Pages: [1] 2 3 4   Go Down

Author Topic: My First Superhet Build -- Planning  (Read 1023 times)

G3EDM

  • Member
  • Posts: 1456
My First Superhet Build -- Planning
« on: June 28, 2021, 06:27:07 AM »

I've gradually been stocking up with parts to build my own 1960s beginner's superhet.

It will be loosely based on the "Miser's Dream", a capable five-tube receiver project in the ARRL Handbook for 1968.

To speed things up and keep it simple, it will initially be single-band only (40m).

The set has an IF of 3300kHz and in the original design, uses a two-crystal IF filter at 3300 and 3301 to get 1kHz selectivity. I will only be using the set for CW, so I'd want more like 500Hz.

I wonder if homebrewing OTs can answer the following question. For excellent filtering, I am thinking of using a YG-3395C filter unit originally fitted in Kenwood transceivers. This has a 500Hz passband at an IF of 3395kHz, so I would have to shift the Miser's Dream's IF up by 95kHz, a trivial change presumably.

My question is how to alter the original filter circuit to accommodate the Kenwood filter. I cannot find any technical description of this filter online, but the pinouts look like this:



The original circuit of the Miser's Dream looks like this:



I presume we can eliminate some of the components in the original filter circuit (not just the crystals). The Kenwood filter has just those two pins, plus a presumably grounded case. How do I make it fit in the Miser circuit?

Edited to add: Am I correct that if the set is used only for CW, the entire AGC circuit is superfluous?

It will probably be several months before I get going on this project, but would like to get my ducks lined up in advance.

73 de Martin, KB1WSY (for now)
« Last Edit: June 28, 2021, 06:32:06 AM by KB1WSY »
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: My First Superhet Build -- Planning
« Reply #1 on: June 28, 2021, 08:31:08 AM »

It will depend on the design termination impedance
of the filter at both ends.

To start with, you could just put in the filter in place
of Y1 and remove Y2.  The lower 470 PF cap becomes
a bypass, the upper one changed to resonate with
L5, and you might not need the RFC,

The ratio of the 180 pF series cap to the upper 470pF
capacitor sets the source impedance for the filter.
the load impedance is the 100K resistor and the
input impedance of the 6AU6.

I would add a DC blocking cap between the filter
and the grid for good measure - some filters
can degrade over time with DC applied.  That
would also let you add a termination resistor if
needed, without messing up the AGC.

Many modern filters are designed for impedances
between 500 and 2000 ohms.  The impedance
match (at both ports) affects the passband ripple,
among other things.


AGC can be useful for saving your ears on CW,
although I also got started riding the manual RF
gain control instead.  Common problems of the
era included the BFO signal affecting the AGC,
and insufficient BFO injection, especially on
receivers that didn’t have a real product detector.
A well-designed receiver can avoid these, but
for simpler / cheaper receivers it often wasn’t
worth the effort.

KM1H

  • Member
  • Posts: 11155
Re: My First Superhet Build -- Planning
« Reply #2 on: June 28, 2021, 08:39:19 AM »

A huge problem with that design is using the 7360 mixer without a RF stage ahead of it. When that tube does not see a steady input load it can go crazy with overload and false signals.
For one band that should be fairly easy using a resonant close to 50 Ohm antenna.

Carl
Logged

G3EDM

  • Member
  • Posts: 1456
Re: My First Superhet Build -- Planning
« Reply #3 on: June 28, 2021, 09:04:47 AM »

It will depend on the design termination impedance of the filter at both ends.

Now I am starting to remember that stuff. I have a whole book somewhere about filter design, I must dig it out to make sure I understand what is going on.

I would add a DC blocking cap between the filter and the grid for good measure - some filters can degrade over time with DC applied. That would also let you add a termination resistor if needed, without messing up the AGC.

The Kenwood filter I am eyeing is designed for a solid state transceiver, but I was assuming it would function OK in this case too. The alternative is to find the two crystals from the original design (but changed to be only 500Hz apart) but that is not so easy nowadays and anyway I assume the performance of the Kenwood filter would be better than the original design, once the circuit is adapted for it.

I'm quite happy to experiment with this set to get it as good as possible, within the obvious constrains of a simple single-superhet. It is almost bound to be better than my existing little three-transistor Novice regen receiver.

(Concerning AGC.) A well-designed receiver can avoid these, but for simpler / cheaper receivers it often wasn’t worth the effort.

The AGC in this design is only applied on a single stage, and the ARRL text admits that this makes it "less effective than if several stages were gain-controlled". It is one-half of a double-triode tube (the other half is the BFO) so I can just leave it unbuilt to start with and later add the AGC components if I feel the need for it.

A huge problem with that design is using the 7360 mixer without a RF stage ahead of it. When that tube does not see a steady input load it can go crazy with overload and false signals. For one band that should be fairly easy using a resonant close to 50 Ohm antenna.

I'm using a plain 40m dipole so that should be a pretty good load, nothing fancy.

(This paragraph has been edited later.) In the ARRL design there is a Q-multiplier tube in front of the mixer, does that not to some extent fulfil some functions of an RF stage, including isolation from the antenna? (Sorry, that got chopped off on the left of the scanned schematic.)

Thank you both!

73 de Martin, KB1WSY (for now)

« Last Edit: June 28, 2021, 09:08:17 AM by KB1WSY »
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: My First Superhet Build -- Planning
« Reply #4 on: June 28, 2021, 02:50:42 PM »

I don't know the current circumstances, but I've read a lot of
older accounts of the high levels of RF on the 40m band in Europe
due to the use of the high end for SW BC.  That means that
your front end and mixer need to be able to handle very strong
signals.  I don't know if a dipole with a Q-multiplier in the
front end will be adequate to reduce the overload - that's
something that I'd get input from other British hams.

If you take out the AGC, just make sure that the RF gain
provides a suitable voltage for the tube without that part
of the circuit.

Because the original IF doesn't appear to use custom
transformers, shifting the frequency shouldn't be a
problem should you find a pair of suitable crystals.  The
older FT-243 crystals can be opened up and ground to
shift the frequency higher, so you could adjust them as
needed (although it isn't a simple screwdriver adjustment).

KM1H

  • Member
  • Posts: 11155
Re: My First Superhet Build -- Planning
« Reply #5 on: June 28, 2021, 03:36:15 PM »

For just one band Id consider a FT-243 2 or 4 crystal filter front end. Ive read about the variable bandwidth circuit but havent used one yet.

Whatever is used that mixer needs a steady load on the input. I use a pair of 7360 mixers in the 75A4 Ive had since 1965.

Carl
Logged

G3EDM

  • Member
  • Posts: 1456
Re: My First Superhet Build -- Planning
« Reply #6 on: June 29, 2021, 01:45:32 AM »

I don't know the current circumstances, but I've read a lot of older accounts of the high levels of RF on the 40m band in Europe due to the use of the high end for SW BC. That means that your front end and mixer need to be able to handle very strong signals.

I am managing OK on 40m at my southern England QTH with the little three-transistor regen, but that is on a long wire (the dipole, and the pole it will be hoisted on, are under construction). I am not getting any breakthrough from SW BC. The only overload I'm getting is with the stronger SSB ham signals, which I can cope with by using a custom attenuator that I built: a switchable antenna trimmer and potentiometer combination, wired across the antenna input. I'm not planning to operate phone in any case, and the sideband signals are far up the band and not affecting the CW portion at the bottom.

The little regenerative set is plenty sensitive, but there are two obvious limitations with such primitive technology: (1) the white noise from the regeneration, and (2) lack of selectivity on CW. The second of these two factors is the one I am most interested in improving, hence the focus on building or obtaining a good filter for the planned homebrew superhet. When I get on the air, I want to use the regen for the first burst of (arduous) QSOs but I will need better "ears" pretty quickly. The only caveat is all of my equipment must be homebrewed.

In terms of grinding my own crystals, I have a whole pile of several dozen old FT243s (and older form factors) but only one of them is anywhere near 3300Khz. I assume you would want crystals that are a bit lower in frequency, to grind them "up". The one I have is gigantic, a military format with a pull handle at the top, and I think it is 3272.5 or something like that. None of my other crystals is even within 1Mhz below 3300.

I will think about it, but it strikes me as fiddly work considering that I want two crystals only 500Hz apart. I don't have any test equipment to measure/generate oscillation to that degree of accuracy. Great fun, if you can get it to work!

Edited to add: I just checked the resurrected AF4K crystal online shop, and it's the same issue: a big gap in the available frequencies, jumping straight from 2MHz to 3.5MHz with nothing in between.

I remember some years ago reading an account by N2EY about building his hollow state transceiver. He found a whole bunch of crystal filters at a hamfest, and built the rig around their rated frequency, rather than the other way around. That is probably a saner way to go about it. There are lots and lots of filters available at 10.7Mhz but I assume they are usually used in double-supers. Also a fair number in the 8Mhz+ range, often as drop-ins for Elecraft rigs.

BTW I am not wedded to the "Miser's Dream" design. Just thinking aloud at this stage. Using a proven ARRL design has been my path most of the time until now, but it doesn't always make sense, especially when parts are unobtainium.

Edited to add: By adapting/designing my own receiver, using book learning and help from this forum, I would learn a lot more than by just blindly building an existing ARRL design exactly. I had a great experience some years back doing something similar, building a more elaborate regenerative set using subminiature (battery-powered) "pencil" tubes. That set will also see action in my shack soon, although it is currently still at the "breadboard" stage (huge and unwieldy). It performs better than the ARRL three-transistor Novice set.

73 de Martin, KB1WSY (for now)
« Last Edit: June 29, 2021, 02:03:29 AM by KB1WSY »
Logged

G3EDM

  • Member
  • Posts: 1456
Re: My First Superhet Build -- Planning
« Reply #7 on: June 29, 2021, 02:23:27 AM »

I have found the schematic for the Kenwood TS-520SE transceiver that the previously referenced CW filter was designed for, and here is the relevant portion of the IF board (the filter is labeled XF2):



According to the manual: "The SSB crystal filter has a diode switching at each of the input and output ends. If the CW crystal filter (YG-3395C, optional) is installed, the diode switching circuit can select one of the crystal filters with the MODE switch and CW NARROW/WIDE switch interlocked to it."

I still can't find a spec sheet for this filter, nor any indication of its impedance, but at least the above gives some context as to how it was used in the original setting.

73 de Martin, KB1WSY (for now)
Logged

W9IQ

  • Member
  • Posts: 8866
Re: My First Superhet Build -- Planning
« Reply #8 on: June 29, 2021, 03:24:14 AM »

It would appear to have about a 2K input impedance and an 8k output impedance.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

VK6HP

  • Member
  • Posts: 1222
Re: My First Superhet Build -- Planning
« Reply #9 on: June 29, 2021, 03:52:54 AM »

If you want a simple, authentic valve/tube CW receiver, well described, you could do worse than look at this one:

https://www.frostburg.edu/personal/latta/ee/6x2rcvr/6x2mechanicalconstruction/6x2mechanicalconstruction.html

You could build it as a single-band version if you really wanted but there's not much extra complexity in including both 80 and 40 m coverage.

I probably wouldn't use the tube rectifier, but that's a personal choice. 

73, Peter.

Logged

G3EDM

  • Member
  • Posts: 1456
Re: My First Superhet Build -- Planning
« Reply #10 on: June 29, 2021, 05:32:35 AM »

It would appear to have about a 2K input impedance and an 8k output impedance.
- Glenn W9IQ

Thanks, Glenn!

If you want a simple, authentic valve/tube CW receiver, well described, you could do worse than look at this one:

https://www.frostburg.edu/personal/latta/ee/6x2rcvr/6x2mechanicalconstruction/6x2mechanicalconstruction.html

You could build it as a single-band version if you really wanted but there's not much extra complexity in including both 80 and 40 m coverage.

I probably wouldn't use the tube rectifier, but that's a personal choice. 

73, Peter.

Thank you Peter, that is very interesting. It is good when someone else has found out how to source the unobtainium parts or figured out how to build them themselves. I probably have some of the Miniductor coil stock that he reproduced. My only quibble is that this is a set designed for both sideband and CW, with a selectivity of about 2KHz. I assume it would be hard to do better with that single-crystal filter design.

It is also an example of the kind of design philosophy I am after, i.e., using the best bits out of a number of vintage designs to build a receiver adapted to one's own needs.

73 de Martin, KB1WSY
Logged

VK6HP

  • Member
  • Posts: 1222
Re: My First Superhet Build -- Planning
« Reply #11 on: June 29, 2021, 07:14:34 AM »

Actually, you're likely to find the simple crystal filter rather more satisfactory for CW than SSB, despite its relatively poor shape factor (ratio of -60 dB to -6 dB bandwidth) compared with more elaborate crystal or mechanical bandpass filters. C2 and C3 set the selectivity, while the phasing control C30 is adjusted to place the parallel resonant (antiresonant) frequency of the filter response on the image of the wanted signal, giving single-signal reception. (See also the author's description and the tweaks for better SSB). In practice, it's much easier to do than to say, and you'll find yourself getting good at it.

By the way the Heathkit HR10 receiver used crystals around 1680 kHz in a 2-crystal filter.  They appear on eBay sometimes. As a proud owner of an HR10B I can say the receivers are generally terrible but maybe you would find recycled crystals useful in a homebrew project.

73, Peter.
« Last Edit: June 29, 2021, 07:18:06 AM by VK6HP »
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: My First Superhet Build -- Planning
« Reply #12 on: June 29, 2021, 07:49:43 AM »

There are any number of articles available on crystal ladder filters,
which use multiple crystals of the same frequency.  That can be
anywhere from 400 kHz to 12 MHz or more, although the lower
frequencies tend to be easier to build for CW bandwidths (and
higher frequencies for SSB).  There is nothing wrong, for example,
with using an IF at 1 or 2 MHz, for example, to take advantage
of standard crystal frequencies.  (The ~4.433 MHz PAL crystals are
probably more common in England than in the US.)

There are also circuits that use a single crystal for CW.  I'd have to
go back through my copies of Amateur Radio Techniques and
the Technical Topics Scrapbooks to find specific examples,
but those resources are also likely to be more available on your
side of the pond.

G3EDM

  • Member
  • Posts: 1456
Re: My First Superhet Build -- Planning
« Reply #13 on: June 29, 2021, 07:54:15 AM »

Actually, you're likely to find the simple crystal filter rather more satisfactory for CW than SSB, despite its relatively poor shape factor (ratio of -60 dB to -6 dB bandwidth) compared with more elaborate crystal or mechanical bandpass filters. C2 and C3 set the selectivity, while the phasing control C30 is adjusted to place the parallel resonant (antiresonant) frequency of the filter response on the image of the wanted signal, giving single-signal reception. (See also the author's description and the tweaks for better SSB). In practice, it's much easier to do than to say, and you'll find yourself getting good at it.
73, Peter.

Ah OK, I did read that on the builder's website but I understand it better after reading your description.

Concerning the multiband factor, I could build it as a 80/40 set and then later add a crystal controlled converter ahead of the receiver to add 20, 15 and 10 (that is another classic ARRL design).

By the way the Heathkit HR10 receiver used crystals around 1680 kHz in a 2-crystal filter.  They appear on eBay sometimes. As a proud owner of an HR10B I can say the receivers are generally terrible but maybe you would find recycled crystals useful in a homebrew project.

I built the HR10-B as a 12-year-old. It was a pretty terrible receiver and I was a bad builder (too impatient). It ended up being sent back to Heathkit to get it working, and when it came back there was a piece of paper saying "repaired numerous cold joints" (in French, because I grew up partly in France and the kit was a Christmas present from my parents).

Still, I had enormous fun with that radio, unlicenced at the time and just listening and logging!

Edited to add: Unfortunately my parents, years later, found the radio in their basement and threw it away!

73 de Martin, KB1WSY (soon to be G3EDM, licence issue is pending)
« Last Edit: June 29, 2021, 07:57:33 AM by KB1WSY »
Logged

WD4HXG

  • Member
  • Posts: 396
Re: My First Superhet Build -- Planning
« Reply #14 on: June 29, 2021, 09:58:46 AM »


I still can't find a spec sheet for this filter, nor any indication of its impedance, but at least the above gives some context as to how it was used in the original setting.


If you have access to a network analyzer or frequency sweeping setup the easy way to determine the input and output impedances is to insert
a pot in series with each the input and output and shunt with a 56 ohm resistor.   Adjust each pot for minimum passband ripple in the filter
response. Then measure the resistance of each pot.  Each pot's resistance will provide the input and output impedance of each port. Usually
the input and output on most crystal filters are close in value.  See the image below:

Logged
Pages: [1] 2 3 4   Go Up