Re: could VFO frequency be below IF ?
- From: "AndyS" <andysharpe@xxxxxxxx>
- Date: 19 Jul 2006 06:39:25 -0700
aadu.adok@xxxxxxxxx wrote:
So I decided to go for 20m band. I had an option to set my VFO either
to 6mhz or 22mhz. I first went with 6mhz. But for my disappointment I
heard nothing but the band noise.
So I'm confused now - is it because the short "antenna" or am I wrong
thinking that VFO 6mhz + IF 8mhz should give me 14mhz?
any ideas?
Andy writes:
Sometimes 20m is deader than a doornail. Nothing.... Other times,
the band is hopping....... Try it again, maybe at dawn or dusk.....
If your system is working properly, you probly just caught 20m at
the wrong time. ...... Kind of like catching the refrigerator in the
middle
of a defrost cycle when you put your beer in to get it cold
....... you just gotta wait .... :>))))
I would also suggest that you get a mixer chart and get familiar
with
how to choose LO and IF frequencies for minimum spurious responses.
There is a mixer spur graph in Ref Data for Radio Engineers, and maybe
in the ARRL handbook.. Also , Minickts labs puts out spur response
charts for their popular mixers such as the SBL-1 and MD108 and it
shows which of the spurs are worst and allows you to adjust your
mixing frequencies accordingly.
The selection of a good frequency scheme is the STARTING point for
a receiver design. I know you are using stuff that you have on hand,
as all of us have done, but if you get familiar with frequency
management
you will be able to account for the signals you hear but don't know
why.....
It's usually due to spurious mixer responses....
Here's a couple rules of thumb, if you have NO OTHER WAY to get the
info you neet.
1) If the LO is either 9/10 of the received freq, ( or 1.1 times
the
received freq for top side injection,), most spurs are low
response.
2) If the LO is near .25 , .50 or .75 the channel freq,
spurs are really bad.
3) The image response is equal to the desired ---- use front end
filtering.
(assuming you don't use an image regect mixer)
4) The "half-IF" response 2LO+/-2RF = IF , is really bad,
always.....
Use front end filtering to get it down also...
Example: RF 7mhz LO 15 mhz Half IF is 11 mhz Image is
23 mhz
Those are just off the top of my head. The best deal is to pull out a
mixer
chart. Personally, I wrote a little BASIC program to go thru all the
combinations
up to the 10th harmonic and print out all the responses that occur in a
bandpass I could specify... it's easy to do and gives you exact
numbers......r
..... but the mixer graph can get you in the ball park quickly.......
Andy W4OAH
.
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- From: aadu . adok
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