[Elecraft] Sall bandwidth of the OP1 SSB filter

[John Grebenkemper, KI6WX]

Roland;
For the last month, I have been working with Eric and Wayne to arrive at a
wider SSB Xtal filter without causing any change in the KSB2 circuit board.
While theory provides some straight-forward ladder filter capacitor sizes to
achieve a wide range of response, practice turns out to be much more
difficult.  You have to pick capacitor sizes that are standard values (for
monolithic ceramic capacitors used in the KSB2 this is 10% values at a 5%
tolerance) and you have to analyze the circuit for capacitor and Xtal
variation.  These restrictions end up eliminating most bandwidth choices.  I
have some tentative choices, but I am still analyzing this to make sure they
will work over a wide set of component variations.  When we finish this, the
data will be published on my web site and I would assume it will also be
available on the Elecraft web site.  I hope to be able to put out several
sets of capacitor values so the builder can decide what bandwith SSB filter
they desire.

On the receive side,  you also have to take into account the response of the
second Xtal filter.  This filter effects the SSB response of the K2, and its
current response tends to narrow the receive bandwidth of the SSB filter.

Elecraft sorts the crystals by resonant frequency and bins them in 10 Hz
increments.  There is no control on the motional inductance and capacitance.
Your resonant frequency and motional inductance values are somewhat higher
than my measurements.  I would appreciate it if you would email me directly
your measurements on all of the individual crystals.  Also, did you directly
measure the Rs of the crystals?  I will add this to my data on the K2
crystals to help determine a reasonable estimate on crystal variation.

For the filter design impedance, you assumed a value of 1500 ohms.  In the
transmit mode, the load impedance on the filter varies from about 1600 ohms
to 900 ohms depending on the setting of the D1/D2 attenuator in the KSB2.
Since generally the attenuator is reducing the output, the impedance tends
to fall on the lower side of these values.  I've also made some measurements
on the NE602 that indicate that its impedance is somewhat less than 1500
ohms.  These variations will effect the filter response.

-John
 KI6WX


> My K2 #2379 is now completed as well as the KSB2 option board. Thanks to
> the fine construction
> notice (Heathkit style) and the numerous informations available on the
> Elecraft WEB site, I had no
> troubles to adjust it and to properly tune the BFO frequencies.
>
> While I still have not made any QSO, I have carried out many
> measurements which show the performances
> of my K2 (TX and RX) are always as good and often better that the
> Elecraft published data.
> As a designer and builder of many homebrew high performance ham
> receivers, I have been specially
> impressed by the measured IP3 and dynamic range reached in such a small
> package.
>
> I am now concerned with a problem related to the IF bandwidth of the SSB
> filter used in the KSB2.
> This BW(-6dB), measured with Spectrogram or in the transmit mode, is
> only about 1.75kHz, a small value
> compared to the 2.2kHz published BW. In order to confirm my home
> measurements I conducted
> some tests on the filter alone with an HP Vector Network Analyzer and
> obtained the same results.
>
> Before building the KSB2, I measured the crystals parameters and
> obtained the following
> data (they are the averaged values of the 7 crystals but the dispersion
> is rather small):
> Series resonance frequency : Fs=4.913866 MHz
> Motional inductance # 80.8 mH
> Motional capacitance # 0.013 pF
> Unloaded Q : Qu # 130000
> Parallel capacity : Cp # 4 pF
>
> In the past few years I have build many ladder crystal filters (5 to 12
> poles CW, SSB and noise
> filter) using the tools and methods provided by Wes Hayward W7ZOI and
> others. Thus I simulated the
> KSB2 crystal filter using these tools and the ANSOFT Harmonica software
> (the "big brother" of the ARRL
> Radio Designer program) : I obtained a 1.8kHz BW with the above crystal
> parameters.
>
> I then computed the capacitors needed for a 0.5dB 2kHz Chebycheff ladder
> filter (1500 ohms I/O
> impedances)and get values different from those of the KSB2: moreover, as
> the 7 crystal frequencies
> have very small dispersion (delta F max=10Hz), I find that I need more
> serial capacitors in the meshes than the
> KSB2 PCB can accept.
> I am now thinking about building a piggyback PCB for an eventual new
> filter but before, I would be
> glad to receive comments about my problem. A feedback from the Elecraft
> K2 designers would be particularly
> appreciated.
>
> 73's
>
> Roland Cordesses/ F2DC
> K2 # 2379
>
>
>
>
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