Building Farhan's PTO -- But Is This Really a PTO?

Farhan has a PTO in his Daylight Again transceiver.  This caught the attention of homebrewers around the world.  Walter down in Orlando built one.  Dean KK4DAS and I have built versions also. Dean has a 3D printer and made the coil support used in my version (above).  Thanks Dean! 

I had a little trouble at first because one of the capacitors I had in there was not NP0, so the device drifted a lot.  But once I straightened that out (I put in a Silver Mica cap in its place) the oscillator became very stable. Mine moves about 25 kHz with each turn of the bronze screw -- this is nicely linear tuning.  But I think I will have to use a San Jian counter to keep track of the frequency. The long plastic tube on mine is there to eliminate had capacitance effects. 

One of the guys in the Vienna Wireless Society questioned whether we should really call this a PTO.  After all, bronze has a permeability equal to that of free space. So we are not really changing the permeability.  Frank Harris points out that the bronze screw really acts as a shorted secondary.  What do you guys think?   Is this a PTO, or do we need another acronym to describe it? 

This has been a really fun project.  I never built an oscillator like this before.  I will probably follow up by building the rest of the Daylight Again rig. 

BTW  WA6OTP has a nice PTO design: http://www.wa6otp.com/pto.htm

Thanks again to Dean, and to Farhan.  

The Cure for Frequency "Bunching Up" in Analog LC VFOs -- It Is Not So Simple. But we have a good calculator. Comments sought!

Bob's calculator shows good tuning linearity with an ordinary SLC capacitor 

One of the complaints about analog LC VFOs is that they have non-linear tuning -- when you turn the dial (usually attached to a variable capacitor) the space between frequencies is NOT constant.  This is especially apparent at the high end of the frequency scale where frequencies (and stations) appear to be severely bunched together, making tuning difficult.  This problem contributes to the defection of some great homebrewers:  They give up on LC VFOs and they switch to digital VFOs.  Sad. 

But there is hope:  Not all LC VFOs tune this way.  Even on rigs from "back in the day," back when the Si5351 hadn't even been thought of,  some LC VFO rigs tuned linearly.  My beloved Drake 2-B and my almost equally beloved HT-37 are good examples.  How did they do this?  How did they escape the dreaded "bunching up?"  

For a while, I thought that it might have had to do with the use of the series tuned Clapp circuit.  But on further noodling, this didn't seem to make much sense. 

Then -- like others -- I thought that it  must be caused by the adroit use of special capacitors. You see, in ordinary variable capacitors when you turn the dial, the capacitance increases linearly.  But in the LC circuit, frequency changes as the inverse of the square of the capacitance. Thus the bunching up. So the solution must come from the use of the special capacitors that compensate for this, that -- because of the shape of their plates --  produce linear tuning.  With these variable caps, frequencies on the dial are spaced out nicely, there is no bunching up. Great right?  

From Terman, Radio Engineers Handbook, 1943, page 123

Over the years, many hams have jumped to the conclusion that rigs with good tuning linearity MUST be using these special caps. For example, in 2013 a ham posted in the Antique Radio forum this message: 

There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-circle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...

SLF and SLW caps have oblong plates.

The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP series of receivers where the ham bands are very
compressed at one end of the tuning range. They used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.

Well, not really.  

-- I now have several VFOs from the extremely linear-tuning FT-101.  But when you open them up to look at the tuning capacitor, it is NOT a Straight Line Frequency capacitor.  

-- Many of us over the years have built VFOs that are quite linear in their tuning without resort to these special capacitors -- we did it with ordinary Straight Line Capacitance caps.

-- When you look at the "How to build VFO" literature in the ham radio books, you see a lot of good recommendations about using solid, brass-vaned caps with ball bearings at either end.  But never do you see circuits that require the use of SLF or SLH capacitors. If they were the key to tuning linearity, we'd see them mentioned in the literature. But we don't.   

So where does the linearity -- or bunching up -- come from?  

The answer comes to us from a really neat calculator from Bob's Electron Bunker: 

http://electronbunker.ca/eb/BandspreadCalc.html

This calculator allows you to select your frequency range, and the tuning range of your variable cap. It then displays for you what the tuning range will look like on your dial.  You can see if there will be bunching up, or if the frequencies will be nicely spread out.  And -- and this is the really cool part -- you can then specify if your capacitor is SLF, SLW, SLC or Midline-Centerline.  This really illustrates the effect of the different capacitor types. 

I used Bob's calculators to do some experiments with various types of capacitors, various frequency ranges, and various combinations of trimmers and padders.   You can see what I did here: 

http://soldersmoke.com/VariableCapsSLCSLF.pdf

One important thing to keep in mind:  The SLF caps were made for AM broadcast receivers that were tuning from 540 to 1600 kc.  That is a 3:1 tuning range.  Most of the time in HF ham radio, we are tuning across a much smaller range, say from 5 MHz to 5.5 MHz.   That is a 1.1:1 tuning range. In those cases where we ARE tuning across a wide tuning range -- for example with a receiver covering 3-9 MHz, the SLF cap can help prevent the bunching up. 

But we can have fairly good linear tuning without resort to SLF caps.  Bob and his calculator point out that by narrowing the frequency range of interest, and by using either smaller range caps (ordinary SLC caps), or SLC caps with trimmers and padders, we can achieve tuning linearity.  And sometimes, when you have achieved this nice tuning linearity with a plain SLC cap, putting a fancy SLF cap makes tuning linearity worse. 

One piece of VFO tribal wisdom that is confirmed by all this:  It is better to use a smaller variable cap with a maximum capacity of about 30 picofarads. 

I think we should spend as much time focusing on VFO tuning linearity as we do on VFO frequency stability.  Bob told me that in the old days, the calculations for various tuning linearity scenarios were difficult.  But now we have Bob's calculator.  When building a VFO, just use Bob's calculator, plugging in the numbers to get a preview of what your tuning linearity will be like. If it is bunched up, you can play with the trimmer and padder values to achieve the tuning linearity you desire.   

Thanks to Bob of Bob's Electron Bunker for this great calculator. 

You can see another discussion of "bunched-up" tuning in the comments section of this article: https://www.nutsvolts.com/?/magazine/article/may2015_Whipple

What do you folks think of this?   Please put comments below. 

Why Do Some VFOs Tune More Linearly Than Others?

This has been one of the major complaints about our beloved analog LC VFOs:   The frequency tuning on these circuits is often not linear.  For given amount of VFO frequency dial turn you can get vastly different changes in frequency.  At one end of the tuning range the frequencies are nicely spaced and tuning is easy.  But at the other end of the tuning range all of the frequencies are bunched together.  This is one of the problems that leads some homebrewers to defect to the sad land of "digital VFOs." 

But wait.  It appears that the old designers found a solution to this problem.  Just look at the tuning dial of my HT-37.  The frequencies are all spaced out evenly.  How did they do that? 

I had been thinking that this success may have resulted from Hallicrafters' engineers using the series-tuned Clapp circuit.  Here the main frequency determining element is a series-tuned LC circuit and not the parallel tuned LC circuit that we see in the more commonly used Colpitts circuit.  

But hold on -- how could that be?  The frequency bunching problem that we attributed to the Colpitts circuit must also exist in the Clapp, right?  I went back to SSDRA where there was a good discussion of Colpitts and Clapp VFOs.  The advantage of the Clapp was said to be in its use of a larger value coil which helped minimize the effects of stray inductances.  But there was no mention of the Clapp offering improved linearity in tuning.  

I have in front of me two transceivers:  The Mythbuster uses a 9 MHz Clapp circuit (see below).  The 17-12 rig uses a Colpitts Circuit.  I checked the tuning linearity of both.  Both appeared quite linear in tuning, with no real difference between the two.  

Then I looked at the tuning capacitor in the Mythbuster 17-12 rig.  It came out of an old Hallicrafters transmitter, probably the HT-44.  I looked closely at the stator and the rotor plates.  Both are curved.  I'm guessing that this may yield a more constant change in capacitance for a given movement of the main tuning dial.  

Next I opened up the VFO on the Mythbuster.  (It is the VFO from an old Yaesu FT-101.)  I couldn't see the stators very well but it appears that their shape is different from the square shape we often see in variable capacitors.  Could it be that this variable capacitor was also made to provide linear tuning?  

Back in 2013 Norm Johnson wrote about all this in the Antique Radios.com forum: 

A capacitor that has uniform increase in capacitance with rotation will have the stations at the high end of the band squeezed together. Another type known as the straight-line frequency variable capacitor has, as you might guess, a characteristic that gives even spacing of frequencies with shaft rotation. These were popular in the 1920's but weren't very good for superhets where you needed to have a dual section capacitor that would tune both the RF and local oscillator, and have them track each other properly. The midline variable capacitor is more compatible with a superhet, and easier to make both sections track properly. This is the type that you see in most receivers from the late 1930's to the end of the tube era. They don't have quite the equal spacing between stations across the band that the old straight-line frequency caps had, but they're much better than the variables that change capacitance linearly with rotation.

I wrote an online calculator that helps in the design of the tuning. It shows what frequency range you'll get with a specific type of variable capacitor, including the effects of padder and trimmer capacitors. It also displays a dial scale that shows how the frequencies are lined up accross the dial.
http://electronbunker.ca/eb/BandspreadCalc.html

Steve W6SSP also provided some really good info back in 2013: 

There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-circle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...

SLF and SLW caps have oblong plates.

The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP series of receivers where the ham bands are very
compressed at one end of the tuning range. They used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.
Steve W6SSP

These two variable caps came out of my junkbox.  Both are Eddystones, made in England.  My guess is that the one on the left is SLF.  But could the one on the right (out of an old regen) be SLW?   

The Drake 2-B also has perfectly linear tuning.  I looked at the manual:  "The tuning condenser is of special design..."   I'm guessing that they used an SLF variable capacitor. The 2-B had no need for ganged capacitors -- the "preselector" was tuned via a separate front panel control. 

I looked at the tuning dials on my Hammarlund HQ-100 receiver.   It is fairly linear in its tuning, but not as linear as the HT-37 or the Drake 2-B; on all of the tuning ranges the frequencies seem to spread out a bit at the lower end.  My guess is that Hammarlund used the midline variable described above by Norm Johnson.  The HQ-100 did use a ganged variable cap, with one section tuning the RF amplifier and the other tuning the local oscillator. 

Mythbuster on the bottom.  17-12 rig on the top

Where Do You Think This Variable Capacitor Came From? What Piece of Gear Did it Come out of? Is it in the Old Catalogs?

 

In an effort to stop the slaughter of innocent Heath QF-1s (for their very nice variable capacitors), I bought this thing on e-bay.  Here is the info I have: 

https://www.ebay.com/itm/255622418189

I checked the resonance of the cap and the coil:   It tunes from about 10.140 MHz to about 13 MHz.   The cap is nice -- brass vanes and with the linear tuning shape.  I don't know about it being from a Hartley oscillator -- no signs of a tickler coil.  There seems to be some sort of a pass-through cap from "Faradon" of Camden N.J.  

Update: 

It tunes from about 9 to 138 pf. The coil that it had on the back of the cap is about 1.6 uH and is tapped (Hartley style). The capacitor is marked on the back: "Licensed Under Pats. MC 15257781258423"

From the e-bay ad: 

"Here's a great project part.  These were likely a VFO assembly for some RF equipment from the late 1920's or 30's.  Essentially it's 4" Type A Velvet Vernier dial (standard 5:1 reduction drive) with a variable capacitor.  The variable condenser is a high quality ceramic frame with brass plates and looks suspiciously like a General Radio house part (also in Cambridge at that time). The variable condenser looks like a SLF shape or similar, about 135pF.  It has a ceramic frame and has an insulated shaft, which was critical to keeping hand capacity down back then.  It appears to be wired for a Hartley oscillator, probably 10 MHz+ give or take (not measured).

The vernier is in excellent condition--smooth, without backlash or sticking.  There are no chips in the skirt and the numbers and nice and clear--these were obviously well cared for over the years.  They have the the hard-to-find dial markers!

There is a rigid metal panel to which the vernier assembly is attached, and this in turn supports the condenser frame.  I suppose you could just that in a project box with an appropriate sized hole for the shaft and have an instant shielded enclosure for a VFO, or maybe screw it to the side of a breadboard.  You can also remove the vernier assembly from the panel and use it separately (e.g., to fix up an old NBD).  Lots of possibilities.  

There is a loose sheet-metal collar behind the rigid panel that friction-fit a (now missing) can that went over the whole assembly. It's easy enough to unscrew the front and remove this piece.  Everything comes apart if you want, including the unobtainium dial pointer."

So where do you guys think this very nice cap came from?  Who made it?  Was it salvaged from a piece of radio gear?   Does it appear in any of the old catalogs?  

Solid-Stating an HT-37 VFO -- Advice Needed

 

Original HT-37 VFO Circuit

A couple of things before I start:  

First, this is not my fault.  The Radio Gods are to blame.  I innocently tried to by an HT-37 tuning capacitor on e-bay, but the seller sent me the entire VFO unit.  The only thing missing was THE TUBE. Clearly, that was a sign, right?  

Second, this is a work in progress.  That is why my diagram (below) is a bit ugly.  I am looking for your input and advice on how I might do this better.  I will understand if religious principles prevent some of you from participating in this endeavor. 

I am trying to solid-state this device WITHOUT major surgery, and without adding any reactive components that would change the resonance or tuning range of the original.  The original circuit tunes from 5 to 5.5 MHz and that is fine with me.  

I started out by just sticking a J-310 FET into pins 1, 2, and 5 of the tube socket.  I put 12 V on the drain and the thing oscillated right where it is supposed to.  That was a good sign.  

Here is what I have done so far: 

Bill's initial solid state conversion of HT-37 VFO

Mechanically, my effort has been very simple.  At first I tried to fashion a more serious male socket for the FET using two broken 7 pin tubes.  This didn't work well.   

So then I just ran three short wires up through the center hold of the tube socket to the connections for pins 1,2, and 5.   I superglued the J-301 to the chassis and made some non-reactive connections: I put a 47 ohm resistor on the source,  and a 220 ohm resistor on the drain. I grounded the drain for RF with a .01 uF cap to ground.    I added a 100k resistor and a diode on the gate.   Oh yea, I put a couple of ferrite bead on the FET gate lead.  (See pictures below.)

Three lead up through the center hole

A rare look inside an HT-37 VFO

The original thermatron circuit has an output bandpass transformer, a 3900 ohm resistor and a coupling cap.  I left them in the circuit, but they are not doing anything. 

The output from the source of the FET looks pretty good.  I can see some VHF on the trace, but I suspect this is from my FM broadcast nemesis at 100.3 FM (one mile away).  On a receiver, I can hear some AF noise on the signal, but this may be the result of the RFI from THE BIG 100 -- WASHINGTON'S CLASSIC ROCK. 

So what do you folks think?  What else could I do, or should I do?  

Daylight Again on the Sunrise Net! Walter KA4KXX Builds a PTO

Dear Bill: 

I had never built a PTO, but after reading Farhan's Daylight Again Transceiver article I cobbled one together with parts and pieces I had on hand. My observations so far are as follows. 

1. The frequency-determining capacitors (shown on the schematic as three 470 pf) are very critical, so I feel the usual experimental cut-and-try technique is a must, even using all NP0 and C0G types. 

2. After I built the 2 MHz version like the article (see first photo), the stability was terrific, but when I tried building the companion Daylight Again crystal filter, I was only able to get a bandwidth of 1.6 kHz, which is too narrow for my taste in an SSB radio, so I decided to build the same filter design but with 11 MHz crystals, where I could easily achieve a 2.8 KHz BW. 

3. Therefore, now I needed a higher frequency VFO, so I merely reduced the capacitance (from about 1200 to 370 pf) without changing the coil and I am very impressed with the performance of my 4 MHz PTO (see second photo). The bandspread easily covers the entire 40M band, CW and Phone. 

4. However, whenever I transmit on the 40M Phone band, I like to first set my VFO within 10 Hz of the operating frequency. That way, if I talk for five minutes or so and get up to 15 Hz of drift (which is quite common with many radios when I operate portable outdoors in the sun and wind), it will not be noticable and I avoid receiving any "you are off-frequency" chastising. But the shortcoming I have with this PTO inductor is that the 1/4-20 bolt has a coarse thread, so it is very difficult for an old fellow like me to get within even 20 Hz of a particular frequency just using this common bolt. Therefore I believe a better choice would be the fine thread 1/4-28 two-inch brass threaded bolt which is available from industrial supply houses like McMaster-Carr. However, for CW use or those with a very steady hand, the 1/4-20 works well enough. 

5. I solved my fine tuning problem by adding a varactor circuit using a common 1N914 diode in series with a 100 pf capacitor, operating from 0 to 6 volts. Another advantage to adding this feature is that since I have not so far enclosed my PTO, I can mount the varactor potentiometer several inches from the PTO so my hand capacitance does not affect the frequency like when tuning with the bolt. 

6. An easy way to "do the math" in my case with the common Sanjian counters is to simply create a small lookup table listing half a dozen common frequencies and stick it on the radio. For example, 90% of the time in the morning I am tuned to my favorite SouthCars Net frequency of 7251, so using a BFO setting of 10,999.900, I simply set the PTO to 3,748.90 on the 6-digit 10 Hz resolution counter I normally use (see third photo). 

7. I am currently using my Daylight Again PTO on a daily basis with an NE602 receiver, and I am thinking of adding a locknut to the bolt so it does not wiggle when I jostle or move the radio, essentially giving me a crystal replacement oscillator that I can use for any single 40M frequency. To date I have been able to listen for hours at a time indoors without even any touch-up of the varactor fine tuning. 

8. Also, if continuous frequency readout is desired without building a noise filter circuit board, a separate power supply for the counter is a solution. For portable operation I use Lithium Polymer radio control model airplane batteries which are light, small, and cheap, so one 12V 2000 mAH battery for the transceiver (allows a half hour of transmitting at 15 watts) and a much smaller 12V 350 mAH battery with a series resistor to reduce the current and brightness of the counter has worked well for me. 

73, Walter KA4KXX 

Orlando, FL

What Coil for the Polyakov Input Circuit? How to calculate a coil value for resonance.

So,  what is the value for L1 and L2?   What coil should I use?  

Michael AG5VG had that question.  And so did I when I built this receiver.  See below for the process I used in answering this question. 

On Tuesday, August 9, 2022 at 10:53:32 PM EDT, Michael S  wrote:

Good Evening Bill,

My name is Michael and I really enjoy your podcast with Pete. I have also spoken with him in regards to the design of a 20M bandpass filter I made for a homebrew rig.  I am currently in the process of making a 20 meter DSB - SC type. Thank you for all the information that you speak and teach about during your podcast. I also enjoy the humor. It's great.

The Polyakov is a simple DC receiver and it amazes me and how the sound quality is. My question is, what is the turns on the toroid for the antenna primary side and the radio secondary and how did you figure out the turns because looking at the schematic it doesn't give that information that I can see. Also how you resonated it with the variable capacitor that looks like a 365pf air variable. 

Thank you for your time and keep up the great work on the podcast and the content on YouTube.

73s,

Michael

AG5VG

My response: 

Good questions Michael.   When I saw the SPRAT article I too was struck by the fact that it didn't give a value for the coil.  But DK2RS did have a large value variable capacitor... And he was billing this as a dual-band (80-40) rig.  So I figured he wanted that LC circuit to resonate as low as 3.5 MHZ and as high as 7.3 MHZ.  So, with a variable cap that goes up to 350 pf, what value L should I use?   I started by calculating the resonant frequency of the frequency mid-way point: 5.1 MHz.   I figured the variable cap should be around 162 pf at the mid-way point.  At this point I went to the on-line resonant frequency calculator: https://www.1728.org/resfreq.htm (a REALLY useful site!).   This site revealed I needed a coil of about 6 uH.  This put me in the ballpark.   But then -- with the site -- I tested it with the values of the variable cap I had on hand.  Mine was 23pf to 372 pf.  (you really need an LC meter to do this kind of thing). 

Again at the resonant freq calculation site:  23 pf and 6uH = 13.5 MHz           372 pf and 6 uH = 3.3688 MHz

This would have been OK, but I wanted to move the frequency range down a bit, so I tried. 6.5 uH 

23 pf and 6.5 uH = 13 MHz                 372 and 6.5uH = 3.23 MHz

Now, how many turns?  First look at the overall coil -- don't worry about taps at this point.    I use the Toroid Turns Calculator: http://toroids.info/

 Start by asking yourself "What core do I have on-hand?    Let's say you have a T-50-2 (red/clear).   The calculator shows you need about 36 turns.  Do-able, but physically kind of tight.  

I found a big core in my junk box.  A T-106-2.  The calculator showed I'd need about 22 turns on this core. It was much easier to get these turns on the larger core.  

You have to measure the core after you wind it to make sure you are at the desired inductance.   One side of the main coil went to ground, the other side to the top of the variable cap. 

 Now for the taps and secondaries:  The schematic shows a tap.  This is usually about 1/4 of the number of turns up from ground.   I picked about 5 turns, and wound a little tap in there at that point -- that tap went to the antenna.  You also have a secondary coil --no value is given, but based on experience I guessed around 5 turns -- I wound these turns on top of the primary one lead went to ground, the other went to the diodes and the switch. 

 The last thing to do is to see if the circuit resonates on both bands that you want to receive.  You can do this with a signal generator, or with the band noise:  Hook up an 80 meter antenna.  Put the cap closer to its max value and tune the cap -- can you hear band noise?  Or can you hear (or see on a 'scope) a signal at 3.5 MHz?   You should be able to peak it with the main cap.  Try to do the same thing on 40 meters -- here the variable cap should be closer to minimum capacity. 

 That's it.  That's how I did it.   You can do it too!   Good luck with the Polyakov.  

 One hint:  Building the VFO is the hard part.  You can get started by using a signal generator in place of the VFO.  Just make sure you have the level right -- around 620 mV input.  

 Good luck -- Let us know if you have trouble. And please let us know how the project goes.   

 73    Bill    N2CQR  

20 meter rig built by AG5VG

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. 

The Polyakov mixer is a "switching mixer."  The book excerpt below shows how I understand these circuits.  The enlightenment came from the Summer 1999 issue of SPRAT (click on the excerpt for an easier read): 

Leon's circuit shows us how a simple switching circuit in which the switches are controlled by the VFO can result in an output that has the sum and difference components. That is the hallmark (and most useful part) of real mixing.  Remember -- we say that mixing happens in non-linear circuits when the passage of one signal depends on what is happening with the other signal.  A switch is as non-linear as you can get! And that switch is being controlled by the VFO.  

In a Direct Conversion receiver we usually run the VFO at the operating frequency. This results in audio just above and just below the operating frequency. 

The Polyakov Direct Conversion circuit is a bit different.  It has the switches (the diodes)  turned on twice each cycle:  When the VFO voltage goes to a positive peak, this turns on one of the diodes.  When the VFO goes to a negative peak, this turns on the other diode.   So in effect the switch is being turned on TWICE each cycle.  So with the Polyakov you run the VFO at HALF the operating frequency.  For a DC receiver designed to run around 7.060 MHz, you build a VFO at around 3.53 MHz.  This has some immediate advantages.  My favorite is that it is easier to get a VFO stable at a lower frequency.  It is easier to stabilize a VFO at 3.53 MHz than it is at 7.060 MHz. 

When you open that SW 1 switch in the Lauser Plus, you no longer have a Polyakov mixer.  Now you just have a diode mixer.   It will be opening and closing once each cycle at the VFO frequency.  DK2RS used this to cover not only the 40 meter band (in Polyakov mode) but also the 80 meter band (in single diode detector mode).  That is why DK2RS has that big variable capacitor in the input circuit -- that LC circuit needs to tune all the way down to 3.5 MHz and all the way up to around 7.3 MHz.  (I used a coil of about 6.5 uH to do this.) 

With just one diode and operating at 80 meters, it works, but not as well as it does in the Polyakov mode on 40.  I can pick up 80 meter signals, but in this mode there seems to be more of an "AM breakthrough" problem. "Experimental Methods in RF Design" on page 8.11  describes what is going on (the last sentence is most relevant here): 

Here are some very good links with information on the Polyakov receiver: 

LA8AK on Polyakov: http://noding.com/la8ak/c21.htm

LA8AK's Home Page: http://www.agder.net/la8ak/index1.htm

LA8AK SK: http://www.agder.net/la8ak/   Almost seventeen years after his death he continues to help his fellow radio amateurs through his web sites.  TNX OM!  FB! 

BH1RBG: https://sites.google.com/site/linuxdigitallab/rf-ham-radio/dc-polyakov-based-first-dc-receiver

Hack-A-Day on Polyakov: https://hackaday.com/2015/11/15/polyakov-direct-digital-synthesis-receiver/

I will post a video tomorrow showing the receiver in operation on 80 meters.  

Three cheers for Vlad Polyakov, RA3AAE

SolderSmoke Podcast #239: Hex DX, VFO Temp Comp, DC RX, Polyakov!, DX-100, Wireless Set, Farhan's "Daylight Again" HDR rig, MAILBAG

N2CQR Hex Beam Aimed at Europe

SolderSmoke #239 is available for download: 

http://soldersmoke.com/soldersmoke239.mp3

TRAVELOGUE: 

James Webb Space Telescope.  Mars returning to opposition in early December.   

BILL'S BENCH

Hex Beam K4KIO - on roof – TV Rotor – 20-17-12  Lots of fun.  Working Japan regularly, Australia, South Africa on long path 17,000 miles.  52 countries SSB since July 11.

VFOs and Temp stabilization.  Dean KK4DAS found my ceramic resonator VFO for DC receiver drifty. He was right.  So I built a real LC Colpitts VFO.  Got me into temp stabilization.  A new hobby!  An obsession.  HT-37 and Ht-32 parts. Ovens?  WU2D’s second VFO video.  Understanding thermal drift and how to address it. Split stator caps.  Cut and try.  

Built a Polyakov DC Receiver. https://soldersmoke.blogspot.com/2010/03/polyakov-plus-dual-band-receiver-with.html  Lauser Plus.  Lauser = Imp or Young Rascal!  DK2RS.  He used a ceramic Resonator VXO at 3.58 MHz.   Mine works great on 40 with VFO running 3.5 -- 3.65 MHz. See schematic below. 

On 40 AM with DX-100 and MMMRX.  DX-100 died.  12BY7 VFO buffer went bad.  How common is failure in this tube type?  Nice QSO with Tim WA1HLR about the DX-100.

Got my Dominican license:  HI7/N2CQR!  SSSS on the way.   Thanks to Radio Club Dominicano and INDOTEL.

Getting more active in the Vienna Wireless Society.  

BOOK REVIEW:  

"The History of the Universe in 21 Stars” by Giles Sparrow.  Written during the pandemic.  Published by Welbeck, in London. https://www.amazon.com/History-Universe-21-Stars-imposters/dp/1787394654  Also:  From “Atoms to Amperes” by F.A. Wilson available for download.  See blog.

SHAMELESS COMMERCE DIVISION:   

Todd K7TFC getting ready to launch “Mostly DIY RF.”   I used his TIA boards in my 1712 rig.  He will have boards like this and much more.  Stay tuned.

I need more viewers on YouTube.  They want 4,000 hours IN A CALENDAR YEAR!  Please watch!

FARHAN’S NEW “DAYLIGHT AGAIN” RIG.  Analog.  VFO.   Comments, observations. We need to get him on the podcast.  Maybe two shows: SDR and HDR. 

PETE'S BENCH

Time very limited. But still sharing lots of tribal wisdom.

Wireless set with tubes!

Tool recommendation – Air compressor

 MAILBAG:

Farhan VU2ESE – Speaking of big antennas “Whenever I look at the huge construction cranes in Hyderabad, I always think how one could make 160m, 4 element yagi using it as a boom..”

Todd K7TFC in Spain, spotting Log Periodics in Madrid.

Andreas DL1AJG:  Can Biologists fix Radios?

Janis AB2RA Wireless Girl.  Expert on Hammarlunds.  And was my first contact with the Tuna Tin 2. She too was HB!

Peter Parker VK3YE on Owen Duffy VK1OD

Lex PH2LB on homebrew radio

Would this really be homebrew?  Mail from H-A-D article on FM receiver

F4IET a DSB rig from France

Ciprian got his ticket YO6DXE    

Josh G3MOT sent us a good video about the Vanguard satellite and IGY.

Dave Wilcox K8WPE bought Chuck Penson’s Heathkit book.

Rogier -- So many great articles and links from PA1ZZ

Bill AH6FC  Aloha. Retiring.  Wants to build.  Mahalo!

Grayson KJ7UM  Working on an Si5351.  Gasp.

Mike KE0TPE viewing YouTube while monitoring 6 meters.   He will have a lot of time to watch!

Chris KD4PBJ spotted Don KM4UDX from VWS FB

Mark WB8YMV building a superhet.  Having trouble with 455 kc IF can filter.

Walter KA4KXX Great comment on the Daylight Again rig. 

Ramakrishnan Now VU2JXN was VU3RDD.  Found lost Kindle with SolderSmoke book on it. Building SDR rig from junk box.  Trouble with the LM386. 

Pete, Farhan and Tony:  Shelves of Shame

Daylight Again by Farhan

The Polyakov receiver I built yesterday (from SPRAT 110, 2002!)

Farhan Takes us Back into the Daylight -- An Analog Rig with a Homebrew Crystal Filter and an LC VFO

 

There is so much radio goodness in this rig and in the blog post that describes it.  Farhan's blog post will keep us busy for a long time.  There is much to learn there.  But perhaps even more important is his larger view of the role of analog circuitry in ham radio.  Here are a couple of excerpts from his introduction: 

 Here is the memo : The analog never died. The world is analog all the way, until you descend into Quantum madness. The antennas are analog, Maxwell died a content, analog man. Our radios, ultimately, are analog machines and we are all analog beasts too. Amateur Radio technology has evolved into the digital domain. However,  it has only made it easier for us to do analog with computers to simulate and print our circuits.  So, it’s time to bid good bye to our Arduinos and Raspberry Pis and build an Analog Radio for ourselves. So let’s see what we can achieve in hindsight, a return to our native land and a rethink of our approaches. The radio is called Daylight Again, a nod to being back at the FDIM in 2022 after a gap of two years. It is named after the Crosby, Stills, Nash and Young’s song that had been humming all the time while put this radio together, emerging after 2 years of lockdown.  This radio that took two days to come together, no actually two years! That’s: parts of it got built and stowed away, thoughts were struck in the shower, questions popped up during early morning cycle rides and notes and circuits were scribbled in the notebook.  I must take the first of many diversion here: I hope you all maintain a notebook. Write down the date and whatever you thought or did on the bench and the result. Nothing is trivial enough to leave out. Wisdom comes to those who write notes.  I started to build this on Saturday the 14th May and I checked into the local SSB net on Monday morning, the 16th May 2022.

AND

Having clean VFO  is the most important way of increasing the dynamic range of your radio. A free running JEFT VFO that has sufficient power and a good Q components, will be unmatched by any synthesized or direct sampling radios. The math is all on the side of the free running VFO. We are talking -150 db/Hz at 10 KHz spacing, by comparison the Si5351 is -125 db/Hz, it is 300 times worse.

That is just part of the intro.  We should all study the rest of Farhan's blog post very carefully and incorporate the wisdom into our new rigs: 

Here is the blog site: 

https://www.vu2ese.com/index.php/2022/08/04/daylight-an-all-analog-radio/

Enough of the darkness.  Step into the daylight my friends. 

Linear Tuning in the HT-37

I just kind of like this picture.  This is the HT-37 dial that came with the HT-37 VFO assembly I recently bought.  Note the retro designation: KILOCYCLES.  And note the nice, even, linear spacing of the VFO. This VFO runs 5 -5.5 MHz.  The circuit is a series-tuned Clapp.  That circuit seems to be one of the secrets of getting linear tuning -- to avoid the common situation of having all the upper frequencies kind of bunched together at the end of the capacitor's tuning range.  I notice that this circuit was used in the Galaxy V VFO and in the VFO of the Yaesu FT-101, both notably linear in their tuning.   

Another HT-37 VFO -- No Temperature Compensation Trimmer Capacitor?

Is that thing beautiful or what?  That is the VFO assembly from an HT-37.  This one includes the fly-wheel mechanism.  It tunes 5 - 5.5 MHz.  I'll probably replace the tube with an FET, but mostly keep it as is for use in a future transceiver.  It is built like a battleship.  Hallicrafters did not mess around with the solidity of VFO construction.  

I was a bit disappointed when I did not see the split stator temperature compensation trimmer cap that was present on the Hallicrafters variable cap that I bought back in February 2021: 

https://soldersmoke.blogspot.com/2022/02/differential-temperature-compensation.html  

I took a look inside my own (beloved) HT-37 and saw that it too lacks the temp compensation trimmer that came with the February 2021 variable cap.  Could it be that the February 2021 seller had the source wrong?  Could he have in fact been selling me the variable cap from an HT-32?  Or could it be that Hallicrafters added this split stator temp compensation capacitor to later versions of the HT-37?  

Hallicrafters patented the split stator temp compensation circuit (U.S. patent #2718617).  Chuck Dachis says in his book about Hallicrafters that the company had perfected this circuit by 1957.  

An HT-32B transmitter was selling for $725 in 1963.  That's $7020 in today's money.   Wow, and that is just for the transmitter!