Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: K3-schematic and 2-pole crystal-filter?  (Read 1853 times)
KM3K
Member

Posts: 295




Ignore
« on: June 25, 2013, 07:35:29 PM »

Hello,

I'm to give a talk to our club next month about "roofing-filters".
So, during my talk, I intend to show schematics for Yaesu's FT-5000 and Elecraft's K3; I choose these two because of their pristine locations on Sherwood's table.

I'd have no problem explaining the 5000's block-diagrams; seems to follow the "expected script".

But, with the K3's schematic:

a) on its block-diagram (pg. 64), the five roofing-filters seem to not be in the expected location of immediately after the 1st-mixer.
As I see it, the received signal-flow is...from the rec'r antenna, thru LC-bandpass-filters, attenuators, +12dB amp, then the 1st-mixer, a +17dB amp, the Noise-Blanker, finally the roofing-filters, and now an IF-amp, etc, etc. Am I interpreting the block-diagram correctly?

b) on both dwg K3-RF-board and dwg KRX3-sub-rec'r, between the 1st-IF and the 2nd-mixer, a 2-pole crystal-ladder-filter is labelled as having a BW=14 KHz. I'm mystified why this part is there, especially since any of the preceding crystal-filters would have a significantly narrower bandwidth. Hopefully, someone could explain this conundrum. Also, I was not able to locate it on the block-diagram.

That's about as far as I got trying to piece together an understanding of the K3-circuitry for now.
Thanks in advance for any replies.

73 Jerry KM3K
Logged
TANAKASAN
Member

Posts: 933




Ignore
« Reply #1 on: June 25, 2013, 09:13:04 PM »

The problem here is that some manufacturers have stretched the definition of 'Roofing Filter' almost to breaking point. It's always been my understanding that a roofing filter sits above the range of frequencies received, so, if your receiver covers 0-30 MHz and you have a 70 MHz first IF frequency then that filter is up in the 'roof'.

However if your first IF frequency is 4.915 MHz, or 10.7 MHz then you have a very strange construction method. Something like having a ground floor, then your roof, then a first floor. An IF filter at these frequencies is not a roofing filter, it is a first IF filter.

Discuss  Smiley

Tanakasan
Logged
KM3K
Member

Posts: 295




Ignore
« Reply #2 on: June 25, 2013, 11:30:16 PM »

The problem here is that some manufacturers have stretched the definition of 'Roofing Filter' almost to breaking point.
It's always been my understanding that a roofing filter sits above the range of frequencies received, so, if your receiver covers 0-30 MHz and you have a 70 MHz first IF frequency then that filter is up in the 'roof'.

However if your first IF frequency is 4.915 MHz, or 10.7 MHz then you have a very strange construction method.
Something like having a ground floor, then your roof, then a first floor.
An IF filter at these frequencies is not a roofing filter, it is a first IF filter.

Discuss  Smiley

Tanakasan

Actually, according to an Elecraft engineer, he feels it is; here is his article on that topic, which I'm copying from "http://www.elecraft.com/K3/Roofing_Filters.htm".

What "Roofing Filter" means to Elecraft

There's been so much discussion about this topic that I'd thought I'd better try to clarify why we used the term when announcing the K3.

A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages.
The narrower this filter is, the less exposure later stages will have.
Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain.

The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers.
Such receivers have an IF above the highest RF band covered; it's typically something in the range of 30 to 70 MHz or higher.
But "roofing" as a term should be interpreted as "protective," not "high in frequency."
A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms.
But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed.
It still provides the same protective function.

When we released the K2 in 1999, we never described our 1st IF crystal filters as roofing filters.
We had only one IF, so the receiver model was simpler; there were no narrow filters at later stages that required protection.

But in 2007, we find that the term is in widespread use.
Average hams now think of roofing filter bandwidths as the standard of comparison between receivers.
This is why manufacturers have jumped through hoops to try to provide the narrowest possible roofing filters.
Many operators have an understanding (justified) that a roofing filter that is wider than the communications bandwidth will not best protect the receiver's later stages.
So the term now seems appropriate to use even in a radio such as the K2, K3, or Orion, all of which use low-frequency IFs (5 to 9 MHz).

In recent years, the roofing filter has become the centerpiece of receiver redesign:
Suppose that manufacturer "A" initially designed their receiver to use a 15- or 20-kHz roofing filter.
Yes, this allows the receiver to handle NBFM and other wide modulation modes; it may also be selected to constrain the signal bandwidth ahead of a noise blanker or spectrum scope.
But it comes at a price.
If you're using CW mode, you'll have much narrower filters selected at the radio's 2nd and 3rd IFs.
Yet the 1st IF roofing filter allows a broad swath of signals into the earlier stages.
You don't need this energy in your passband.
It can cause trouble.

Manufacturer "A," realizing they have a problem with dynamic range at close spacing, then announces that they've had a breakthrough: they can now offer a 6-kHz, or more recently 3-kHz roofing filter.
This will certainly improve the situation for SSB and AM operation, but it still opens the barn door in CW or DATA modes, because the bandwidth is a factor of 10 wider than needed for communications. 

So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high?
It's because they can't make filters any narrower at such a high IF.

Enter the "down-conversion" rig (K2, K3, Orion, etc.).
By converting to a low first IF, the designer can easily create narrow filters that are compatible with the required communications bandwidth.
This is why we are offering filters with bandwidths as low as 200 Hz.

And yes, these are still "roofing" filters, because they limit exposure (bandwidth), thus protecting later stages (in the K3 case, the IF amp, 2nd mixer, and DSP).

73,
Wayne  N6KR  CTO, Elecraft, Inc.

Now, all of the above was very interesting reading to me when I started this project; however, my original questions, which deal with the K3's block-diagram and schematic, still remain unanswered.
73 Jerry KM3K
Logged
G3RZP
Member

Posts: 4501




Ignore
« Reply #3 on: June 26, 2013, 02:08:23 AM »

Some filtering can be desirable to reduce the noise into the second mixer from the preceding amplifiers. Historically, in IC IF stages, a lack of filtering can lead to poor performance in terms of signal to noise ratio, especially on AM. Also, you need to avoid image responses in the second mixer.
Logged
PD2R
Member

Posts: 131




Ignore
« Reply #4 on: June 26, 2013, 07:58:42 AM »

Why not ask Elecraft about how they designed their transceiver? They are very responsive to questions so I would bet you get your answer within 24 hours.
Logged
KM3K
Member

Posts: 295




Ignore
« Reply #5 on: June 26, 2013, 08:08:16 AM »

Why not ask Elecraft about how they designed their transceiver? They are very responsive to questions so I would bet you get your answer within 24 hours.
Earlier today, I had the same thought, went to their website and did email them; too soon to expect an answer back.
Thanks though for your reply.
73 Jerry KM3K
Logged
K7PEH
Member

Posts: 1125




Ignore
« Reply #6 on: June 27, 2013, 05:58:20 PM »

Earlier today, I had the same thought, went to their website and did email them; too soon to expect an answer back.
Thanks though for your reply.
73 Jerry KM3K

Join the Elecraft e-mail reflector (subscribe accessible on Elecraft web site) and ask your questions.  You have dozens (and more) of experts besides Wayne and Eric to answer your questions.  Well posed questions start accumulating answers within minutes during the prime time of the day.
Logged
WX7G
Member

Posts: 5987




Ignore
« Reply #7 on: June 27, 2013, 06:08:43 PM »

I am surprised that the K3 roofing filter is after the noise blanking. However, during 160 meter CW contesting with S-9 +50 dB stations less than 1 kHz away I detect ZERO noise blanker pumping; I cannot tell the station is there. That is why I bought a K3.
Logged
ZL1BBW
Member

Posts: 371




Ignore
« Reply #8 on: June 29, 2013, 12:20:44 AM »

Have a look at http://www.sherweng.com/documents/NC0B-Contest-U-2008-9.pdf  it may upset some people but that is life Smiley
Logged

ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
G3RZP
Member

Posts: 4501




Ignore
« Reply #9 on: June 29, 2013, 03:45:30 AM »

It would have been instructive for many to have shown more about the effects of phase noise, which limits the close in (and sometimes far out) performance.
Logged
KM3K
Member

Posts: 295




Ignore
« Reply #10 on: June 29, 2013, 06:14:52 AM »

Earlier today, I had the same thought, went to their website and did email them; too soon to expect an answer back.
Thanks though for your reply.
73 Jerry KM3K
Join the Elecraft e-mail reflector (subscribe accessible on Elecraft web site) and ask your questions.  You have dozens (and more) of experts besides Wayne and Eric to answer your questions.  Well posed questions start accumulating answers within minutes during the prime time of the day.
Thanks for the idea; just did that now.
'Part a' of my original post has been answered, so I need to know about 'Part b'.

Have a look at http://www.sherweng.com/documents/NC0B-Contest-U-2008-9.pdf  it may upset some people but that is life Smiley
I somehow stumbled across this a few days ago when I was at Sherwood's website and intend to use parts of it in my ppt next month.
Great as it is, it has too many slides for a 40-minute talk and the inevitable questions that come up during those 40-minutes.
73 Jerry KM3K
Logged
K8GU
Member

Posts: 718


WWW

Ignore
« Reply #11 on: July 01, 2013, 06:23:13 AM »

It would have been instructive for many to have shown more about the effects of phase noise, which limits the close in (and sometimes far out) performance.

This is the quote of the year.
Logged
KM3K
Member

Posts: 295




Ignore
« Reply #12 on: July 01, 2013, 07:25:00 AM »

I have an answer to "b" (for the 2-pole crystal-filter) provided by David G3YYD, who also made comments about "a".

Here is a copy of his answers to me:
A) NB is in that position so the hardware noise blanker works on a wider
bandwidth than the narrower roofing filters which can be as low as 200Hz.
Thus preserving the edges of the impulsive waveform.
Note that the K3 has two noise blankers one in hardware and the other in DSP.
Normally I find the DSP one does the best job but occasionally the hardware one is used either
in place of or in conjunction with the DSP one.
b) it reduces the noise bandwidth going into the 2nd mixer and thus into the ADC.
High gain IF stages generate wideband noise.
Excellent engineering practice.
Block diagrams should only show the key stages rather than every stage as they are the initial input before a human looks at the detailed
schematics.
Again excellent engineering practice.
73 David G3YYD

I do take issue with not showing the 2-pole crystal-filter in the block-diagram; IMHO, I think it proper to have it there.
So, as far as I'm concerned, the problems I posted here are answered.
However, I will be posting a new question as a result of reading a W8JI website this morning.
73 Jerry KM3K
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!