<p dir="auto">My issue is about ...</p>
<p dir="auto">Sos output is incorrect for analog filter design. I found a comment in fitler_design.py stating as much, but I would expect an error or clearer documentation if this is a known limitation. Instead the code below will generate incorrect SOS coefficients. The combined SOS result in a  higher order analog highpass fitler, not a bandpass.</p>
<p dir="auto">For this this ticket I suggest adding a simple exception to the library code to make this obvious. After which, we can open another ticket to add the feature of correct SOS analog filter design.</p>
<h4 dir="auto">Reproducing code example:</h4>

<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="BWsos = signal.butter(order,np.array([flow, fhigh]),btype='bandpass', analog=True, output='sos')"><pre class="notranslate"><code class="notranslate">BWsos = signal.butter(order,np.array([flow, fhigh]),btype='bandpass', analog=True, output='sos')
</code></pre></div>
<h4 dir="auto">Scipy/Numpy/Python version information:</h4>
<p dir="auto">version 1.5.4</p>
<p dir="auto">As described in <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="35173341" data-permission-text="Title is private" data-url="https://github.com/scipy/scipy/issues/3717" data-hovercard-type="pull_request" data-hovercard-url="/scipy/scipy/pull/3717/hovercard?comment_id=142460019&amp;comment_type=issue_comment" href="https://github.com/scipy/scipy/pull/3717#issuecomment-142460019">#3717 (comment)</a>, zpk2sos does not work for analog filters since the coefficients of each stage are different.</p>
<p dir="auto"><code class="notranslate">b, a = butter(1, 1, analog=True)</code> produces b = [1], a = [1, 1]  → H(s) = 1/(s+1)</p>
<p dir="auto"><code class="notranslate">sos = tf2sos(b, a)</code> produces <code class="notranslate">[1, 0, 0, 1, 1, 0]</code></p>
<p dir="auto">this stage is b = [1, 0, 0], a = [1, 1, 0] → H(s) = s^2/(s^2+s) = s/(s+1) which is not the same as above.  For digital filters, trailing zeros don't matter, but for analog filters, they do, and <em>leading</em> zeros don't matter.  The correct result should be <code class="notranslate">[0, 0, 1, 0, 1, 1]</code></p>
<p dir="auto">So docstring should probably say it's not meant for analog at all for now?</p>
<p dir="auto">and add analog=True to zpk2sos?</p>