SPRAT 110 (Spring 2002). Rudi Burse DK2RS built a Polyakov receiver for 80 and 40 that he called the Lauser Plus. (Lauser means "young rascal" or "imp" in German.) For the AF amplifier, I just attached one of those cheap LM386 boards that you can get on the internet. With it, I sometimes use some old Iphone headphones, or an amplified computer speaker.
Saturday, May 28, 2022
"Experimental Methods in RF Design" LADPAC Software Available FREE!
We've frequently said that is pays to check the W7ZOI web site. Tony G4WIF did just that and pointed us in that direction, noting that the LADPAC "Ladder Package" software is now available for download from that site.
Homebrewers will really want to have that package on their computers. There are all kinds of useful programs in that package: software for designing crystal filters and feedback amplifiers, a program that allows you to think systematically about receiver gain distribution and dynamic range, and many other useful things.
You can get the program here:
http://w7zoi.net/emerrata.html
Also on Wes's site is this May 22, 2022 picture of Farhan VU2ESE with EMRFD co-author Bob Larkin, W7PUA.
Thanks Wes!
Tuesday, December 28, 2021
How to Fix the Spur Problem in my 17 Meter SSB Transmitter?
In the 2004 QST article I discuss a problem I had with "spotting" or "netting." This is something of a lost art, something that you had to do back in the pre-transceiver days, when running a separate transmitter and receiver. This was how you got the transmitter on the receiver's frequency. Essentially you would turn on the carrier oscillator and the VFO and let a little signal get out, enough to allow you to tune the VFO until you heard zero beat on the receiver. My problem was that around one particular frequency, I would hear several zero-beats. This made netting the receiver and the transmitter hard to do.
Important note: This is really just a problem with the "netting" or "spotting" procedure -- the problematic spur does not show up in any significant way in the output of the transmitter. I can't see it on my TinySA. But it is strong enough to be heard in the unmuted receiver sitting right next to the transmitter. And that creates the netting problem.
In the QST article, I said that I noticed that the problem seemed to be centered around 18.116 MHz. As I approached this frequency, the tones -- desired and unwanted -- seemed to converge. That was an important clue. In the article I said I thought that I could eliminate the problem with just one trimmer cap to ground in the carrier oscillator, but looking back I don't think that this really fixed the problem.
I recently took a fresh look at it. Exactly which frequencies were causing the unwanted signals that appeared in my receiver?
I used an Excel Spread sheet to find the culprits.
Tuesday, November 2, 2021
Farhan's Amazing Knack Story: From a Boyhood SW Receiver to the Design of the sBITX SDR
-- Farhan talks about his practice of taking the pictures of new rigs with the new rig sitting atop the book that was most important in its design and construction. FB.
-- I was really blown away by Farhan's presentation of how the uBITX advertisement was inspired by and in many ways based on the Heathkit ad for an HW-101. Amazing.
-- I learned a lot from Farhan's discussion of SDR theory. I pledge to spend more time with this. I really like Farhan's hybrid HDR/SDR approach.
-- But I have a question: Farhan seems to say that we'd need a big expensive GOOGL computer to do the direct sampling HF SDR. But doesn't the little RTL-SDR do just that? Without a GOOGL?
-- Great to see Wes's AFTIA being used in the sBITX.
-- Really cool that Farhan has his mind on VHF transverters when designing the sBITX. I liked use of the TCXO module to free up one of the Si5351 clock outputs. FB. And great to include an idea from Hans in this rig.
Thanks very much to Farhan (who stayed up until 3 am to do this!) and to the RSGB for hosting.
Friday, May 28, 2021
Farhan's sBITX -- Combining SDR with the Traditional Superhet
Monday, November 30, 2020
Adam N0ZIB's Direct Conversion Transceiver
This is obviously very cool, but looking ahead I think Adam should think about adding one more mixer, changing the bias on the TX amps, and adding a mic amp. Boom: A Double Sideband Transceiver.
Pete wrote: When I was in the US Navy and a particular unit did something outstanding – the Command ship would raise the Bravo Zulu Flag for a job well. Don’t know if you can see it there in MO but I have raised the BZ flag to you. Outstanding and congratulations.
Bill and Pete:
Just finished a DC transceiver using Arduino nano, SI5351 (my sincerest apologies, Bill), diode ring mixer and lm386 audio amp. The transmit portion is a two-stage class AB pre-amp (from EMRFD page 2.32), which is driving an IRF510 final (biased at 2.08 volts) from Pete’s design. Output is about 5watts into a CWAZ low pass filter, based on the design from here: https://www.arrl.org/files/file/Technology/tis/info/pdf/9902044.pdf
I’m using a manual TX/RX switch which is doing multiple things. It brings the Nano A1 LOW, offsetting the transmit frequency 600 Hz for CW, grounds the audio input to prevent deafness (learned that one the hard way), and it engages a relay that switches the antenna from the receiver to the transmit, and also turns on the transmitter stages. Keying is through the first stage of the pre-amp.
I still have some tidying up to do, and I’m not sure the LPF works so well using two component inductors instead of all toroids, but I finished it today and made my first QSO into Ontario almost 1000 miles away. It’s been great fun!
73,
Adam
N0ZIB
Missouri
Thursday, October 15, 2020
Too Simple? Deficiency of the Lafayette HA-600A Product Detector?
I've been having a lot of fun with the Lafayette HA-600A receiver that I picked up earlier this month. Adding to the mirth, I noticed that on SSB, the signals sound a bit scratchy, a bit distorted, not-quite-right. (I'm not being facetious; this is an interesting problem and it might give me a chance to actually improve a piece of gear that I -- as a teenager -- had been afraid to work on.)
Before digging into the circuitry, I engaged in some front panel troubleshooting: I switched to AM and tuned in a strong local AM broadcast signal. It sounded great -- it had no sign of the distortion I was hearing on SSB. This was an important hint -- the only difference between the circuitry used on AM and the circuitry used on SSB is the detector and the BFO. In the AM mode a simple diode detector is used. In SSB a product detector and BFO is used. The BFO sounded fine and looked good on the scope. This caused me to focus on the product detector as the culprit.
Check out the schematic above. Tr-5 is the product detector. It is really, really simple. (See Einstein quote below.) It is a single-transistor mixer with BFO energy going into the base and IF energy going into the emitter. Output is taken from the collector and sent to the audio amplifiers. (A complete schematic for the receiver can be seen here: https://nvhrbiblio.nl/schema/Lafayette_HA600A.pdf )
I had never before seen a product detector like this. One such detector is described in Experimental Methods for RF Design (page 5.3) but the authors devoted just one paragraph to the circuity, noting that, "We have not performed careful measurement on this mixer." The lack of enthusiasm is palpable, and probably justified.
A Google search shows there is not a lot of literature on single BJT product detectors. There is a good 1968 article in Ham Radio Magazine: http://marc.retronik.fr/AmateurRadio/SSB/Single-Sideband_Detectors_%5BHAM-Radio_1968_8p%5D.pdf It describes a somewhat different circuit used in the Gonset Sidewinder. The author notes that this circuit has "not been popular."
To test my suspicion that the product detector is the problem, I set up a little experiment. I loosely coupled the output of a signal generator to the IF circuitry of the HA-600A. I put the sign gen exactly on the frequency of the BFO. Then, I switched the receiver to AM, turning off the BFO and putting the AM diode detector to work. I was able to tune in the SSB signals without the kind of distortion I had heard when using the product detector.
So what do you folks think? Is the product detector the culprit? Or could the problem be in the AGC? Should I start plotting a change in the detector circuitry? Might a diode ring work better?
Monday, August 24, 2020
Experimental Methods Book on Clearance sale -- $20
And it includes a .pdf copy of Solid State Design for the Radio Amateur.