Richards,
(I apologize to the group; this is a very long post. Delete it now if that
bothers you. - Rick)
It has always been the tradition that if you want to have a general coverage
(0.5 to 30 MHz) RX, you place your first IF high above 30 MHz. (Always
begins in the 70's. It was different before that.)
Several radios have it at 70 MHz, some at 60 MHz and a few even as low as 40
MHz.
The problem is, at these high frequencies, you can't build as good of
crystal filters as you can as with filters of lower frequencies (for
instance 9 MHz or 5 MHz, where earlier ham-band-only radios had their first
IF).
When I say "not as good as", that is an understatement!
Basically, it is the steepness of the bandpass curve which is worse.
This significantly affects the ultimate attenuation of the filter.
The measurements that are affected by this are the close in Blocking Dynamic
Range (5 KHz or less), not the wide ones at 20 KHz.
Without going back to the 50's or 60's, starting in the 70's, ham band
radios typically used 9 MHz first IF or 5 Mhz first IF (earlier ones used
455 kHz). At any of these frequencies, it was possible to build 4-pole or
8-pole crystal filters which had very sharp (steep) bandpass curves.
Filters at much higher frequencies had very wide bandpass curves.
Since all of the JA sourced radios had a high first IF, with upwards
conversion, it was thought this is the way to go.
In doing so, you were able to build a good general coverage receiver, but
you sacrificed ham band performance by having an inferior (broader skirts)
first IF filter.
The Orion I was the first radio to come out with a compromise.
It used two receivers, one of each.
The main RX had downwards conversion, which had outstanding performance
results.
The secondary RX had upwards conversion, which had good, but not outstanding
performance.
This gave us the best of both worlds.
The JA companies responded with marketing, saying they had two identical
good receivers.
The expensive ones were very good but worse than the Orion, and they were
certainly more expensive.
The reason: you just cannot obtain the ultimate results using upwards
conversion.
With the Omni VII, Ten-Tec challenged the existing theory.
You could almost say, they bent the rules.
By paying VERY close attention to individual stage gain in the front end of
the RX, they were able to use upwards conversion up front, then downwards to
9 MHz with sharp (roofing) filters at this IF. After that, good DSP filters
(which everyone has now days) provided the necessary sharp filtering.
The results were surprisingly good. Very good. Better than we expected.
BUT NOT AS GOOD AS A DOWNWARDS CONVERSION RECEIVER!
With the K3, elecraft really bent the rules.
They were the first ones to use downwards conversion, yet still offer
general coverage RX.
BUT not so fast, in order to be able to use general coverage, you had to buy
an optional 6 kHz roofing filter AND an optional Band Pass Filter (BPF).
THEN you got general coverage to 30 MHz... but, since it's first IF is at
8.215 MHz, you cannot receive signals that are close to that frequency.
They will be drowned out by the radio's own local oscillator. That's one
reason why they also say degraded performance on some frequencies.
With the Eagle, there is no indication what the Band Pass Filters will be
like for General Coverage.
All I can say for sure is that it won't be able to hear well on or about 9
MHz, because the radio's own 9 MHz local oscillator will drown out whatever
external signal is there.
Let's say Ten-Tec, for cost reasons, did not provision any special BPF for
GC.
Without the preselection, receiver performance degrades.
Work-around: purchase a general coverage preselector, such as those sold by
MJF, which were "borrowed" from the Bavarian Contest Club (without
compensation).
This solves your preselection problem, and all that remains is the degraded
performance at 9 MHz. So what? There is nothing on 9 MHz anyway.
My biggest question to you Richards, is, why do you even want a general
coverage receiver in your main ham rig?
There are so many tade-offs that it simply is not worth it.
Buy yourself a good used GC RX for $300, or one of these new SDR radios for
a few dollars more, and keep it independent of your ham radio transceiver.
If you insist on having the GC RX within your transceiver, unless you pay
$4000, you will have degraded performance on the ham bands !!!!!!!!!!
You just can't win the Indy 500 in a Chevy Malibu.
Hope this helps.
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of Richards
Sent: Saturday, September 25, 2010 9:20 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] The Eagle
OK... thanks for the uptake.
Do they do this on the Omni VII ? Reason I ask is, I am a former SWL
interested in receiver radios before joining the ham clan, and the Omni
VII has a great gen coverage receiver which is seems so superior to my
previous "better" Kenwood and other SW receivers that if SW performance
outside the ham bands is equally good on the Eagle as it is on the Omni
VII - it will be plenty good.
Parenthetically, I appreciate your taking a moment to explain these
things, as it is a real handicap to have come to ham radio later in
life, after a career in something other than engineering, and there is
so @#$% much to learn about how these radios work. That is the
boon and bane of the hobby -- boon because I enjoy learning stuff, and
bane because there is so much to learn!
Guys like you, Gerry, Carl, Bob, Jim B, Ken, Art, and so many others
have proven good Elmers.
Thanks.
================================= JHR ===============================
On 9/25/2010 21:54, Barry N1EU wrote:
> It seems a no-brainer that they're using bandpass filters for the ham
bands.
> ____________________________________________________________________
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|