Checking crystal filters,

[Mike Scott-7]

I received a bad 6 KHz filter and it was replaced immediately upon my
discovery. In my case the upper and lower sidebands had significantly
different pass band characteristics so it was easy to tell by ear and then
measured with "Spectrogram" software on my PC.

I have a 500 Hz filter and would not know how to test it for IMD. I don't
have the test equipment to support the test. Does someone know of something
simple that can be done to check roofing filters?

Mike Scott - AE6WA
Tarzana, CA (DM04 / near LA)
K3-100 #508/ KX1  #1311


----------------------------------------------------------------------
Date: Wed, 18 Jun 2008 15:16:19 -0400
From: Jerry Flanders <[hidden email]>
Subject: Re: [Elecraft] RSGB RadCom K3 review posted on RSGB Members
        web page
To: "Eric Swartz - WA6HHQ, Elecraft" <[hidden email]>
Cc: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 02:44 PM 6/18/2008, Eric Swartz - WA6HHQ, Elecraft wrote:
>Looks like he clearly had a bad 500 Hz filter. It does not match our
>measurements here or Sherwood's.  We'll make sure the customer gets
>a replacement.

Just curious - do you guys check filters before shipping them? Most
of us out here wouldn't have a clue if we had a bad filter - we would
just suffer with poor performance without ever knowing why.

Jerry W4UK

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[AD6XY]

Traditionally this is a particularly hard measurement to make unless the filter performance is very poor.

Without professional test gear you will need to obtain or build two very high quality signal sources, (say identical 7MHz crystal oscillators) at least one of which needs to be tunable. These have to be buffered and then very carefully combined. I think there was a design in the ARRL solid state design book back in the 1980s. Then you can look for the 3rd order IMD products at audio with something like Spectraview - hoping that your sound card IMD performance is better than the radio, which it probably is not if the radio is working.

You need test sources with very low phase noise, more than 100dB down at the spacing you are testing at. A typical signal generator is probably not good enough for this measurement. Well designed crystal oscillators should be OK.

Maybe it is easier just to use the radio. When listening, if you can not detect any intermods, then you don't have a problem with them.

Mike

[Brett Howard]

Doesn't the radio have a 2-tone generator built into it?

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of AD6XY - Mike
Sent: Thursday, June 19, 2008 8:12 AM
To: [hidden email]
Subject: Re: [K3] [Elecraft] Checking crystal filters,


Traditionally this is a particularly hard measurement to make unless the
filter performance is very poor.

Without professional test gear you will need to obtain or build two very
high quality signal sources, (say identical 7MHz crystal oscillators) at
least one of which needs to be tunable. These have to be buffered and then
very carefully combined. I think there was a design in the ARRL solid state
design book back in the 1980s. Then you can look for the 3rd order IMD
products at audio with something like Spectraview - hoping that your sound
card IMD performance is better than the radio, which it probably is not if
the radio is working.

You need test sources with very low phase noise, more than 100dB down at the
spacing you are testing at. A typical signal generator is probably not good
enough for this measurement. Well designed crystal oscillators should be OK.

Maybe it is easier just to use the radio. When listening, if you can not
detect any intermods, then you don't have a problem with them.

Mike
--
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