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1-7 of 7 messages
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DeMaw's Simple Superhet
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by KA5KAB on October 1, 2009
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Hi All,
I recently built the simple superhet receiver from Doug DeMaw's QRP Notebook and it works surprisingly well. I would like to use this receiver on other bands, mainly 40 and 30 meter CW. To accomplish this he suggests using crystal controlled converters ahead of the receiver.
Being fairly new to the homebrew game I am having some trouble understanding how the converter would work. From what I understand the converter would take the incoming signal, say 7 mhz, and convert it to 3.5 mhz which the receiver is set up for (the original design was for the the phone portion of the 80 meter band, but I moved it down into the CW portion of the band). Am I correct in this assumption?
He calls for using an overtone crystal oscillator fed into a dual gate MOSFET which would mix it with the incoming 7 mhz signal and that is fed to the input of the receiver. So, if I am correct I would need a 3.5 mhz crystal or a 10.5 mhz crystal for the converter's oscillator. I am thinking that the 3.5 crystal wouldn't be a good choice since the receiver is set up to receive that frequency.
My question is, could I use a LC tuned circuit in place of the crystal, since I am not sure I can get crystals for the frequencies I need for the converter at a reasonable price, or can someone suggest another method for converting the desired bands (40 and 30 meters) to the 80 meter band? Also, using a converter would I set it up for the lowest frequency of the band or the highest? How does this affect receiver tuning?
I have included a link with all the info on the receiver and converters:
http://www.oselectronics.com/downloads/Design%20tips%20for%20Simple%20Superhet%20Receivers.pdf
Thanks in advance for any help and advice,
73
Carl KA5KAB
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RE: DeMaw's Simple Superhet
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by N2EY on October 1, 2009
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You don't want to use an LC oscillator unless you absolutely have to. The reason is stability; LC oscillators are harder to stabilize, thermally and mechanically, and the situation gets worse as you go up in frequency.
To choose a crystal frequency, you just need to do simple addition and subtraction. Which is all the converter really does.
Suppose we have a basic 80/75 meter receiver that tunes 3.5 to 4 MHz and we want to listen to other bands. The xtal oscillator can be above or below the band we want to convert.
Using 40 meters (7.0 to 7.3 MHz) as an example:
Using a below-the-band crystal:
7.0 MHz - 3.5 MHz = 3.5 MHz
7.3 MHz - 3.5 MHz = 3.8 MHz
Using an above-the-band crystal:
11.0 MHz - 7.0 MHz = 4.0 MHz
11.0 MHz - 7.3 MHz = 3.7 MHz
The 3.5 MHz xtal wouldn't be the best choice because at the low end you'd hear it very loud. The 11.0 MHz xtal makes 40 tune "backwards" compared to 80.
Note that since 40 is narrower than 80, there are other possibilities. For example:
Using a below-the-band crystal:
7.0 MHz - 3.3 MHz = 3.7 MHz
7.3 MHz - 3.3 MHz = 4.0 MHz
Using an above-the-band crystal:
10.8 MHz - 7.0 MHz = 3.8 MHz
10.8 MHz - 7.3 MHz = 3.5 MHz
Note that the 3.3 MHz example moves the oscillator out of the band.
Other bands are left as an exercise for the reader.
Inexpensive crystals show up at places such as Mouser, Hosfelt and Dan's Small Parts. 11 MHz is a pretty common one.
Any questions?
73 de Jim, N2EY
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RE: DeMaw's Simple Superhet
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by WB6BYU on October 1, 2009
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There have been a number of converter circuits published
over the years. Some receivers in the ARRL Handbook
(quite possibly including one by DeMaw) were based on a
single band receiver with switched converters for the
higher bands. There may even be a circuit in his book -
I haven't looked through it recently.
A simple receive converter could use the SA612 chip, which
includes both oscillator and mixer in an 8-pin package.
The strong signal performance isn't wonderful, but it is
a quick and easy circuit to build.
Most converter circuits include an RF amp, crystal oscillator,
and mixer. For low HF the RF amp may not be needed. A
simple implementation (converting down to the AM broadcast
band) appears here:
http://home.alphalink.com.au/~parkerp/projects/canning.htm
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RE: DeMaw's Simple Superhet
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by WB6THE on October 1, 2009
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Instead of a crystal or an LC oscillator you could use a
digital VFO with offset capability so that you read the
desired receive frequency on its display but the VFO actually
generates the required sum or difference frequency. I'm doing
just that with a simple superhet that I've built. The VFO that
I use is from N3ZI at www.pongrance.com
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RE: DeMaw's Simple Superhet
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by N2EY on October 1, 2009
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I just checked the Hosfelt website
www.hosfelt.com
They have 11 MHz and 18 MHz xtals for under a dollar each.
11 MHz will convert 40 meters and 18 MHz will convert 20 meters with a receiver that tunes 3.5 to 4 MHz. Both bands will tune "backwards" but that's not a big problem, really.
---
The N3ZI DDS VFO looks really interesting! Thanks for posting the info.
73 de Jim, N2EY
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RE: DeMaw's Simple Superhet
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by KA5KAB on October 10, 2009
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Just wanted to thank everyone for their suggestions and input. It has been very helpful and also educational.
73,
Carl KA5KAB
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