Lash up Direct Conversion Receiver

[pwdrive]

I stand to be corrected....Mini Circuits do do them they are 50 ohm jobs so could be used for cathode mixer injection without to much bother, not sure about matching to the anode of a harmonic amp, maybe simple L/C matching (L section)

Lawrence.

[Skywave]

Using an L/C section to match the output impedance of the H.G. valve to that LPF won't work, since that L/C section will interfere with the characteristics of the LPF. What you need is a cathode-follower between the HG valve anode and that LPF and, in turn, that LPF will need to see a 50 ohm load. And that, in turn, may require a grounded-grid stage to produce that impedance transformation (but not, obviously, if you will be using cathode-injection at the mixer. Just choose 50 ohms for the cathode bias resistor and design the rest of the cctry. for that valve around that value). Now both of those impedance-transforming sections should be obtainable with a double-triode, but very good screening between the two valve halves will be required.

In summary: you will only obtain the stated spec. of that LPF is it 'sees' a 50 ohm source and 'sees' a 50 ohm load. And its input must be well-screened from its output.

Al.

[pwdrive]

Thanks Al, I didn't realise that a high to low L/C impedance transformation would effect the characteristics of the LPF but now thinking about it i suppose it would as we are dealing with both L and C reactances. Do you think it would be possible using a transformer match from the HG to the LPF or is a purely resistive match via cathode follower the only realistic option if I use one of those LPFs, I am still unsure if I need the LPF to extract harmonics only insofar as providing a fairly flat response to the harmonics within the LPFs passband or is there something else to it? I appreciate all this brain stretching.

Lawrence.

[Skywave]

O.K. a mini-tutorial on filters. I have tried to keep this brief: there is a lot more to filters than what follows . . . .

All filters are designed to have a characteristic impedance. The word 'impedance' here is a bit misleading, since the word 'resistance' is more accurate. There are a number of characteristic response curves that can be obtained for any filter requirement, depending on the 'base model' that is used: the type of characteristic required determines that choice. Amongst these choices are the three main classic types: Butterworth, Chebychev and Bessel. Amongst the others, a Cluer response is quite popular these days. The Butterworth gives a reasonably flat pass-band response, and its attenuation rolls off at a fairly moderate rate: typically 10 dB / decade. It's popular for non-critical applications and is often the easiest to design. A Bessel has a uniform phase response, but has a really slow attenuation rate: almost only ever used for filtering pulse-type waveforms. The Chebychev has the fastest attenuation rate of those three main types, but has a ripple in the pass-band. To design any filter, certain parameters must be known: pass-band and stop-band freqs., the resistance that the filter operates between, the atten. in the pass-band, the amount of atten. required at a given freq. outside of the stop-band - and in the case of the Chebychev, the amount of ripple that is acceptable. Many filters do give a response outside of the pass-band - and this is usually a level that is significantly less than that in the pass-band, and sometimes at more than one freq. - the Chebychev and especially the Cluer being prime examples - and sometimes this response level can be 'input' as a chosen parameter. And that's where the trouble / skill kicks in, since many of those 'input' choices are not independent. And even after the maths has been done, it's possible to end up with values of L and C that are not realisable in practice: things like 0.1 pF or 100 Henries! In which case, it's back to the drawing board.

As you say, there are several filter design utilities out there on the 'Net. If you play around with them, aiming to design a filter, you'll get a better appreciation of what I've written above. (I did my filter design 'apprenticeship' long before PCs were in such an abundance!   )

As far as using a transformer to achieve a match, it really depends on its impedance characteristics - and especially the uniform (or otherwise) behaviour of those characteristics - over the band of freqs. of interest: pass-band and stop-band. That's why a purely resistive source / load is often chosen: makes the sums easier; eliminates a few more unknowns.
An example of using such a transformer is, of course, in an I.F. amplifier. The 'seen' impedance of the L/C ccts. will be high, and in a valve cct., the source and load impedances will be high also - and usually resistive. The fact that the pass-band (in this case a BPF is required) is relatively narrow compared to the centre freq., makes the design a lot easier. However, when we come to transistor I.F. amplifiers, the O/P Z of the driving transistor is often too low to effect a good match. Hence, the collector is often tapped down the primary winding in order to prevent the characteristic Z of the filter from being upset too much. In both cases, (valve and transistor) the fact that there is a mutual coupling, pri. / sec. does not alter the requirement for matching: it just enables a BPF response of the required shape to be obtained: the driving source simply 'sees' a load that is frequency dependent with a particular operating impedance and a particular pass-band / stop-band combination.

HTH

Al.

[pwdrive]

Thanks for the tutorial Al, some stuff I knew, some I didn't, very informative post. I will use the simplest method for the harmonics matching imp. assuming that I can generate them in the first place.....By the way if I ever get this project up and running proper I'm going to call it "The Skywave" if thats ok with you or maybe "the Skydrodyne"

Lawrence

[Skywave]

"The Skywave": fine, just fine. On reflection, after that last 'tutorial' on filters, I could do with being brought back down to Earth! However, once your project is completed and working, if you find that you get occasional forms of fading and other ionospheric disturbances that spoil your reception, please don't put that down to interference from Skywave!

Al.

[pwdrive]

Cheers Al, 12.50 am and my eyelids are starting to head South now so signing off 'till tomorrow.

Lawrenzzzzzzz

[Skywave]

Hi Lawrence,
I didn't answer your Q. in post #30: my plumbing job: now fixed.

It could be a long story, but I'll try to be be brief. Some time ago we had our central heating boiler replaced and a complete re-fit of the bathroom. As a consequence, the water pressure in the domestic hot water system was much, much higher (old system was a gravity-fed affair). The increased water pressure meant that when taps were turned on, water came out so fast that it tended to splash about a great deal and go everywhere!    The fitment of adjustable isolating valves (so that the pressure at the taps could be reduced to a sensible figure) solved the problem: bathroom, downstairs W.C. and kitchen sink, H & C taps.

Al.

[pwdrive]

Hi Lawrence,
I didn't answer your Q. in post #30: my plumbing job: now fixed.

It could be a long story, but I'll try to be be brief. Some time ago we had our central heating boiler replaced and a complete re-fit of the bathroom. As a consequence, the water pressure in the domestic hot water system was much, much higher (old system was a gravity-fed affair). The increased water pressure meant that when taps were turned on, water came out so fast that it tended to splash about a great deal and go everywhere!    The fitment of adjustable isolating valves (so that the pressure at the taps could be reduced to a sensible figure) solved the problem: bathroom, downstairs W.C. and kitchen sink, H & C taps.

Al.

Yep, been there done that, all our tap/toilet and anything else that might be servicable has isolating valves fitted technicalyl I think they reduce the flow more than pressure but they do the job. Our hot water is gravity but as we are in a very low single story building the bottom of the cold water cistern is only about 3 ft higher than our hot water tank so high pressure out of the hot taps does not apply...literaly...

Lawrence.

[pwdrive]

All this horrible weather has prompted me to do a bit more experimenting with my birds nest regenerodyne, I have been working on the tunable regen stage, originally I used an old SW coil that was originally designed to tune to my design center freqency (3 mhz) at the top end of the tuning capacitor setting (High L/C Ratio) it worked ok in reception terms but tended to drift a bit and pull when tuning the RF stage and/or going in and out of regen.
I decided to go for a low L/C Ratio so looked for another spare coil, I eventually found one that tuned to around 5.5 mhz with the tuning capacitor I have fully engaged, I decided to screw this frequency down by using a couple of larger value fixed capacitors, in the end the coil ended up with two 500pf micas across it (.001 uf), this gave me a center frequency of 2.75 mhz which was near enough to the initial 3 mhz idea and a nice low L/C Ratio this greatly improved the drift characteristics and when receiving SSB transmissions there was no discernable pull when going in and out of regen and only the slightest amount of pull when tuning the RF stage.
The tuning of this latest arrangement was done with the tuning capacitor I have been using all along in series with a padder.
Because I don't have a reduction drive at the moment I padded the tuning capacitor to give a 100 khz spead, the final design will have a 500 khz spread through a suitable reduction drive. Interesting looking these old coils I have, most of them were originally in the anode circuit of an RF amp, the tuned secondary going to the grid of a mixer, the primary winding in inductance terms is very large compared to the secondary, in fact they are wave wound and look similar to MW or LW coils, the reason for this as far as I can acertain is that the large inductance combined with its own self capacitance and stray circuit capacitance resonates just below the lowest frequency of the secondary tuned circuit thus compensating for the lower gain due to the secondary having a low L/C Ratio when tuned to the low end of its band.
In the above arrangement the regen winding has to be quite closely coupled to the tuned winding to get it to go into full oscillation (a must for strong SSB signals) across the designed 500 khz spread.
I have successfully tried different RF coils and can receive SSB on all bands tried, 3.5 mhz through to 18 mhz, I have not tried Top Band yet as there is normally nothing on there in my location but being a low band (1.7 mhz) I cannot forsee any problems should I choose to do so.
The advantage of using a first converter followed by a tunable regen came into it's own when trying the different ranges as the regen action and the tuning rate remain the same which ever band you are listening to.
I also did some checks for image rejection, I adjusted the center tuning frequency of the regen so that when the local osc frequency (my sig gen) was set to be above the regen frequency and tuning the 7 mhz amature band the image frequency (approx 5.5 mhz higher) corresponded to a very strong commercial broadcast up around 12 to 13 mhz, there was no discernable breakthrough the 5.5 mhz spacing between the amature band and the commercial station doing its stuff, the other advantage of this system on that particular band is that to change reception from the amature frequencies to the commercial frequencies on the higher image just needs the RF tuning to be retuned and peaked, you can obviously go the other way and set the local osc below the desired frequency to be received then tune the RF down again to see what's down there.
I was going to try a tapped cathode circuit for the regen as well but I will leave that for the moment.
Hope any regen builders find some of this usefull.

Lawrence.