It is a compromise to get rid of problems with images. With the
original minima, the IF was near the middle of the coverage range. This
presented several problems. One was difficulty of getting the desired
selectivity with a home made crystal filter at this high a frequency .
Another was that you had to notch out the IF frequency from the input
signals, and you mostly lost one of the HF bands. Then there are
always problems with images. Going to a higher IF allows you to have
continuous coverage in your desired tuning range. It eliminates most
problems with images. Then the second conversion brings you down to a
lower frequency where it is fairly easy to build a crystal filter with
the desired selectivity.
There has been some comment about using a 45Mhz first IF. This is a
common IF used in several VHF transceivers, and there are fairly
inexpensive 15Khz bandwidth crystal filters available in the surplus
market that would eliminate the problems with trying to align the first
IF LC filter. One comment I saw suggesting using a higher first IF had
to do with the roll off of the input low pass filter. This can easily
be taken care of by going to a slightly different configuration for the
low pass filter. I have not checked the values, but it is either a
Butterworth or Chebyshev inductor input low pass filter. Going to
something like a M-derived filter will give you the flat response to 30
Mhz. desired and more than adequate attenuation at 45Mhz. Download a
copy of Elsie filter software and play with some different filters and
see the difference.
When I get a couple other projects finished, I will probably build a
general coverage receiver based on the HF-1 design. Don't really need
another transceiver right now, but a GC receiver makes a very useful
piece of test gear.
DuWayne
KV4QB.blogspot.com
On 7/25/2016 11:36 AM, Mark Smith (Redacted sender smittyhalibut for
DMARC) wrote:
Thank you Ben for that. I looked at the schematics and understand them. I've been a ham since 1991, but am just getting into RF electronics and circuit design. Schematics like that one make it so much easier to understand what's going on. Thank you!
One question, that's not specific to your design. I've never understood the benefit of more than two conversions, one IF. Why does the HF1 convert to 45MHz, loosely filter with LC, then convert again to 10MHz and filter with crystals? How is that better than converting to 45MHz, filtering with crystals, then converting directly to base band?
There are also a couple more amp stages in there, but those are probably just to make up for losses in the filters and conversions, and could be added anyway without the extra conversion.
To be clear: I'm not saying you're wrong, I'm totally ignorant and asking for education. :-)
Thanks!
-Mark
On Jul 25, 2016, at 7:23 AM, Ben Aupperlee <beninturkye@xxxxxxxxx <mailto:beninturkye@xxxxxxxxx>> wrote:
Hello Mark,
There is now an HF1, with a high first IF of 45 MHz and even a newer version where i am working on with an if of 60 MHz, with better filters.
Regards Ben (in Turkije)
-------- Origineel bericht --------
Van:"Mark Smith (Redacted sender "smittyhalibut" for DMARC)"
Verzonden:Mon, 25 Jul 2016 17:15:29 +0300
Aan:minima@xxxxxxxxxxxxx <//freelists.org>
Onderwerp:[minima] Re: for minima project
Forgive my ignorance. "The old minima project?" Is there a "new minima project?"
This is the one I've been working off of:
http://www.phonestack.com/farhan/minima.html
Is there something else I should be looking at too, or instead?
-Mark
On Jul 25, 2016, at 5:33 AM, allison <ajp166@xxxxxxxxxxx <mailto:ajp166@xxxxxxxxxxx>> wrote:
On 7/25/16 4:19 AM, Computer Burgas wrote:
SI570 and the 5331 all provide a square wave output which is rich in harmonics. The frequency counter may be
In old minima project, after series of tests I found that Si570 has too many harmonics and frequency counter reported me them rather than the fundamental frequency, which means that the mixer and quartz filter probably being down work properly.
such that it has higher sensitivity at harmonic frequencies or your not coupled adequately.
Also the cost of Si570 in ebay is 2-3 times higher than , for example - AD9851. Moreover BFO also realized unstable frequency. I decided to work towards replacing VFO and BFO with the 2 pcs of AD9851 DDS generators. So with this I will get freq accuracy and easy programmability of LSB and USB in BFO and VFO.From cost perspective ok. However you need a micro to set it and DDS have spurs. Minimally the DDS will require gain to drive the DBM adequately.
Old minima project will still require better filters and more of them to avoid out of band unwanted signals.
Allison
