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Author Topic: My First Superhet Build -- Planning  (Read 1025 times)

WB6BYU

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Re: My First Superhet Build -- Planning
« Reply #45 on: January 21, 2022, 09:34:54 AM »

IF transformers are less critical if you are using a
crystal filter.  Many solid state rigs don’t use them
at all.

You can change the capacitor over a reasonable
range, particularly adding capacitance in parallel
to lower the frequency.  (Some QRP rigs for 40m
or 80m used 10.7 MHz IF transformers that way.)
But raising the frequency may be problematic:
a 455 kHz transformer winding may have too much
self-capacitance to be used at 1500 kHz.

But traditional IF transformers used double-tuned
circuits for selectivity, and a crystal filter will
provide far sharper response than any reasonable
number of tuned circuits at 1500 kHz.  So you can
use other interstage coupling methods instead.
You may still want at least one tuned stage near
the product detector to help reduce noise generated
in the IF strip, but a single parallel-tuned circuit
may well be adequate, which could be any sort
of coil, either slug-tuned, or using a trimmer cap.


A crystal-controlled converter should work well.
That makes stability less of a problem on the higher bands.


The additional loss of a crystal filter can be made up for
either at IF or AF.  It may just be a matter of turning
up the AF gain a bit.  Don’t worry about it until you
confirm that it is a problem.

WB8VLC

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Re: My First Superhet Build -- Planning
« Reply #46 on: January 21, 2022, 09:46:32 AM »

Pick up a nanoVNA and sweep the filter yourself then make some matching transformers to help in calculating the Zi/Zo of the new filter.

 I'm working on reviving my old Tempo One novice rig and it uses a  single SSB width ~2.7 K IF filter and I prefer CW so I'm adding a CW filter to it similar to what you are doing.


For me it's easy since the Tempo 1 uses a 9 MHz IF and I have an assortment of 9meg IF filters lying around from old hamfests purchases.

 The NanoVNA, for what I bought it for which was 49 dollars, has been the best piece of test equipment for this project.

I'm only using the NanoVNA with home brew transformers that will be wound to have a 50ohm Zin to whatever gives the best match and then I can work backwards and calculate the filters optimum Zi/Zo and put this into the Tempo 1 original IF strip with some mods.

My swap meet IF filters are over 40 years old, with markings that have worn off years ago and I'm also working in the dark with no specs for them.
« Last Edit: January 21, 2022, 09:51:33 AM by WB8VLC »
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KA4DPO

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Re: My First Superhet Build -- Planning
« Reply #47 on: January 21, 2022, 11:57:21 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)

Martin, try using two 3300 KHZ crystals in the filter circuit and place a gimmick capacitor, or a very small value mica compression cap across the lower one in the schematic.  You should be able to easily pull it 500 HZ

As for the front end, Carl is right, it really should have an RF stage, but it will work without it as long as you don't encounter any very strong signals.
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