Lash up Direct Conversion Receiver

[pwdrive]

Hi Lawrence,
O.K., I've briefly read that and stored the cct. diag.; will read your report and study the diag. at a more convenient time when I can give it my full attention that it deserves: at present, my 'in-tray' is rather full! Then I'll come back to you here and talk about it.
Anyway, well done in seeing it through to this stage. I know how daunting and demanding 'home-brew R & D' can be!

TTFN,
Al / May 14

Thanks Al, as and when.

Lawrence.

[Skywave]

O.K., a few thoughts.

I see that the input tuning uses a 300 pF variable cap. and that the regenerator tuning uses a 300 pF variable cap. too (albeit padded down: C6, 100 pF). Hence, question: are these two caps. ganged - or does that not matter, on account of the fact that the R.F. tuning is broad enough so that, in effect, it becomes a simple 'peak' control?

I see that you use a 6AC7 from the freq. converter and that you say that it is prone to overload on strong signals. Since the 6AC7 is a sharp cut-off R.F. pentode, did you consider installing a remote cut-off, vari-µ valve instead - like a 6K7G? That way, a simple manual R.F. gain control could be incorporated.

Overall though, an interesting cct. We need to think about how we can get a relatively simple osc. design going to replace the ext. sig. gen.: I'm thinking about that.

Al.

[pwdrive]

O.K., a few thoughts.

I see that the input tuning uses a 300 pF variable cap. and that the regenerator tuning uses a 300 pF variable cap. too (albeit padded down: C6, 100 pF). Hence, question: are these two caps. ganged - or does that not matter, on account of the fact that the R.F. tuning is broad enough so that, in effect, it becomes a simple 'peak' control?

I see that you use a 6AC7 from the freq. converter and that you say that it is prone to overload on strong signals. Since the 6AC7 is a sharp cut-off R.F. pentode, did you consider installing a remote cut-off, vari-µ valve instead - like a 6K7G? That way, a simple manual R.F. gain control could be incorporated.

Overall though, an interesting cct. We need to think about how we can get a relatively simple osc. design going to replace the ext. sig. gen.: I'm thinking about that.

Al.

Thanks for the reply Al, the RF tuning and the tunable detector are not ganged as in the circuit as posted the tunable det only has a range of 600 kcs ( I have since done another det coil that has a range of 1.1 mhz) that would mean a lot of switchable or plug in coils for the RF...to much I think.
The RF coils I am using are from a scrap set, each has its own aerial winding bottom coupled to a tuned winding, with four of these coils I can go from MW BCB up to about 20 mhz so the short answer is yes the RF tuning capacitor does just that ie: tune/peak, it's not fiddly to use, it does not need a reduction drive, it will have a find your way around scale fitted in the final design.
It performs well considering what it is and it is stable, listening to a couple of VK's this morning on 20, no JA's yet though....
I take your point about the valves, I am a bit limited for choice at the moment, the 6AC7 is the only valve I have that will take cathode injection mixing, I tried a 6K7 originaly for the mixer but it is a poor performer in that role.
The 6K7 works well as the regen det.
The 6AC7 does not appear to overload from a signal point of view but the maximum osc injection voltage is quite critical.
I have a couple of pentagrid mixer valves but as you know they need a lot of osc volts in order for them to mix properly.
So far as an on board osc goes for the final design there are not many choices.

Don't bother and use the signal generator....easy option.

Usual triode hexode/pentagrid.....don't really want to go that route, I can forsee drift problems over 10 mhz I need it to be really stable for SSB.

Switched or plug in Xtals.....good solution but restricts coverage to probably just the amature bands......possible.

Cheat...make the receiver a hybrid...DDS VFO with knob and shaft encoder....tempted.

Harmonic generator using a 1 mhz xtal...just select the harmonic required to set the band then tune away....this was my original idea, I like the thought of this but can it be done......on the positive side the mixer in the receiver only needs a low level input (currently running it with 25 mv injection)

Lawrence.

[pwdrive]

Did a bit more on this today, pinched a 6SA7 pentagrid from my beloved Hallicrafters SX28 and tried it out in the regen as the mixer, works very well, signal to G3 and Sig. Gen to G1, it needs the full output from the Sig. Gen. though (200 mv) Also used the other half of the 6SC7 double triode (other half is used as an audio amp) I plumbed in this spare half as a crude RF amp, tuned grid input & choke with capacitor output coupled to the mixer tuned grid input circuit by a few turns over the cold end of it's tuning coil.
Amazingly this RF amp lash up works up to 15 mhz but from then on it becomes a transmitter......No doubt due to all the long wires I used to connect it up with.
Managed to gang the two RF tuned coils I and got them to track ok.
Changed the mixer to regen coupling from coil to choke/capacitor, this gives a lot more gain, a relatively large value coupling capacitor can be used (200 pf) as there is only the anode capacitance and the choke self capacitance to take into account at the mixer output, I will do some listening tests tonight and tommorow if the band conditions are favorable.

Lawrence.

[pwdrive]

Put the new arrangement through its paces last night, the results were very good although not much DX around, managed to copy a W2 from long Island and several UK QRPer's (England & Scotland)
The addition of a tuned RF amp has enabled me to drop the tunable IF down in frequency and still retain a reasonable image ratio, the tunable regen det now tunes from 2 to 2.5 mhz,
The lower resulting L/C ratio has enable me to increase the value of the regen grid coupling capacitor from 4.7 pf to approx 27 pf whilst still maintaining no discernable frequency drift when going in and out of full regen (important for SSB)
I have also used a screened lead from the regens grid coupling capacitor to it's top cap grid in order to reduce the inherent hum pickup which is inevitable in a test bed lashup job. The down side of this is that it forms a capacitive divider for the grid input so some attenuation is present but it has only a small effect on the receivers performance, if a G1 top cap valve is used in the final design then that whole section can be screened so no problems.
If I get time I will post up the latest circuit.

Lawrence.

[pwdrive]

Here is the circuit I've come up with for the trial RF amp version, I will probably go for cascode RF in the final setup. for practical reasons itt was not possible to use the primary of the mixer tuned circuit coil in a HT circuit, hence the choke/capacitor coupling
The screen voltage on the mixer is quite critical so to that end I have used a preset pot. the screen (G2/G4) should sit somewhere around the 20 volts level.
The resistor across the regen pot was fitted to pan the control out a bit as I only had a 50k pot to hand.
The performance of this receiver is getting better each time so far.
Any comments/suggestions welcome.

Lawrence.

[Skywave]

Hi Lawrence. I see that you're making good progress with your project: well done.
I'm sure that you don't need any words of encouragement from me on this, but I feel I need to write the following . . . .

This thread is unusual since it does not come into the usual "this doesn't work, what do I do now?" category. As such, it doesn't readily invite responses from others and you're probably aware of that: your posts are in the nature of news bulletins, which is just fine. Obviously I can't speak for anyone else, but I do read every report that you send in - and read them with interest. As a case-hardened experimenter myself, I well know that with projects like this it is easy to 'run out of steam' during their development due to many reasons: sometimes mere enthusiasm is not enough! Nevertheless, you do appear to have that in abundance here, and although I do not know if you hold an Amateur Radio ticket *, your progress in this project is very much in the classical mould of traditional Amateur Radio. So to conclude: please keep 'at it'. One day, hopefully, this project will transpire into a fully-completed radio (bread-board to finished product): I look forward to that final event.
So please keep going: you've put a lot of effort into this; I'm sure you'll get there in the end.

*you may have mentioned this once: if so, I've forgotten: my memory is none too good these days!

Aside: this post crossed with Lawrence's immediate post above.

Al. / May 20th., 2012 //

[Skywave]

Lawrence,
Just a rider to my last post. What is the aerial you are using for your radio project? It certainly seems to pull in the 'weak ones'.

Al.

[Skywave]

Any comments/suggestions welcome.
Lawrence.

Two:
(1) the R.F. choke in the anode of the R.F. amp.: I wouldn't do that. Why? Because the capacity of that choke will resonate with its inductance and create a tuned cct. at some frequency: this may affect the performance of the R.F. amp.; probably adversely. I would have connected L3 in the anode cct and used L4 as the feed to the control grid of the converter.
(2) You've used a triode for an R.F. amp. Isn't that inviting instability, not to mention an undesirable loading on its input L/C cct.? (Miller effect). Would not a pentode or cascode triode pair have been a better choice?

Just two thoughts; not criticisms.

Al.

[pwdrive]

Hi Al, no I never took the Amature or the PMG exam, I was young and got distracted, not taking the exam is something I always regreted but there we go.
My aerial (brace yourself) consists of various strands of this and that twisted together to give approx 30 ft it comes out of the top of the window (single story) through a Budliah Bush and then hangs on a nail on the back of one of the outbuildings, its average hieght above the ground is 6 ft approx....'nough said...could do better, will do better, maybe after I have relplaced the roof (next big job) any long distance DX I have received so far at this location has mainly been on the Grey Line especially South America. We are not far from the sea, I don't know if that has anything to do with reception. Since you replied I have plumbed in the 6AC7 as an RF amp which seems to have dispersed the Miller effect!.

Lawrence.