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Deltahet is alive!!

After 4 iterations of my Wadley loop receiver, all my hard work was a disaster. So, ripped it all apart and learned from my mistakes. It now has more in common with the original Pogson Deltahet than my interpretation of it, hence the new topic.

Here is a link to the very first signals received, here on 9 megs, some weird god station, always good for strong signals during a bad time of day, but no miracles were performed despite the theological BS.

It was still a battle to get this far, and birdies remain a problem, especially at the 1 megacycle boundaries. This is not suprising as the harmonic comb generator is as yet unsheilded. Another issue was getting valve mixers to have enough conversion gain at these frequencies, I had long forgotten just how hard it can be.

Ideally I need an extra amplifying stage in the 37.5 meg IF, I found that using a hi slope pentode with grid leak injection worked fine and saved another current hungry valve, the mains transformer is working close to its limits. ... we have 15 valves!! I tried cathode injection on the Wadley loop input, here the comb feeds into the cathode, the VFO into the grid, this too works very well. Another 300mA saving in heater current was using a ECF80 , the triode portion as the VHF oscillator, the pentode section as the third mixer. The first mixer remains problematic, this one converts the RF from the preselector to the 40 meg first IF. Currently this is using a 12AT7 twin triode, but efficiency seems low. The RF amp is a 6BA6, as are the 2 455kc IFs. I stuck with the ECH81 for the tuneable IF, as I am very familiar with these. The detector is an anode bend for AM, and will see the BFO injected into its cathode for SSB/CW. There are still problems with the 40 meg bandpass filter, and am struggling to find a good coupling to prevent double humping, and signal loss.

There is no preselection yet, other than a random tuned circuit temporarily hooked up to the first mixer.

AGC seems to work fine, this uses a voltage doubler using germanium diodes, giving a negative 10 volts on s9 signsls. The voltages to the BFO, VHF oscillator and associated mixers are regulated by an OA2 neon., giving a stabilised 110 volts.

The s meter uses a bridge circuit, one side is the cathode of the third IF valve, the other side derived from a voltage doubler from the heater supply. This needs improving, it drifts all over the place as the set heats up.

Another two quick videos graphically show the workings of the Wadley loop.

The next few days I will tidy up what I have so far and try to pinpoint the weaknesses. At least with sound, it helps to track these issues.

15 comments

15 Comments

w5jag

Jan 22

I have looked at them with my Red Pitaya and I agree, the Si5351 output is filthy compared to a conventional L/C oscillator.

My first iteration of my double conversion superhet used dual gate MOSFET's for the mixers, and between all of the filth and the cross tallk, it was completely unacceptable.

However, I find that double balanced diode mixers, like the commercial mini circuits types, are pretty much immune to all of this, and work well with nothing more than a coupling cap between the Si5351 and the DBM.

The true DDS modules looked at on my mediocre test equipment have a strong harmonic content, but are otherwise pretty clean. In my homebrew SSB transceiver, I use a DDS module and, on the transmit side, I pre filter the 23 MHz LO with a SAW filter to get rid of the harmonics before they get into the mixer. The transmitter has a very clean output - it easily meets FCC requirements at its power level with only a five pole low pass filter at its output.

I think if you wanted to use a DBM with a Si5351, you would probably be okay for a BFO / product detector.

73,

Win W5JAG

w5jag

Jan 18

Wow.

That is absolutely incredible, Dayle. Fantastic work.

If solid state can be tolerated, QRP Labs make a very compact fixed frequency Si5351 synthesizer/controller for $20 ish USD. Programs with DIP switches. I have one and it works great - the project it was to be used in is abandoned however.

I agree low frequency crystals are hard to source, and no guarantee they are on frequency after this time.

73,

Win W5JAG

dayleedwards

Jan 21

Replying to

I dont know if anyone has actually looked at the spectral "purity" of these oscillator modules, they are really filthy , at least the ones I have attempted to use for different projects are. The BFO could use a simple coil at 455, but the long term drift with chassis heating would be a problem, exaggerated by the narrow crystal filter bandwidth. One way around this is to add a BFO frequency control I guess, adjusted as the set warms to keep it in the passband, but this is a messy solution from an operators standpoint.

Right now I will shelve the project until I find some xtals.

At some point I did intend to do a longer video with it receiving, and a direct comparison with either my Icom R71, NRD93 or NRD515, not only regarding drift, but images and stability, as it really can give a very good account of itself. Sensitivity is in the sub microvolt range, dynamic range is limited by relatively low AGC control voltage over just 3 stages, first RF, second IF, and one third IF stage. An added front end attenuator will fix this. This limitation I suspect is a direct result of having a 1 meg wide first IF, a requirement to tune each 1 meg wide band segment, giving the second mixer a very hard time. The relatively higher operating voltage of valves helps with these dynamics .

Now I hope to find time to revisit my supergainer.... I have a few ideas......

dayleedwards

18 jam lalu

Replying to

The Deltahet now has SSB reception.... a short video on its performance compared as best I can to the NRD515, given the weak daytime signals and solar controller interference.

https://youtu.be/U3xJq8pfjYY

for the full explanation

dayleedwards

Jan 18

I am slowly making progress between fixing cars and cutting down trees that are growing so fast they are blocking my solar array....

The Deltahet is working very well, several more things to do, especially adding SSB capability.

The mechanicals are largely completed, wondering if there is enough panel space for my EM81 magic eye tube, and just where to put it.

A few pics.....

Here we are locked and tuned to the 7 meg band. Managed to add an FRG7 type lock indicator, here I cheated and used two germanium diodes and a BC547 transistor, Im running short of room, and another valve seemed wasteful. Also had major problems with way too much harmonic generation, so ditched the vacuum state approach and settled for a small transistor harmonic generator module. This outputs a few millivolts only and cured the 1 meg boundary breakthrough. This I will revisit at some stage.

There is still a front end preselector coil to wind, 1.6 to 4 megs, this will be next. The vfo coil is as yet without its screening can. There is room behind the speaker to add the BFO and product detector, will probably use another ECH81 here. I need 455kc crystals, these are VERY hard to source. My experiments using ceramic resonators are unreliable, they have HUGE frequency tolerances, and dont like the heat, like 14 valves a glowing, at 1.5 watts of heat each, plus the hard working power transformer in a confined space.....

Its looking a tad messy and busy underneath, mainly due to having to isolate and bypass every stage to prevent interaction. I may have gone overboard. The bent tin sheild is sheilding the 40 meg bandpass filter, made from a scrap of tinplate from a can of CRC. I managed to switch the band indicator LEDs with the same switch contacts that switch the serial coil primaries, even rotary switches seem to becoming rarer by the day.

I wasted 4 hours replacing the three gang kilohertz tuning capacitor.... note to self. When drilling holes in an aluminium chassis, cover the tuning gangs, else they will become scratchy forever after......

I know its easy to over praise ones performance on homebuilt designs, but this thing receives radio Pyongyang 20 minutes before either my NRD93, or my NRD515, both very sensitive receivers, have any listenable audio.......

I have picked up yet another FRG7 Yaesu, rusty and unloved...... Ill need to , ummm, upgrade that one too I guess

LarryDee

Dec 26, 2022

Sorry to not add to the technical aspect of your article. I don't have any experience in this area of tubes and this design. Great stuff, interesting to learn about all this. I really appreciate the contributions made here on this site, like dayleedwards here. Thanks

dayleedwards

Dec 26, 2022

Replying to

thanks Larry, I must admit, I could put more effort into writing this up better. For instance, the spectrum analyser vid I have just realised has no explanation of what we are looking at.

In the first video you will see two "pips". The one rising and falling on the left hand side is the injection frequency of 37.5 megs, the one on the right is the VFO. The spectrum scan is from 36 megs on the left, 70 megs on the far right. As the VFO increases its frequency and moves right, as it passes each one meg harmonic, ie, at 41, 42, 43, .....68, 69, 70, the pip on the left will rise, and then fall as the VFO moves on to the next harmonic. The next video is a closeup of that 37.5 meg signal, or mixer product. Every time it peaks, that indicates the VFO is near a 1 meg harmonic. So, if we use that VFO into a mixer and up convert the wanted RF signal, and then use that same 37.5 injection freq to down convert it again, but offset at the last IF,, remebering BOTH mixers are referenced to the same VFO, then if that VFO increases its frequency say 100kcs, then the second mixer will also see that 100kcs shift, but in the opposite direction. As long as the VFO product remains in the bandpass of that 37.5 meg filter, then it will cancel any drift. Basically we have 30 frequencies, each 1 meg apart, similar to having a bank of crystals at 1, 2,3,4,5,6....28,29,30 megs.

Now, I thought that any nasties such as synthesiser hash would disappear with this method Unfortunately no, the injection voltage has added noise from two mixing processes, and two amplifying stages, nothing comes free......

Interestingly too, these frequencies I am quoting are not cast in stone, and can be almost anything. Im not sure offhand what Racal used, but the Barlow Wadley design used 44 megs as the first signal IF and I think 41.5 megs for the Wadley loop. The only requirement really is to have the tuneable IF somewhere between the two frequencies, so one conversion is on the high side, the other on the low. I think the 2 to 3 meg tuneable IF was chosen to simpllfy the circuitry and obviously the low frequency VFO here is more stable.

It would be so much easier doing this with semiconductors. I envisage using a FM all in one radio chip for the Wadley loop, and a similar AM all in one for the tuneable IF. I think that will be my next project...........

coildog

Dec 26, 2022

Replying to

Great explanation! I have only a vague (and probably wrong) understanding of how the wadly loop is functioning, only that it is a clever design to cancel out VFO drift.

dayleedwards

Dec 23, 2022

Here is another video of a 9 meg band scan after the debugging process of removing spurs and birdies. Performance is now smoother, the birdie on the one meg boundaries remains, but WWV at 10 megs can still be received, so its down in the microvolt region Im guessing.

https://www.youtube.com/watch?v=aP5ppbreLlM

dayleedwards

Dec 23, 2022

Replying to

That was the 9 meg BC band. Still fiddling with the AGC, that S meter is all over the place, it needs more damping. Reception here in NZ is always DX and usually difficult, multiple skips and selective fading, sometimes affected by the aurora too. Once the preselector front end is completed, and multiple tweeks, Ill do a longer video and a schematic, in the unlikely event someone would like to replicate it. Its been a time consuming build I have to say, but satisfying in that it keeps me off the streets, away from a life of crime, drugs, booze and loose women. At age 70, I need something more.........

Thank you for your encouraging comments.

coildog

Dec 23, 2022

Replying to

Yeah, I see what you mean. Those loose women are bad ...! 😁

I'm 68 and spend a lot of my time in a small shed in the garden. Building simple radioes that I always wanted to try, but never got around to do before.

NZ is quite a ways from other countries. I believe even Australia has long since closed their short wave broadcasts. That leaves China as the only country pretty much dominating the airwaves. Although, I sometimes can hear Radio Japan and Thailand on the 31m band here in Norway.

Looking foreward to hear what your radio can pick up.

BTW: You've got some really nice tall coil cans in your radio!